Chapter 13

Discuss the mechanism of the glass membrane electrode response for pH measurements.

What is the alkaline error and the acid error of a glass membrane \(\mathrm{pH}\) electrode?

Describe the different types of ion-selective electrodes. Include in your discussion the construction of the electrodes, differences in membranes, and their usefulness.

What is the Nicolsky equation?

A 50 -mL solution that is \(0.10 M\) in both chloride and iodide ions is titrated with \(0.10 M\) silver nitrate. (a) Calculate the percent iodide remaining unprecipitated when silver chloride begins to precipitate. (b) Calculate the potential of a silver electrode versus the SCE when silver chloride begins to precipitate and compare this with the theoretical potential corresponding to end point for the titration of iodide. (c) Calculate the potential at the end point for chloride. For simplicity, in lieu of activities, use concentrations in calculations.

The potential of a hydrogen electrode in an acid solution is \(-0.465 \mathrm{~V}\) when measured against an SCE reference electrode. What would the potential be measured against a normal calomel electrode \((1 M \mathrm{KCl}) ?\)

The quinhydrone electrode can be used for the potentiometric determination of \(\mathrm{pH}\). The solution to be measured is saturated with quinhydrone, an equimolar mixture of quinone \((\mathrm{Q})\) and hydroquinone \((\mathrm{HQ})\), and the potential of the solution is measured with a platinum electrode. The half-reaction and its standard potential are as follows: What is the \(\mathrm{pH}\) of a solution saturated with quinhydrone if the potential of a platinum electrode in the solution, measured against a saturated calomel electrode, is \(-0.205 \mathrm{~V}\) ? Assume the liquid- junction potential to be zero.

A potassium ion-selective electrode is used to measure the concentration of potassium ion in a solution that contains \(6.0 \times 10^{-3} M\) cesium (activity). From Table \(13.3,\) the electrode responds equally to either ion \(\left(K_{\mathrm{KCs}}=1\right)\). If the potential versus a reference electrode is \(-18.3 \mathrm{mV}\) for a \(5.0 \times 10^{-3} M \mathrm{KCl}\) solution and \(+20.9 \mathrm{mV}\) in the sample solution, what is the activity of \(\mathrm{K}^{+}\) in the sample? Assume Nernstian response.

The nitrate concentration in an industrial effluent is determined using a nitrate ion-selective electrode. Standards and samples are diluted 20 -fold with \(0.1 M \mathrm{~K}_{2} \mathrm{SO}_{4}\) to maintain constant ionic strength. Nitrate standards of 0.0050 and \(0.0100 M\) give potential readings of -108.6 and \(-125.2 \mathrm{mV},\) respectively. The sample gives a reading of \(-119.6 \mathrm{mV}\). What is the concentration of nitrate in the sample?

The perchlorate concentration in a sample containing \(0.015 M\) iodide is determined using a perchlorate ion-selective electrode. All samples and standards are diluted 1: 10 with \(0.2 M \mathrm{KCl}\) to maintain constant ionic strength. A \(0.00100 M \mathrm{KClO}_{4}\) standard gives a reading of \(-27.2 \mathrm{mV}\), and a \(0.0100 M\) KI standard gives a reading of \(+32.8 \mathrm{mV}\). The sample solution gives a reading of \(-15.5 \mathrm{mV}\). Assuming Nernstian response, what is the concentration of perchlorate in the sample?

The selectivity coefficient for a cation-selective electrode for \(\mathrm{B}^{+}\) with respect to \(\mathrm{A}^{+}\) is determined from measurements of two solutions containing different activities of the two ions. The following potential readings were obtained: (1) \(2.00 \times 10^{-4} M \mathrm{~A}^{+}+1.00 \times 10^{-3} M \mathrm{~B}^{+}\) \(+237.8 \mathrm{mV} ;\) and \((2) 4.00 \times 10^{-4} M \mathrm{~A}^{+}+1.00 \times 10^{-3} M \mathrm{~B}^{+},+248.2 \mathrm{mV} .\) Calculate \(K_{\mathrm{AB}}\). The electrode response is \(56.7 \mathrm{mV} /\) decade.