3IARCH 15, 1940
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ANAL\-TICAL EDITION
to make multiple exposures, as with astigmatic spectrographs. A further possible use is in absorption work, where two separate exposures, one through the solvent and one through the solution, can be taken in place of the single exposure with a split beam. Hence, it would obviate use of a cumbersome piece of apparatus. Still another use is in miscellaneous test work in the laboratory, such as the determination of relative spectral sensitivity of plates, relative efficiency of‘ condensing systems, and selective absorption of screens and neutral filters. DISTANCE ALONG ARC GAP-
MM
Acknowledgment
FIGURE 5
As Pfund pointed out, other metals can be used as the anode. The present author has tried several having spectra similar to iron. Cobalt and nickel, particularly the latter, behaved very well, chromium refused to fuse to a bead, and manganese proved entirely unsteady. No tests were made for reproducibility.
Applications of Iron Arc Besides the use to which the standard iron arc is put in this laboratory, it should prove highly suitable for plate calibration in conjunction with the usual internal standard procedure of spectrochemical analysis, particularly when one is forced
The author’s thanks are due to Howard F. Carl, of this station, for suggesting the iron arc as a standard and for the x-ray work.
Literature Cited (1) Ayrton, Hertha, “The Electric Arc”, London, Electrician Printing and Publishing Co., 1902. (2) Forsythe, W. E., “Measurement of Radiant Energy”, p. 246, New York, McGraw-Hill Book Co., 1937. (3) Nottingham, W. B., J . Am. Inst. Elec. Engrs., 42, 12 (1923). (4) Pfund, A. H., Astrophys. J., 27, 296 (1908). (5) Slavin, Morris, IXD. ENG.CHEM.,Anal. Ed., 10, 407 (1938). (6) Strock, L. W., “Spectrum Analysis with the Carbon Arc Cathode Layer”, London, Adam Hilger, 1936.
PUBLISHED by permission of ment of t h e Interior.
the Director, Bureau of Mines, U. S. Depsrt-
( S o t subject t o copyright.)
An Empirical Mercurimetric Method for Zinc ALBERT C. TITUS AND JACK S. OLSES University of U t a h , S a l t Lake City, U t a h
I
?i T H E application of the usual volumetric ferrocyanide
method to the determination of zinc dependable results can be obtained only with practice and by standardizing the procedure very carefully. However, a previously existing gravimetric method R as perfected by Vosburgh, Cooper, Clayton, and Pfann (6) in which the zinc was precipitated as its mercuric thiocyanate. By converting this to a volumetric basis time could be saved in an otherwise excellent method. Jamieson (2) dissolved the precipitate in an excess of potassium iodate in acid solution, extracted the liberated iodine by an ether layer, and titrated in this two-phase system with sodium thiosulfate until the disappearance of the purple color in the ether layer. The liberation of free hydrocyanic acid tends to make this method undesirable. Another conversion to the volumetric basis was that of Kolthoff ( 5 ) ,who determined the excess of precipitant in aliquot portions of the supernatant liquid above the white crystalline zinc mercuric thiocyanate, using a standard mercuric-ion solution with ferric-ion indicator. It would seem preferable t o determine some constituent of the pure precipitate rather than the excess of precipitating agent in the filtrate, since the latter would contain various possibly interfering ions derived from the sample being analyzed. The authors base their method upon the mercurimetric determination of the excess iodide remaining in a solution made by dissolving the filtered out zinc mercuric thiocyanate in a known amount of potassium iodide solution This empirical method necessitates the use of a simple straight-line equation to convert the milliequivalents of iodide apparently used up to milliequivalents of zinc. Its appli-
cation should be particularly useful in the routine determination of large numbers of zinc samples Were the method stoichiometric, the following equations would represent the course taken by the reactions: 4 I(excess)
+ ZnHg(SCN)d = Zn’+ + HgId-- + 4 SCX‘(precipitate) Hg++ + 4 I- = HgI,-(standard solution)
The indicator for the last reaction is self-contained in the system: Hg++ HgId-- = 2 HgI? (red precipitate)
+
In practice two sources of error of opposite sign appear to be the chief reasons for the empirical relationship. The first is a positive error caused by the appearance of the end point before an equivalent amount of mercuric ion has been added, causing the calculated amount of used up iodide to be too the error is high As has been pointed out by llolthoff (8, 4, quantitatively related to the square root of the Hg14-- molarity at the equivalence point. By control of the volume a t the end point and of the amount of iodide added, the concentration of the HgIa-- is kept essentially constant in all runs, whether no zinc is present or a large amount is being determined. In this way the positive error is kept constant. The other and negative error is proportional to the amount of zinc being determined, since it is due to the thiocyanate ion equivalent to the former. Kear the equivalence point the iodide-ion molarity decreases rapidly and so becomes too
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small t o prevent the formation again of some Hg(SCN)4--, leading t o the addition of too much mercuric ion as the standard solution.
