A Standard for the Measurement of the pH of Blood and Other Physiological Media

A buffer solution containing potassium dihydrogen phosphate (0.008695 molal) and disodium hydrogen phosphate (0.03043 molal) is proposed as a pH standard for the physiologically important range, pH 7 to 8. The proposed standard solution is prepared by dissolving 1.179 g (air weight) of potassium dihydrogen phosphate and 4.303 g (air weight) of disodium hydrogen phosphate in ammonia-free water and diluting to 1 liter at 25 °C. The ionic strength is 0.1. Standard pH values (pH_s) were assigned to this reference solution at temperatures from 0 to 50 °C by means of emf measurements of hydrogen-silver chloride cells without liquid junction. The activity coefficient of chloride ion, upon which the assignment of pH_s depends, was evaluated by means of a recently proposed convention. By this means, standard values precise to ± 0.001 unit could be derived from the emf data. At 25 °C pH_s is 7.414, and at 38 °C it is 7.382. The operational consistency of these standard values with those for the 0.025-m equimolal phosphate buffer (one of the NBS primary standards) was demonstrated.