1

Analytical Chemistry Laboratory Techniques and Instrumentation

I. Basic Laboratory Tools and Operations: Terms to remember

Exercise 1. Match the following terms with the items below.

a. Reagent grade chemicals h. Analytical balance o. “TC 20oC”

b.Primary-standard grade chemicals i. macrobalance p. Desiccator

c.Special purpose reagent chemicals j. Semimicroanalytical q. Drierite

d. Tare k. Microanalytical r. Decantation

e. Weighing by difference l. Transfer pipets s. MSDS

f. Parallax m. Measuring pipets

g. Meniscus n. Micropipets

Items

1. mass of the empty vessel

2. the error that occurs when your eye is not at the same height as the top of the liquid

3. type of balance whose maximum capacity is 1g-1kg precision at max.cap.of at least 1/105

4. type of balance whose maximum load is 10-30g precision of 0.01 mg.

5. type of balance whose max. load is 1-3 g precision of 0.001 mg.

6. type of balance whose max. load is 160-200g precision of 0.l mg.

7. weighing technique used routinely and is necessary for hygroscopic reagents.

8. concave formed at the surface of most liquids.

9. means "to contain at 20°C"

10. also known as Mohr pipet and is calibrated like a buret, used to deliver a variable volume.

11. type of pipet which is calibrated to deliver one fixed volume.

12. a closed chamber containing a drying agent called a dessicant.

13. A type of dessicant

14. Conform to the minimum standards set forth by the Reagent Chemical Committee of the

American Chemical Society (ACS) and are used wherever possible in analytical work.

15. Reagents that have been carefully analyzed by the supplier and the assay is printed on the container label. ·

16. Chemicals prepared for a specific application

17. A listing of hazards and safety precautions for a chemical sold in the US. It also gives first aid

procedures and instructions for handling spills.

18. process of pouring a liquid gently so as to not disturb a solid in the bottom of the container.

II. Steps in a typical Quantitative Analysis

1. Select method -level of accuracy required, no. of samples to be analyzed, complexity and no of

components in the sample.

2. Sampling- involves obtaining a small mass of a material whose composition accurately

represents the bulk of the material being sampled; source of greatest error

3. Prepare a laboratory sample- decrease particle size to assure homogeneity, storage, drying

/moisture determination.

4. Define replicate samples- portions of a material of approximately the same size that are carried through an analytical procedure at the same time and in the same way.

5. Dissolve the samples- water, acids, bases, oxidizing agents, reducing agents, combination of

reagents, ignition, and high temperature fusion of sample in presence of fluxes

6. Eliminating interferences- an interference is a species that causes an error by enhancing or

attenuating (making smaller) the quantity being measured in an analysis. Specific techniques­

work for only one analyte; selective techniques- work only for only a few analytes.

7. Calibration and Measurement- Calibration- empirical determination of the relationship between the quantity measured (X) in an analysis and the analyte concentration CA. Ideally CA = kX where k is a

proportionality.

8. Calculate results

9. Estimate the reliability of results: An analytical result without an estimate of reliability is of no

value.

III. Experimental Error and Basic Statistics

Terms to remember: mean, median, systematic error, random error, precision, accuracy, absolute

error, relative error, standard reference materials, blank, outlier, bias, gross error

Exercise 2: Identify the terms described.

1. contains the solvent and all of the reagents in an analysis but none of the sample.

2. a substance prepared and sold by the National Institute of Standards and Technology and certified

to contain specified concentrations of one or more analytes.

3. measures the systematic error associated with an analysis. It has a negative sign if it causes the results

to be low and a positive sign otherwise.

4. occasional result in replicate measurements that obviously differs significantly from the rest of

the results.

5. the absolute error divided by the true value.

6. difference between the measured value and the true value. It bears a sign.

7. closeness of a result to its true or accepted value.

8. closeness of data to other data that have been obtained in exactly the same way.

9. middle value in a set of data that has been arranged in order of size. Middle value for odd number

of results, mean of the middle pair for even number of results.

10. average value

Exercise 3. Identify types of errors

1. Instrument, method, personal. Originates from a fixed cause; affects accuracy of results.

2. Originates from indeterminate processes, affects precision of results

3. Occur only occasionally and are often large; lead to outlier results

Basic Statistic Equations:

Standard deviation, √∑

Variance = s2

Student's t test: used to compare one set of measurements with another to decide whether or not they are

the same. This is often used to decide whether two sets of replicate measurements obtained using two

different methods each with its own standard deviation, give "the same” or "different" results within a stated confidence level.

