Colorimetric Determination of Germanium as Molybdigermanic A c i d R.
E. KITSON WITH M. G. MELLON Purdue University, Lafayette, Ind.
A spectrophotometric study of the molybdigermanic acid method for determining germanium shows the procedure works well when the yellow color is developed in 5 N acetic acid solution. Preferably the molybdate solution is added to the acidified germanate solution. For 5-cm. thickness the range i s 0 to 30 p.p.m., through which
Beer's law applies. Other elements forming colored heteropoly solutions, such as arsenic and silicon, interfere with the determination of germanium b y this procedure. Picric acid or buffered potassium dichromate solutions are suitable for use as permanent standards for visual comparison.
c
RMANATES, phosphates, silicates, and arsenates have used, its concentrat,ion, the molybdate concentration, and the G ' l ong been known to form heteropoly compounds with order of mixing the reagents. molybdates, tungstates, and vanadates. These complexes, of Prelirpinary experiments showed that solutions prepared which ammonium molybdiphosphate, (NH,)*[P(MO~O~~)~].~H~O, according to the procedure of Alimarin and Ivanov-Emin ( 1 ) is a well-known example, have found many applications faded rapidly. For transmittancy readings made at 440 mp in analytical chemistry (10). Colorimetric uses are based upon the fading error amounted to 2% in less than a minute after color the colors of the soluble complexes or of their reduction products. development. It was obvious that a more stable solution would Molybdigermanic acid, H q [Ge(Mo3O1~)~].zHIO, was first have t o be found to use the method with a photometer. prepared by Schwarz and Giese (8) and by Grosscup (S), who In order to study the effect of acidity and molybdate connoted its intense yellow hue and its solubility in water and certain centration on the intensity and stability of the color, 5 ml. of organic solvents. Although Grosscup suggested its use in the standard germanium solution were pipetted into a 50-ml. colorimetry, Alimarin and Ivanov-Emin (1) were the first to volumetric flask, to this was added a freshly prepared mixture apply it. Several methods based upon reduction of this acid to a containing kno1v-n amounts of acid and molybdate, and the molybdenum blue have appeared in the literature since 1930 solution was then diluted to the mark and mixed. Trans(a, 4, 6, 7), but little work has been done with any of them, mittancy readings were taken at 440p at definite time interval8 Since so few have used the acid itself, the present investigaafter the color development. tion was undertaken to extend our knowledge of the method, These experiments showed that the intensity of the color special attention being given to determining the optimum condeveloped with sulfuric, nitric, hydrochloric, perchloric, or ditions for developing and stabilizing the color and to observing trichloroacetic acid was extremely sensitive to acid concentration. the effect of diverse ions upon it. The maximum color was developed when the solutions were 0.1 to 0.3 N in acid, but the exact range depends on the molybdate APPARATUS AND SOLUTIONS concentration. Throughout the range of maximum color the solutions fade rapidly. More stable solutions are secured at Transmittancy measurements were made in 5.00-cm. cells with a General Electric recording spectrophotometer adjusted higher acid concentrations, but at this acidity the intensity of the for a spectral band width of 10 mp. All numerical calculations h a 1 color is so sensitive to acid concentration that large errors were based on transmittancy readings of the photometer dial a t are produced by extremely small variations in acidity, and the 440 mp rather than on the recorded curve. A glass electrode was color intensity is much lower than the maximum possible with used for making pH measurements. the system. Germanium tetrachloride (c.P.) was hydrolyzed to the dioxide by treatment with water. This material was purified by a proIncreasing the amount of molybdate in the presence of the cedure similar to that of Johnson and Dennis (6). The standard strong acids broadens the range for maximum color developgermanium solution was prepared by fusion of 0.3602 gram of the ment, but has little effect on the stability of the system. dry dioxide with 2.0 grams of sodium carbonate, dissolution in The color intensity of solutions prepared with. acetic acid water, and dilution to 1000 ml. Dilutions of this stock solution containing 0.1 mg. of germanium per ml. were used. increases rapidly until the acidity is about 3.5 N . Above this Ammonium molybdate solutions were made by dissolving the concentration, the color intensity increases slowly. Variations C.P. salt, (NHJ&IO,OM.