Question: A fluoride electrode was used to determine

> Why is it necessary to buffer solutions in organic voltammetry?

> Why is the reference electrode placed near the working electrode in a three-electrode cell?

> Why is a high supporting electrolyte concentration used in most electroanalytical procedures?

> Define (a) Voltammograms. (b) Hydrodynamic voltammetry. (c) Nernst diffusion layer. (d) Dropping mercury electrode. (e) Half-wave potential. (f) Limiting current.

> Distinguish between (a) Voltammetry and amperometry. (b) Linear-scan voltammetry and cyclic voltammetry. (c) Differential-pulse voltammetry and square-wave voltammetry. (d) A rotating disk electrode and a ring-disk electrode. (e) A limiting current and

> (a) What are the advantages of performing voltammetry with microelectrodes? (b) Is it possible for an electrode to be too small? Explain your answer

> List several sources of uncertainty in pH measurements with a glass/calomel electrode system.

> An unknown cadmium (II) solution was analysed polarographically by the method of standard additions. A 25.00-mL sample of the unknown solution produced a diffusion current of 1.86 μA. Following addition of a 5.00-mL aliquot of 2.12 x 10-3 M Cd2+ standard

> It has been suggested that many polarograms can be obtained on a solution without depleting the electroactive analyte. Suppose that in a polarographic experiment we monitor the limiting current for 45 minutes in 60 mL of 0.08 M Cu2+. If the average curre

> Sulfate ion can be determined by an amperometric titration procedure using Pb2+ as the titrant. If the potential of a rotating mercury film electrode is adjusted to -1.00 V versus SCE, the current can be used to monitor the Pb2+ concentration during the

> Quinone undergoes a reversible reduction at a voltammetric working electrode. The reaction is (a) Assume that the diffusion coefficients for quinone and hydroquinone are approximately the same, and calculate the approximate halfwave potential (versus SC

> Suggest how Equation 21-13 could be used to determine the number of electrons n involved in a reversible reaction at an electrode.

> List the advantages and disadvantages of the hanging mercury drop electrode compared with platinum or carbon electrodes.

> What experimental variables affect concentration polarization in an electrochemical cell?

> How does a current in an electrochemical cell affect its potential?

> Describe three mechanisms responsible for the transport of dissolved species to and from an electrode surface.

> Briefly define (a) Ohmic potential. (b) Overvoltage. (c) Controlled-potential electrolysis. (d) Coulometric titration. (e) Current efficiency. (f) Potentiostat.

> Why is it necessary for the glass in the membrane of a pH-sensitive electrode to be appreciably hygroscopic?

> Briefly distinguish between (a) Concentration polarization and kinetic polarization. (b) A reference electrode and a working electrode. (c) Diffusion and migration. (d) An ampere and a coulomb. (e) The electrolysis circuit and the control circuit for con

> The nitrobenzene in 300 mg of an organic mixture was reduced to phenylhydroxylamine at a constant potential of −0.96 V (versus SCE) applied to a mercury cathode: The sample was dissolved in 100 mL of methanol. After electrolysis for 30

> Electrolytically generated I2 was used to determine the amount of H2S in 100.0 mL of brackish water. Following addition of excess KI, a titration at a constant current of 56.8 mA required 9.13 minutes. The reaction was Express the results of the analysi

> The CN− concentration of 10.0 mL of a plating solution was determined by titration with electrogenerated hydrogen ion to a methyl orange end point. A color change occurred after 3 minutes and 55 s with a current of 57.5 mA. Calculate the number of grams

> A 0.1330-g sample of a purified organic acid was neutralized by the hydroxide ion produced in 5 minutes and 24 s by a constant current of 300 mA. Calculate the equivalent mass of the acid in grams.

> Calculate the time needed for a constant current of 1.25 A to deposit 0.550 g of (a) Tl(III) as the element on a cathode. (b) Tl(I) as Tl2O3 on an anode. (c) Tl(I) as the element on a cathode.

> Calculate the time needed for a constant current of 0.8510 A to deposit 0.250 g of Co(II) as (a) Elemental cobalt on the surface of a cathode. (b) Co3O4 on an anode. Assume 100% current efficiency for both gases.