√

√

tcalculated compared with t from statistical table for n1 + n2 -2 degrees of freedom. If tcalculated is greater than

t t a b l e at the 95% confidence level, the two results are considered to be different.

Q-test - this is used to help decide whether to retain or discard a questionable datum. To apply the

Q test arrange the data in order of increasing value and calculate Q, defined as

where Xq = questionable result

Xn = nearest value

w = spread or range

If Q calculated > Q table, the questionable point should be discarded

Confidence interval: an expression stating that the true mean, , is likely to lie within a certain

distance from the measured mean, x . The confidence interval is given by

√ where s is the measured standard deviation, n is the number of observations, t is the

student's t taken from table.

Exercise 4: Traces of toxic, man-made hexachlorohexanes in North Sea sediments were extracted by a

conventional method and by a new procedure and measured by chromatography. Is the mean concentration

found by new procedure significantly different from that of the conventional procedure? t95% = 2.228 for 10

degrees of freedom.

.

Method Concentration found (pg/g)

Conventional 34.4 ± 3.6 (n=6)

New Procedure 42.9 ± 1.2 (n =6)

Exercise 5: Using Q test, decide whether the value 216 should be rejected from the set of

results192, 216, 202, 195 and 204. For 5 measurements, the Q table at the 95% confidence level is 0.710.

IV. Uncertainties in Experimental Results

• Expressed as standard deviation or confidence interval

• Based on series of replicate measurements

• Applies only to random error

Summary of rules for propagation of uncertainty

Function Uncertainty Function Uncertainty

√

√

√

√

• Retain one or more insignificant figures to avoid introduction of round-off errors

into later calculations.

Exercise 6: Calculate the absolute uncertainty and % RSD of the results in the following

arithmetic operations. Express final answer with the correct number of significant figures.

A. 1.76 (± 0.03) + 1.39 (± 0.02) - 0.59 (± 0.02) = 3.06 ±?

B. 1.76(±0.03) x 1.89(±0.02) = 5.6 ?

0.59(±0.02)

C. 1.76(±0.03)-0.59(±0.02) =

0.6190 ± ?

1.89(±0.02)

D. Find [H+] and its uncertainty for pH= 5.21 ± 0.03

V. Calibration Methods

Calibration curve: shows the response of an analytical method to known quantities of analyte.

External calibration method.

• Solutions containing known concentrations of analyte (standard solutions) are prepared covering

a convenient range of concentrations.

• Measure response of the analytical procedure to these standards.

• Subtract average response of the blank samples from each measured response to obtain corrected response. The blank measures the response of the procedure when no analyte is present

• Make a graph of corrected response vs. concentration of analyte standards.

• Use least-squares procedure to find the best straight line through the linear portion of the data.

The equation of the linear calibration line is y = mx +b, where m= slope, b=y-intercept,

y=response, x=concentration.

• Analysis of unknown. Read response of unknown. Obtain corrected absorbance.

Terms to remember:

Linear range of an analytical method is the analyte concentration range over which response is

proportional to concentration.

Dynamic range: concentration range over which there is a measurable response to analyte, even if the

response is not linear.

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Standard Addition Method- Known quantities of analyte are added to the unknown. From the increase in signal we

deduce how much analyte was in the original unknown.

Terms to remember:

Matrix- everything in the unknown other than analyte.

Matrix effect- change in the analytical signal caused by anything in the sample other than analyte.

Spike- analyte deliberately added.

1. Single standard addition: When signal is directly proportional to analyte concentration.

Case a. Analytical signal is measured before and after addition of standard to a solution containing

analyte.

Rearranging gives

where Cx = Concentration of analyte solution Vx

Cs = concentration of standard solution

Vs = volume of standard solution add4d

Ax= Analytical signal of solution with analyte

Ax+s = Analytical signal of solution with analyte + standard

VT =total volume= Vx + Vs

Exercise 7: Serum containing Na+ gave a signal of 4.27 mV in an atomic emission analysis. Then 5.00 mL of 2.08 M NaCl were added to 95.0 mL of serum. This spiked serum gave a signal of 7.98 mV. Find the original concentration of Na+ in the serum.

