JOURNAL OP CEIWICALEDUCATION
1428
NovEmsn, 1927
THE TEST FOR CALCIUM WITH AMMONIUM OXALATE ROBERT B.
ConSv
AND
H. W. ROGERS, CORNELLUNNERSITY, IFAACA, NEW Y O R ~
Introduction Several methods are employed in elementary qualitative analysis for the separation of strontium from calcium and the subsequent detection of the latter. The one which appears to be in most general use is that involving the precipitation of strontium as sulfate and its removal by filtration, followed by the detection of calcium as calcium oxalate by adding ammonium oxalate to the filtrate. While using this method in elementary classes the attention of the writers was called to the fact that if strontium is present considerable uncertainty is introduced into the calcium test, in as much as an appreciable precipitate of strontium oxalate is almost always obtained. Calcium is, therefore, often reported when absent since the white strontium oxalate is indistinguishable in appearance from the corresponding calcium salt. Such confusion may be obviated to some extent through confirmation by means of a flame test. Nevertheless, it would appear to be much more satisfactory, especially from a pedagogic standpoint, if conditions could be so adjusted that the strontium oxalate precipitate would not form a t all, and at the same time the sensitivity of the calcium test be not appreciably affected. The great diierence in the solubilities of the oxalates of the two metals suggests that such an adjustment might not be impossible. As early as 1886 Bloxaml pointed out that after strontium has supposedly been removed with sulfuric acid, the solution will still give a precipitate on the addition of an oxalate. Arzruni2 remarked that the filtrate from the precipitation of strontium by a soluble sulfate will yield a precipitate of strontium oxalate. Ebler3noticed that in the separation of the alkaline earth metals strontium oxalate may come down with calcium oxalate when the filtrate from the sulfate precipitate is tested for calcium. He recommended diluting the filtrate with an equal volume of water before adding ammonium oxalate. Experimental Part To prevent the precipitation of strontium oxalate after the removal of strontium as strontium sulfate, the strontium-ion concentration in the filtrate must be reduced. This diminution might be effected (1) by allowing ample time for the complete precipitation of the sulfate and (2) by dilution. The effects of these two factors were studied qualitatively in the following experiments. Bloxam, Chem. Nevs, 54, 16 (1886). Arznmi, Ber., 5, 1044 (1872). a Ebler, Zeit. anal. Chem., 48, 175 (1909).
VOL. 4. NO.
11
Tesr SOR C A LWITR ~ AMMONIUM OXALATE
1429
To a 0.05 normal solution of SrCla an equal volume of 0.5 normal solution of K&04 was added. The mixture was heated to boiling and the precipitate of SrS04 allowed to settle. Five minutes after precipitation a portion of the supernatant liquid was filtered off and tested as follows. To 3 cc. of this filtrate 2 cc. of 0.5 normal (NHJzCaOnwere added. The solutions were mixed and allowed to stand. In 10 minutes a precipitate appeared, and at the end of 20 minutes a fairly heavy precipitate (about 5 mg.) was present. Spectroscopic examination showed it to consist of strontium oxalate containing only a trace of calcium. A second portion of the filtrate was diluted with an equal volume of were added. water. To 3 cc. of the diluted solution 2 cc. of (NH4)2C204 An appreciable precipitate appeared on standing for 20 minutes. A portion diluted to three times its original volume gave a slight precipitate with ammonium oxalate after standing for 40 minutes. Samples diluted further gave no visible precipitates whatever. At the end of 10 minutes a second sample of the supernatant liquid was filtered off from the SrSO, precipitate and subjected to similar tests. In the case of the undiluted sample, the precipitate was not quite as heavy as before. That diluted to twice its volume gave a faint precipitate, but one which would hardly be considered a conclusive test for calcium. The sample diluted to three times its volume gave no visible precipitate at all. Several additional samples of the supernatant liquid above the SrSO4 precipitate were removed a t 10-minute intervals and tested as outlined above. Standing for a time longer than 10-15 minutes made no appreciable difference in the results. From these experiments it appears that the interference of strontium in testing for calcium with ammonium oxalate can be entirely avoided if the solution is allowed to stand for 15-20 minutes after the precipitation of SrS04 and if the filtrate from this precipitate is diluted to three times its volume before making the calcium test. Although dilution effectively prevents the precipitation of SrCeO4 in the test for calcium, it has little effect on the sensitivity of the latter because of the exceedingly small solubility of CaC204. Experimental analyses showed that calcium is readily detected in a solution containing 1' mg. of the metal in the presence of 100 mg. of strontium. A parallel analysis made on a solution containing 100 mg. of strontium but no calcium gave no precipitate whatever in the calcium test. Theoretical confirmation for the method suggested may be obtained from calculations based on the solubility products of strontium and calcium oxalates using the values published by Kohlra~sch.~ This modification has been used by the writers in their classes for some time with very gratifying results. 4
Kohlrausch, Zeit. phys. C k m . , 50,356 (1905); 64, 129 (1908).
In the analysis of the alkaline earth group the removal of strontium as sulfate is incomplete to the extent that the filtrate from the sulfate precipitate will still yield a precipitate of strontium oxalate upon the addition of ammonium oxalate. In the customary procedure this precipitate may readily be confused with the test for calcium. Strontium oxalate is not precipitated if 15-20 minutes are allowed for the removal of strontium sulfate and if the resulting filtrate is diluted to three times its volume before adding ammonium oxalate. Such dilution does not appreciably affect the sensitivity of the calcium test.
