Use of Digital Display Instruments as Demonstration Aids

Submitted by: Charles G. Moseley T h e Ohio Sta te University-Lima Campus

Lima, 45804 Checked by: Leonard C. Crotz

Uniuersity of Wisconsin- Waukesha Waukesha, Wisconsin

Various scientific instruments are now available with large, easily visible digital displays. The author has found that balances and pH meters of this type are very useful for lecture, recitation, or laboratory demonstrations of topics involving mass or pH, respectively. The demonstrations provide a supplemental empirical verification for some of the mass- and pH-related topics covered in first-year chemistry courses and thus aid students in their study. Students in our freshman chemistry course for nonscience majors seem to find the demonstrations esoeciallv heloful. The laree dieital disolav . . " . "

(ahout 13 mm) of a typical instrument is visible from 30 f t awav and allows the student to immediatelv see the actual empirical mass or pH value being discussed. The viewing ranee can easilv be extended for use in laree rooms hv clamping a magnifier in front of the displafor partial& darkening the room. Recentlv. in addition. a demonstration digital pHmeter with a very l&e display (12 cm) has become available,' and a method of adapting non-digital pH meters for lecture demonstrations has appeared.2 A discussion of a number of demonstrations which the author has found useful follows.

Balance Demonstrations

Relationship Between Certain Metric Mass Units

The significance of the digital display of the balance is first demonstrated by utilizing a series of masses of known values. Simply placing a labeled metric weight3 on the balance pan causes rapid display of the equivalent mass in grams. Useful demonstration weights are: 500 mg, 2 X 500 mg, 0.5 kg, 1 kg, and 1 kg + 0.5 kg.

Relationship between Certain English Weight and Metric Mass Units

Relationships between selected English weiaht units and grams can be demonstrated by placing on thebalance pan either labeled avoirdupois weights%r the contents of packages of free-flowine solid foods such as instant coffee. eelatin. and . - sugar.4 In either case, the equivalent mass in grams is quickly displayed. Useful examples are: 1 oz weight (28.4 g); 3 oz gel- atin (85.2 g); 4 oz instant coffee (113.6 g); 1 lb sugar (453.6 g); and 2 lb weight (907.2 g).

Density

The density relationships for various liquids can be effec- tively demonstrated by placing an empty graduated cylinder on the balance pan, setting the display to zero grams, adding a volume of lialiid to the vessel. and observine the relative mass and volume values. Useful examples are: 100.0 ml hexane (66.0 g, 0.66 glml); 100.0 ml rubbing alcohol (78.5 g, 0.785 g/

ediled by

GEORGE L. GILBERT Denison University

Granville, Ohio 43023

ml); 50.0 ml water (50.0 g, 1.00 g/ml); 100.0 ml water (100 g, 1.00 g/ml); and 10.0 ml mercury (135.3 g, 13.5 glml).

Densitv determinations for various solid ohiects can be demonstiated by placing a graduated cylinder containing a volume of liquid (usually, but not necessarily, water) on the balance pan, setting the display to zero grams, adding to the cylinder a solid object (with density greater than the liquid), and observing the relative increases in mass and volume. Se- quential density determinations can easily be made in the same cylinder for the liquid to be used in the determination and for various solid objects if the display is set to zero grams before each addition is made and a running record of volume is kept. Usina different shapes and sizes of the same solid substance c l e h y demonstr&es that density is independent of these variables. Useful solid obiects are: Lucite plastic pieces (1.16 g cmq); black r111her stopprrs rl.26 g/cm"): green ruhher stoppers r 1.49 E rm:'): and marbles r2.?2 g cm?.

Of course, a more &curate determination of density for either a solid or liquid can be made by plotting mass (ordinate) against volume (ihsrissa) for severildifirent sized samples uithe substance and then decerming the slope of the resulting line.

