(a) When the difference in pH across the membrane of a glass electrode at 25C is 4.63 pH

The apparatus in Figure 14-7 was used to monitor the titration of 50.0 mL of 0.100 M AgNO3 with 0.200 M NaBr. Calculate the cell voltage at each volume of NaBr, and sketch the titration curve: 1.0, 12.5, 24.0, 24.9, 25.1, 26.0, and 35.0 mL.

The apparatus in the figure can follow the course of an EDTA titration and was used to generate the curves in Figure 11-10. The heart of the cell is a pool of liquid Hg in contact with the solution and with a Pt wire. A small amount of HgY2 added to the analyte equilibrates with a very tiny amount of Hg2: (A) The redox equilibrium is established rapidly at the surface of the Hg electrode, so the Nernst equation for the cell Hg2  2e Hg(l) Kf  [HgY2] [Hg2][Y4]  1021.5 Hg2  Y4 HgY2 Pt wi

A solid-state fluoride ion-selective electrode responds to F but not to HF. It

One glass-membrane sodium ion-selective electrode has a selectivity coefficient . When this electrode was immersed in 1.00 mM NaCl at pH 8.00, a potential of 38 mV (versus S.C.E.) was recorded. (a) Neglecting activity coefficients, calculate the potential with Equation 14-10 if the electrode were immersed in 5.00 mM NaCl at pH 8.00. (b) What would the potential be for 1.00 mM NaCl at pH 3.87? You can see that pH is a critical variable for the sodium electrode.

An ammonia gas-sensing electrode gave the following calibration points when all solutions contained 1 M NaOH. NH3 (M) E (mV) NH3 (M) E (mV) 1.00  105 268.0 5.00  104 368.0 5.00  105 310.0 1.00  103 386.4 1.00  104 326.8 5.00  103 427.6 A dry food sample weighing 312.4 mg was digested by the Kjeldahl procedure (Section 10-8) to convert all nitrogen into NH4 . The digestion solution was diluted to 1.00 L, and 20.0 mL were transferred KPot Na,H  36 to a 100-mL volumetric flask. The 20.0-mL aliquot was treated with 10.0 mL of 10.0 M NaOH plus enough NaI to complex the Hg catalyst from the digestion and diluted to 100.0 mL. When measured with the ammonia electrode, this solution gave a reading of 339.3 mV. Calculate the wt% nitrogen in the food sample. 14-F.

H2S from cigarette smoke was collected by bubbling smoke through aqueous NaOH and measured with a sulfide ion-selective electrode. Standard additions of volume VS containing Na2S at concentration cS  1.78 mM were then made to V0  25.0 mL of unknown and the electrode response, E, was measured. VS (mL) E (V) VS (mL) E (V) 0 0.046 5 3.00 0.030 0 1.00 0.040 7 4.00 0.026 5 2.00 0.034 4 From a separate calibration curve, it was found that  0.985 in Equation 14-14. Using T  298.15 K and n  2 (the charge of S2), prepare a standard addition graph with Equation 14-15 and find the concentration of sulfide in the unknown. P

(a) Write the half-reactions for the silver-silver chloride and calomel reference electrodes. (b) Predict the voltage for the following cell.

Convert the following potentials. The Ag | AgCl and calomel reference electrodes are saturated with KCl. (a) 0.523 V versus S.H.E.  ? versus Ag | AgCl (b) 0.111 V versus Ag | AgCl  ? versus S.H.E. (c) 0.222 V versus S.C.E.  ? versus S.H.E. (d) 0.023 V versus Ag | AgCl  ? versus S.C.E. (e) 0.023 V versus S.C.E.  ? versus Ag | AgCl 14

Suppose that the silver-silver chloride electrode in Figure 14-2 is replaced by a saturated calomel electrode. Calculate the cell voltage if [Fe2] / [Fe3]  2.5  103 .

From the following potentials, calculate the activity of Cl in 1 M KCl.

For a silver-silver chloride electrode, the following potentials are observed: From these potentials, find the activity of Cl in saturated KCl. Calculate E for a calomel electrode saturated with KCl, given that E for the calomel electrode is 0.268 V. (Your answer will not be exactly the value 0.241 used in this book.) E

A cell was prepared by dipping a Cu wire and a saturated calomel electrode into 0.10 M CuSO4 solution. The Cu wire was attached to the positive terminal of a potentiometer and the calomel electrode was attached to the negative terminal. (a) Write a half-reaction for the Cu electrode. (b) Write the Nernst equation for the Cu electrode. (c) Calculate the cell voltage.

