digital photography and printing · ing of a gas through a small opening into a relative vacuum....

®FEBRUARY 2006

Digital Photographyand Printing

Also Inside:

Your vintage car couldrust away!

CMatter02.06 1/24/06 10:37 AM Page 21

Q: My parents just bought new tires fortheir car, and the salesman told them theyshould inflate them with nitrogen gasinstead of regular air. Is paying extra fornitrogen gas in car tires worth it?

A: This is a very intriguing question. My ini-tial reaction is to assume that salespeople arejust looking for one more way to squeeze abuck out of gullible customers. After all, air is78% nitrogen. Why would inflating your cartires with pure nitrogen make that big of a dif-ference? But this question is particularly trickyfor me because it pertains to chemistry, whichI have taught for the past 22 years. And cars,though I have driven for the past 29 years, Iknow next to nothing about. I do, however,know where to go for advice about cars: thefamous Magliozzi Brothers, Tom and Ray,a.k.a. “Click and Clack, the Tappet Brothers”!(Their NPR radio show “Car Talk” is aweekend favorite.)

Their Car Talk website(http://www.cartalk.com) revealsthat they have addressed the nitrogenquestion quite a few times, mostrecently in April 2005. Their advice:For the average driver, filling tires withnitrogen gas is not worth the addedcost. Still, for a chemist, the “nitrogenin tires” question has an intriguing mixof silly and plausible claims swirling about it.

Race car drivers do it. Why? Air con-tains about 78% nitrogen, 20% oxygen andsmall quantities of other gases, including,most importantly, water vapor. On humiddays, air contains considerably more water

vapor than it does on dry days. What's more,because of the extensive hydrogen bonding,water vapor deviates quite dramatically fromideal gas behavior. Thus, as it heats up, anair-filled tire tends to show much more incon-sistency in pressure changes than does a tirefilled with pure nitrogen gas. In car racing,contact between the tire and the track iseverything, and a fraction of a psi (poundsper square inch) in the tire can make the dif-ference between winning, losing, or spinningout of control. NASCAR mechanics figuredout years ago that the pressure in a nitrogen-filled tire was far more consistent and reliablethan the pressure in air-filled tires. Does thatnecessarily mean that the average Joe orJane on the street should swap out the air intheir tires for nitrogen gas? Is it worth spend-ing the extra money? Again, probably not.

A large number of web sites disagree.But, consider this: These sites tend to be theones selling nitrogen. As such, the informationthey offer should be scrutinized and not merely

accepted as factual.One argument is

that nitrogen is far lessreactive than oxygenand thus does lessdamage to the insideof the tire. This all maybe true, but it has beenmy experience that tiretreads wear off longbefore any kind of

deterioration from the inside could make a dif-ference. Maybe nitrogen causes less oxidationof the wheel rims? But again, is this really evenan issue?

They also contend that nitrogen-filledtires dissipate heat more quickly and thus run

at a lower temperature. Unless this effectcan be attributed to the water vapor, itmakes very little sense, since nitrogen andoxygen have very similar thermal conduc-tivities (0.02583 and 0.02658 W/mKrespectively) and specific heat capacities(29.1 and 29.4 J/molK, respectively).

2 ChemMatters, FEBRUARY 2006

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COVER PHOTOGRAPHY BY MIKE CIESIELSKI

By Bob BeckerQuestion From the Classroom

http://chemistry.org/education/chemmatters.html

COREL

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All the same, one of their more convinc-ing arguments pertains to nitrogen's ability tomaintain tire pressure. Nitrogen reportedlyleaks out of tires more slowly than air. If youhave discussed Graham's law in chemistryclass this year, then you would probablyexpect the opposite to be true. Because nitro-gen has a somewhat lower molecular weightthan oxygen, its average molecular velocityshould be greater and it should thereforeescape or effuse through a small hole morereadily. But effusion has to do with the pass-ing of a gas through a small opening into arelative vacuum.

For the leaking out of a gas through athick permeable layer of rubber, there is some-thing else to consider. Molecular size andpolarity play important roles too.

Gas permeability of rubber is believed toinvolve dissolution of the gas in the polymer,diffusion through the thickness of the rubber,and loss on the other side. The N-N bond ismuch shorter than the O-O bond, and thehighest-energy electrons of O2 are more polar-izable than for N2 (O2 has a higher boilingpoint than N2 for much the same reason). Thegreater polarizability of oxygen should make itslightly more soluble than nitrogen in rubbers.(For example, oxygen is more soluble inorganic solvents than is nitrogen.)

This would mean that a nitrogen-filled tireshould hold its pressure longer than one filledwith air. How much longer? Here the websitesdisagreed, and it is important to note that notone of the commercial websites actually citedany scientific studies in their arguments.

An issue that far outweighs the nitrogenissue is this: An estimated 80% of the tires onthe road today are not properly inflated; they’reeither underinflated or overinflated. Both con-ditions can shorten the lifetime of a tire.

Underinflationlowers gasmileage andover inflationcan danger-ously lowerthe amount oftraction yourcar has on theroad. Drivers

should check their tire pressure every monthor so, and add or remove air as needed.

®

Vol. 24, No. 1 FEBRUARY 2006

ChemMatters, FEBRUARY 2006 3

Question From the Classroom 2Is paying extra for nitrogen gas in car tires worth it?

ChemSumerThe Chemistry of Digital Photographyand Printing 4Once upon a time, people put stuff called film in the

their cameras. First, they paid for it. Then they took

photos, but couldn’t view them on a screen. No delet-

ing, no computer editing—they paid for strangers to

develop every miserable photo, hoping that a few were

OK! So primitive! So last-century!

ChemMysteryReal or Fake? The James Ossuary Case 8Is it a priceless artifact? A bone box with a controversial

inscription, is put to the test—a test that includes 18O isotopic

analysis.

Super Fibers 11Carbon nanotubes could be the key to spinning the future’s

hottest threads.

Salting Roads: The Solution forWinter Driving 14In northern parts of the United States, salt is the

winter miracle that keeps life moving. But after the

ice is cleared, you’ll find crystals of it on the ground or

coating the undersides of cars. What’s the chemistry

involved?

Flaking Away 17From Ferraris to Ford Pintos, nearly every car fights a losing

battle with rust.

Chem.matters.links 20

CMTEACHERS! FIND YOUR COMPLETE

TEACHER’S GUIDE FOR THIS ISSUE ATwww.chemistry.org/education/chemmatters.html.

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4 ChemMatters, FEBRUARY 2006 http://chemistry.org/education/chemmatters.html

Imagine needing eight hours totake a single photograph! That’s how long it tookFrench scientist and inventor Joseph Niepce to take theworld’s first photograph in 1826. And the end resultdidn’t win any prizes—it was a

grainy image of some buildings viewedfrom a third-floor window. We havecome a long way since then! Today, anyamateur photographer can produce aglossy full-color photo in a matter ofminutes using a digital camera andcomputer. In just the past 10 years, dig-ital photography has taken the world bystorm, threatening to do to film whatthe DVD has done to video.

