elegant determinations of submicrogram amounts of metals (22, 26). For halides, however, 10 pg. of chloride has been determined spectrophotometrically (93) but other ions interfere; 2 p.p.b. of chloride, 1 p.p.b, of bromide, and 3 p.p.b. of iodide can be detected under the most favorable circumstances on the MS7 mass spectrometer (20), the accuracy being 1 5 0 to 100%; and 0.03 ng. of bromide and 1 ng. of chloride can theoretically be detected by neutron activation with a n accuracy of i50%, but require irradiation at 10l2 neutrons minute-’ for 150 hours followed by radiochemical processing. Clearly, even without troubling about adsorption losses, the inexpensive microcoulometric DEP stands a t a considerable advantage in precision, sensitivity, convenience, and speed. LITERATURE CITED
(1) Bishop, E., Analyst 83,212 (1958). (2) Ibid., 85, 422 (1960). (3) Bishop, E., Mikrochim. Acta 1956, 6lQ
-_1.
(4) Ibid., 1960, 803. ( 5 ) Bishop, E., Dhaneshwar, Analvst 87.207 (1962). ( 6 ) I b i k , p. 845. (7) Ibid., 88, 424 (1963). (8) Ibid., p. 433. (9) Ibid., p. 442.
R. G.,
~
(10) Bishop, E., Dhaneshwar, R. G., Proc. 1st Australian Electrochem. Conf. p. 481, Pergamon, Oxford, 1963. (11) Bishop, E., Dhaneshwar, R. G., Short, G. D., Proc. Feigl Anniversary SymposiuwL, Birmingham 1962, p. 236, Elsevier, Amsterdam, 1963. (12) Bishop, E., Dhaneshwar, R. G., Short, G. D., Proc. 1st Australian Electrochem. Ccmf., p. 802, Pergamon, Oxford, 1963. (13) Bishop, E., Short, G. D., AXAL. CHEM.36, 730 (1964). (14) Bishop, E., Short, G. D., Analyst 87,467 (1962). (15) Bishop, E., Sutton, J. R. B., Anal. Chim. Acta 22,590 (1960). (16) Cooke, W. D., Reilley, C. N., Furman, N. H., ANAL. CHEY. 24, 205 (1952). (17) Dhaneshwar, R. G., Ph.D. thesis,
University of Exeter, Exeter, England,
1962. (18) Dutoit, P., von Weisse, G., J . Chim. Phys. 9, 578 (1911). (19) Duyckaerts, G., Anal. Chim. Acta 8,57 (1953). (20) Elliott, R. M . , Craig, R. D., Errock, G. A., “Analysis of Solids by Mass
Spectrometry,” Associated Electrical Industries, Ltd., Manchester, 1960. (21) Foulk, C. W., Bawden, A. T., J . Am. Chem. SOC.48,2045 (1926). (22) Gardiner, K. W., Rogers, L. B., ANAL.CHEM.25, 1393 (1953). (23) Kemula, W., Hulanicki, A., Janowski, A., Talanta 7,65 (1960). (24) Lingane, J. J., “Electroanalytical Chemistry,” 2nd ed., Interscience, New York, 1958. (25) Ibid., p. 599.
(26) Lord, S. S., O’Neil, R. C., Rogers, L. B., ANAL.CHEM.24,209 (1952). (27) Mattoqk, G., “pH Measurement and Titration, p. 158, Heywood, London, 1961. (28) Meites, L., ANAL.CHEM.24, 1057 (1952). (29) Merriam, E. S., Ph.D. dissertation,
Universitats Georg-Augusts, Gijttingen, Germany, 1906. (30) Monk, R. G., Steed, K. C., Anal.
Chim. Acta 26.305 (1962). (31) Same, R.’ G. ‘van,’ Fenwick, F., J . Am. Chem. SOC. 4 7 , 9 (1925). (32) Reilley, C. N., Cooke, W. D., Furnian, S . H., ANAL.CHEM.23, 1223 (1951). (33) Sandved, K., Backer, J., Ti&. Kemi Bergv. 5, 224 (1925). (34) Short, G. D., Ph.D. thesis, University of Exeter, Exeter, England, 1963. (35) Short, G. D., Bishop, E., Analyst 87,724 (1962). (36) Ibid., p. 860. (37) Short, G. D., Bishop, E., “Study of
the Current-Potential Relationships at Indicating Electrodes,” Electronic Instruments Ltd. Research Reports, University of Exeter, KO. 2, December 1961, S o . 3, August 1962. (38) Willard, H. H., Fenwick, F., J . Am. Chem. SOC.44,2504 (1922).
