Electrochemistry (CHEM 4700) Potentiometry
1. What is meant by the term “potentiometry”? Explain why the use of fluoride ion-selective electrode is an example of potentiometry.
2. Define the terms “ion-selective electrode” and “glass-membrane electrode”. Use a typical pH electrode to illustrate both of these ideas. Draw and indicate the main features of the pH electrode.
3. Briefly explain selectivity coefficient (k) of ion-selective electrodes. What range of values of k are ion-selective electrodes considered to: (a) be ideal electrodes (b) respond better to analyte ions (c) respond better to interfering ions
4. Name the three groups of ion-selective electrodes and give two examples of each.
5. Give three applications of ion-selective electrodes.
6. Describe how a modern pH electrode is constructed. Explain why this type of electrode is also known as “combination electrode.”
7. Explain how pH is measured by a pH electrode. What role does the pH glass membrane play in this process?
8. List the sources of error associated with the pH glass electrodes.
9. What is solid-state ion-selective electrode? How is this similar to a glass-membrane ion-selective electrode? How is it different?
10. How different is sodium ion-selective electrode from a typical pH electrode? How are these two types of electrodes similar?
11. Calculate the ionic strength of the following, assuming complete dissociation of salts: (a) 0.32 M Ca(NO3)2 0.96 M (b) 0.45 mM AlCl3 2.7 M (c) 0.24 M Na2SO4 0.72 M
12. Determine the ionic strength for each of the following mixtures. In each case, assume that all of the solids dissolve and dissociate into their respective ions. (a) 0.10 M NaCl plus 0.20 M KI 0.30 M (b) 0.500 M NaCl plus 0.0500 M MgCl2 0.65 M (c) 0.050 M MgSO4 plus 0.050 M Na2SO4 0.35 M (d) 0.050 g KBr plus 0.100 g KCl in 1.00 L water 1.8 x 10-3 M
13. Use the Debye-Hückell equation to estimate each of the following activity coefficients (γ) (assume that each completely dissolves to form the listed ions): (a) γ of H+ and NO3- in a 0.0050 M HNO3 solution (Ion size: H+ = 900 and NO3- = 300). (b) γ of Ba2+ and Cl- in a 0.010 M BaCl2 solution (Ion size: Ba2+ = 500 and Cl- = 300). (c) γ of K+ and OH- in a 0.020 M KOH solution (Ion size: K+ = 300 and OH- = 350).
(a) 0.993 and 0.925 (b) 0.531 and 0.840 (c) 0.864 and 0.867
14. The activity of a potassium ion as measured by a potasium ion-selective electrode is 0.0674 M. If the solution ionic strength is 0.500, what is the molarity of the potasium ion in the solution (ionic size of potasium = 300)? Activity coefficient = 0.613, molarity = 0.110 M
15. The amount of protein in a wheat flour sample is to be measured. A 0.3476 gram portion is dissolved in concentrated sulfuric acid and heated to boiling in the presence of copper ion, which serves as a catalyst to destroy the biomolecules in the sample and convert the protein nitrogen into ammonium ion. After cooling, raising the pH with NaOH, and diluting the solution to 100 mL, an ammonia-selective electrode is used to measure the ammonium concentration to be 0.32 mM. Protein is typically 16% nitrogen. Calculate the percent protein in this sample. Explain how an unscrupulous grain dealer could make the grain look richer in protein by adding melamine (C3H6N6). 0.81% . The amount of ammonium ion will increase, which will increase the amount of nitrogen and hence the percent protein will be artificially high.
16. A cyanide ion-selective electrode obeys the equation, E = K - 0.05916log[CN-]. The potential was -0.230 V when the electrode was immersed in 1.00 mM NaCN. (a) Evaluate the constant in the equation. -0.407 (b) Using the result from part (a), find [CN-] if E = -0.300 V. 0.0155 M (c) Without using the constant from part (a), find [CN-] if E = -0.300 V. 1.18 x 105 M