> A solution is 0.200 M in each of two reducible cations, A and B. Removal of the more reducible species (A) is deemed complete when [A] has been decreased to 1.00 × 10−5 M. What minimum difference in standard electrode poten

> Electrogravimetric analysis with control of the cathode potential is proposed as a means for separating Bi3+ and Sn2+ in a solution that is 0.250 M in each ion and buffered to pH 1.95. (a) Calculate the theoretical cathode potential at the start of depos

> Describe the source of pH dependence in a glass-membrane electrode.

> A solution is is 0.0350 M in BiO+ and 0.0250 M in Co2+ and has a pH of 2.50. (a) What is the concentration of the more easily reduced cation at the onset of deposition of the less reducible one? (b) What is the potential of the cathode when the concentra

> A solution is 0.200 M in Co2+ and 0.0650 M in Cd2+. Calculate (a) The Co2+ concentration in the solution as the first cadmium starts to deposit. (b) The cathode potential needed to lower the Co2+ concentration to 1.00 x 10-5 M. (c) Based on (a) and (b) a

> Nickel is to be deposited on a platinum cathode (area = 120 cm2) from a solution that is 0.250 M in Ni2+ and buffered to a pH of 2.50. Oxygen is evolved at a partial pressure of 1.00 atm at a platinum anode with an area of 75 cm2. The cell has a resistan

> Copper is to be deposited from a solution that is 0.250 M in Cu(II) and is buffered to a pH of 4.00. Oxygen is evolved from the anode at a partial pressure of 730 torr. The cell has a resistance of 3.60 Ω, and the temperature is 25°C. Calculate (a) The t

> The cell Sn|Sn2+ (6.18 x 10-4 M) || Cd2+(5.95 x 10-2 M)|Cd has a resistance of 3.95 Ω. Calculate the initial potential that will be needed for a current of 0.062 A in this cell.

> Calculate the initial potential needed for a current of 0.065 A in the cell if this cell has a resistance of 4.50 Ω.

> Calculate the theoretical potential at 25°C needed to initiate the deposition of (a) Copper from a solution that is 0.250 M in Cu2+ and buffered to a pH of 3.00. Oxygen is evolved at the anode at 1.00 atm. (b) Tin from a solution that is 0.150

> Why is an auxiliary reagent always required in a coulometric titration?

> Why is the working electrode normally isolated from the counter electrode in a controlled-potential coulometric analysis?

> What is the purpose of a depolarizer?

> What is meant by Nernstian behavior in an indicator electrode?

> How do electro gravimetric and coulometric methods differ from potentiometric methods? Consider currents, voltages, and instrumentation in your answer.

> What is a supporting electrolyte, and what is its role in electrochemistry?

> Describe conditions that favor kinetic polarization in an electrochemical cell.

> How do concentration polarization and kinetic polarization resemble one another? How do they differ?

> Quinone can be reduced to hydroquinone with an excess of electrolytically generated Sn(II): The polarity of the working electrode is then reversed, and the excess Sn(II) is oxidized with Br2 generated in a coulometric titration: Sn2+ + Br2 ( Sn4+ + 2Br-

> Traces of aniline, C6H5NH2, in drinking water can be determined by reaction with an excess of electrolytically generated Br2: The polarity of the working electrode is then reversed, and the excess Br2 is determined by a coulometric titration involving t

> Construct a coulometric titration curve of 100.0 mL of a 1 M H2SO4 solution containing Fe(II) titrated with Ce(IV) generated from 0.075 M Ce(III). The titration is monitored by potentiometry. The initial amount of Fe(II) present is 0.05182 mmol. A consta

> At a potential of −1.0 V (versus SCE), CCl4 in methanol is reduced to CHCl3 at a mercury cathode: 2CCl4 + 2H+ + 2e- + 2Hg(l) ( 2CHCl3 + Hg2Cl2(s) At -1.80 V, the CHCl3 further reacts to give CH4: 2CHCl3 + 6H+ + 6e- + 6Hg(l) ( 2CH4 + 3Hg

> The phenol content of water downstream from a coking furnace was determined by coulometric analysis. A 100-mL sample was rendered slightly acidic, and an excess of KBr was introduced. To produce Br2 for the reaction a steady current of 0.0703 A for 5 mi

> You need to choose between determining an analyte by measuring an electrode potential or by performing a titration. Explain which you would choose if you needed to know (a) The absolute amount of the analyte to a few parts per thousand. (b) The activity

> A lithium ion-selective electrode gave the potentials given next for the following standard solutions of LiCl and two samples of unknown concentration: (a) Construct a calibration curve of potential versus log aLi+, and determine if the electrode follow