~H~O, in warm water. Since the soluin the molybdate concentration have little effect on the color. tions developed a turbidity on standing, they were prepared The stability of the solutions increases with acidity, solutions fresh every 48 hours. The various acids used were of the usual reagent quality. more than 2 N in acetic acid being stable for about 5 minutes. Standard solutions of the recrystallized salts used to observe Solutions more than 6 AT possess a slight yellow color even in the effect of diverse ions on the color reaction contained 10 mg. the absence of germanium. The intensity of this color increases of the ion per ml. of solution. Nitrate, acetate, or sulfate salts with acidity. The optimum acetic acid concentration is about of the cations, and potassium, sodium, or ammonium salts of the anions were used. 5 N. Recrystallized picric acid, and potassium chromate and dichromate were used in preparing solutions for permanent standards. Having selected acetic acid as best for the color development, Except for storage in Pyrex bottles, no special attempt was the following experimental procedure was used to study the effect made to avoid contamination by silica. Since most of the work of variables on the color reaction. The desired amount of gerwas comparative in nature, this should have introduced no serious manate solution, usually 5 ml., was transferred to a 50-ml. error. I n cases where it was desired to avoid contamination all volumetric flask, followed by 15 ml. of glacial acetic acid and solutions were used within 24 hours after preparation. enough water to make the total about 40 ml. After 5 ml. of 2.5% ammonium molybdate solution had been added, the system was diluted t o the mark with water and mixed. Transmittancy COLOR REACTION measurements were made within 15 minutes after the molybdate The yellow color which forms when molybdate solutions are addition. added to acidified germanate solutions depends upon the formaACID CONCENTRATION, The optimum amount of acid is tion of a heteropoly acid. The intensity and stability of the color are functions of the germanium concentration, the acid 15 ml. of glacial acetic acid in a final volume of 50 ml. At lower 4
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February, 1944
ANALYTICAL EDITION
acidities the initial color intensity is not quite so high, and the solution is less stable. At higher acidities considerable color develops in the blank solution, which necessitates a correction. MOLYBDATECONCENTRATION. After enough ammonium molybdate has been added to develop the full color, an excess has little effect. Five milliliters of a 2.5% solution are adequate for amounts of germanium up to 75 p.p.m. ORDEROF ADDINGREAGENTS.The greatest stability and intensity of color are obtained by adding the molybdate solution to the acid and then adding this mixture to the germanate solution. Solutions of equal color intensity but of slightly less stability are produced by adding the molybdate solution to the acidified germanate solution. The latter procedure is recommended because of its greater simplicity. COLORSTABILITY.Solutions prepared by adding ammonium molybdate to the acidified germanate solutions fade slowly, the error not exceeding 2% within 15 minutes. GERMANIUM CONCENTRATION. The range of germanium concenkration which can be measured with 5-cm. cells is 0 t o 30 p.p.m., if transmittancy measurements are made a t 440 mp (see Figure 1). Beer's law is valid over the entire range. With 1-cm. cells the range is from 1 to 75 p.p.m. of germanium, and the solutions conform to Beer's law up to 40 p.p.m.
129
by Alimarin and Ivanov-Emin ( 1 ) or of small amounts of nitric acid in addition to the acetic acid, increases the permissible arsenic concentration. However, in neither case was the increase sufficient to compensate for the resulting loss of stability and reproducibility. Of the 62 diverse ions studied, the following did not interfere when present in concentrations 100 times that of the germanium : acetate, benzoate, bromide, chlorate, chloride, chlorostannic, citrate, cyanide, formate, iodide, lactate, nitrate, nitrite, oxalate, perchlorate, sulfate, sulfite, thiocyanate, tungstate, bismuth, cadmium, lithium, magnesium, manganese, mercuric, mercurous, potassium, and sodium. Table I summarizes the data for the ions which interfere, and Figure 2 shows typical transmittancy curves for several solutions containing interfering ions.
Table I. Added as KaHAsOr KasAsOd Na2B40: NatCOs XanOHdOa KaCraOT NaF KHzPOd NapPlOl NatSaOs NatSiOa HzSnClr KVOs
Effect of Diverse Ions Present P.p.m. 5 (As) 2 (A01 500 200
Error
%
Amount Permisaible P.p.m.