Case b. Two solutions are prepared; one solution contains analyte only and the other contains analyte

and standard. Both solutions are diluted to .the same volume before measurement of analytical signal.

Rearranging gives

6

Exercise 8: A 4.97 -g petroleum specimen was decomposed by wet-ashing and subsequently diluted to

500 mL in a volumetric flask. Cobalt was determined by treating 25.00 mL aliquots of this diluted

solution as follows:

Sample Co(II), 3.00 ppm Ligand H2O Total Volume Absorbance

25.00mL 0.00 mL 20.00mL 5.00mL 50.00mL 0.398

' 25.00mL 5.00 mL 20.00mL 0.00 mL 50.00mL 0.510

Assume that the Co(II)/ligand chelate obeys Beer’s Law. Calculate the % of cobalt in the original sample.

2. Multiple Standard Addition

A Graphic procedure for standard addition with constant volume of sample. Technique used if

chemical analysis consumes solution.

• Pipet equal volumes of unknown into several volumetric flasks

• Add increasing volumes of standard to each flask and dilute to mark.

• Analyze solutions and construct graph of analytical signal (dependent y-variable) v s . concentration

of added analyte, after it has been mixed with sample, [S] (independent x-variable)

The x-intercept of the extrapoted line is the concentration of unknown, [X]r after it has been

diluted to the final volume.

Therefore [X] i = [X]f x Vf /Vo

• Or fit points with a least-square line y = mx + b. The x-intercept is obtained by setting y = 0:

0 = mx + b; x= -b/m = [X]f ; [X]i = [X]f x Vf/Vo

B. Successive standard addition to one solution.

• Technique used when chemical analysis does not consume solution. (Ex. In electrical potential

measurement)

• Measure signal at 0 addition.

• Make a standard addition which increases total volume of sample, measure signal again.

• Repeat process several times until original signal has increased by a factor of 1.5 to 3.

(

) (

)

is the corrected response. x-intercept = [X]o = original concentration of unknown.

Internal Standards

• A known amount of a compound, different from analyte is added to the unknown

Signal from analyte is compared with signal from the internal standard to find out how much

analyte is present.

• Use in chromatographic analysis. Chromatographic separation of unknown X and internal

standard S. Relative areas of the signals from X and Sallow us to find out how much X is in the

mixture.

• It is necessary first to measure the relative response of the detector to each compound.

• To use an internal standard, we prepare a known mixture of standard and analyte to measure the

relative response of the detector to the two species.

•

[X]f and [S]f are concentration of analyte and standard after they have been mixed together.

Exercise 9: In a preliminary experiment a solution containing 0.0837 M X and 0.0666 M S gave peak areas of

Ax = 423 and As= 347. (Areas are measured in arbitrary units by the instrument’s computer.). To analyze the

unknown, 10.0 mL of 0.146 M S were added to 10.0 mL of unknown and the mixture was diluted to 25.0 mL

in a volumetric flask. From the chromatogram, Ax = 553 and As = 582. Find the concentration of X in the

unknown

VI. Titrimetric Methods

Titration Titrant Primary Standard Indicator (pH

range)

Color

change/condition

1. Acid-base NaOH KHP H2Ph(8.2-9.8) Colorless to pink

HCl Na2CO3 Methyl

Orange(3.1-4.4)

Bromocresol

green(3.8-5.4)

Red to yellow

orange

Yellow to blue

2. Redox

Iodimetry

Iodin

soln.

Anhydrous

Na2S2O3

starch Colorless to deep

blue

Iodometry Na2S2O3 KIO3

(strongly acidic)

Cu, CuSO4

starch Deep blue to

colorless

Permanganimetry KMnO4 K2C2O4 KMnO4 self

Indicator

From _ to purple

Dichromate

Titration

K2Cr2O7 FAS(ferrous

ammonium sulfate)

Diphenylamine-

sulfonic acid

From reduced form

colorless to oxidized

form-violet.

3. Complexometry EDTA Titration

Na2EDTA CaCO3

MgCO3

Zn or Zn salt

Cu or Cu salt

EBT Wine red to blue (at pH 10)

4. Precipitation

Mohr Titration

Direct Chloride

determination

AgNO3 NaCl K2CrO4 Formation of red-

orange AgCrO4

precipitate (pH 7 to

10)

Volhard Titration

(Indirect

determination of

halide: Ag(I) is

added in excess

KSCN NaCl Fe(III) Formation of red

FeSCN2+ complex

Fajans Titration

(Adsorption Indicator Method)

AgNO3 NaCl Fluorescein The silver

fluoresceinate adsorbed on the

surface of the silver

chloride precipitate.

