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OSCAR E. LANFORD AND SAMUEL J. KIEHL
THE FOURTH IONIZATION CONSTANT OF FERROCYANIC ACID OSCAR E. LANFORD
AND
SAMUEL J. KIEHL
Department of Chemistry,Columbia University, New York, New York Received A4ay 20, 1940
The fourth ionization constant of ferrocyanic acid may be determined by direct measurement of the activity of the hydrogen ion in solutions of ferrocyanide and hydrochloric acid whose respective concentrations are accurately known. Such measurements of pH can be made by means of the glass electrode. For by its use thr interference of oxidation potential, which hinders the use of the ordinary hydrogen electrode in such systems, is thereby avoided. Furthermore, ferrocyanic acid is a tetrabasic acid with but one point of inflection, corresponding t o the fourth hydrogen, in its electrometric titration curve.' Consequently, at constant temperature, provided the concentration of hydrochloric acid is less than the concentration of total ferrocyanide, the respective pH for each solution depends upon the logarithm of the ratio of thc concentration of the ferrocyanide ion to the concentration of the hydroferrocyanide ion, the logarithm of the ratio of their respective activities, and the PI(. Recently the fourth ionization constant of ferrocyanic acid was calculated by Kolthoff and Tomsicek (3) from their data which demonstrated the variation of the oxidation potential of the ferricyanide-ferrocyanide system with the activity of the hydrogcn ion. THEORETICAL BASIS
By letting Cqand Carepresent the ferrocyanide-ion and the hydroferrocyanide-ion concentrations, f 4 and f3 their respective activity coefficients, and aH+the activity of the hydrogen ion, the fourth ionization constant may be expressed as follows:
If we consider the limiting law of Debye-Huckel to be valid for dilutr solutions,
- log14 = 3 . 5 G fs 1 This fact is corroborated by the authors' unpublished data upon the clectrometric titration of especially prepared pure fcrrocynnic acid with standard sodium hydroxide.
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in this system, in which p designates the ionic strength; then, instead of the above equation, the following expression may be used: PK4 = pH
- log c4 + 3 . 5 ~ 5
Now let
+
Then pK4 would equal ( A 3.5 v'i), provided the limiting law is valid in this case. As a matter of fact, our limiting slope was about 4.3 instead of 3.5,-a fairly good agreement for ions of high charge. But in any case A may be plotted again& v'i and, when