20
50 50
2 (PZOd 2 (Plod
10
4 500 2
500 20 50 46 50 500 50
20 50 500 50 100 500 500 200 500 500
20
200 500
PERMANENT STANDARDS
DIVERSEIONS.Germanium can be separated from most elements by distillation as the tetrachloride. Of the several elements which may volatilize with the germanium, arsenic is the only one capable of forming a colored heteropoly compound. Although it is possible to separate arsenic from germanium by careful distillation, the effect of arsenic on the color development was carefully studied. With 5 p.p.m. of germanium, the largest permissible concentration of arsenic is 2 p.p.m. I n an effort to extend the tolerance for arsenic, various modifications of the procedure were tried. The use of excess ammonium molybdate, as suggested
Since the yellow molybdigermanic acid color fades rather rapidly, permanent standards are desirable. Alimarin and Ivanov-Emin (1) proposed solutions of picric acid or potassium chromate for this purpose. Swank and Mellon (9) used buffered solutions of potassium chromate or dichromate for the analogous molybdisilicic acid. In order to determine the suitability of these solutions as color standards, and to determine the concentrations equivalent to a definite germanium concentration, a solution of molybdigermanic acid was matched, by means of a Duboscq comparator, with picric acid, potassium chromate, and buffered potassium dichromate solutions. The amounts of the various compounds, in milligrams per liter, equivalent to 10 p.p.m. of germanium were 32.0 for potassium dichromate (buffered with 0.57, borax), 46.4 for potassium chromate (unbuffered), and 4.0 for picric acid. These color matches, checked visually in 30-cm. Nessler tubes, gave the transmittancy curves shown in Figure 3. The colors of solutions of picric acid or buffered potassium dichromate cannot be differentiated by the eye from rnolybdigermanic acid, and there is little difference in the transmittancy curves. Cnbuffered potassium chromate solutions possess a
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INDUSTRIAL AND ENGINEERING CHEMISTRY
hue different from the other three. As this difference makes accurate visual comparison difficult, these solutions are not recommended for permanent standards. Solutions containing t,hree times the amount of potassium dichromate or picric acid required to match 10 p.p.m. of germanium matched visually and spectrophotometrically a solution containing 30 p.p.m. of germanium Therefore, either picric acid solutions, or solutions of potassium chromate or dichromate buffered to pH 9, are considered suitable permanent standards.
Vol. 16, No. 2
of the germanium is unnecessary, neutralize the solution with dilute acid or base, dilute to a definite volume, and proceed as described under measurement. If it is necessary to separate the germanium by distillation a' the tetrachloride, follow by precipitation as the disulfide from a solution 6 N in sulfuric acid. Such sulfide precipitates, after thorough washing should then be dissolved in the smallest possible amount of dikilled aqueous ammonia. Transfer the yellow solution to a platinum dish, decolorize with 30% hydrogen peroxide, and add 1 to 2 ml. excess. Boil the solution gently to destroy excess peroxide, cool, neutralize with dilute sulfurir acid, and dilute to a definite volume.
DISCUSSION
The proposed procedure offers a rapid and reliable method for the determination of small amounts of germanium. Its principal disadvantages are the interference of other elements which form colored heteropoly solutions, and the yellow color o f the molybdigermanic acid solution. The former necessitates separation of germanium from silicon and arsenic, with which it is commonly associated, before an accurate determination ran he made. Comparison of this rork with that of Alimarin and IvanovEmin (1) reveals several differences in the results, most of which can be attributed to the greater sensitivity of the photoelectric instrument used in the present study as compared to the earlier visual instrument.
MEASUREMENT.Transfer an aliquot of this solution, containing 1 to 3 mg. of germanium, to a 100-ml. volumetric flask. Add 30 ml. of glacial acetic acid, dilute the solution to about 80 ml. with water, and then add 10 ml. of a freshly prepared 2.5% solution of ammonium molybdate. Dilute to the mark, mix well, and measure or compare the color immediately by any of the usual means. Standards for comparison may be prepared similarly, or permanent standards containing picric acid or potassium dichromate buffered to pH 9 may be used. With permanent standards a blank should be run to correct for silica in the reagents. LITERATURE CITED
The choice of picric acid or buffered potassium dichromate for permanent standards may be left to the analyst using the method. The dichromate is usually available as a primfir? oxidimetric standard. R E C O M M E N D E D PROCEDURE
Dissolve the sample by appropriate means, if TREATMENT. necessary, and remove or inhibit any interfering ions to bring their concentration within the limits set,in Table I. If separation
(1) Alimarin and Ivanov-Emin, Mikrochemie, 21, 1 (1936). (2) Geilman and Brunger, Biochem. Z., 275, 375 (1935). (3) Grosscup, J. Am. Chem. SOC.,52, 5154 (1930). ENG.CHEW.,ANAL.ED., 14, 71.5 (4) Hybbinette and Sandell, IND. (1942). (5) Johnson and Dennis, J. Am. C h m . Soc., 47, 790 (1925). (6) Komarovskii and Poleuktov, Mikrochemie, 18, 66 (1935). (7) Poleuktov, Zavodskaya Lab., 5, 27 (1935). (8) Schcrarz and Giese, Ber., 63B,2428 (1930). (9) Swank and Meilon, IND.ENG.CHEM.,ANAL.ED., 6, 348 (1934 (10) Wright and Mellon. Proc. Indiana Acad. Sci., 50, 110 (1940).
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ABSTRACTED from a thesis presented by R . E. Kitson t o the Graduate School of Purdue University in partial fulfillment of the requirements for the degrpe of master of science. dune. 1942