Explain why a silver electrode can be an indicator electrode for Ag and for halides.

A 10.0-mL solution of 0.050 0 M AgNO3 was titrated with 0.025 0 M NaBr in the cell Find the cell voltage for 0.1 and 30.0 mL of titrant.

A solution containing 50.0 mL of 0.100 M EDTA buffered to pH 10.00 was titrated with 50.0 mL of 0.020 0 M Hg(ClO4)2 in the cell shown in Exercise 14-B: From the cell voltage E  0.027 V, find the formation constant of Hg(EDTA)2. 1

Consider the cell , whose voltage is 0.126 V. The cell solution contains 2.00 mmol of Fe(NH4)2(SO4)2, 1.00 mmol of FeCl3, 4.00 mmol of Na2EDTA, and lots of buffer, pH 6.78, in a volume of 1.00 L. (a) Write a reaction for the right half-cell. (b) Find the quotient [Fe2]/[Fe3] in the cell solution. (This expression gives the ratio of uncomplexed ions.) (c) Find the quotient of formation constants: (Kf for FeEDTA)/ (Kf for FeEDTA2). S.

CN)2 25.0 mL of 0.400 M acid, HA, with pKa  9.50 25.0 mL of KOH solution The measured voltage was 0.440 V. Calculate the molarity of the KOH solution. Assume that essentially all copper(I) is Cu(CN)2 . A little HCN comes from the reaction of KCN with HA. Neglect the small amount of HA consumed by reaction with HCN. For the right half-cell, the reaction is . Suggested procedure: From E, find [CN]. From [CN], find pH. From pH, figure out how much OH had been added. Jun

What causes a junction potential? How does this potential limit the accuracy of potentiometric analyses? Identify a cell in the illustrations in Section 13-2 that has no junction potential.

Why is the 0.1 M HCl | 0.1 M KCl junction potential of opposite sign and greater magnitude than the 0.1 M NaCl | 0.1 M KCl potential in Table 14-2?

Which side of the liquid junction 0.1 M KNO3 | 0.1 M NaCl will be negative?

Refer to the footnote in Table 14-1. How many seconds will it take for (a) H and (b) NO 3 to migrate a distance of 12.0 cm in a field of 7.80  103 V/m?

Suppose that an ideal hypothetical cell such as that in Figure 13-7 were set up to measure E for the half-reaction . (a) Calculate the equilibrium constant for the net cell reaction. (b) If there were a junction potential of 2 mV (increasing E from 0.799 to 0.801 V), by what percentage would the calculated equilibrium constant increase? (c) Answer parts (a) and (b), using 0.100 V instead of 0.799 V for the value of E for the silver reaction. 1

Explain how the cell Ag(s) | AgCl(s) | 0.1 M HCl | 0.1 M KCl | AgCl(s) | Ag(s) can be used to measure the 0.1 M HCl | 0.1 M KCl junction potential.

Henderson equation. The junction potential, Ej , between solutions  and can be estimated with the Henderson equation: where zi is the charge of species i, ui is the mobility of species i (Table 14-1), Ci() is the concentration of species i in phase , and Ci( ) is the concentration in phase . (Activity coefficients are neglected in this equation.)

Describe how you would calibrate a pH electrode and measure the pH of blood (~7.5) at 37C. Use the standard buffers in Table 14-3.

List the sources of error associated with pH measurement with the glass electrode.

If electrode C in Figure 14-18 were placed in a solution of pH 11.0, what would the pH reading be?

Which National Institute of Standards and Technology buffer(s) would you use to calibrate an electrode for pH measurements in the range 34?

Why do glass pH electrodes tend to indicate a pH lower than the actual pH in strongly basic solution?

Suppose that the Ag | AgCl outer electrode in Figure 14-11 is filled with 0.1 M NaCl instead of saturated KCl. Suppose that the electrode is calibrated in a dilute buffer containing 0.1 M KCl at pH 6.54 at 25C. The electrode is then dipped in a second buffer at the same pH and same temperature, but containing 3.5 M KCl. Use Table 14-2 to estimate how much the indicated pH will change.

