Determination of Germanium in Silicate Rocks INNA-GRETA HYBBINETTE AND E. B. SANDELL Rolideri RTining Company, Stockholm, Sweden, and School of Chemistry, University of Rfinriesota, Jfinneapolis, 31inn.
T
HE method here described for the determillation of
Apparatus
germanium in silicate rocks and the rock-forming silicates involves decomposition of the sample with sulfuric-nitrichydrofluoric acid, isolation of germanium by distillation of the tetrachloride, arid its colorimetric estimation in the distillate with ferrous sulfate-ammonium molybdate. The latter reagent has been used b y Poluektov (3) for the determination of gernianium after isolation as the sulfide. The composition of the reagent has been modified for the present purpose. In :iddition to germanium, phosphorus, silicon, and arsenic yield complex molybdic acids which are reduced to niolybdenuni blue by ferrous sulfate. Silicon and arsenic may give rise to error in the procedure. E r e n after double evaporation of the decomposed sample to fumes Kith sulfuric acid, traces of fluoride remain, which in the subsequent distillation pass over as fluosilicic acid and lead t'o high results. This difficulty can be overcome by distilling the sulfuric acid solution of the decomposed sample until all fluorine has been expelled, as shon-n by a negative reaction v i t h ammonium molybdateferrous sulfate. Hydrochloric acid may then be added and the germaniuni distilled. The interference of arsenic is not serious in samples for which the method is intended. I n order t o keep the procedure as simple as possible no special method (1) for the separation of arsenic and germanium has been included here. The arsenic content of igneous rocks is so low (ordinarily less than 0.001 per cent) that the amount' of the element finding its way into the distillate is insignificant if it is originally present in the quinquevalent state, as it will be after the decomposition. A4rock sample containing 0.01 per cent of added arsenic shon-ed an increased germanium content of 0.00004 per cent ( S o . 6, Table I), a deviation which is within the limit. of error of t'he method. The possible effect of selenium was not investigated because it occurs only in vanishingly small amount's (of the order per cent) in igneous rocks. There is no reason to believe that amounts considerably larger t,hanthis would interfere if selenium should find its way into the distillate. The presence of small amounts of chloride in the sample does not lead to the loss of any significant amount of germanium in the decomposition ( S o . 5, Table I), as already shown (1). Beer's law is obeyed by the blue product given by the'amnionium molybdate--ferrous sulfate reagent u p t o a concentration of 1.5 micrograms of germanium per milliliter, but a t higher concentrations the color intensity is less than that demanded by the concent'ration under our conditions. Care must be taken t o avoid contamination of the final solutions by silica. The sodium hydroxide solution in which Dhe germanium distillate is caught must be contained in a paraffined vessel. It is important to use sodium hydroxide containing minimal amounts of silicate. A blank should show only a trace of blue color. Exactly the same amount of sodium hydroxide solution must be used for the unknown, standard, and blank solutions. \Then it 1.0-gram sample is taken for analysis, less than 0.0001 per cent of germanium can be detected with certainty if a filter photometer with a 1-cni. cell is used. Since the average germanium content of igneous rocks is 0.0004 per cent according to Goldschmidt and co-workers ( 2 ) , the proposed method appears to have sufficient sensitivity for the determination of the element in the samples for which i t is int.ended.
The all-glass (Pyrex) distilling apparatus of Scherrer (4) may conveniently be used for the distillation of the germanium tetrachloride. The flask should preferably have a volume of 100 ml. The end of the condenser tube should be d r a m to a narrow point. A test tube, drawn t'o a narrow taper at t'he lower end, is used as a receiver to hold the sodium hydroxide solution in which the germanium tetrachloride is absorbed; it should have a capacity of at least 12 ml. and should be well paraffined to prevent attack by the sodium hydroxide solution.
