Determination of mean activity coefficients with ion-selective electrodes (original) (raw)

1983, Analytical Chemistry

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 determlned with ion-selective electrodes. For this study, NaNO,, KNO,, and Ca(NO,), were used as model compounds. Their activity coefficients In aqueous solution at 25 "C were determlned both in dilute solutions and at molalities as hlgh as 2 to 4 mol kg-' by using electrodes responsive to Ida+, K+, Ca2+, and NO3-and different accepted evaiuatlon teclhniques. The results were compared wlth the literature data for these salts. A general nonllnear regression method Incorporating the extended Debye-Huckel equation was developed. By use of experlmentai and simulated data, It was found that reliable activlty coefflcients can only be obtained when randm errors do not exceed f0.5 mV, even though the electrode function is strictly linear. Otherwise a strong correlation between the various parameters renders the results inaccurate.

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