Most of your family photos were probably taken theold-fashioned way, with film that had to be taken to aphoto shop to be developed. There is a fascinatingbunch of chemistry involved in this process. All photo-

graphic film is coated with a thinlayer of a silver halide compound,such as silver bromide (AgBr).When light strikes this layer, animage is recorded on film, which ismade visible during the developingprocess. If you have ever beeninside a darkroom, you have proba-bly seen all sorts of mysteriouschemicals such as developers, fix-ers, and baths. Even if you don’t

The Chemistry ofDigital Photographyand Printing

ChemSumer

Once upon a time, people putstuff called film in the their

cameras. First, they paid for it.Then they took photos, butcouldn’t preview them on a

screen. No deleting, no computerediting—they paid strangers todevelop every miserable photo,

hoping that a few were OK! So primitive! So last-century!

By Brian Rohrig

The world’s first photograph!

CMatter02.06 1/24/06 10:37 AM


quite know how it all works, you can stillappreciate the fact that a lot of chemistrygoes into developing pictures.

Does digital processing mark the end ofchemistry in photography? There is actuallyplenty of fascinating chemistry going on—it’sjust on a much smaller scale.

Sensing lightAll cameras work by focusing light

through lenses to create an image. A conven-tional camera records this image on film. A dig-

ital camera records thisimage on a permanent partof the camera known as asensor. A typical sensor ina digital camera measuresonly 4.4 mm × 6.6 mm.This is about the size of afingernail.

Sensor technologyhas enabled manufacturersto make digital cameras sosmall they can even beincorporated into cellphones. Similar sensordevices are used in faxmachines, scanners, copymachines, and bar codereaders at the grocerycheckout.

The sensor is a semi-conductor. Silicon is thematerial of choice for mostsemiconductors. This isironic because siliconbarely conducts electricityat all in its pure form. But

if a small amount of impurity is added,through a process known as doping, then sili-con becomes a fair conductor of electricity.

The sensor in a digital camera com-prises many tiny semiconductors known asdiodes. Diodes allow current to flow in onedirection, but not another. Diodes are com-posed of two different types of doped siliconlayers sandwiched together. One type of sili-con is doped with phosphorus or arsenic.Both of these elements contain five valenceelectrons. Because silicon atoms only havefour valence electrons, the doping agentsprovide the extra electrons that movethroughout the material. With its excess ofelectrons, this type of silicon is known as n-type, with the “n” referring to the negativecharge resulting from the free electrons.

Another type of silicon isdoped with either boron orgallium, which only have threevalence electrons. These dop-ing agents create a deficiencyof electrons in the structure,since silicon atoms have fourvalence electrons. This elec-tron deficiency creates elec-tron “holes” in the structure.Silicon doped with these defi-cient atoms is referred to as p-type silicon, with the “p”standing for the positivecharge resulting from the defi-ciency of electrons.

When placed together,these two types of silicon forma diode, the one-directionalconductor described above.Think of a diode as a one-waystreet for electrons. At the p-njunction, a positive chargebuilds on the n side, and a neg-ative charge on the p side untilthe internal electric field counteracts the ten-dency of the electrons to fill the holes. Theinternal electric field then permits current topass in one direction.

PhotositesEach diode in a sensor is a photosite.

Each photosite represents one picture ele-ment—better known as a pixel. The greater

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As more and more holes are filled with electrons, a region, neitherP nor N, forms, called the depletion zone. Holes are shown as .Electrons are shown as .

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SuperCCD SR structure diagram, one microlens, one color filter, two photodiodes per photosite.

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http://chemistry.org/education/chemmatters.html6 ChemMatters, FEBRUARY 2006

the number of pixels, the greater theresolution and overall quality of thepictures you take. For example, a typi-cal digital camera may have a resolu-tion of 640 × 480 pixels, for a totalresolution of 307,200 pixels. The bestdigital cameras on the market todayhave a resolution of more than 10 mil-lion pixels (10 megapixels). For com-parison, you should take pride in yourpersonal sensor. The human eye con-tains 120 million pixels!

The pixels of any photo can beclearly seen through the low power ofa microscope. The larger the pixel sizein a photo, the poorer the quality, aslarger pixels mean fewer pixels withina certain area. If you compare a normalcolor photo with a newspaper photo,you can see a huge difference in pixel size.Newspaper photos will have larger pixels,representing poorer quality.

When you take a picture with a digitalcamera, each tiny photosite on the sensor isexposed to light. When a photon is absorbedby the semiconductor, it promotes an elec-tron to a higher energy level. What thismeans is that the high-energy electron actslike an electron that was added by doping: Itis free to move about the semiconductor.Normally, the electron would just relax backto its lower-energy state. However, if it isnear the p-n junction, it is attracted to thepositive side, and migrates there, where it iscollected.

As more photons strike a photosite,more electrons are knocked free. The greaterthe intensity of the light that strikes a photo-site, the more electrons accumulate. A usefulanalogy is to think of the photosite as a tree,the photons as balls that you throw into thetree, and the electrons as leaves on the tree.Suppose that every time you throw a ball intoa tree, a leaf is knocked loose. The more ballsthat you throw into the tree, the more leaveswill accumulate on the ground below. A pho-tosite that has been exposed to very brightlight will contain far more electrons than onethat has been exposed to dimmer light. Gen-

erally, each image sensor can record 256 dif-ferent shades of gray, ranging from purewhite to pure black.

Seeing in colorSo then, how do digital cameras take

color photos, if the sensors can only recordshades of gray? The trick is to use filters, thatcombine to produce any color imaginable.Most cameras use the 3-color system to pro-duce color. The three primary light colors arered, green, and blue. Together, these threecolors make white. Any other color can beproduced by mixing together various shadesof these three colors. To accomplish this feat,

either a red, blue, or green filter is placed overeach photosite on the sensor of a camera. Themost common pattern is known as the Bayerpattern, which alternates a row of red andgreen filters with a row of blue and green fil-

ters. This configuration gives you twice asmany green filters as blue or red. Because thehuman eye is not sensitive to all three colorsequally, extra green filters must be used toproduce the best color for our eyes.

Next, the information at each of thesephotosites is converted to digital form. Bythemselves, electrons that accumulate at eachphotosite do not represent digital informationthat can be read by your computer. So everydigital camera carries its own built-in com-puter that converts information to digital formand stores it on your memory card.

PrintingOnce an image is recorded digitally by a

camera and downloaded onto a computer, itcan be printed. Or, it can be manipulatedusing software on a computer and thenprinted. The ability to choose, alter, and cropphotos on screen before printing gives even acasual photographer unprecedented power toprint only the images they want.

There are two basic types of printersthat can print photos: laser and inkjet. Thelaser printer works by using static electricity.The underlying principle involves positivelycharged toner sticking to negatively chargedpaper, since opposite charges attract. A laser

Information from photosites is converted to digital form and stored on memory cards for later retrieval.

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beam projects a negatively chargedimage of whatever is to be printedonto the light-sensitive drum. Thedrum is then coated with positivelycharged toner, which is attracted tothe negatively charged image on thedrum. An analogy would be writing amessage on the outside of a coffee canwith glue, and then rolling it in flour. Theflour will stick to the glue but not to the“unglued” parts of the can.

