Calcium hydroxide as a highly alkaline pH standard

Abstract

The Natl. Bureau of Standards1 conventional activity scale of pH is defined by a series of standard buffer solutions prepared from certified materials issued as NBS Standard Samples. The 5 standards thus far established cover the pH range 1.68 to 9.18 at 25[degree]C. To increase the accuracy of measurements at high pH, a highly alkaline standard is needed. A solution of calcium hydroxide saturated at 25[degree]C is recommended as the 6th pH standard. No weighing is necessary, for the solution is easily prepared by shaking finely granular calcium hydroxide with water. The material must not be contaminated with soluble alkalies, but the presence of insoluble carbonate is of no concern. The filtered solution supersaturates readily and can usually be used from 0[degree] to 60[degree]C without the separation of solid phase. Electro-motive-force measurements of 29 cells containing mixtures of calcium hydroxide and potassium chloride were made in the range 0[degree] to 60[degree]C, and standard pH values were assigned to 0.0203-, 0.02-, 0.019-, 0.0175-, and 0.015-[image] solutions of calcium hydroxide without added chloride. The saturated solution is about 0.0203 [image] and has a pH of 12.45 at 25[degree]C. The buffer capacity is high (0.09 mole/pH). Like most other alkaline solutions, however, this standard has a rather large dilution value ([long dash]0.28 pH unit) and a large temperature coefficient of pH ([long dash]0.033 pH unit/degree C).