Determination of Mean Activity Coefficients with Ion-Selective Electrodes


Electromotive force (emf) measurements of cells without liquid junction have been used to shed light on the factors Influencing the accuracy of mean ion activity coefficients determined with ion-selective electrodes. For this study, NaNO3, KNO3, and Ca(NO3)2 were used as model compounds. Their activity coefficients in aqueous solution at 25 °C were determined both in dilute solutions and at molalities as high as 2 to 4 mol kg−1 by using electrodes responsive to Na+, K+, Ca2+, and NO3− and different accepted evaluation techniques. The results were compared with the literature data for these salts. A general nonlinear regression method incorporating the extended Debye-Hückel equation was developed. By use of experimental and simulated data, it was found that reliable activity coefficients can only be obtained when random errors do not exceed ±0.5 mV, even though the electrode function is strictly linear. Otherwise a strong correlation between the various parameters renders the results inaccurate. © 1983, American Chemical Society. All rights reserved.

Publication Title

Analytical Chemistry