A piece of paper then passes over acharged roller, giving it an even strongernegative charge than the drum. The drumthen rolls over the sheet of paper. Thestrongly negatively charged piece of paperpulls off the positive toner from the drum.Finally, the paper passes through a pair ofheated rollers known as the fuser, whichfuses the toner to the paper. After the paperattracts the toner from the drum, a dischargelamp bathes the drum in bright light, erasingthe original electrical image.

Color printers work the same way,except the above process is repeated fourtimes. Four types of toner are used: cyan(bluish), magenta (reddish), yellow, and black.By combining tiny dots of these four colors,nearly every other color can be created.

A photocopier works according to thesame basic principle, except the electrostaticimage that forms on the drum is formed bybright light that reflects off the paper to becopied. The drum is manufactured with a pho-toconductive material on its surface thatmakes it sensitive to light. White areas of thepaper are reflected onto the drum. The blackink on the paper to be copied absorbs light, soparts of the drum do not receive an electricalcharge. These uncharged parts of the drumwill form the photocopy. Just like in a laserprinter, the negatively charged toner isattracted to the positively charged imageimprinted by light on the drum. A stronglypositively charged piece of paper then attractsthe toner from the drum. Your copy is com-

plete once it passes through the heated rollersof the fuser.

Inkjet printers, as the name implies, workby spraying tiny droplets of ink onto the sur-face of the paper. Each drop is very tiny, beingonly about 50–60 micrometers in diameter. A

micrometer (!m) is a thousandth of a mil-limeter. A human hair has a diameter of

about 70 !m. Thereare two maintypes of inkjet

printers onthe markettoday. Bub-ble jet print-

ers use heatto vaporize inkto form a bub-ble. This

expanding bubbleforces some of the

ink onto the paper.

When the bubble pops, a vacuum is created,causing more ink to flow from the cartridgeinto the print head. A piezoelectric printerworks using piezo crystals (such as quartz).Piezoelectric crystals generate an electric fieldwhen distorted, but conversely, they can bedistorted by an electric field. Thus, to get thenozzle to deform and eject the ink, an electricfield is applied. This electric charge causes thenozzle to vibrate, forcing ink out on the paper.

Digital photos can be printed usingeither laser or inkjet printers, but inkjet print-ers tend to produce better quality photos. Aninkjet photo printer will generally use six col-ors as opposed to the four that are normally

employed in a color printer. Other types ofphoto printers use a dye sublimation tech-nique. Sublimation is the process of chang-ing phase from a solid to a gas, skipping theliquid phase altogether. Heat is used tovaporize solid dyes, which permeate thepaper before they return to the solid form.Thermal autochrome photo printers requirethe use of special paper that already containsthe ink. A print head delivers variousamounts of heat to the paper, causing vari-ous pigments to appear.

Amazingly, experts agree that digitalphotography is still in its infancy. We will no

doubt see huge advances in digital quality andconvenience in the near future. Will digitalcameras completely replace conventionalcameras? There are photographers whoremain devoted to the artistic and visualeffects of developed film and darkroom pro-cessing. For most of us, it’s nice to know wehave plenty of options available for recordinglasting images of our big moments. And it’sall due to—you guessed it—chemistry!


Inkjet printers work by spraying tiny droplets in ink on the surface of the paper and tend to producebetter quality photos.

Brian Rohrig teaches chemistry at Jonathan AlderHigh School in Plain City, OH. His most recentChemMatters article “There’s Chemistry in GolfBalls” appeared in the October 2005 issue.

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Real or Fake?

ChemMystery

CSI (criminal scene investiga-tion) is not limited to murder.Recently, a crime team was

assembled to examine a controversialand potentially priceless bone box andblack stone tablet. Although the bonebox may have once held skeletalremains, the team was not interestedin DNA or fingerprint evidence. Theyfocused on a grimy buildup, calledpatina, on the surface and in theinscriptions of the box and tablet. Ifproven to be fake, the patina couldexpose a forgery ring that has fakedinscriptions on ancient objects foundin museums all around the world.

The inscription on the bone boxclaims that it once held the bones ofthe brother of Jesus, while the tabletreports ancient repairs to Solomon’sTemple in Jerusalem. Too good to betrue? The Israel Antiquities Authoritythinks so. But what did the crimeteam find?

Dr. Elizabetta Boaretto at theWeizmann Institute radiocarbon14C–dated the tablet, using samplesof patina from the inscription. Patinais the coating that builds up onancient objects over long periods oftime. The main component of thepatina in the inscription is calciumcarbonate (CaCO3), called calcite bygeochemists. Calcite contains car-bon, which can be 14C tested. Dr.Boaretto reported that the patinadates to 390–200 BC. This makes ittempting to declare the tablet authen-tic, but 14C dating is not enough toprove it is real. Expert forgers knowthat scientists use 14C dating toauthenticate pieces, so they concoctpatinas using ancient carbon (char-coal) found at archaeological digs.Dr. Boaretto reported that 14C-datingthe bone box would not provide anybetter proof of authenticity than sheobtained for the tablet.

The James Ossuary Case

Above: Bone box that may have held the bones of the brother of Jesus.

By Lois Fruen

Plants and animals take insmall amounts the radioactiveisotope 14C when alive. Whenan organism dies, replenish-ment of 14C stops and 14Csteadily decreases at a knownrate. The longer the organismis dead, the less 14C remains.One important fakery note—a pencil made yesterday froma tree cut down 2000 yearsago will appear to be 2000years old. But a tree cut downlast year will not!

Carbon Dating

CMatter02.06 1/24/06 10:37 AM


Next, Professor Yuval Goren at Tel AvivUniversity examined the tablet and the bonebox. He tested hardness and density and didmicroscopic analysis of the mineral composi-tion. He determined that the tablet isgreywacke, a stone found in western Cyprusand northern Syria, not Israel. Because thetablet is supposed to be from the 9th century

BC, it seems less likely that imported stonewould have been used instead of local stone.With further investigation, Professor Gorenfound that the surface patina is a silicate thatfirmly adheres to the surface. He pointed outthat it was unlikely that a silicate patinaformed in the calcite environment ofJerusalem. Even more suspicious, the patinain the inscription is different from the surfacepatina. It is calcite and soft enough to be easi-ly removed with a toothpick. This softness is ahallmark of recent patina formation. He con-cluded that the inscription was added inrecent times.

Professor Goren also analyzed the bonebox and determined that it was made of nativelimestone (CaCO3)—a stone that was com-monly used to make bone boxes in the 1stcentury AD. That finding and his analysis ofthe surface patina convinced him that the boxis genuine. It was the patina in the inscriptionthat concerned him. On close inspection, hefound the inscription cut through the surfacepatina, so he was suspicious that it had beenfaked in modern times.

