JULY, 1953
349
THE SEPARATION OF CALCIUM AND STRONTIUM IN SEMIMICRO QUALITATIVE ANALYSIS RICHARD B. HAHN Wayne University, Detroit, Michigan
THEcations of Group IV in qualitative analysis in- precipitate is washed two times with hot water. These clude the barium, strontium, and calcium ions. In all procedures barium is separated first by precipitation as barium chromate in an acetic acid solution. No difficulty is encountered usually in this step. The following step involving the separation of strontium and calcium is the troublesome one. , Current methods for the separation of strontium from calcium usually give mediocre results in the hands of the beginning student. These methods include precipitation as chromate in alcohol-water solution, precipitation as sulfate and precipitation as ferrocyanide. Methods using anhydrous solvents, as hutyl cellosolve or acetone, to dissolve calcium nitrate are likewise unsatisfactory in the hands of students. A method which gives an excellent separation of calcium and strontium was devised by Willard and Goodspeed.' This involves the precipitation of strontium nitrate in 80 per cent nitric acid solution. By this method the above authors were able to separate quantitatively strontium from calcium. The only drawback encountered in this method is the use of 100 per cent (fuming) nitric acid which is somewhat hazardous. This di5culty can he overcome by the use of ordinary 72 per cent (concentrated) nitric acid. The method is no longer quantitative with acid of this concentration, but an excellent qualitative separation is achieved. The loss of strontium amounts only to a few tenths of a milligram which is insignificant in qualitative analysis. The following procedure is suggested: A. Precipitation of Group I V and the Separation of Barium. This is carried out in the usual manner: BaCOa, SrC03, and CaC03 are precipitated by the addition of ammonium carbonate. This precipitate is dissolved in acetic acid and barium chromate precipitated by the addition of potassium chromate. B. Reprecipitation of CaC03 and SrC03. The solution from the separation of barium is rendered basic by the addition of ammonium hydroxide. Ammonium carbonate is added, the solution is warmed and centrifuged. The supernatant liquid is discarded and the 'WILLARD,H. H., AND E.W. GOODSPEED, Anal. Chem., 8,414 (1936).
washings are discarded. C. Separation of Calcium and Strontium. The precipitate of CaC03 and SrCOa (free of liquid) in the tube is dissolved by the cautious, dropwise addition of 72 per cent (concentrated) nitric acid. After effervescence has ceased, an additional 5 ml. of concentrated nitric acid is added. The mixture is stirred vigorously for a minute or two, then the tube is placed in a beaker containing cold water and allowed to stand for 10 minutes with occasional stirring. (Caution, do not allow any water to get into the tube. This will dilute the acid and render the Sr(N0a)z precipitate soluble.) The mixture is centrifuged and the supernatant liquid decanted into a beaker containing 20 ml. of water. This solution is saved for the calcium test. The precipitate is stirred up with 1 ml. of 72 per cent nitric acid, the mixture centrifuged, and the liquid discarded. D , IdentiJication of Strontium. The precipitate, Sr (NOs)*,is dissolved in 3 ml. of hot water. A flame test is made on this solution (Sr = crimson). If barium were incompletely separated it would also be precipitated as an insoluble nitrate. Five drops of 1 M ammonium sulfate is added to the solution and the mixture heated to boiling. The formation of a white flocculent precipitate of SrS04 confirms the presence of strontium. E. Identification of Calcium. Concentrated (15 M ) ammonium hydroxide is added cautiously to the dilute nitric acid solution from the stontium separation until the solution is basic (litmus test). Two ml. of saturated (0.25 M) ammonium oxalate are added to this basic solution. A finely divided white precipitate of CaC2O4indicates the presence of calcium. The above procedure has been used in the author's classes during the past year. The accuracy has been above 95 per cent with this new procedure compared with accuracies below 70 per cent with other procedures. As little as 1 mg. of Sr can he detected in the presence of 50 mg. of Ca and vice versa, using this procedure. The students were reminded about the corrosive nature of concentrated nitric acid, hence no accidents have arisen in using this reagent.
