THE DETECTION OF STRONTIUM IN THE PRESENCE OF CALCIUM A. W. AVENS, CORNELL UNIVERSITY, ITHACA, NEWYORK M., ~ J. MURRAY, LYNCHBURG COLLEGE, LYNCBBURG, V~GINIA
The method for the detection of strontium, using saturated CaS04 solution, has b e a modij5ed to decrease supersaturation and to increase the sensitiwity of the test. The effect of chromate on the precipz'tation of SrS04 i s also discussed.
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A method which is commonly used for the detection of strontium in the presence of calcium is to add to the solution containing these two ions some saturated CaSO, solution. The mixture is then warmed and allowed to stand for some time. The appearance of a turbidity is taken as an iudication of the presence of strontium. This procedure was employed by the writers with large laboratory sections for a number of years. I t was found, however, that in the hands of students a test was not always obtained even when the quantity of strontium present was shown to be much in excess of the amount necessary for the precipitation of SrSOa. An investigation has proved that the test can he made appreciably more sensitive and much more reliable if carried out in the following manner. The test tube in which the test is being made is placed in a beaker of boiling water for ten minutes. From time to time the inside of the tube is scratched with a glass rod. This process not only breaks down supersaturation, but also yields a precipitate of SrSOl which is distmctly more visible because crystallization begins on the scratched portions of the tube. In this way it is much easier to observe than when the precipitate is scattered throughout the body of the liquid. A number of experiments were performed to determine the sensitivity of the test using the ordinary method and the modified procedure. The solutions to be tested were mixed in each case with equal volumes of saturated CaS04 solution. Solutions containing as high as 0.5 mg. of strontium per cc. were tested. In every case digestion produced precipitates which were decidedly more noticeable. When the solution containing 0.1 mg. of strontium per cc. of the original solution was treated by the new method, a very slight turbidity was obtained. By the ordinary TABLE I Comparison of Methods, Chromate Absent ME. of Slronliunr
per cr. of Soiufron lo Bc Tcrlad
0.5 0.4
0.2 0.1 0.05
RcsUI~S
Old Method
Good test
N m Mdhod
Very good test Very good test
Slight turbidity Slight turbidity No test No test
Good test Very slight turbidity No test 522
DETECTION OF STRONTIUM
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process the test was negative. When the quantity of strontium was 0.2 mg. per cc. the test was very distinct by the new method but only a very slight turbidity was obtained by the old method. Further experiments showed that the presence of acetic acid in small amounts does not alter the sensitivity of the test, hut that if chromates are present, SrSO, is more d i c u l t to precipitate. Nevertheless, the modified procedure was found to be very much better. If the solution to be tested contains K2Cr0d and a little acetic acid, to prevent the precipitation of SrCr04, the new method gives a good test when 0.2 mg. of strontium per cc. is present, but no test is given unless digestion is carried out. With 0.4 mg. per cc. the old method gave only a very slight turbidity while the new method gave a very good test. TABLE I1 Comparison of Methods, Chromate Present Composilian of Solvlion lo Be Teslcd mc. of mp. of mg. of Strontium p n cc. K K I O ~ Hc1H.03
0.4 0.2 0.1
20 20 20
10 10 10
Rcsulls
Old Mcllrod
New Mathod
Very slight turbidity No test No test
Very good test Good test No test
This new method has been employed for more than a year with a large number of students in qualitative analysis and has been found to be highly satisfactory. In the hands of an inexperienced analyst, the difference in results obtained by the two methods is much more striking than even the above experiments would indicate.