Dr. Avner Ayalon from the GeologicalSurvey of Israel took the investigation further.Using mass spectroscopy, he analyzed theoxygen isotope composition (!18O) of patinafrom both the bone box and the tablet. Oxygenisotope analysis measures the ratio of 18O to16O in a sample, compares the ratio to a stan-dard, and then expresses the finding as per

mil !18O. In this notation, the lower caseGreek letter delta (!) signifies the relationshipbetween the measured 18O and the 18O foundin a standard. You can think of the delta valueas the number of atoms per 1000 that theheavy isotope in the sample differs from theheavy isotope in the standard. To calculate avalue, investigators use this formula:

To find the !18O, Dr.Ayalon reacted samples ofpatina with phosphoric acid(H3PO4). This reaction pro-duces CO2 gas, seen in thereaction below. The CO2 gaswas ionized in the massspectrometer, and then the

oxygen isotopes were separated by mass by astrong magnetic field.

Dr. Ayalon compared !18O for patinason the bone box to !18O of patinas from 1stcentury AD bone boxes that he knew wereauthentic. He found !18O of authentic boxes

were between –4.1 and –6.7 per mil !18O.These matched results for surface patina onthe bone box in question. But, !18O readingsof patina from the inscription were between–5.8 and –10.2 per mil !18O, with all but oneof the readings falling outside of the accept-able range. He concluded that the box itselfwas real, but the inscription was forged.

Oxygen isotope analysis was the nail inthe coffin for the bonebox. It convinced Dr.Ayalon that the inscriptionwas forged. The reasonhad to do with the tem-perature at which the pati-

na in the inscription formed, which he deter-mined from !18O values. Natural calcite pati-na forms in much the same way crusty scaleslowly builds up in hot-water pipes and insidekettles, sealing oxygen isotopes in as thepatina dries. The process of calcite patinationstarts when groundwater picks up CO2 fromair trapped in soil. When this CO2-rich

groundwater comes in contact with calcite, itdissolves it. Although calcite is not very solu-ble in pure water, the higher the concentra-tion of dissolved CO2 in water, the more cal-cite will dissolve, shifting the equilibrium tothe right.

When calcite-rich groundwater isexposed to air in a burial cave, the equilibri-um shifts back, releasing CO2 gas into thecave and precipitating calcite.

If the patina on the bone box is authen-tic, the patina could have formed in one oftwo ways. First, it could have recrystallizedfrom water that seeped into the burial cave.Water entering the burial cave would haveabsorbed CO2 gas from the cave environ-ment, formed a thin film of water on thebone box, and reacted with the surface.(This reaction is the same as above becausethe box is made of limestone, CaCO3). Whenthe film of water evaporated, it would haveleft behind a white film of calcite patina onthe surface of the box. However, if the bone

Dr. Avner Ayalon from the Geological Survey of Israel examines thecontroversial bone box.

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The Reason 18O is concentratedin the calcium carbonate relativeto water is because of quantummechanical effects: The carbon-ate has more vibrations whoseenergies depend on the mass ofthe oxygen atoms, and the car-bonate with the 18O isotope haslower-energy vibrations and over-all energy. 18O prefers to be in thecarbonate. The importance of thisenergy difference is lessened athigher temperature because it’san exothermic reaction—thereaction becomes less favorableas the temperature increases.

18 O /16 O( )sample18 O /16 O( )standard

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3CaCO3 + 2H3PO4 ! 3CO2 (g) + 3H2O + Ca3(PO4)2calcite phosphoric acid

CaCO3 (s) + CO2 (aq) + H2O (1) ! Ca2+ (aq) + 2HCO3- (aq)

calcite dissolved CO2

CO2 (g) + H2O (1)!

!

CMatter02.06 1/24/06 10:37 AM


box had been buried in a shallow grave, thepatina would have precipitated directly fromthe groundwater water onto the surface of thebox by CO2 degassing from the groundwater.

In either case, if groundwater or seep-age water were cold, the patina would havecontained more 18O than 16O isotopes, givinghigher !18O readings.

Conversely, calcite patinas that formfrom hot groundwater have lower !18O val-ues. The very low !18O readings obtained forthe patina found in the inscription of the bonebox suggest the patina formed from very hotwater. In fact, Dr. Ayalon reported that thewater temperature had to be between 40 and50° C. Because groundwater temperatures incaves and shallow graves in the Jerusalemarea are between 18 and 20° C, it would havebeen impossible for a patina with such a low!18O values to have precipitated naturally.

On the basis of this evidence, Dr. UziDahri, Deputy Director of the IsraelAntiquities Authority, declared that theinscription was a forgery. To further supporthis case, he reported that fluorine was foundin the patina in the same percentage as fluo-rine added to tap water in Jerusalem to pre-vent tooth decay. Since drinking water wasnot fluoridated until modern times, Dr. Dahriconcluded that the patina had been fakedusing modern-day tap water. Things werenot looking good for the bone box.

Next, Dr. Ayalon checked the patina onthe tablet. From the !18O, he determined thattwo different calcite patinas had been used inthe inscription. The first gave !18O valuesbetween –7.3 and –8.4 per mil !18O, while thesecond had higher values between –0.9 and–1.7. Dr. Ayalon speculated that the patina wasconcocted by grinding a carbonate material likechalk with a carbonate that contained fossils.He suggested that the forger then dissolved themixture in hot water and spread it onto the sur-face of the inscription. Ms. Orna Cohen, anexpert conservator who specializes in identifi-cation and restoration of ancient patinas, con-curred with him that the patina was forged. Shereported that when the patina was removedfrom the inscription, it appeared freshly cut.

The forgery case seemed airtight. TheIsraeli police arrested the owner of the bonebox, who is an antiquities dealer, on a chargeof knowingly conspiring with intent todefraud. And that's when things took a turn.

Muddying the watersDr. James Harrell, professor of

Archaeological Geology at the University ofToledo, suggested that the low !18O readingscould have come from a cleanser that was usedto clean the bone box. He pointed out that antiq-uities dealers and collectors often clean artifactsto increase value, and the patina in the inscrip-tion “looks and feels exactly like what one wouldexpect from a powdered cleanser”. Cleanserscontain ground-up limestone (CaCO3) abrasivesand baking soda (NaHCO3), which serves as thecleansing agent. Both limestone and bakingsoda react with H3PO4 to produce CO2 used in!18O analysis. Intrigued, Dr. Harrell decided tohave an oxygen isotope analysis done on fourwidely used cleansers from Israel.

The Georgia Center for Applied IsotopeStudies tested the four Israeli cleansers withinteresting results. Three of the four cleansersproduced results lower than the –4.1 to –6.7per mil !18O range set by the Geological Surveyof Israel for authentic carbonate patina. And,the most popular cleanser had a !18O of –8.5per mil, consistent with the patina in theinscription of the bone box. The low !18O read-

ings, which had convinced the Israel AntiquitiesAuthority that the bone box was forged, couldhave come from cleanser!

Further doubt was raised by AndreLemaire of the Sorbonne in France. He sug-gested that the fluorine found in the inscrip-tion of the bone box could have resulted fromcleaning with tap water. He also suggestedthat fluorine might be present in patinas ofmany authentic antiquities, since the antiqui-ties may have been exposed to groundwatercontaining modern-day runoff.

