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ANALYTICAL EDITION
hand, care must be taken not to bake the residue and thus decompose the calcium chloride, since in that case it will not be possible to obtain a clear solution in water. A safer procedure is to finish the evaporation over a water bath. For extensive manipulative notes on the remainder of the method the reader is referred t o the recent paper on the direct magnesium-uranyl acetate method for sodium ( I ) . No difficulties will be encountered, however, if the details as given above are followed. Results The average results obtained on several representative samples of reagent-grade calcium carbonate by both the old and new methods are shown in Table I. I n each case the actual sodium in the extraction residue was determined by the new method in order to ascertain the actual amount of alkali extracted from the samples by boiling with distilled water. From the data shown in the table it is obvious that the extraction method gives neither true nor comparative results for the alkali content of reagent-grade calcium carbonate. The minute amounts of sodium found in the residue obtained by evaporating the water extract demonstrate clearly the futility of attempting t o remove and determine adsorbed alkali salts by a washing process. The residues obtained by extraction consist chiefly of calcium carbonate dissolved by the relatively large volume of water used in the method. The results also show that there is a considerable variation in the sodium content of the samples of reagent-grade calcium carbonate from different sources, although the usual method of testing the material would not indicate this variation.
Vol. 1, No. 4
T a b l e I-Comparative Results in Determination of S o d i u m in Samples of R e a g e n t Calcium C a r b o n a t e by Extraction M e t h o d a n d by Magnesium-Uranyl Acetate M e t h o d SODIUM ALKALI SODIUM ALKALI CONTENT CON- ACTUALLY CONTENT SODIUM C!AI.Cn. TENT PRESENT I N LABEL CONTENT TO EXTRAC-EXTRACSTATE- NEW CARBON- TION TION SAMPLE MENT METHOD ATE METHOD RE31DUEa
Per cent Per cen! Per cent Standard stock material None 0.13 0.30 Standard stock material . 0 . 0 5 0.12 0.28 Specially prepared lot Low 0.02 0.05 Standard stock material 0 . 0 1 0.008 0.018 Selected lot from standard stock material 0.053 0.006 0.014 a Expressed in percentage of sample taken.
Per cent
Pdr cent 0.002
0.02 0.02 0.04 0.01
0.003 0,001
