RESEARCH
Membrane Electrodes Respond to Anions Selectivity shown by electrodes makes them useful for direct potentiometric and titrimetric determination of I" and SO;2 Two anion-selective membrane electrodes have been used in a number of analytical applications by Dr. Garry A. Rechnitz of the University of Pennsylvania (Philadelphia). One electrode can be used for determination of iodide. The other can be used for determining sulfate, Dr. Rechnitz told the Division of Analytical Chemistry at the ACS winter meeting in Phoenix. The Penn chemist's interest in anion-selective membrane electrodes stems from his work with cation-sensitive glass electrodes for use in analytical measurements. He has used cation-sensitive glass electrodes for direct measurement of cations such as sodium, potassium, and silver in solution. Anion-selective membranes of the type studied by Dr. Rechnitz were first prepared and studied by Dr. E. Pungor and his co-workers at the University of Chemical Engineering in Veszprem, Hungary. The membranes in these electrodes consist of sparingly soluble salts imbedded in a silicone rubber matrix. The electrode used for the determination of iodide contains silver iodide. The electrode used to determine sulfate contains barium sulfate. The way that these heterogeneous membrane electrodes operate is not fully understood, Dr. Rechnitz says. The measured e.m.f. arises principally at the membrane-solution interfaces. And the e.m.f. reflects changes in phase boundary potential set up at the electrode-sample solution interface in
response to changes of anion activity in solution. Applications. Dr. Rechnitz has studied several analytical applications for the anion-selective membrane electrodes. For example, he has used them for direct potentiometric measurement of sulfate or iodide in solution. He has also used them as indicator electrodes for potentiometric titration of sulfate or iodide. In addition, he has explored the possibility of using the electrodes in continuous analysis and in automatic titrators. The membrane electrodes can be used with commercial pH meters for the direct potentiometric measurement of the anion in solution. Dr. Rechnitz has quantitatively evaluated the selectivity characteristics of the iodide electrode in the presence of chloride, bromide, and other common anions such as sulfate, phosphate, perchlorate, and ferrocyanide. This evaluation shows that the iodide membrane electrode can be used for the determination of iodide in the presence of fairly large excess concentrations of the other anions. The voltages obtained with both iodide and sulfate membrane electrodes lie in nearly a straight-line relationship to the logarithm of the anion concentration at about 0.1 to 10~5M, Dr. Rechnitz finds. The uncertainty of e.m.f. measurement of the membrane electrodes is between 1 and 2 mv., giving a precision not quite as satisfactory as that obtained with glass electrodes, Dr. Rechnitz notes.
Iodide Membrane Electrode Shows High Apparent Selectivity Anion Pair Selectivity Ratio
Effective Amv.
Br~/I -
Fe(CN) 6 - 4 /|-
2.1 X 1 0 2
3.3 X 103
140
211
CI ~/l ~ P 0 4 - 3 / | -
CI04-/|- S04-2/|-
1.7 X 105 4.8 X 105 1.6 X 106 3.2 X 107
314
341
372
451
Data obtained at 30° C. and at 0.1M anion concentrations show high apparent selectivity of the iodide membrane electrode in the presence of other anions. The selectivity ratio indicates the concentration ratio required to give the same electrode e.m.f. as a unit concentration of iodide. The effective Amv. is the difference in e.m.f. obtained when a 0.1M iodide solution is compared to a 0.1M solution of the other anion
24
C&EN
J A N . 31, 1966
SPECIFIC. Dr. G. A. Rechnitz examines a membrane electrode containing silver iodide imbedded in a silicone rubber matrix. The electrode is specific for determining iodide concentration
He has used both membrane electrodes as indicator electrodes for potentiometric titrations. Iodide can be titrated with silver ion, and sulfate ion can be titrated with barium ion. Dr. Rechnitz has obtained titration curves with very sharp breaks for both titrations. The break in the titration curve for 10 - 3 M potassium iodide is about 300 mv. in magnitude. The Penn chemist also finds that the membrane electrodes are suitable for direct determination of anions in continuous streams. The half time of the electrode response is usually less than 10 seconds, he notes. Dr. Rechnitz believes that the membrane electrodes show great promise for use in automatic titrators because of their rapid response. He is currently studying the possibility of determining other anions using membrane electrodes.