Reagents and Apparatus Nitric acid and water were properly purified. In all work check runs were made with separately distilled water. ZIXCKITRATE. This was standardized indirectly as explained below. Any impurities which might react like zinc would not be harmful, since the solution was merely used in checking the precision of the equation for the general method and the straightness of the curve represented by the equation. POTASSIUM IODIDE.The very useful discovery of Bouyer ( I ) that magnesium hydroxide exerts a stabilizing effect,on potassium iodide solutions was most helpful in enabling the authors effectively to prevent a slow oxidation to iodine which would otherwise have taken place. To about 20 liters of the solution made from the reagent material there was added about 1 gram of magnesium oxide. The solution was found by the gravimetric silver method, by t x o series of four runs each, to be 0.1097 * 0.0005 N. Appropriate blanks were negligible. In the application of the zinc method the normality of the iodide must not vary more than a very few per cent from 0.11, since it controls the Hg14-concentration. The normality is obtained by dividing the gravimetrically obtained molarity by two. The latter must be known to a very few tenths of a per cent. ~ ~ E R C U R I NITRATE C SOLUTIOSS.For each solution reagent mercuric nitrate was dissolved in pure nitric acid, follon-ed by appropriate dilution. Solution I1 x a s 0.20 N in nitric acid and was found to be 0.1081 * 0.0002 K in mercuric ion by tT7-oseries of three and four runs, respectively, against standard potassium thiocyanate. Solution I11 v a s 0.17 N in the acid and in tn-o series of three runs each its strength in mercuric ion was found to be 0.1088 * 0.0005 N . Ferric alum vias the indicator. PoTAssInar THIOCYASATE.Reagent material was used in making a solution which was standardized against t n o ,different silver nitrate solutions in the presence of nitric acid, using the same sample of indicator employed for the mercuric nitrate standardizations. The first solution of silver nitrate was made from a commercial sample of "pure recrystallized silver nitrate" and the second from silver nitrate made by recrystallization of more of the first sample. In each case the powdered material was dried a t 110" C. and made to volume. Two sets of three runs each showed that the final normality of the t,hiocyanate was 0.1080 * 0.0004. PoTAssInni MERCURIC THIOCYANATE. Ammonium thiocyanate was dissolved and recrystallized, after filtration, and the crystals were washed in filtered alcohol. To a solution of this ammonium thiocyanate, reagent grade mercuric nitrate was added t o form mercuric thiocyanate. For each liter of find 0.1 N solution there were used 16 grams of the mercuric thiocyanate which was dissolved in a concentrated solution made from 11 grams of reagent potassium thiocyanate. Some hydrolysis occurred during preparation but after filtering no further change was observed. The solution contained 10 per cent excess of the potassium thiocyanate.
Method for Determination of Zinc 1. Take a sample containing 0.04 to 0.17 gram of zinc as the sulfate or nitrate in about 50 ml. of acidic solution. (The final normality after precipitation of the zinc can be as high as 0.1 in acid.) 2. Add 75 ml. of 0.1 N potassium mercuric thiocyanate slowly with mechanical stirring. Let stand 1 hour. 3. Filter, and wash with 50 to 75 ml. of 0.001 molar (0.002 N ) , potassium mercuric thiocyanate cooled to 0" C. Discard the filtrate and washings. (The filter paper used was KO.201, H. Reeve Angel BE: Co., New York.) 4. Punch a hole in the bottom of the filter paper and wash the precipitate through. (The comparatively large crystals mshed cleanly away from the surface of the paper in this work.) However, if desired the potassium iodide used in the fifth step can next be allowed to run over the DaDer as it is delivered from the pipet. Washing can then folio