Mass Relations in Chemlcal Reactions Usine the balance to show that either no mass chance occurs

durinKa reartiun or rhat a specific mass rhange occ& ran be used in rlemonstratinr certain of rhe laws of chemical change. For example, the lawof conservation of mass is illustratedby showing that no observable mass change occurs when solutions of sodium chloride and silver nitrate are combined to form silver chloride" or that a flash bulb has the same mass before and after f l a ~ h i n g . ~

The law of definite proportions can be demonstrated by carrvine out a reaction which eenerates a volatile gas on the ~ ~~ . u - "

balance pan and then relating the loss of mass to the mass of the reactant(s). One reaction which eives eood reoroducible . . - "

results is the thermal decomposition of potassium chlorate in the presence of manganese dioxide catalyst to give potassium chloride and oxygen as indicated below.

2KC103 % ZKCl + 302 A

The reaction can he carried out by supporting a large test tube (25 X 200 mm) containing a small quantity (< 4 g) of thor- oughly mixed potassium chlorate-manganese dioxide ( 5 1

' Type 80, Chemtrix, Inc., Hillshoro, Oregon. Stanko, J. A,, Haas, C. G., and Lotz, J. R., J. CHEM. EDUC., 54,

365 (1977). Wseful demonstration weights are available from most scientific

supply firms. 'In practice, the food is best poured into a beaker on the balance

pan with the display set on zero. This reaction is used to illustrate the law of conservation of mass

in Jones, M. M., et al., "Chemistry, Man, and Society," 2nd ed., W. B. Saunders Ca., Philadelphia, 1976, p. 56.

"his reaction is used to illustrate the law of conservation of mass in Ouellette, R. J., "Introductory Chemistry," 2nd ed., Harper & Row Publishers, New York, 1975, p. 45.

Volume 55. Number 6. June 1978 / 381

mass ratio) on an asbestos board (8 X 8 in.) on the balance pan. For large capacity balances the test tube can be supported by a buret clamp and ring stand; however, for low capacity (< 1 Ke) balances the test tube can simolv be attached to a buret clamp and placed on the asbestos board on the balance pan with the end of the test tube extending out as far as ~ossible from the side of the balance. In the la& case the sideof the I~alanre over which thp test tuhp hanas should be orotected from heat by an asbestos board. ~ e a c n ~ the mixtbre in the test tube with a burner causes complete evolution of the oxygen within five minutes. That the weight loss observed is due to oxygen evolution is easily demonstrated by placing a glowing wood splint near the mouth of the test tuhe. The di- rect relationship between the mass of the potassium chlo- rate-manganese dioxide mixture used and the mass of oxygen lost is easily related to the constant ratio of the masses of po- tassium chloride and oxygen in potassium chlorate.

pH Meter Demonstrations

The concept of oH is a purely formal concept. Some meaning to the rrad& of a d i R i t n l p ~ meter ran fjrst bees- tahlishcd 11y the utilization of known arid and base solutions to show that the pH bears an inverse relationship to the hy- dronium ion concentration of the solutions. When colorless solutions are involved in the demonstrations, indicators can be used in conjunction with the digital pH meter for added effect.

pH of Known Solutions The pH is determined for 0.1 N HCI, 0.0001 N HC1, pure

water, O.M)01 N NaOH, and 0.1 N NaOH, and the relatianship between pH and hydronium ion concentration is discussed.

pH of Familiar Substances The pH of various familiar substances is effectively dem-

onstrated Ly the digital pH meter. Useful substances are: vinegar, lemon juice, milk, sour milk, tomato juice, carbonated beverage, sodium bicarbonate solution, and household am- monia cleaner.

Titration A dieital DH meter shows the Dromess of a twical acid-hme - . . - . .

titration much more effectively than does an indicator or pH paper. The rapid chanee of DH near the end point and the DH i t which an indicator changes are easily se;n.

Buffers Using a digital pH meter, the effect of adding acid or base

to a buffered solution is easily compared to the effect on pure water or the effect on an unbuffered solution. Standard ace- tate or phosphate buffers may be used or, for more relevance, a buffered headache tablet may be used as the buffer sys- t e n 7

7 Friedman, Norman, J. CHEM. EDUC., 52,605 (1975).

382 1 Journal of Chemical Education