(a) When the difference in pH across the membrane of a glass electrode at 25C is 4.63 pH units, how much voltage is generated by the pH gradient? (b) What would the voltage be for the same pH difference at 37C? 1

When calibrating a glass electrode, 0.025 m potassium dihydrogen phosphate/0.025 m disodium hydrogen phosphate buffer (Table 14-3) gave a reading of 18.3 mV at 20C and 0.05 m potassium hydrogen phthalate buffer gave a reading of 146.3 mV. What is the pH of an unknown giving a reading of 50.0 mV? What is the slope of the calibration curve (mV/pH unit) and what is the theoretical slope at 20C? Find the value of in Equation 14-6. 14

Activity problem. The 0.025 0 m KH2PO4/0.025 0 m Na2HPO4 buffer (6) in Table 14-3 has a pH of 6.865 at 25C. (a) Show that the ionic strength of the buffer is   0.100 m. (b) From the pH and K2 for phosphoric acid, find the quotient of activity coefficients, , at   0.100 m. (c) You have the urgent need to prepare a pH 7.000 buffer to be used as a calibration standard.45 You can use the activity coefficient ratio from part (b) to accurately prepare such a buffer if the ionic strength is kept at 0.100 m. What molalities of KH2PO4 and Na2HPO4 should be mixed to obtain a pH of 7.000 and   0.100 m?

Explain the principle of operation of ion-selective electrodes.

What does the selectivity coefficient tell us? Is it better to have a large or a small selectivity coefficient?

What makes a liquid-based ion-selective electrode specific for one analyte?

Why is it preferable to use a metal ion buffer to achieve pM  8 rather than just dissolving enough M to give a 108 M solution?

To determine the concentration of a dilute analyte with an ion-selective electrode, why do we use standards with a constant, high concentration of an inert salt?

A cyanide ion-selective electrode obeys the equation The potential was 0.230 V when the electrode was immersed in 1.00 mM NaCN. (a) Evaluate the constant in the equation. (b) Using the result from part (a), find [CN] if E  0.300 V. (c) Without using the constant from part (a), find [CN] if E  0.300 V. 14-34

By how many volts will the potential of an ideal Mg2 ionselective electrode change if the electrode is removed from 1.00  104 M MgCl2 and placed in 1.00  103 M MgCl2 at 25C? 14

When measured with a F ion-selective electrode with a Nernstian response at 25C, the potential due to F in unfluoridated groundwater in Foxboro, Massachusetts, was 40.0 mV more positive than the potential of tap water in Providence, Rhode Island. Providence maintains its fluoridated water at the recommended level of 1.00 0.05 mg F/L. What is the concentration of F in mg/L in groundwater in Foxboro? (Disregard the uncertainty.) 14-3

The selectivities of a Li ion-selective electrode are indicated on the following diagram. Which alkali metal (Group 1) ion causes the most interference? Which alkaline earth (Group 2) ion causes the most interference? How much greater must be [K] than [Li] for the two ions to give equal response?

A metal ion buffer was prepared from 0.030 M ML and 0.020 M L, where ML is a metal-ligand complex and L is free ligand. Calculate the concentration of free metal ion, M, in this buffer.

Calibration curve and propagation of uncertainty for exponents. The following data were obtained when a Ca2 ion-selective electrode was immersed in standard solutions whose ionic strength was constant at 2.0 M. Ca2 (M) E (mV) 3.38  105 74.8 3.38  104 46.4 3.38  103 18.7 3.38  102 10.0 3.38  101 37.7 (a) Prepare a calibration curve and find the least-squares slope and intercept and their standard deviations. (b) Calculate the value of in Equation 14-13. (c) For a measured potential, the calibration curve gives us log[Ca2]. We can compute . Using rules for propagation of uncertainty in Table 3-1, calculate [Ca2] and its associated uncertainty of a sample that gave a reading of 22.5 (0.3) mV in four replicate measurements. 14-39. T

The selectivity coefficient, , for a Li ion-selective electrode is 4  104 . When this electrode is placed in 3.44  104 M Li solution at pH 7.2, the potential is 0.333 V versus S.C.E. What would be the potential if the pH were lowered to 1.1 and the ionic strength were kept constant? 14

Standard addition. A particular CO2 compound electrode like the one in Figure 14-28 obeys the equation E  constant [ RT (ln 10)/2F] log[CO2], where R is the gas constant, T is temperature (303.15 K), F is the Faraday constant, and  0.933 (measured from a calibration curve). [CO2] represents all forms of dissolved carbon dioxide at the pH of the experiment, which was 5.0. Standard additions of volume VS containing a standard concentration cS  0.020 0 M NaHCO3 were made to an unknown solution whose initial volume was V0  55.0 mL. VS (mL) E (V) VS (mL) E (V) 0 0.079 0 0.300 0.058 8 0.100 0.072 4 0.800 0.050 9 0.200 0.065 3 Prepare a graph with Equation 14-15 and find [CO2] in the unknown.

