抄録
1. The value of pH of a phosphate solution, which contains both the primary and secondary phosphates in an equi-molar concentration, gives the value of the apparent second dissociation exponent, pK', of the reaction.
H2PO4-_??_H++HPO4--
The determination was repeated by means of the quinhydrone electrode method. Then from this result the thermodynamic dissociation exponent, pK, of the same reaction, calculated with the aid of Debye-Hückel's formula, was found to be 7.04 at 18°C, 7.02 at 25°C and 7.01 at 37°C. This value at 18°C does not agree with Cohn's figure, 7.16 at the same temperature.
2. The value of pK' determined as described above is applicable to a phosphate solution of the same concentration, but differing in the ratio of the two phosphates, except when the ratio is very extreme. In the latter case, when the ratio is 1:9 or 9:1, the value of pK' found experimentally becomes smaller, owing to the change in the ionic strength and the influence of the individual properties of the ions.
3. When the phosphate solution contains a neutral salt such as NaCl, KCl, MgCl2 or MgSO4, the value of pK' determined in this solution becomes smaller owing to the increase in the ionic strength and also to the individual properties of co-existing ions. The value of pK' diminishes with increase in the ionic strength. The effect of ionic properties is sometimes very marked, amounting to 0.35 at an ionic strength of 0.1. The ions act in order of
K<Na<Mg
and
phosphate<SO4<Cl
Referring to these facts it is conceivable that the value of the thermodynamic dissociation constant of the phosphate reaction mentioned above, which is deduced from the experimental data by the aid of Debye - Hückel's equation, includes a systematic error owing to the effect of the individual properties of ions contained in the solution examined. The value of the apparent dissociation constant is not a simple function of the ionic strength, but suffers from the same error, so that the accurate value of it should be determined with the solution to be examined.
4. A new method of determining the value of pK' of the second dissociation in a phosphate solution is described. The value for human urine is determined by this method and its mean value, 6.62 at 18°C, is obtained. The value at the body temperature is estimated to be in a range of 6.4-6.6.
