Calcium Hydroxide as a Highly Alkaline pH Standard

The National Bureau of Standards conventional activity scale of pH is defined by a series of standard buffer solutions prepared from certified materials issued as N BS Stan dard Samples. The five standards thus far established cover the pH range 1.68 to 9.18 at 25° . In order to increa e t he accuracy of m easurements at high pH, a highly alkaline standard is needed. A solut ion of calci um hydroxide saturated at 25° C is recommended as the sixth pH standard. K 0 weighing is necessary, for the solution is easily prepared by shaking fin cly 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° to 60° C without the separat ion of solid phase. E lectromotive-force m easurem ents of 29 cells containing mixtures of calcium h ydroxide and potassi um chloride were made in the range 0° to 60° C, and standard pH values were assigned to 0.0203-, 0.020.019-, 0.0175-, and 0.015-M solutions of calcium hydroxid e without added chloride. the saturated solution is about 0.0203 M and has a pH of 12.45 at 25° C. The buffer capacity is high (0.09 mole/pH). Like most other alkaline solutions, however , this standard has a rather large dilution value (-0.28 pH unit) and a large temperature coefficient of pH (0.033 pH uni t/deg C).