Special Solutions h I h l O N l U M h'lOLYBDATE. Dissolve 6.00 grams of ammonium molybdate tetrahydrate in about 35 ml. of water, add a cooled mixture of 16.0 ml. of concentrated sulfuric acid (sp. gr. 1.84) with 35 ml. of water, dilute the solution to 100 ml. with water, and preserve in a paraffined bottle. FERROUS AMJIOSIUM SULFATE.Dissolve 10.0 grams of the hydrated salt in water containing 1.5 ml. of 6 N sulfuric acid and dilute t o 500 ml. SODIC11 ACETATE. Dissolve 67.5 grams of sodium acetate trihydrate in water and dilute to 200 ml. Keep t'he solution in a paraffin-lined bottle. A m I o N I u h r MOLYBDATE-FERROUS . h r v o ~ ~ u aSULFATE r REAGEST. This solution is unstable and must be prepared immediately before use. Add successively with shaking 10 ml. of ammonium molybdate solution, 10 ml. of ferrous ammonium sulfate solution, and 25 ml. of sodium acetate solution to 50 ml. of water and dilute to 100 ml. with water. Allow the reagent to stand 5 minutes before use. SODIUM HYDROXIDE. Dissolve 25 grams of analytical quality reagent as lox as possible in silica in 100 ml. of water and store the solution in a paraffined bottle. STANDARD GERRZANICJY SOLUTION, 0.01 P E R C E N T GERMASIUM OR GERhfANIUM DIOXIDE. Dissolve a weighed amount of pure dry germanium dioxide in a few milliliters of water to which a drop or two of dilute sodium hydroxide has been added, neutralize with dilute sulfuric or hydrochloric acid, add a drop or two of acid in excess, and make up t o volume.
Procedure Weigh 1.0 gram of rock powder into a platinum dish and add a few milliliters of water, 6 ml. of 1 to 1 sulfuric acid, 0.5 to 1 ml. of concentrated nitric acid, and 10 ml. of hydrofluoric acid, Evaporate slowly on a hot plate until the sulfuric acid just starts t o fume, cool, add a few milliliters of water, and again evaporate to fumes. Repeat the addition of water and evaporation at least twice, never letting the mixture fume strongly. Transfer the decomposed sample to the distilling flask with 1 to 1 sulfuric acid (chloride-free) and water. The total volume of the solution should be approximately 50 ml., of which 35 ml. should be 1 to 1 sulfuric acid. REMOI-AL OF RESIDUAL HYDROFLUORIC ACID. To remove traces of fluoride, bubble air slowly through the solution and raise the temperature t o 140' C,. Maintain the temperature at this figure during the distillation by the slow addition of water through the funnel. In this manner distill over and discard 150 ml. of liquid. A l l o ~ the distilling flask t o cool, add 15 ml. of water and, while passing air through the solution, distill until a temperature of 120" is reached. Catch the distillate in a paraffined drawnout test tube containing 1 ml. of 25 per cent' sodium hydroxide solution and 1 ml. of water. Neutralize the distillate with 1 to 1 hydrochloric acid, using phenolphthalein as indicator, add a drop of sodium hydroxide, and then acidify with acetic acid and add the mixed ammonium molybdate-ferrous ammonium sulfate reagent as described below, but use the whole solution instead of an aliquot. Prepare the comparison solution by adding the reagent to a solution obtained by mixing sodium hydroxide and hydrochloric acid to give a mixt,ure having the same composition as the neutralized distillate. If the distillate shows no color when conipared against the reference solution, all hydrofluoric acid has been expelled from the solution in the distilling flask and germanium may be dist,illed over as described in the next section; if a color 715
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INDUSTRIAL AND ENGINEERING CHEMISTRY
is shown, distillagain at 140" m already described until a negative result is obtained. DISTILLATION OF GERMANIUM TETRACHLORIDE. Add 2 ml. of 1 to 1 hydrochloric acid and 2 ml. of water to the cool contents of the distilling flask, and distill while bubiling air through the solution until the temperature reaches 120 . Collect the distillate in the drawn-out test tube containing 2.00 ml. of 25 per cent sodium hydroxide solution. At the end of the distillation, neutralize the solution with l to l hydrochloric acid, transfer t o a 25-ml. volumetric flask, and make up to volume with water. Allow the distilling flask to cool, add 2 ml. of 1 to 1 hydrochloric acid and 2 ml. of water, and distill as before. Xeutralize t,he solution, transfer to a 25-ml. volumetric flask, and make up to the mark with water. With small amounts of germanium practically all is found in the first distillate, but with quantities in the neighborhood of 25 micrograms a small amount is found in.the sacond distillate. COLORIMETRIC DETERMINATIOK OF GERMANIUM. Prepare two standard germanium solutions as follows. Measure out two portions of germanium oxide solution containing, for example, 5 and 10 micrograms of germanium, and add 10 ml. of water, 1.5 ml. of 1 to 1 hydrochloric acid, and 2.00 ml. of 25 per cent sodium hydroxide solution. A-eutralize the solutions carefully with 1 to 1 hydrochloric acid (phenolphthalein as indicator) and dilute to 25 ml. In a similar manner prepare a blank solution for the color comparison by neutralizing 2.00 ml. of sodium hydroxide and diluting to 25 ml. Take a 10-ml. aliquot of each standard, of the two germanium distillates, and of the blank. Make each solution basic with a drop of 25 per cent sodium hydroxide and then acidify with 0.10 ml. of 1 to 1 acetic acid. Add 10.0 ml. of ammonium molybdateferrous ammonium sulfate reagent, mix, and dilute to 25 ml. with water. After 15 minutes compare the germanium distillates and the standards against the blank solution in a filt'er photometer, using a red filter. The standard series method of comparison may also be used. The color intensity of the solutions increases slowly on standing and the readings should, therefore, be made after the period of standing specified. Although germanium in appreciable amounts is unlikely to occur in the reagents, it is nevertheless advisable to run a blank through the procedure, especially since any contamination by silica will then be revealed.