The solution

surrounding the solid

turns red.

Exercise 10: Predict the effect of the given condition on the indicated (parameter/calculated result).

1. The coin sample which was weighed has a higher temperature than the balance

(Mass of coin).

2. The coins were weighed by difference. (average weight of coins)

3. Calcium oxalate was precipitated at pH 3.0 instead of pH 4.0. (% CaO)

4. The permanganate solution was not filtered prior to its standardization and was used as is a week

later for the analysis of the unknown sample. (Volume of permanganate for the unknown

sample).

5. Bromthymol blue (pH range 6.0-8.0) was used as indicator for the first endpoint in the titration of carbonate

mixtures rather than H2Ph. (Volume of HCl, first endpt.).

6. Zn metal was used to standardize the EDTA solution for total hardness determination of water samples.

(Total hardness)

7. In the iodometric determination of Cu(II), starch was added at the start of the titration of the standard

Cu . (N of standard thiosulfate solution).

VII. Solvent Extraction: separation method based on difference in solubility of solute in two immiscible solvents

Nernst Distribution law for solute species A:

Distribution constant or partition coefficient:

Distribution coefficient or Distribution ratio:

where CA = total conc’n of A

Fraction of solute transferred to organic phase, p:

Fraction of solute remaining in aqueous phase, q:

Fraction of solute in aqueous phase after n transfers, qn:

% solute remaining in aqueous solution after n transfers: %A = qn x 100

Concentration of solute A in aqueous phase after n transfers, [A]n: [A]n =

x [A]o

Exercise 11: The distribution coefficient for Z between n-hexane and water is 6.25. Calculate % Z

remaining in 25.0 mL of water that was originally 0.0600 M in Z after extraction with five 5.00-mL portions.

VIII. Instrumental Methods of Analysis

Analytical Method Excitation Signal

Source

Sample

Cell

Detector/Sensor Qualitative

Parameter

Quantitative Parameter Measured

l.Spectro

UV Absorption

Spectrophotometry

Deuterium/Hydrogen

lamp

Quartz Phototubes,

PMT, silicon

photodiodes,

diode-array,

photovoltaic

cells

λ max

(wavelength

of optimum

absorption)

Absorbance

(Beer's Law)

Vis Absorption

Spectrophotometry

Tungsten-halogen

lamp

Quartz,

glass or

plastic cell

Phototubes,

PMT, silicon

photodiodes,

diode-array,

photovoltaic

λ max

(wavelength

of optimum

absorption)

Absorbance

(Beer's Law)

cells

Flame-AAS

Atomic Absorption

Spectrophotometry

Hollow Cathode

Lamp Flame (e.g.

acetylene- air)

Photomultiplier

tube(PMT)

Absorbance

(Beer's Law)

Electrothermal-

AAS

Hollow cathode

lamp

Graphite

furnace

PMT Absorbance

(Beer's Law)

Atomic Emission Flame or

ICP

PMT λ max

(wavelength

of optimum

emission)

Emission

Intensity, I

Spectrofluorometry Xenon lamp Quartz cell PMT λ max

(wavelength

of optimum

emission)

Fluorescence

Intensity, F

(F=kC)

2. Chrom

GC:Gas Chrom (Isothermal and Temperature Programming)

Flame

ionization (FID) Thermal Conductivity (TCD) Electron

Capture (ECD)

Retention time

Peak area or

peak height

(PA or PH=

kC)

HPLC (Isocratic

and Gradient)

Appropriate

HPLC flow cell for a given detector

UV-Vis,

Refractive

index (RI)

Electrochemical

detector

Retention

time

Peak area or peak

height (PA or

PH=

kC)

Criterion Figure of merit

1. Precision Absolute standard deviation; RSD, CV, variance

2. Bias Absolute systematic error, relative systematic error

3.Sensitivity Calibration sensitivity, analytical sensitivity

4. Detection limit 3𝜎 blank/m

5.Concentration range LOQ to LOL

6. selectivity Selectivity coefficient

3.