Recently, !13C data for the bone box pati-na have been released. !13C is a comparison of13C to 12C isotopes in a sample and is deter-mined using mass spectroscopy. Readings forthe surface patina on the bone box varied from

–1.2 to –7.7 per mil !13C. However, unlike the!18O results, the !13C readings for the inscrip-tion patina were almost identical to the valuesfor the surface patina, ranging from –1.1 to–7.4 per mil !13C.

Are the bone box and stone tablet fakes?If they are, they could expose a forgery con-spiracy that has been operating for years. Infact, the Israel Antiquities Authority has warnedthat many forgeries from this conspiracy couldbe in museums in Israel and around the world.The forgery trial was scheduled to begin onSeptember 4, 2005. But, even if the defendantsare found not guilty or the case is dismissed,doubt will linger about the authenticity of thetablet and bone box. Scientists will continue todisagree about analyses and interpretations ofresults. Methods to test antiquities willimprove, and it is likely that new examinationswill be undertaken.

More importantly, even if one day theinscription on the bone box were to be provenauthentic, the names James, Joseph, andJesus were common in the 1st century AD, sothe inscription could refer to a family otherthan the biblical one. You can follow the ongo-ing controversy on the bone box by clicking onthe “Update” section at www.bib-arch.org.

Lois Fruen teaches chemistry at the Breck Schoolin Minneapolis, MN. Her most recent article “LiquidCrystal Displays” appeared in the October 2005issue of ChemMatters.

The inscription on the bone box reads, “Ya’akov bar Yosef ahui d'Yeshua,” which translates “James, son of Joseph, brother of Jesus”.

Stone tablet that reports ancient repairs toSolomon’s Temple in Jerusalem.

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magine you’re a soldier of the future. Asyou scan the horizon for possible threatsto your platoon, the day heats up, but youstay cool. Tiny air conditioners placedstrategically within your shirt turn up the

juice before sweat makes its first appearance.But that’s just the start of the laundry

list of things your new standard-issue attiremight do.

For example, sensors integrated into thefabric could take minute-by-minute healthreadings to make sure you’re fit, calm, andwell hydrated. Radios woven right into thecloth might communicate your position andstatus back to the home base. Your camou-flage could instantly morph to hide you in anybackground, not just the usual forest greensor desert browns. All of these powerful appli-cations might be fueled by light-weight capac-itors, or batteries, twisted right into thethreads. To top it all off, your shirts and pantsmight stop projectiles on their own, withoutthe help of extra bullet proof materials.

Think these revolutionary duds are “mis-sion impossible”? Think again. Scientists arecurrently working on developing super fibersmade of powerful carbon nanotubes. Theseultra-thin threads can be less than one-ten

thousandth the width of a human hair, andthey come with a bevy of interesting chemicalproperties such as super strength, and superelectrical and thermal conductivity. Woveninto a fabric, these fibers could turn any articleof clothing into extraordinary wired attire.

The research still has a long way to gobefore you'll be able to pick up your ownsuper shirt at the army surplus store—chemists need to work out some toughkinks in manufacturing the rightkind of nanotubes at the rightlength for these applications. Butwith the science marching swiftlyahead, an army of super-tailoredsoldiers won’t be long behind.

Strong characterDespite scientists’ long-range

scheming over possible applica-tions, carbon nanotubes are still arelatively new technology. Theywere discovered by chance in 1991by researcher Sumio Iijima atJapan-based NEC Laboratorieswhile he was examining other carbonstructures for unrelated applications.

Iijima and his colleagues knew they hadsomething special. They immediately saw howthe structure of the tubes, which look likerolled-up chicken wire at the molecular level,gives them unique chemical features that sci-entists haven’t seen in any other material.

Carbon nanotubescould be the key to spinning the future’shottest threads.

COURTESY OF OAK RIDGE NATIONAL LABORATORY

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According to MatteoPasquali, a chemical engineerat Rice University in Houston,TX, like every covalent net-work solid, every atom in acarbon nanotube shares elec-trons with its neighbors.Sometimes, this propertygives them the ability to con-duct electricity extremely effi-ciently. The tubes’ honeycomblattice and cylindrical structurealso allow them to channelheat effectively and retain theirshape.

Carbon nanotubes con-duct heat better than anyknown material and are manytimes stronger than any knownfiber. Plus, they are extremelylightweight, making them per-fect for adding these specialqualities to other materialswithout adding extra pounds.

“To some extent,” saysPasquali, “they’re the HolyGrail of fibers.” But much likethe original Holy Grail, headds, good carbon nanotubesare extremely tricky to find.

When Iijima’s group dis-covered nanotubes, theynoticed that the tubes sponta-neously arise from a varietyof combustion reactions.For example, nano-tubes emerge everytime you light a candle.But this run-of-the-millproduction makes nano-tubes that aren’t suitable foranything useful, says Pasquali.

First, many combustionreactions produce a randommishmash of different nan-otube structures—the hexa-gons in the tubes’chicken-wire structure may berolled up at different angles,for example, giving them dif-ferent properties. This slip-shod production also spewsout various assortments ofnanotube types. Single-wallednanotubes, the hose-likestructures that seem to have

the most benefits, are often mixed in withmultiwall nanotubes, which look like tubesrolled up within tubes. To get the most reli-able properties, scientists need to work witha uniform batch of tubes that perfectly matcheach other.

Second, the manufacturing techniquesavailable today—such as knocking carbonoff of a surface with a laser, for example, ordischarging bits of carbon by zapping car-bon rods with electricity—can only producenanotubes that are usually only a fewmicrometers long. “If you interrupt the nan-otube, then you interrupt their properties,”says Ray Baughman of the University ofTexas at Dallas. The most bang for chemists’buck lies in learning how to manufactureindividual, extra-long nanotubes. However,many scientists agree that it will be a bigstretch to produce single nanotubes muchlonger than the current limits with today’stechnology.

Do the twistTo get enough material for

usable super fibers, scientistshave a few tricks up theirsleeves. The bestidea for now,says R.Byron

Pipes ofPurdue Uni-

versity in WestLafayette, IN, is to take

many short nanotubes andbundle them into yarns. Although theresulting yarn has less than 1% of the theo-retical strength, heat conductivity, and elec-trical conductivity, the end product still hassome intriguing possibilities.

“For many centuries, man had only dis-continuous fibers at his disposal, like flaxand cotton. But we’ve made yarn, rope, andcloth, twisting fibers so they become

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Carbon nanotube fibers are 4 times stronger than spider silk and 17 times stronger than the Kevlar used in bullet-proof vests.

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strong,” says Pipes. “Most people think thatif the elements aren't continuous then it’snot strong, but that's not true.”

For example, Baughman and hiscoworkers have manufactured carbon nan-otube yarns by distributing billions of nan-otubes into a detergent solution. Thescientists keep the tubes from bunchingtogether by blasting the solution with high-frequency sound waves. Feeding a thinstream of the solution into a whirling bath,the scientists have twisted yarns up to 200meters long and as thickas a human hair,but muchstronger.These

super-fibers are 17

times as tough asthe Kevlar used in bullet

proof vests, and 4 times astough as spider silk—the

strongest known natural fiber.Pasquali and his colleagues are using

another solution-based way to make theirown nanotube yarns. Dumping their individ-ual nanotubes into sulfuric acid, theresearchers found that electrical chargeswithin the acid distributed the tubes evenlywithout the high-frequency sound waves.Pasquali’s team simply pressed the nan-otube solution through a syringe into acoagulating bath, pushing out meters ofsuper-strong nanotube cables.

