NICKEL PLATED IN LEATHER CASE 21/- Post Free is not ONLY a LINEN TESTER not ONLY a SCREW THREAD TESTER and GAUGE, not only a BRINELL IMPRESSION MEASURER, BUT ALL THESE and MANY MORE COMBINED. W.5. Naval and Military Telescopio Gun Sights, Theodolites, Tacheometers, Levels, Finers' Dials, Prismatic Compasses, Heliographs, Equatorials, Driving Clocks, Micrometers, Coelostats, Transits, Mirrors, Flats, Naval, Military and Astronomical Telescopes, Prismatic Binoculars, Sextants, etc., eto. The recent revival of interest in the history of the Sciences makes the book appeal to the general scientific public. It is also intended for use in Schools, where the History of Science occupies a place in the curriculum. THE CHEMICAL NEWS, VOL CXXXI. No. 3410. MERTON HOUSE, SALISBURY SQUARE, LONDON, E.C.4. TELEPHONES: Administrative Central 6521 Printing Works, Hop 2404. CRITICAL STUDIES ON METHODS OF ANALYSIS. XVI.-ZINC. By LEON A. CONGDON, A. B. GUSS, AND F. A. WINTER. (Contribution from the Department of Chemistry, Syracuse University.) (Continued from Page 100.) as of Six modifications of the gravimetric phosphate method for zinc were tried experimentally and the order of their average accuracy may be stated as follows, the most accurate coming first:-1, Findlay and Cumming's modification of the DakinTamm method weighing the zinc Zn,P,O,; 2, Dakin's modification Tamm's method, weighing the zinc as ZnNH,PO, after heating the precipitate on a water bath for one hour before filtering; 3, modification of Dakin's method by weighing the zinc at ZnNH,PO, after letting the precipitate stand four hours and filtering while cold; 4, modification of Dakin's method, weighing the zinc as ZnNH PO, after letting the precipitate stand 24 hours and filtering while cold; 5, modification of Losekam and Meyer's modification of Tamm's method, weighing the zinc as Zn,P,O,, after letting the precipitate stand 5 hours and washing the precipitate with a warm dilute solution of NH1NO,, instead of hot water as suggested by Losekann and Meyer; and 6, Losekann and Meyer's modification of Tamm's method weighing the zinc as Zn,P2O,, using hot water to wash the precipitate of ZnNH,PO,. All these methods gave an accuracy of less than 0.20% of the theoretical amount of zinc in the sample examined, with the exception of Losekann and Meyer's method, which, if followed as given by them, will give 1.5% less than theoretical, and as modified, will give 0.27% to 0.5% less than theoretical. 2 7 SPECIAL NOTICE. All MSS., Books for Review, Business Letters and Communications of every description should be sent to the offices as under, and not to any other address. CHEMICAL NEWS, MERTON HOUSE, SALISBURY SQUARE, LONDON, E.C.4. Three modifications of the gravimetric sulfide method for zinc were experimentally performed and the order of their accuracy can be laid down as follows:-1, Anthony Guyard's method of precipitating the zinc as zinc sulfocarbonate by ammonium sulfocarbonate, the precipitate heated at 100° to 110°, then heated gently to constant weight and weighed as ZnS; 2, by precipita ting the zinc with HS in sodium acetate solution as zinc sulfide, drying at 100° to 105° on a weighed filter paper and weighed as zinc sulfide; 3, by precipitating the zinc with HS in ammonium acetate solution as zinc sulfide,drying at 100° to 105° on a weighed filter paper and weighnig as ZnS. Only one of these methods gave results within 0.2% of the theoretical, the last two mentioned giving results within 0.4% less than the theoretical. Three modifications of estimating zinc as sulfate were undertaken and their accuracy may be classified as follows:-1, Precipitating zinc by ZnS by HS in potassium acetate solution and the sulfide changed to a sulfate by H2SO, and weighed as sulfate; 2, Precipitating zinc as ZnS by H2S in ammonium acetate solution and the sulfide changed to a sulfate by H2SO, and weighed as sulfate; 3, precipitating the zinc as ZnS by (NH4)2S and the sulfide changed to a sulfate by H2SO, and weighed as ZnSO,. These three methods were within 0.2% of theoretical on the average. Three modifications of determining zinc as an oxide were examined and their accuracy may be stated as follows:-1, zinc precipitated as ZnPy, (SCN), by ammonium sulfocyanide and pyridine in neutral solution, the precipitate ignited to ZnO to constant weight and weighed as such; 2, zinc precipitated as ZnS by sulfur in a solution of KOH, ignited to the oxide by a muffle and weighed as ZnO; 3, zinc precipitated as ZnS by HS in potassium acetate solution and strongly heated to constant weight to the oxide and weighed as ZnO. These methods gave results within 0.2% of theoretical with the exception of the last named method which was within 0.4% less than theory. Three modifications of the volumetric K,Fe(CN) method by use of different indicators were tried and the accuracy classed as follows:-1, using uranium acetate as indicator; 2, using uranium nitrate as indicator; and 3, using uranium chloride as indicator. To this may be added fourth in accuracy, the Percy Walker volumetric neutralisation method by adding an excess of standard acid to the ZnNH,PO, precipitate and titration of the excess of acid with standard alkali. These methods gave an accuracy within 0.2% of the theory with the exception of Percy Walker's neutralisation method which, while giving good checks, gave results a little over 0.4% higher than the theory. 4.