EXERCISE in INDIRECT ANALYSIS M. G. MELLON Purdue University. Lafayette, Indiana
A
LTHOUGH methods of chemical analysis involving indirect determinations are usually avoided, if possible, there seems still to besufficientnecessity for resorting to such procedures to warrant bringing them to the attention of students, noting the applicabilitv of the more imoortant tvoes tocrether with their inherent limitations. In order to give experience with one such determination an adaption has been made of the familiar and important method for determining the alkalies, sodium and potassium. In the belief that the practice followed in this laboratory may be of interest to others, a brief statement is presented concerning the preparation of the sample and the methods of determination suggested.
lations involved indicates the desirability, after separating the flint, of determining either of the non-common elements, sodium or potassium, or the common element, chlorine, in order to calculate thepercentagesof the two metals. To accomplish this one may proceed as indicated below.
BARBERAN^ KOLTHOPP. ihid., 50, 1625 (1928); 51,3233 (1929).
the different constituents.
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PROCEDURE
Weigh a dried sample of about 2 g. and transfer to a 250-ml. beaker. Dissolve the sodium and potassium chlorides in about 50 ml. of water by heating nearly to boiling. Transfer the insoluble matter to a weighed Gooch crucible, or to a funnel fitted with a good grade of paper, and wash well with hot water. If a Gooch crucible is used, the residue should be dried a t 105OC. SAMPLE If a paper is used, i t should be ignited in the usual The samples are prepared by mixing in a ball mill dif- manner. In either case the residue constitutes the inferent proportions of sodium and potassium chlorides soluble matter. and flint, each constituent having been separately The filtrate contains the sodium and potassium ground to pass a 100-mesh sieve. Any flint finer than chlorides, the amount being the differencebetween the 200 mesh is discarded as experience has indicated that weights of the sample and the insoluble matter. Conpart of such finely divided material passes through tinue by diluting the filtrate to 250 ml. in a volumetric filtering media, such as thick mats of asbestos and the flask, and use a portion of 25 ml.* of this for determining best grades of paper procurable. The flint serves sodium, potassium, or chlorine. In proceeding by an merely as a diluting material to obtain a suitable spread indirect method, involving the determination of one of in the composition of the samples. The ratio of the these elements and the calculation of one or both of the percentages of sodium and potassium are kept within alkalies, it is evident one has a choice of methods. The the limits 3: 1 and 1:3. following are suggested as the most suitable, although With the recent introduction of a direct method' for they should not be expected to yield equally precise determining sodium, it is now possible, of course, to results: potassium gravimetrically as KCIOl or Kzdetermine directly each of the constituents of this PtCle; chlorine gravimetrically as AgCl or titrametrimixture. Where the sample serves for an indirect cally with silver nitrate using either a direct or back tidetermination, several theoretically possible procedures tration; and sodium as NaMg(UOz)s(C2HaOz)~.6'/zHzO. may be used. Consideration of the nature of the calcu- * The amount of sample and of the aliquot part of the filtrate CALEYAND FOULK.I. Am. C h . Soc., 51, 1664 (1929); may need to be varied to minimize errors involved in determining