Next big thingResearchers are now testing the new

fibers to see exactly what kind of physicalproperties they possess. If they can tweakthe properties of these yarns to even one-tenth of an individual nanotube, then scien-tists will be in business for a number ofopportunities.

Clothes for a futuristic soldier will beonly the tip of the iceberg, says Baughman.

They could play a majorpart in building strongervests for police officers andartificial muscles that twitchwith electricity. These aretwo super fiber possibilitiesthat he and his team aretalking about.

Many scientists aredreaming much bigger; forexample, some researchersbelieve that an extra-longcable spun of carbon superfibers might somedaystretch all of the way tospace, tethering an orbitingspacecraft to the earth.Once the initial thin cord isestablished, a small climb-ing machine could

strengthen and thickenthe line, eventuallycreating a space ele-

vator that couldcheaply carry people andequipment into space.

On a more practicalnote, Pasquali thinks thatstretching electrical cablesmade of super fibers fromcoast to coast—or evencontinent to continent—might save an enormousamount of electricityaround the world. “One ofthe big problems withpower is that, for the mostpart, you can’t store it,” saysPasquali. “You continuouslyhave to produce the power that’s needed.”

Right now, loads of electricity arewasted as power plants churn away throughthe night, supplying electricity for just a fewnight owls. But at night in one part of theworld, it’s daytime in another. If scientistscould move electricity instantly from onehalf of the world to another, energy produc-tion’s efficiency would dramaticallyimprove.

Traditional cables won’t work for suchan expansive application—right now, theylose about 50% of the energy that’s pro-duced through resistance while just movingit around. However, a cable woven of car-bon super fibers could be a perfect fit forthe job, notes Pasquali.

Although the phenomenon has beenonly demonstrated over a distance of amicrometer, it’s an exciting result. He esti-mates that some of the applications of car-bon super fibers could be here in a mere 10to 20 years.

In the meantime, he and other carbonnanotube chemists will keep spinning longerand longer superfibers, weaving theirdreams toward reality.

Christen Brownlee is a contributing editor toChemMatters. Her article “Flaking Away” alsoappears in this issue.


Some researchers believe that carbon superfibers may somedaystretch to space.

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S a l t i n g R o a d s


By Doris R. Kimbrough

ost of you are probably planningto be in a car at some point thiswinter, so be sure to wear yourseatbelt, obey the speed limit,and be extra cautious in “winterdriving conditions”. It is actually

physics that makes being in a car on an icyroad hazardous; friction is an important part ofkeeping a car under control. However, yourstate and local transportation departmentsmake use of some pretty interesting chemistryto keep roads safer for travelers in the winter-time. In addition to plowing, one of the waysthat highway workers keep roads clear of iceand snow is by spreading salt on the roads.Even though salt can cause rust and corrosionon cars, bridges, and other parts of the high-way, it more than makes up for this costlydamage by saving lives. Let’s take a look athow it works.

Freezing pointdepression

Pure water freezes at 0°C; adding salt towater depresses or lowers the freezing pointbelow zero. When you remove heat fromwater (or any substance), the molecules slowdown. The freezing process occurs when themolecules stop sliding and tumbling all overeach other (liquid phase) and settle into fixedpositions in a large network called a crystallattice, which is the solid phase. The mole-cules are still moving, but in the solid phasethat motion involves bonds stretching andcompressing or the atoms wiggling a little bit.This is called vibrational motion.

When a solution of salt in water iscooled to a low enough temperature, thewater molecules begin to stick together in anorganized way to form solid crystals. Thecrystal framework tends not to include thesalt ions because the ions would disturb this

organization. So when you cool a solutionenough, the ice crystals that start to form aremade of pure frozen water. You can actuallypurify salty water like this, by freezing a por-tion of the solution and washing the saltywater off of the ice crystals and then thawingthe ice to produce pure water. Eventually, ifyou cooled it enough, the whole solution willfreeze, but it does not have a sharply definedfreezing temperature. Getting the water mole-cules organized into a crystal from a solutionrequires that you remove more energy (actu-ally free energy) than if you are freezing purewater, so the water in a solution typicallydoes not start freezing until it reaches alower temperature than the normal freezingpoint. This is true of all solutions, not justthose made with water.

The difference in temperature betweenwhere the pure solvent (water in our case)freezes and where the solution starts to freezeis called the freezing point depression. How low

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you go depends upon what the solvent is andhow concentrated the solution is. The moreconcentrated the solution, the lower the freez-ing point. This is why if you freeze a solutioncompletely, you have to keep lowering the tem-perature. As the pure water freezesaway from the solution, the con-centration of the solution remainingincreases. How does your Depart-ment of Transportation use thischemistry to keep roadways clearof snow and ice in the wintertime?

Salting roadsHighway workers use salt in

two ways: 1) to melt ice that isalready on the roadway and 2) toprevent ice from forming on theroadway. The second one is a littleeasier to understand, so we willstart there. Let’s say a snowstormis forecast for your town. Municipal workersget out and spread salt on the roads. As thestorm hits, snow starts to fall, but the roadsurface is warmer than the air, so the firstflakes melt. As they melt, the salt dissolves inthe liquid water. Now you have a solution ofsalty water, which has a lower freezing pointthan pure snow, so that even though the addi-tional snow might cool the road enough to“stick” to the road surface, the temperaturewill not get cold enough to freeze the solutionthe way it could freeze pure water. In the end,the real snow removal is done by the plows,but salt plays a crucial role in preventing snowand ice from bonding to the pavement.

“Hold it!” you say. Suppose the tempera-

ture does get cold enough to freeze the solu-tion. Or suppose that enough snow falls sothat the salt water solution gets too dilute towork, what happens then? In both of thosecases, snow could build up on the road. This

is why your highway workers are out thereround-the-clock, plowing and spreading moresalt as long as the storm is in the area. Com-munities with really cold temperatures, likeparts of Canada and Alaska, where it can getto –20°C (below zero on the Fahrenheit scale),don’t even bother with salting the roads,because it doesn’t help. They typically plow offas much as they can and use gravel or sand toadd traction.

How does it work if the road already hassnow or ice on it? You may have seen howspreading salt on an icy sidewalk will causethe snow or ice to melt. The very beginning ofthe melting process begins where solid icecontacts solid salt. At the surface of ice crys-

tals, water molecules do not have the samestable arrangement they have on the interior;they are more mobile and more reactive. Sothe surface ice reacts with the surface of thesalt crystals, allowing a small amount of salty


Water freezes at lower temperatures when it issaltier. But solubility of salt in water decreaseswith decreasing temperature. So at some point, asolution is too cold to hold the salt that keeps itfrom freezing. The eutectic temperature is thelowest temperature at which a mixture of two ormore substances can stay liquefied. For a NaCland water mixture, the eutectic temperature is–21.1 °C. For road ice, –10 °C is the practicallimit for salt.