TABLEI. DETERMINATION O F GERMANIUM I N SILICATE ROCKS Sample
Germanium Present0
Germanium Found
%
'%
%
0.0005 0.0009 0.0029 0.0050 0.00028 0.00030 0.0007 0.0013 0.0052
0.0000 -0.0001 -0.ooo1
1 Graniteb 2 Granite 3 Granite 4 Granite 5 Granitec 6 Granited 7 Diabase' 8 Diabase 9 Diabase
0.0005 0.0010
0.0030 0.0058 0.00026 0.00026 0.0008 0.0014 0.0056
Error
-0.0008
+o, 00002
+- 0.00004 0.0001
-0.0001 - 0.0004 S u m of germanium originally present i n samples as determined by method described (0.00026% in, granite. 0.000147, i n diabase) a n d t h a t added. b Percentage composition: SiOz, 76.8; AlaOa! 13.2; FezOa, 0.3; FeO, 0.4; Mg-0, 0.2: CaO, 0.7; NalO, 3.8; Kz0, 4.5; TiOz. 0.08; PzOs,0.02; M n O ,
Acknowledgments The authors are indebted to Theodore C. Blegen, Dean of the Graduate School, and S. C. Lind, Dean of the Institute of Technology, University of Minnesota, for making laboratory facilities available for this work.
Literature Cited (1) Aitkenhead, W. C., and Middleton, A. R., IXD. ENG.CHEM., ANAL.ED.,10, 633 (1938). (2) Goldschmidt, V. M., and Peters, C., Nachr. Ges. Wiss. Gottingen, Math.-physik. Klasse, Fachgruppen, 1933, 141; Goldschmidt, V. M., Hauptmann, H., and Peters, C., Naturuiasenschaften, 21, 363 (1933). (3) Poluektov, N. S., Mikrochemie, 18, 48 (1935). (4) Schemer, J. A , , J . Research Nail. Bur. Standards, 16, 255 (1936).
A Volumetric Method for Determining Tin Based on the Formation of a Dioxalatothiometastannate HOBART H. WILLARD AND T A F T Y. TORIBARA University of Michigan, A n n Arbor, Mich.
W
HEELER (1) recognized that stannic tin in oxalic acid solution forms a fairly stable compound with
sulfur when hydrogen sulfide is passed into such a solution. H e made use of this fact in devising a volumetric method of determining tin in bronze by titrating the sulfur with iodine. The tin was separated from other metals by addition of phosphoric acid, the precipitate was dissolved in concentrated sulfuric acid, and the solution was neutralized with ammonia, using methyl orange indicator. Six or 7 grams of oxalic acid were added per 0.2 gram of tin and the solution was boiled until clear. The hot solution, 100 ml. in volume, was saturated with hydrogen sulfide, diluted to 260 ml., and cooled to room temperature, and a stream of air was bubbled through it 15 t o 20 minutes to remove the excess of hydrogen sulfide. A slight excess of standard iodine was added and back-titrated with thiosulfate.
Difficulties i n the Use of the Existing Method I n attempting t o verify Wheeler's work, it was found that conditions had t o be adjusted so carefully in order to obtain the theoretical relationship between tin and sulfur that the method was not practical. It was necessary to pass hydrogen sulfide into the solution for at least 20 minutes a t a rate of 1 liter per minute to ensure complete absorption. Using air,
nitrogen, and carbon dioxide as agents to sweep the excess hydrogen sulfide out of the solution, the results obtained were dependent upon the gas used and the time of its passage. The passage of air at a rate of 1.5 to 2 liters per minute for more than 15 minutes always gave low results, whereas nitrogen and carbon dioxide at the same rate gave low results when passed for much over 20 minutes. At the end of the passage of the gas, there was still an odor of hydrogen sulfide. Since it was difficult to regulate the passage of gas in the same manner for each determination, it was likewise difficult to compare the times of passage on a quantitative basis.
ON TABLEI. EFFECTOF PASSISGNITROGEN RATIO
Time Min.
0 10 30 50 80 110
THE
S/Sn Ratio
SULFUR-TIS