Electroanalytical

Ion-selective electrode (ISE) Method (Direct Potentiometry)

Concentration gradient at the electrode-solution

interlace

Any

appropriate

sample

container

ISEs Potential/pX

(Nernst

equation)

Potentiometric Titration(Indirect Potentiometry)

Addition of titrant

causing a change in

the potential of the

system

Any

appropriate

sample

container

Indicator

electrodes such

As Pt, Ag and

Au

Volume of titrant at equivalence point

Polarography Applied potential Polarographic cell

DME E1/2 Diffusion

current

ASV Applied potential Voltammetric

cell

GCE, Pt TFME Anodic peak

potential

Anodic peak

current or

Performance Characteristics of Instrumental Method; Figures of Merit.

UV-Vis- Spectroscopy- interaction of matter with the uv-vis region of the electromagnetic spectrum.

• Visible wavelength region: 380-750 nm

• Ultraviolet region: 200-380 nm

• Types of Instruments used for absorption measurements:

1. Spectrophotometer- employ grating or a prism monochromator to provide a narrow band of

radiation for measurements. (scanning)

2. Photometers- use an absorption filter or an interference filter for wavelength selection

• Designs:

1. Single beam- employs a fixed beam of radiation that irradiates first the solvent and then the analyte

solution.

2. Double beam- the solvent and solution are irradiated simultaneously

3. Multichannel or diode array spectrophotometers- detect the entire spectral range simultaneously.

Can produce a spectrum in < 1 second.

• Quantitative analysis based on Beer's Law

• Quantitative Approaches

1. Two component analysis

2. Standard addition Method

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Exercise 12. The absorption spectra of two colored substances HMR and HMO are determined and the

following data obtained in a 1.00-cm cell:

Solution Concentration A at 375nm A at 650nm

HMR alone 4.75 X 10-4M 0.726 0.0950

HMO alone 1.68 X 10-4 M 0.112 0.625

HMR+HMO unknown 0.595 0.925

Calculate concentration of HMR and HMO in the unknown solution.

Exercise 13. Draw a schematic diagram of an instrument used for UV-vis measurements. Give a short description

for each instrument component.

Molecular Photoluminescence

• Involves emission of certain wavelengths of light by some substances after electronic excitation by either UV

or vis light.

• Types:

1. fluorescence- emission of a photon during a transition between states with the same spin quantum numbers.

2. phosphorescence- emission of a photon during a transition between states with different spin quantum

numbers.

• Quantitative analysis based on I = KPoC

Exercise 14: Draw a schematic diagram of an instrument used to measure a sample luminescence. Give a short

description for each component.

Atomic Absorption Spectroscopy

• Involves interaction of monoatomic particles with UV-vis light

• Involves electronic transitions in which one or more of the electrons of the atoms is raised to a higher

energy level.

Instrument Components

• Hollow Cathode lamps

• Flame or electrothermal atomizer

• Monochromator

• Detector- radiation detectors.

• Quantitative analysis based on Beer's Law. Atomic absorption cannot provide qualitative information sinceit does not provide complete absorption spectra because of the discontinuous nature of radiation sources that must be used

Exercise 15: Draw a schematic diagram of an instrument used to measure atomic absorption. Give a short

description for each component.

Atomic Emission Spectrometry

• Provide both qualitative and quantitative information about an analyte.

• Identification of the elements present is based upon the peak wavelengths which are unique for each

element.

• ICP (Inductively Coupled Plasma) is the most popular source for emission spectrometry. Flame emission is

also used.

Plasma- a hot partially ionized gas. It contains relatively high concentrations of ions and

electrons.

• Atomic emission and atomic absorption instruments are similar except that no hollow cathode

lamp is required for emission measurement.

Exercise 16: Draw a schematic diagram of a typical atomic emission spectrometer. Give a short

description for each component.

Potentiometry

• Based on the measurements of a potential difference between two electrodes immersed in an analyte solution.

1. Direct Potentiometry (ISE) - measure activities of ions

2. Potentiometric Titration-determines volume at equivalence point without use of an indicator.

• Commonly used indicator electrodes 1.

1. Ion selective electrodes

2. Inert electrodes (Redox electrodes) - the only role of this type of electrode is to provide or accept electrons • Commonly used reference electrodes

1. Saturated Calomel electrode

2. Silver/silver chloride electrode

• Quantitative analysis based on Nernst Equation.