> The F− concentration of a solution was determined by measurements with a liquid-membrane electrode. The electrode system developed a potential of 0.5021 V when immersed in 25.00 mL of the sample, and 0.4213 V after the addition of 2.00 mL of 5.45 x 10−2

> The Na+ concentration of a solution was determined by measurements with a sodium ion-selective electrode. The electrode system developed a potential of −0.2462 V when immersed in 10.0 mL of the solution of unknown concentration. After addition of 1.00 mL

> The titration of Fe(II) with permanganate yields a particularly asymmetrical titration curve because of the different number of electrons involved in the two half-reactions. Consider the titration of 25.00 mL of 0.1 M Fe(II) with 0 1. MMnO−4. The H+ conc

> A 40.00-mL aliquot of 0.05000 M HNO2 is diluted to 75.00 mL and titrated with 0 0800 MCe4+ The pH of the solution is maintained at 1.00 throughout the titration; the formal potential of the cerium system is 1.44 V. (a) Calculate the potential of the indi

> Calculate the potential of a silver indicator electrode versus the standard calomel electrode after the addition of 5.00, 15.00, 25.00, 30.00, 35.00, 39.00, 39.50, 36.60, 39.70, 39.80, 39.90, 39.95, 39.99, 40.00, 40.01, 40.05, 40.10, 40.20, 40.30, 40.40,

> A 0.4021-g sample of a purified organic acid was dissolved in water and titrated potentiometrically. A plot of the data revealed a single end point after 18.62 mL of 0.1243 M NaOH had been introduced. Calculate the molecular mass of the acid.

> The cell SCE||H+(a = x)| glass electrode has a potential of 0.2106 V when the solution in the right-hand compartment is a buffer of pH 4.006. The following potentials are obtained when the buffer is replaced with unknowns: (a) −0.2902V and (b) +0.1241V.

> The cell SCE||Ag2CrO4(sat'd), (x M)|Ag is used for the determination of pCrO4. Calculate pCrO4 when the cell potential is 0.389 V.

> Briefly describe or define a) Liquid junction potential. b) Boundary potential. c) Asymmetry potential. d) Combination electrode

> Why is atomic emission more sensitive to flame instability than atomic absorption?

> Define (a) Atomization. (b) Pressure broadening. (c) Doppler broadening. (d) Aerosol. (e) Plasma. (f) Nebulization. (g) Hollow-cathode lamp. (h) Sputtering. (i) Additive interference. (j) Ionization interference. (k) Chemical interference. (l) Radiation

> Describe the basic differences among atomic emission, atomic absorption, and atomic fluorescence spectroscopy.

> Lead was determined in a brass sample by atomic absorption and the method of standard additions. The original sample was dissolved and diluted to 50.0 mL. This solution was introduced into an AA spectrometer and an absorbance of 0.42 was obtained. To the

> Calculate (a) pI if the cell in Problem 19-17(a) has a potential of -196 mV. (b) pCl if the cell in Problem 19-17(b) has a potential of -0.137 V. (c) pPO4 if the cell in Problem 19-17(c) has a potential of 0.211 V. (d) pCN if the cell in Problem 19-17(d)

> The copper in an aqueous sample was determined by atomic absorption flame spectrometry. First, 10.0 mL of the unknown were pipetted into each of five 50.0-mL volumetric flasks. Various volumes of a standard containing 12.2 ppm Cu were added to the flasks

> The chromium in a series of steel samples was determined by ICP emission spectroscopy. The spectrometer was calibrated with a series of standards containing 0, 2.0, 4.0, 6.0, and 8.0 μg K2Cr2O7 per milliliter. The instrument readings for th

> A 5.00-mL sample of blood was treated with trichloroacetic acid to precipitate proteins. After centrifugation, the resulting solution was brought to pH 3 and extracted with two 5-mL portions of methyl isobutyl ketone containing the lead-complexing agent

> In the atomic absorption determination of uranium, there is a linear relationship between the absorbance at 351.5 nm and concentration from 500 to 2000 ppm of U. At concentrations much lower than 500 ppm, the relationship becomes nonlinear unless about 2

> Discuss the differences that result in ICP atomic emission when the plasma is viewed axially rather than radially.

> Why is the ICP rarely used for atomic absorption measurements?

> Name four characteristics of inductively coupled plasmas that make them suitable for atomic emission spectrometry.