-PROCEDURE FOR MOST ACCURATE Zinc freed from interfering elements may be determined most accurately by one of the following methods which will give the best concordant results in the hands of the average analyst. (a) Authors' modification of A. Thiel's method:-Take nearly 0.2 gram of the sample in about 50 cc. of solution, add 10 cc. of a 10% solution of potassium acetate. Then add HS under pressure, filter on filter paper and place filter paper with precipitate in a weighed silica crucible. Add HCl to dissolve the sulfide and then a small amount of dilute H,SO,. Heat with a Bunsen burner. After all the carbon has been burned off and the ppte heated, add a very small amount of concentrated H.SO, to the ignited mass to convert to the sulfate any of the zinc that might have turned to an oxide. The difficulty of the method is to carefully drive off the excess of H2SO,, otherwise it might spatter, and also carefully ignite so as not to change the sulfate to an oxide. The zinc is weighed as ZnSO1. In Thiel's original method, he weighs as sulfide and heats the sulfide to 120° in a current of hydrogen to get rid of the last traces of H,S. He also used ammonium acetate instead of potassium acetate solution as medium for HS precipitation. The method as it now stands is nearly a new method of the authors, and a glance at the table of results will show concordant reliable findings as compared to other methods experimented upon. (b) G. Spacu's (31) gravimetric method where zine is weighed as the oxide is as follows:- Add a slight excess of NH,SCN to a neutral solution of a zine salt, then a few drops of pyridine slowly and with constant and vigorous stirring. Collect the white precipitate of ZnPy (SCN)2, after letting stand 15 minutes or more and wash the precipitate with a cold solution of a mixture made up as follows- 0.3 gram of NH,SCN, 0.2 gram (NII),SO1, dissolved in 100 cc. water. Dry the precipitate and the filter, and ignite over a Teclu burner until constant weight is obtained and weigh as ZnO. This procedure will also separate Zn from Mg and the alkaline earths. In the investigation of this method it was found necessary to heat the precipitate at least half an hour before constant weight was obtained, and to ensure the zinc was all in the form of an oxide, otherwise high results will be obtained. Best results were obtained by using 0.2 to 0.3 gram sample (in this case the sample was zinc nitrate). The method was found exceedingly accurate in the hands of average analyst. It is quite necessary that the solution precipitated be neutral, as best results are obtained under these conditions. It is also found that mercury, copper, lead, and cadmium form similar compounds quantitatively, and hence must be separated from the solution precipitated. 5. SUMMARY AND CONCLUSIONS. (a) A study of the literature for the estimation of zinc is given with over one hundred original references. (b) A sample of c.p. zinc nitrate of known purity was analysed by nineteen different modifications for the estimation of the zinc content, and compared with the theoretical percentage of zinc in the sample. (c) The most accurate and concordant method found was that developed by the authors modified from A. Thiel's ammonium aetate sulfide method, wherefrom the authors practicaly develop a new method by precipitating the zinc as sulfide in potassium acetate solution, changing the sulfide to a sulfate by H.SO, and weighing the zinc as ZnSO,. Thiel's original method was not found very accurate, but gave results a little over 0.3% less than the theoretical. (d) Another accurate concordant result method found was that of Spacu (31). where the zinc is precipitated in neutral solution by NH SCN and pyridine, and the fflnPy (SCN), resulting is ignited to ZnO 2 by the Teelu burner and weighed as ZnO. This method is applicable in the presence of magnesium, but it is found that Cu, Pb, Hg, and Cd interfere as they also form similar quantitative compounds. (e) The nineteen methods or modifications experimentally studied were as follows-Six modifications of the phosphate method; three modifications of the sulfide method; three modifications of the sulfate method; three modifications of the oxide method; three modifications of the volumetric K,Fe(CN) method; and Percy Walker's volumetric neutralisation method. (f) The following methods for zinc were studied from the theoretical and practical standpoint As phosphate; sulfide; sulfate; oxide; carbonate; mercury thiocyanate; by electrolysis; volumetric potassium ferrocyanide; volumetric sulfide; metric thiocyanide; volumetric oxidation methods: volumetric neutralisation methods; volumetric potassium cyanide methods; and by miscellaneous methods such as indirect gasometric, colourimetric and turbidimetric methods. volu (g) All of the nineteen methods or modifications experimentally examined gave results within 0.2% with the exception of the following Losekam and Meyer's modification of Tamm's phosphate method; A. Thiel's original H.S method in ammonium or sodium acetate solution; A. Thiel's method by changing the ZnS to the oxide, after precipitation by HS in potassium acetate solution. All of Thiel's methods gave results a little over 0.3% less than theoretical, but if modified by the authors' method as outlined in this paper and weigher as sulfate, it was found that this gives very concordant results. Losekam and Meyer's method as phosphate,if carried out as given in their original paper, was found to give over 1.5% less than theoretical. An attempt was made to modify this method by using NH,NO, solution instead of water to wash the precipitate, which brought results nearer theoretical, giving values from 0.27 to 0.5% less than theoretical. Austin, M., Amer. Jour. Sci. (4), (1889) VIII., 206. (9) (10) (11) (12) (13) (1896), XV., 866. Langmuir, A. C., Jour. Amer. Chem. Dakin, H. D., Zeit. Anal. Chem. (14) Hassreidter, V., Zeit. Angew Chem. (1911), XXIV., 2491; ibid. (1912), XXV., 1005. (15) Taylor, Frank O., Jour. Ind. Eng. Chem. (1911), III., 866-7; ibid. 1912), IV., 467. (16) Findlay, T. M. and Cummings, A. C., Proc. Chem. Soc. (1903), CIII., REFERENCES. 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