DETECTION of BARIUM ION in the PRESENCE of PHOSPHATE ION T. W. DAVIS AND NATHAN KORNBLUM New York University, University Heights, New York City
A
STANDARD method of removing phosphate ion before the analysis of Group I11 is to buffer an a d d solution with ammonium acetate and precipitate a mixture of ferric, aluminum, and chromic phosphates, together with basic acetates of these three ions.' It was noted that a beginning class a t New York University seldom or never reported barium in Group IV in quantities of 8 to 20 mg. per 10 ml. sample of "unknown" when phosphate ion was thus removed. The percentage of correct reports of the presence of barium in one year was less than ten. The class in which the record was noted numbered forty-four students, mostly freshman chemical engineers and arts students majoring in chemistry. Since barium phosphate is soluble under the conditions of removing ferric phosphate, it is evident that the barium is lost through adsorption. A procedure was introduced, therefore, providing for the isolation and detection of barium from the phosphate precipitate. This involves treatment with sodium peroxide to dissolve aluminum and chromic phosphates, and subsequent solution of the ferric phosphate and ferric basic acetate residue in 6 N hydrochloric acid. Ammonium sulfate (1 N) is added to this hydrochloric acid solution, and the mixture is heated to boiling. A white precipitate of barium sulfate indicates the presence of Ba++, and this is confirmed through a flame test made in accord with the following procedure. The barium sulfate is filtered and washed thoroughly with hot water. A platinum wire is wrapped around the paper containing the precipitate, and after drying over a small flame, the paper is charred and burned off. A residue of barium sulfide remains. This is moistened with hydrochloric acid and introduced into the Bunsen flame. A green flame confirms the presence of barium. One mg. of Ba++ per 10 ml. of solution can be positively detected in this way-an immediate precipitate being secured with ammonium sulfate-but it is difficult to obtain a good flame with this amount. With the old procedure, the smallest amount of barium which could be identified in the presence of phosphate was 20 mg. per 10-ml. sample, and this happened to be the maximum concentration appearing in our unknown solutions. As a result of the introduction of the new of qualitative analy. LMnLEK,F , W,, ',A laboratory
sir," D. Appleton-Century Co., New York City. 1930, P. 82. REEDY,J. H., "Elementary qualitative analysis," 2nd ed., McGraw-Hill Book C? New York City,,l932, pp. 63-4. C~TXAN L.,J.. Qualltatlve chemical analys~s: The Macmillan Co., New York City, 1931, pp. 440-2. NOYES.A. A., "Qualitative chemical analysis." 9th ed., The Macmillan Co., New York City, 1928, p. 105.
procedure, the percentage of correct reports of the presence of barium has increased from less than ten to seventy-five. It is interesting to note that barium was not erroneously reported in spite of the fact that considerable difliculty was anticipated with beginners handling the technic of ashimg filters and using the residue for a flame test. A small amount of sodium is always picked up from the fingers in wrapping the platinum wire around the filter paper, but this does not ordinarily obscure the barium, nor lead to frequently imagined green flames. Strontium is not missed in the presence of phosphate, with the usual procedure, unless the amount of phosphate ion is rather large (about 100 mg. per 10 ml.) or the amount of strontium small. Students had no difficulty finding 10 mg. of strontium, but consistently missed 5 mg. No new procedure, however, has been adopted to provide for the detection of these smaller amounts of strontium. The number of correct reports of the presence of barium when phosphate is present is given in the table subjoined. The record is taken from a beginning class of forty-one students. We include in the table data on the detection of barium and strontium when phosphate is absent but when Group I11 is present. The figures cited indicate that the loss of Ba++ resulting from oxidation of Group I11 sulfides is in most cases unimp~rtant.~Since the strontium test is not ordinarily regarded as entailing any special difficulties, a record of the reports for strontium appears also for comparison. The amounts of Ba++ and ST++ in the solutions varied from 5 mg. to-20 mg., and the PO4--- from 25 mg. to 100mg. per 10 ml. TABLE
DBTSCT~ON W ~ T BPBOSIBATB ABSBNT Mmcd Fomd Mmrd
Ba+* AND Sr*' WITB PBOSPB*~B PRESENT
B= + *
Fomd 21 reports ' 7 reportr
9 report$
2 report*
sr + *
18 r e p m
9 report$
6 reports
9 reports
CONCLUSION
With a given class, we might regard the reports for strontium as setting a standard with which to compare reports for the other ions, since the strontium test usually causes no great difficulty. The figures in the table shok that the modified barium procedure leads to a higher percentage of correct reports than is found for strontium, And at all events, the modified procedure is far more satisfactory than the usual procedure for detecting barium in the presence of phosphate.
CURTMAN
587
9
AND
FRANKEL, 3. Am. Chenr. Soc., 33,724 (1911).
