[Read at the Fourth Science Congress, Auckland, January, 1929; received by Editor, 9th April, 1929; issued separately, 15th August, 1929.]

Determinations have been made of the solubility of freshly ignited calcium oxide in sodium hydroxide and calcium chloride solutions.

There did not appear to be any convenient method of separately estimating free sodium hyroxide and calcium oxide in solution other than by titration of the total alkali present, the calcium oxide being found by difference from the titration figures of the mixed alkali solution and the sodium hydroxide solution. The calcium oxide was added in excess to a quantity of the sodium hydroxide solution, the mixture being frequently shaken throughout the day and then left to stand overnight. A known volume of the filtered solution was titrated with N/2 sulphuric acid. The temperature of saturation was taken as that found just before filtering. In the case of the sodium hydroxide solution the temperature was 13° C. Table 1 gives the data obtained, these being also shown graphically in Fig. 1.

From Fig. 1 it is seen that a minimum exists in the solubility in about 1.4 per cent. sodium hydroxide (0.35N); the solubility falls rapidly on the addition of small amounts of alkali but increases only slowly after passing the point of least solubility.

In dealing with the calcium chloride solutions the total alkalinity was taken as calcium oxide. For the determination the same

procedure was employed as with the sodium hydroxide solutions only that an aliquot of the filtered solution was added to an excess of standard acid, the excess acid being then estimated.

In the strong chloride solutions there is some doubt as to the composition of the solid phase, as it has been reported in the literature that an insoluble basic chloride is formed; above a concentration of 6 per cent. calcium chloride it is possible for this compound to be present. No analyses were made in this series of experiments.

The experimental results are given in Table 2, and shown graphically in Fig. 2. The temperature in this series of experiments varied between 16° C and 20° C.

From Fig. 2 it is seen that the solubility curve exhibits a distinct minimum at a concentration of about 1 per cent. (0.19N) calcium chloride.

The data presented here are interesting in that both sets of the data show the existence of a minimum solubility for the calcium oxide. This appears to have been overlooked previously, probably because very dilute solutions do not seem to have been employed for determinations of the solubility.

Summary.

The solubility of calcium oxide in sodium hydroxide and calcium chloride solutions is shown to pass through a minimum value in about 1.4 per cent. (0.35N) and 1.0 per cent. (0.19N) solution respectively.

Acknowledgement.

The authors wish to express their thanks to The New Zealand Refrigerating Company Limited for permission to publish this paper, the results of which were carried out in their laboratory.