Standard addition with confidence interval. Ammonia in seawater was measured with an ammonia-selective electrode. A 100.0 mL aliquot of seawater was treated with 1.00 mL of 10 M NaOH to convert NH 4 to NH3. Therefore, V0  101.0 mL. A reading was then taken with the electrode. Then a series of 10.00 mL aliquots of standard NH 4Cl were added with results in the table. VS (mL) E (V) VS (mL) E (V) 0 0.084 4 30.00 0.039 4 10.00 0.058 1 40.00 0.034 7 20.00 0.046 9 SOURCE: Data derived from H. Van Ryswyk, E. W. Hall, S. J. Petesch, and A. E. Wiedeman, Extending the Marine Microcosm Laboratory, J. Chem. Ed. 2007, 84, 306. The standard contains 100.0 ppm (mg/L) of nitrogen in the form of NH 4Cl. A separate experiment determined that the electrode slope RT (ln 10)/F is 0.056 6 V. (a) Prepare a standard addition graph. Find the concentration and 95% confidence interval for ammonia nitrogen (ppm) in the 100.0 mL of seawater. KPot

Data below come from the graph in Box 14-2, for which the separate solutions method was used to measure selectivity coeffi- cients for a sodium ion-selective electrode at 21.5C. Use Equation 14-11 to calculate log KPot for each line below.

The H ion-selective electrode on the Phoenix Mars Lander has selectivity coefficients and . Let A be the primary ion sensed by the electrode and let its charge be zA. Let X be an interfering ion with charge zX. The relative error in primary ion activity due to an interfering ion is46 This expression applies for errors less than ~10%. If the pH is 8.0 ( ) and , what is the relative error in measuring ? If the pH is 8.0 and , what is the relative error in measuring ?

A Ca2 ion-selective electrode was calibrated in metal ion buffers with ionic strength fixed at 0.50 M. Using the following electrode readings, write an equation for the response of the electrode to Ca2 and Mg2. [Ca2] (M) [Mg2] (M) mV 1.00  106 0 52.6 2.43  104 0 16.1 1.00  106 3.68  103 38.0 14-45

The Pb2 ion buffer used inside the electrode for the colored curve in Figure 14-27 was prepared by mixing 1.0 mL of 0.10 M Pb(NO3)2 with 100.0 mL of 0.050 M Na2EDTA. At the measured pH of 4.34, (Equation 11-4). Show that [Pb2]  1.4  1012 M.

The Pb2 ion buffer used inside the electrode for the colored curve in Figure 14-27 was prepared by mixing 1.0 mL of 0.10 M Pb(NO3)2 with 100.0 mL of 0.050 M Na2EDTA. At the measured pH of 4.34, (Equation 11-4). Show that [Pb2]  1.4  1012 M. range 1015  [Hg2]  1011 M was obtained with HgBr2(s)  KBr(aq) (based on pKsp for HgBr2  17.43). The resulting calibration curve is shown in the figure. Calibration points for the HgCl2 / KCl buffer are not in line with the other data. Suggest a possible explanation. 14

Activity problem. Citric acid is a triprotic acid (H3A) whose anion (A3) forms stable complexes with many metal ions. When a Ca2 ion-selective electrode with a slope of 29.58 mV was immersed in a solution having , the reading was 2.06 mV. Calcium citrate solution was prepared by mixing equal volumes of solutions 1 and 2. Solution 1: Solution 2: The calcium citrate solution gave an electrode reading of 25.90 mV. (a) Refer to the discussion with Figure B-2 in Appendix B. Calculate the activity of Ca2 in the calcium citrate solution. (b) Calculate the formation constant, Kf, for CaA. Assume that the size of CaA is 500 pm. At pH 8.00 and   0.10 M, the fraction of free citrate in the form A3 is 0.998. Soli

What does analyte do to a chemical-sensing field effect transistor to produce a signal related to the activity of analyte?