> In flame AA with a hydrogen/oxygen flame, the absorbance for calcium decreases in the presence of large concentrations of phosphate ion. a) Suggest an explanation for this observation. b) Suggest three possible methods for overcoming the potential inte

> Why are the lines from a hollow-cathode lamp generally narrower than the lines emitted by atoms in a flame?

> Why are higher resolution monochromators found in ICP atomic emission spectrometers than in flame atomic absorption spectrometers?

> Generate an equation that relates pAnion to Ecell for each of the cells in Problem 19-17. Cells in Problem 19-17: (a) pI. (b) pCl. (c) pPO4. (d) pCN.

> Why is source modulation used in atomic absorption spectroscopy?

> Why are ionization interferences usually not as severe in the ICP as they are in flames?

> Why do some absorbing compounds show no fluorescence?

> Which compound in each of the following pairs would you expect to have a greater fluorescence quantum yield? Explain. a) b)

> Why are fluorescence methods potentially more sensitive than absorption methods?

> Briefly describe or define (a) Fluorescence. (b) Non-radiative relaxation. (c) Internal conversion. (d) Chemiluminescence. (e) Stokes shift. (f) Secondary absorption. (g) Inner-filter effect. (h) Triplet state.

> The determination in Problem 25-12 was modified to use the standard additions method. In this case, a 2.196-g tablet was dissolved in sufficient 0.10 M HCl to give 1.000 L. Dilution of a 20.00-mL aliquot to 100 mL produced a solution that gave a reading

> Quinine in a 1.664-g antimalarial tablet was dissolved in sufficient 0.10 M HCl to give 500 mL of solution. A 15.00-mL aliquot was then diluted to 100.0 mL with the acid. The fluorescence intensity for the diluted sample at 347.5 nm provided a reading of

> The volumes of a 1.10 ppm standard solution of Zn2+ shown in the following table were pipetted into separatory funnels each containing 5.00 mL of an unknown zinc solution. Each was extracted with three 5-mL aliquots of CCl4 containing an excess of 8-hydr

> The reduced form of nicotinamide adenine dinucleotide (NADH) is an important and highly fluorescent coenzyme. It has an absorption maximum of 340 nm and an emission maximum at 465 nm. Standard solutions of NADH gave the following fluorescence intensities

> Use the shorthand notation to describe a cell consisting of a saturated calomel reference electrode and a silver indicator electrode for the measurement of (a) pI. (b) pCl. (c) pPO4. (d) pCN.

> Why are fluorometers often more useful than spectrofluorometers for quantitative analysis?

> Why are phosphorescence lifetimes much longer than fluorescence lifetimes?

> Describe the components of a filter fluorometer and a spectrofluorometer.

> Explain why fluoresence emission often occurs at a longer wavelength than absorption.

> Describe the characteristics of organic compounds that fluoresce.

> What is(are) advantage(s) of the multiple standard additions method over the single-point standard addition method?

> What experimental variables must be controlled to assure reproducible absorbance data?

> What minimum requirement is needed to obtain reproducible results with a single-beam spectrophotom?

> Describe the differences between the following pairs of terms, and list any particular advantages of one over the other: a) Spectrophotometers and photometers. b) Single-beam and double-beam instruments for absorbance measurements. c) Conventional and

> Predict the shape of photometric titration curves (after correction for volume change) if—at the wavelength selected—the molar absorptivities for the analyte A, the titrant T, and the product P are as follows:

> a) Calculate E0 for the process b) Use the shorthand notation to describe a cell consisting of a Ag/AgCl reference electrode and a lead indicator electrode that could be used for the measurement of pCl. c) Generate an equation that relates the potential

> The equilibrium constant for the conjugate acid-base pair is 8.00 x 10-5. From the additional information a) Calculate the absorbance at 430 nm and 600 nm for the following indicator concentrations: 3.00 × 10-4 M, 2.00 × 10-4

> Estimate the frequencies of the absorption maxima in the IR spectrum of methylene chloride shown in Figure 24F-2. From these frequencies, assign molecular vibrations of methylene chloride to each of the bands. Notice that some of the group frequencies th

> Mercury(II) forms a 1:1 complex with triphenyltetrazolium chloride (TTC) that exhibits an absorption maximum at 255 nm.21 The mercury(II) in a soil sample was extracted into an organic solvent containing an excess of TTC, and the resulting solution was d