Solubility of Strontium Chromate and the Detection of Strontium T. W. DAVIS, New York University, New York, N. Y.
THE
removal of strontium chromate in the qualitative analysis of the alkaline earth group is a very slow process. The salt is generally precipitated by adding alcohol to a hot alkaline solution containing strontium chromate and then the mixture is thoroughly cooled before it is filtered. Noyes' original directions for this process are very satisfactory, in the sense that they allow the detection of rather small amounts of strontium in the presence of much calcium, but the filtering and the washing of the precipitate are very time-consuming. If the filtration and washing could be carried out with hot solutions, without loss of sensitivity or reliability, the procedure would be far more convenient. The cooling of the alcoholic mixture containing strontium chromate seems to be advised for two reasons: (1) Calcium chromate precipitated with the strontium compound should redissolve on cooling, for its solubility increases a t lower temperatures, and (2) the precipitation of the strontium chromate itself should be more complete a t lower temperatures than a t higher. One thinks in this connection of the next step in the procedure, which consists of dissolving the strontium chromate in hot water. Both reasons for filtering the mixture cold, however, are spurious. I n the first place, if one follows the original Noyes directions, which prescribe minimal excess amounts of both chromate and alcohol, no calcium will precipitate unless the amount present in the solution exceeds 500 mg., yet as little as 1 mg. of strontium will precipitate under the same conditions. These claims have been confirmed by the author's direct observations. If one uses a modification of Noyes' procedure, such as that of Miller (9), as little as 80 mg. of calcium in the original sample will lead to a precipitate during the strontium removal. But if calcium chromate precipitates a t all, it cannot be redissolved in the course of several hours' shaking a t room temperatures. The extraordinary slowness with which calcium chromate dissolves has been noted by others. Kohlrausch (6) reported, for example, that a solution was still undersaturated after more than 15 months of contact with anhydrous calcium salt. The cooling of the strontium chromate mixture neither eliminates possible precipitation of calcium chromate nor promotes a more complete removal of strontium chromate itself. Contrary to earlier figures appearing in the literature ( l a ) ,the solubility of strontium chromate in water diminishes a t higher temperatures and one should heat rather than cool a solution to promote more thorough precipitation. The solubility trends may be different in another solvent, but the probabilities are that strontium chromate is less soluble in hot aqueous alcohol than in cold. Because there are no data in the literature on this point, the solubilities of strontium chromate in alcoholic solutions were investigated before attempting to modify the well-established Noyes procedure (11).
Determination of Solubilities The preparation of strontium chromate and the procedure followed in determining the solubility by iodometric titration of chromate have already been described ( 2 ) . The concentration of alcohol in the mixed solvents was established by density measurements. The best strontium chromate which could be prepared from ammonium chromate and strontium chloride was 99.6 per cent pure as determined by titration of the chromate, and this was used for the solubility experiments. 709
Numerous modifications in the preparative procedure were made without any improvement in the product. The dissolving of the strontium salt like the dissolving of the calcium compound is very slow, saturation not being attained after a year's rotation of a gram of solid with about 100 grams of water a t 25". The changes in the liquid after the first month, however, amount to a few per cent only. The solubilities were all run from undersaturation except for two determinations in pure water a t 50" and 75" that were made from supersaturation, which was checked by analysis before rotating with solid. Some of the water solutions were examined for strontium content as well as chromate, the strontium being precipitated as sulfate in presence of alcohol. The agreement in every case was within 2 or 3 per cent with no evident trends
TABLEI. SOLUBILITY OF STRONTIUM CHROMATE Medium
250 G./100
Water 19.7% alcohol
4 1 . 8 % alcohol
92.5% alcohol
0.096 0.011
50' Q.
0.0004 -0,00005
G./1008.
0.090
0.012 0.0009 10,00005
750 G./~OOQ.
0,080 0.008 0.0003