Exercise 17: An aqueous solution is to be analyzed for its free fluoride ion concentration by direct potentiometry. A 100.0-mL aliquot of this solution was measured with a fluoride ISE electrode and gave a reading of -120.0 mV vs. a suitable reference electrode. Exactly 1.00 mL of a 0.100 M solution of KF is added to the test solution with stirring and the potential changed to -132 mV. Calculate the fluoride ion concentration in the sample.

Exercise 18.The benzoic acid (C6H5COOH) extracted from a 100.0 g banana catsup was titrated potentiometrically with 0.05555 M NaOH. Given below is a portion of the potentiometric data for the determination of % Na benzoate in the catsup sample. The extracted benzoic acid was diluted to 100.0 mL and was then titrated with std. NaOH.

NaOH, mL pH NaOH, mL pH

14.40 5.25 15.00 6.75

14.60 5.32 15.20 9.16

14.80 ' 5.89 15.40 10.04

A Tabulate the 1st and 2nd derivative point

B. Determine the equivalence point from the 2nd derivative graph or points.

C. Calculate % sodium benzoate (NaC6H5COO) of the sample.

D. Determine Ka of benzoic acid.

Voltammetry

An electroanalytical technique in which a varying potential is applied to an indicator electrode and the current that is

generated is monitored as a function of the applied potential. The graph that results is called a voltammogram.

• Typical indicator electrodes used

13

1. Dropping Mercury Electrode (DME) -most common. Voltammetry at DME is termed

polarography.

2. Pt wire

3. Other noble metal electrodes (Au)

• Voltammetric equipment consists of:

1. Potentiostat - which applies a varying potential to the indicator electrode

2. Voltammetric cell- consisting of the indicator electrode, reference electrode and an auxiliary

electrode

• Terms to remember: half-wave potential, limiting current, diffusion current, residual current • Quantitative analysis is based on Ilkovic Equation, where m in mg/s, D in cm2/s, id in µA, c in mM.

(id)max = 708 n D1/2 m

2/3 t1/6 c (id)ave = 607 n D1/2 m2/3 t1/6 c

Exercise 19: A 5 x 10-3 M solution of CaCl2 in 0.1 M KCl shows a diffusion current at -0.8 V versus SCE of

50.0 µA. The mercury is dropping at a rate of 18.0 drops per minute. Ten drops are collected and found to

weigh 3.82 x 10-2 g.

a) Calculate the diffusion coefficient D.

b) If the capillary were replaced by another, for which the drop-time is 3.0 sec., and 10 drops weigh

4.20 x 10-2 g, what will be the new value of the diffusion current?

Chromatography

• Refers to any separation method in which the components are distributed between a stationary

phase and a moving (mobile) phase.

• Base on types of mobile and stationary phases, classified as

1. liquid chromatography

2. gas chromatography

3. supercritical-fluid chromatography

Resolution Equation: R =

√

• Optimize each term to increase resolution

• k' = capacity factor = t'R /to (also known as retention factor, k)

• α = selectivity = t'R(B) / t'R (A)

• N =theoretical plate number= 16 (tR/W)2

• R = Resolution = t/ 1/2(W(A) + W(B) )

Gas-liquid Chromatography

Components:

l. Carrier gas- includes He, Ar, N2 and H2

2. flow controller or pressure regulator

3. Injection port

4. Column-conventional "packed" and Capillary column

5. Detector- BCD, FID and TCD

Liquid Chromatography

Components:

1. Eluent Reservoir

2. Pump

3. Sample Injector

4. Column

5. Detector- Refractive index, electrochemical, UV/vis

14

Separation Modes

1. Normal phase

2. Reversed phase

3. Adsorption

4. Size exclusion

5. Ion-exchange

Quantitation techniques:

1. Internal standard calibration

2. Area normalization

Supercritical fluid chromatography

• a hybrid of gas and liquid chromatography, MP is a supercritical fluid (physical state of a substance

that is held above its critical temperature) - usually CO2

• density of a supercritical fluid is 0.2 to 0.5 g/cm3 200 to 400 times greater than that of the corresponding

gas, and approaches that of the substance in its liquid state will dissolve large nonvolatile molecules

GC: temperature programming I HPLC: gradient elution or solvent programming I SFC: pressure

programming

Calculate the ppm Cr m the sample.