Sodium chloride and calcium chloride.

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solution to form. This first step is relatively slow, but then the growingsolution continues to dissolve more salt and melt more ice. Passingvehicles warm the slush through friction, which speeds the dissolutionof the ice and may crush the salt and ice together, which will increasethe surface areas of the particles in contact with each other. Somecommunities use “prewetted” salt (usually rock salt with a CaCl2 solu-tion sprayed on it) to speed the process.

Road salt history, fun facts, and current technology

Putting salt on the roads to lessen the buildup of snow and icebegan in the 1930s, and by the 1960s, it was used by most communi-ties where snow and ice are a problem. Concerns about the effect of theuse of sodium chloride (common table salt) on the environment haveprompted some state and local road crews to explore the use of moreenvironmentally friendly salts, such as magnesium chloride and calciumchloride. These two salts have the advantage of being more effective atlowering the freezing point and there is some interestingchemistry behind this benefit.

As you probably know, sodium chloride has the formulaNaCl. When it dissolves in water, it dissociates into its ions:Na+ and Cl", producing two ions in solution for every NaCl for-

mula unit. Magnesium chloride (MgCl2) and calcium chloride (CaCl2)dissociate to three ions each because the metal has a 2+ charge andthere are two chlorides per metal ion:

MgCl2(s) ! Mg2+(aq) + 2Cl-(aq)CaCl2(s) ! Ca2+(aq) + 2Cl-(aq)

It is the number of dissolved particles that determine the extent ofthe lowering of the freezing point of a solution. So although NaCl pro-duces two ions, MgCl2 and CaCl2 each produce three, making themmore effective. Other variations include mixtures of the magnesium andcalcium chlorides, as well as magnesium and calcium acetates,Mg(C2H3O2)2 and Ca(C2H3O2)2.

The technology of salting roads has become fairly sophisticated.Often, these salts are dissolved in water or some other solvent so thatthey can be sprayed onto the road surface. Having the deicing sub-stance in a solution (i.e., fluid) form makes it possible to pump throughhoses, which allows for a more targeted application. In addition, variousanticorrosive substances are added to protect highways and cars fromthe damage the salts can cause over time. Some 15 million tons of deic-ing salt is used each year in the United States and about 4–5 milliontons in Canada.

You may have seen signs that warn about bridges freezing beforeroad surfaces. This is because bridges are more exposed and not insu-lated by the ground from underneath like the rest of the highway. Somehigh-tech highway bridges have been constructed with deicingsprayers built right into the pavement, complete with sensors thatdetect when conditions are right (e.g., cold temperatures, high windspeeds, high humidity) for ice to form. The sensors detect the possibleformation of ice, and the deicing sprayers go to work to keep the road-way from freezing.

Highway engineers have been working withother interesting variations. One deicing materialthat is currently on the market mixes magnesiumchloride with sugar cane or sugar beet molasses.

The sticky molasses keeps the magnesium chloridefrom getting blown or washed off the road surface.

There are also substances that are added directly tothe top layer of concrete or asphalt when the road isbuilt or repaved that help prevent ice from forming.Highway workers can then get away with using less saltthan before, which is cheaper, easier on the environ-ment, and helpful in preventing corrosion. Scientistsand engineers continue to develop new ways to keepwinter highways safe while minimizing expense and

environmental harm. Just another way that chem-istry is keeping you out of harm’s way.

Doris Kimbrough teaches chemistry at the University of Colorado-Denver. Herlast article, “Einstein’s Miraculous Year”, appeared in the December 2005 issue.


Salt solutionFreeze point

Phase diagram for NaCl

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Solid salt+ Ice

Solution concentration (% by weight)

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Ice +Salt solution

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Practical Melt Temp. Eutectic Temp.CaCl2 "32°C "56°CMgCl2 "15°C "33°C

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he old-style Volkswagen Beetle: Isit a classic car, or an endangeredspecies? The answer depends onwhere you live. Although there arefew classic cars hanging aroundthe northern states or on the

coastlines, plenty of vintage automobiles stillexist in the mild southern climates and inAmerica’s interior states.

The reason that Volkswagen Bugs andother older cars are dropping like flies isn’t thetypical habitat loss or human encroachmentthat's plaguing other endangered species.Classic car fleets are constantly shrinking dueto a chemical reaction that you’re no doubtalready familiar with: rusting.

But why does rust unequally strike carsin the snowy states and coastal towns butleave vehicles elsewhere virtually untouched?And more importantly, how can you keepyour beloved grocery-getter safe, no matter

what parking place you call home? Read onto get the lowdown on how rust works andwhat measures you can take to stop corro-sion in its tracks.

Electron swap meetLike all types of corrosion, rust is

actually a chemical bargain, withtwo reactions in one: reduc-tion, in which someatoms gain electrons,and oxidation, inwhich other atomslose electrons. Withall those electronsflowing from oneplace to another,rust-making is alsoconsidered an electro-chemical reaction.According to John Scully, a

From Ferraris to Ford Pintos,almost every car is fighting a losing battle to rust.


TBy Christen Brownlee

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18 ChemMatters, FEBRUARY 2006

corrosion expert at the University of Virginia inCharlottesville, the redox reaction that formsrust needs just three ingredients to take place:an anode, or metal that readily gives up elec-trons; a cathode, or substance (in this case,oxygen) that easily accepts electrons; and anelectrolyte solution, which shuttles ionsbetween cathode and anode.

Most cars are made mostly of steel, atough mixture of iron, carbon, and smallamounts of other ingredients like manganese,silicon, phosphorus, and sulfur. It’s the ironpart of steel that corrodes to make rust.

Iron doesn’t hold onto its electronsvery tightly, says Scully, making it the per-fect anode for an electrochemical reaction totake place. Other metal atoms in the steelmixture, or even another point on the pieceof iron, make excellent cathodes. Steel has anonuniform surface because the chemicalcomposition is not completely homoge-neous. Also, physical strains leave stresspoints in the metal. These defects createanodic regions where the iron is more easilyoxidized than it is at others (cathodicregions). However, without a bridge to con-nect potential anodes and cathodes, rustingwould be such a time-consuming processthat cars would virtually last forever.

The water on the steel surface is a sol-vent for ions produced when the iron metal atthe anodic region loses electrons (is oxidizedto form ferrous ions) and the electrons areconducted through the metal to the cathodicregion where they react with water and oxy-gen from the air to form hydroxide ions, asshown in these equations:

2Fe ! 2Fe2+ + 4e- (oxidation at anodic sites)

4e- + O2 + 2H2O ! 4OH-

(reduction at cathodic sites)

The ions in this electrolyte solution canmigrate together and react to form ferroushydroxide, which reacts further with oxygenfrom the air to oxidize the ferrous ion andform insoluble ferric oxide, the chemical namefor rust, as shown in these equations:

Fe2+ + 2OH- ! Fe(OH)2

(formation of ferrous hydroxide)

4Fe(OH)2 + O2 ! 2Fe2O3 + 4H2O(oxidation to ferric oxide or red ‘rust’)

The movement of ions through the electrolytesolution completes the electric circuit thatallows the electrons from iron to move fromthe anode to the cathode.