RECEIVEDfor review August 9, 1963. Accepted January 17, 1964. Paper C3-24 presented a t the XIXth IUPAC Congress, London, July 1963. Kumber 12 of a series. Work supported by Electronic Instruments Ltd. R. G. D. acknowledges study leave from the Indian Atomic Energy Establishment and receipt of a Colombo Plan grant.
Absolute Differential Electrolytic Potentiometry as Index of Poising Capacity EDMUND BISHOP and GLYN D. SHORT’ Washington Singer laborotories, The University, Exeter, Devon, England Absolute
or
single
measurement to classical potentiometry and can b e used to monitor the poising of a solution with respect to an active ion or the oxidation-reduction ratio. Potentiometric pH measurement can be supplemented by buffer capacity measurement by DEP, which shows maximum sensitivity in unpoised media wherein potentiometry is a t o disadvantage. Consequently, absolute DEP offers a sensitive indication of buffering or poising substances in the presence of substantial amounts of neutral salts. This is illustrated in the determination of salts of weak acids with weak bases in the presence of salts of strong acids with strong bases, for which conductimetric methods ore insensitive. The determination of 7 to 700 p.p.m, of ammonium acetate in salt solutions is described, and current-potential relationships and the effects of stirring, ionic strength, and atmosphere are discussed.
DEP is complementary
730
ANALYTICAL CHEMISTRY
D
electrolytic potentiometry (DEP) as a method for the study of electrode processes and titrimetric reactions has been mentioned and a bibliography has been given in a previous communication (1). The method is complementary to classical potentiometry: Titrinietrically i t gives the first differential of the zero-current potentiometric titration curve for reversible reactions, and will give second differential curves if combined with time differentiation (6). A single zero-current potentiometric measurement gives an indication of the concentration of the active ion to which the electrode responds-e.g., a pH value-or of the redox ratio of a n electron transfer reaction. A single D E P measurement gives an indication of the poising of the solution with respect to the active ion or to electron transfer-e.g., the buffer capacity. Thus, a measurement of the glass electrode potential in a solution indicates the p H value, but gives no IFFEREXTIAL
indication of the nature of the sub stances present, whereas a DEP measurement distinguishes sharply and sensitively between poised and unpoised solutions, as between pure water and a buffer solution of the same pH. Confining the discussion to acid-base reactions and p H responsive electrodes, buffer capacity can be defined as the change in pH of a solution wrought by the addition of a small amount of strong acid. Strictly this increment should be spread over the working point and is tantamount to the difference in p H between two identical samples of the solution to one of which is added a small increment of strong acid and to the other an equivalent increment of strong base. If the increments are kept small and equal, the value of A ~ H / [H+] A permits intercomparison of the buffer capacity of diverse solutions. 1 Present address, Department of Chemistry, Massachusetts Institute of Technology, Cambridge 39, Mass.
The full quantitz,tive interpretation of the mechanism of DEP is in the course of publication (3, 10, 11, and to be communicated), but a qualitative explanation will show how the method can be applied to measurement of buffer capacity. The m:thod consists in polarizing two indicator electrodes immersed in the stirred solution with a minute heavily statilized current and measuring the potential difference between them. Whatever the electrode reaction mechanism :4, 5 , 9 ) , the net result is that a t the anode, hydroxyl ion is coulometrically coiisumed and an equivalent amount of hydrogen ion is generated. The rate of dispersion of hydrogen ion by diffusion and other migration processec rapidly reaches equilibrium with the generation rate so that at the electrode surface the hydrogen ion concentration is slightly greater than that in the bulk of the stirred solution. ht the anode, therefore, the effect is equivalent t o the addition of an increment of strong acid. analogous situation a t the catiode is equivalent t o the addition of an increment of strong base, and if the current density is the same at both electrodes, the increments will be equal. This is not quite true in practice because the diffusion coefficient of the hydrogen ion diffusing a n a y from the anode and toward the cathode is greater than the diffusion coefficient of hydroxyl ion diffusing in the counter direction at the electrodes, but the effect is small and at constant current density the net increment is constant. A D E P measurement at fixed current density will therefore afford a comparison of the buffer capacity of diverse sc lutions. Moreover, zero-current electrodes are least reliable and most sluggish in response in unpoised solutions ( 2 , 5 , 7 , 8 ) , b u t DEP attains its maximum sensitivity under these conditions. The lower the concentration of the buffering agent, the more sensii ive is its effect on the DEP signal, so that single measurement D E P affords a sensitive method for the direct determination of traces of such material\. Noreover, the presence of large excesses of salts of strong acids and strong base