Exercise 21. To prepare a solution of NaCl, you weigh out 2.634 ( 0.002)g and dissolve it in a

volumetric flask whose volume is 100.00( 0.08) mL. Express the molarity of solution, along with its

uncertainty with correct no. of significant figures. MW NaCl = 58.4425 ( 0.0009) g/mol.

Exercise 22. The chromium in an aqueous sample was determined by pipetting 10.0 mL of the unknown

into each of five 50.0 mL volumetric flasks. Various volumes of a standard containing 12.2 ppm Cr were

added to the flasks, and the solutions were then diluted to volume.

Unknown Standard, mL Absorbance 10.0 0.0 0.201 10.0 10.0 0.292 10.0 20.0 0.378 10.0 30.0 0.467 10.0 40.0 0.554

Calculate the ppm Cr in the sample. Exercise 23: A mixture of methyl esters of fatty acids was chromatographed on a Carbowax 20M giving The following peak areas and detector response factors:

ester Peak, area, cm2 Response factor Methyl-n-butyrate 2.95 0.81

Methyl-iso-valerate 0.86 0.88 Methyl-n-octonoate 1.66 0.98 Methyl-n-decanoate 4.52 1.00

Calculate the percentage composition of the mixture by area normalization.

Exercise 20. Standard Addition: An unknown sample of Ni2+ gave a current of 2.36𝞵A in an electrochemical

analysis. When 0.500 mL of solution containing 0.0287 M Ni2+ was added to 25.0 mL of unknown, the

current increased to 3.79 𝞵A. Find the concentration of Ni2+ in the unknown.

15

Exercise 24. The following calibration data were obtained by an instrumental method for the

determination of the species X in aqueous solution.

Conc. X, ppm No.Replications, N Mean Analytical

Signal, S Standard

Deviation, ppm

0.00 25 0.031 0.0079

2.00 5 0.173 0.0094

6.00 5 0.422 0.0084

10.00 5 0.702 0.0084

14.00 5 0.956 0.0085

18.00 5 1.248 0.0110

A Calculate the calibration sensitivity.

B. Calculate the analytical sensitivity at each concentration.

C. What is the detection limit for the method

Exercise 25. Internal Standard: A solution was prepared by mixing 5.00 mL of unknown (element X) with 2.00 mL of solution containing 4.13 µg of standard (element S) per milliliter and diluting to 10.0 mL. The measured signal ratio in an atomic absorption experiment was (signal due to X)/(signal due to S) = 0.808. In a separate experiment, it was found that for equal concentrations of X and S, the signal due to X was 1.31 times more intense than the signal due to S. Find the concentration of X in the unknown.

IX. Other Techniques

1. Nuclear Magnetic Resonance (NMR) spectroscopy is based on the measurement of absorption of

electromagnetic radiation in the radio frequency region. Nuclei absorb electromagnetic radiation in a strong

magnetic field as a result of the energy splitting that is induced by the magnetic field.

2. Mass Spectrometry (MS) is a technique in which gaseous molecules are ionized, accelerated by an electric

field, separated according to their mass-to-charge ratio, and the amount of each species is detected.

3. Nuclear and Related Techniques a. X-ray fluorescence (XRF) spectroscopy is based on the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays. The technique is used for elemental analysis and chemical analysis. b. Neutron activation analysis (NAA) is based on the detection and measurement of characteristic gamma rays emitted from radioactive isotopes produced in the sample upon irradiation with neutrons. The emitted radiation is a 'fingerprint' of the element, and the amount of radiation given off at a certain energy is indicative of the amount of the element present in the sample. 4. Polarimetry measures the extent to which a substance interacts with plane polarized light (light which consists of waves that vibrate only in one plane); whether it rotates plane polarized light to the left or to the right (optically active), or not at all. 5. Refractometry measures how light is refracted when it passes through a given substance. The amount by which the light is refracted determines the refractive index. Refractive index can be used to identify an unknown liquid compound, or it can be used as a means of measuring the purity (if a liquid compound by comparing it to literature values. Refractive index is defined as the ratio of the velocity of light in air to the velocity of light in the medium being measured. 6. Turbidimetry is a method for determining the concentration of a substance in a solution by measuring the loss in intensity of a light beam through a solution that contains suspended particulate matter.