But all this still doesn’t explain whycolder climates and coastal areas get anunfair share of rust. The magic ingredient thatboth areas share—which is missing in thebasic recipe for rust—is a high abundance ofsalt. Coastal areas have plenty of salt sailingthrough the air from ocean spray, and witheach cold, snowy winter, northern statessmear tons of rock salt on roads to lower thefreezing point of water and help keep roadsfree or ice and snow. [See “Salting Roads:The Solution for Winter Driving” in this issue]

Salt speeds rust’s redox reaction alongby making water a better conductor. “Saltallows the anode and cathode to be in toucheven better,” says Scully, making corrosionhappen even quicker. Also, chloride ions formvery stable complex ions with Fe3+, whichhelps dissolve iron and accelerate corrosion.

Costly corrosionScully points out that iron can’t help

rusting—existing as an oxide in its thermody-namically favored state. In fact, the metalrarely exists in a pure state in nature. Before itbecomes a side panel in your car, engineersmust convert rusty iron ore into a pure metal.

With rust being iron’s favored state, it isof little use trying to fix rust after it hasalready happened. By putting energy intorust, it’s possible to plate metal back onto a

car, says Scully. However, it’s also incrediblycostly and impractical. Plus, by the timemost car owners notice a rust spot, a signifi-cant amount of iron has already sloughed offof the car, lost to wind and rain.

One solution to stopping iron’s thermo-dynamic conversion, says Scully, would be tomake cars out of a metal that doesn’t cor-rode, such as gold or silver. “Converting toan oxide isn’t thermodynamically favorable

A redox reaction on wheels. The bumper of this vehicle wears the product of the reaction between ironand oxygen—rust!

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reaction for these metals, so they won’t cor-rode spontaneously,” he says. “Archaeolo-gists can dig up gold coins that have survivedthrough the ages without corroding.”

But while driving a gold car might makeyou feel like a million bucks, making such avehicle would cost substantially more. Not tomention the fact that these soft metals wouldbe unable to support the car’s weight andwould squash like putty in a collision!

Even using noncorroding metals that arecheaper than gold or silver, such as stainlesssteel that Deloreans are made of, is still moreexpensive than using the plain steel that mostcars are made of today.

The best way to prevent corrosion is stillthe cheapest. A good coating of paintremoves the connection between anode andcathode by preventing water from makingcontact with steel. Without water, rustingslows down to a snail’s pace.

Today’s high tech paints have evolvedfar from being just a simple barrier, saysScully. Researchers are currently working onpaints that release rust inhibitors on demandwhen paint’s seal on steel is breached, forexample, when the paint on a car is scrapedor scratched. Other “smart paints” that oozetogether to close gaps whenever a car’s pan-els get scratched are also in the works.

Although rust is a big deal for car own-ers, it's an even bigger deal for industries thatrely on machines with metal parts, rangingfrom farm tools to factory equipment tofighter jets. According to Scully, corrosioncosts the United States a whopping annualtoll of about $276 billion in lost goods andservices. With the exorbitant cost of new mil-itary equipment, the Department of Defense(DOD) is one of the largest investors inantirust research, says Scully. Scientists atthe DOD hope they can keep the aging Black-hawk helicopters and B52 bombers that arecurrently in use running smoothly fordecades to come—a cost savings of millionsof dollars per machine.

But one military asset sometimesharmed by corrosion is extremely difficult toput a price on, says Scully. “If a soldier goesto war and the rifle he’s using to protect him-self doesn't fire when he needs it to, how doyou estimate the cost of corrosion then?”

Christen Brownlee is a contributing editor toChemMatters. Her article “Super Fibers” alsoappears in this issue.

This student investigates the role of salt on therate of rusting by putting nails in various saltwater solutions.

It’s not only cars that are rusting away. Corrosion costs the United States a whopping $276 billion per year!

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With a flash of light,nanotubes ignite!

Shine light on some materialsand they will make an audiblesound. This interesting phenome-non is known as a photoacoustic oroptoacoustic effect.

The photoacoustic effect wasfirst discovered by Alexander Gra-ham Bell and reported to the world in1880. While working on the photo-phone, he inadvertently discoveredthat if a beam of light was focusedon a selenium cell and then rapidlyinterrupted by a rotating disk, the cellproduced an audible sound. He fur-ther demonstrated that the strengthof the photoacoustic effect is depen-dent on the strength of lightabsorbed by the material.

Shine light on some newnanoscale materials and they’ll notonly sound off, but they actuallyignite under an ordinary cameraflash. Enhanced photothermal prop-erties are one of the many unex-pected properties that researchersare discovering as they delve intothe realm of nanoscale materials.

Single-walled carbonnanotubes

Last year, researchers atRensselear Polytechnic Institute in

Troy, NY, reported in Science thatsingle-walled carbon nanotubes(SWNTs) of a dry, “fluffy” natureignite when exposed to a conven-tional flash at close range.

Similar materials such as C60,fluffy carbon soot, and multiwalledcarbon nanotubes do not ignite.

The researchers believe thenanotubes must have an enhancedability to absorb light and convertit to heat (a photothermal effect).In the “fluffy” bundles, the heatdoes not dissipate and reaches anestimated 1,500°C—more thanenough to ignite the SWNTs. Otherresearchers have hypothesizedthat part of this effect might relyon metal impurities present fromthe manufacture of the SWNTs.

See SWNTs ignite and burnafter exposure to an ordinary cam-era flash.http://www.sciencemag.org/cgi/content/full/296/5568/705/DC1

Silicon nanowiresIn an article in Nano Letters,

researchers at the University of Sci-ence and Technology in Hong Konghave described a similar phenome-non involving enhanced photother-mal effect in Silicon (Si) nanowires.When these nanowires are hit by a

light, they make an audiblepop. If the flash has suffi-cient power the Si nanowiresignite. Like SWNTs, theresearchers see evidencethat the Si nanowires experi-ence temperatures ofapproximately 1500 °C during the flash.

The figures on the rightshow Si nanowires before andafter a flash under an inertatmosphere. The nanowiresdon’t ignite, but they get hotenough to melt Si. Theauthors believe that “the opticalabsorption in Si nanowires wasenhanced by their special struc-ture.” The same effect is notobserved in bulk silicon material,and yet the absorption spectrum ofSi nanowires and pure Si singlecrystals is similar. The mechanismis unclear, but the Si nanowiresmust clearly “trap an incredibleamount of energy from the flash.”The ignition of the Si nanowires isdistinct from carbon nanotubesbecause there is no metal impurityin Si nanowires.

Because of the unexpectedability to confine photoenergy, Sinanowires may find uses in “smartignition systems, self-destructing

systems for electronic devices,nanosensors, and nanostructural or nanophase reformation for nanotechnology.”

See Si nanowires resist burn-ing in gas flame, only to ignite aftera flash of light at:http://www.chemistry.org/portal/resources?id=3f136aba6e8711d7f03f6ed9fe800100

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