The Determination of Rhenium 11.
The Geilmann Reaction
LOREN C. HURD AND BERNARD J. BABLER University of Wisconsin, Madison, Wis.
T
HE Geilmann (2) color reaction for rhenium is brought
about by adding hydrochloric acid, stannous chloride, and potassium thiocyanate to a solution of a perrhenate. The stannous chloride presumably reduces the rhenium to the hexavalent state where it reacts with the thiocyanate to yield an intensely colored complex. The actual compound formed is said to be ReO(CNS)4 (1, 9). Geilmann and coworkers have found that the lower limit of sensitivity lies somewhere in the neighborhood of 0 . 5 ~per 10 ml. (ly = 0.001 mg.). The reaction has been adapted to the quantitative determination of rhenium in much the same manner as the corresponding molybdenum reaction has been utilized 26Y
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in the Maag and McCollum (7) determination. Although molybdenum interferes when the Geilinann determination is applied directly to the analysis of minerals and concentrates, the method represents, with slight modification and when used in conjunction with a preliminary separation, the best available to date. The procedure recommended by the original authors and similar to the analogous molybdenum method was as follows: To the near1 neutral solution of rhenium as the perrhenate were added 10 m? of 20 per cent hydrochloric acid and 2 mi. of 10 per cent potassium thiocyanate. This was diluted to 50 ml. and treated with 10 ml. of 2 per cent stannous chloride. After shaking for 0.5 minute, 20 ml. of ether were added and the yellow complex was extracted. Residual traces of the complex were removed by a second extraction. It was early recognized that the intensity of color produced was dependent upon a number of factors, Chief among these were concentrations of reagents and time. By comparing the intensities of colors produced under varying conditions with the color of an aged ether extract of the complex, they were able to establish optimum concentrations and to arrive at what seemed to be the proper time interval between the addition of stannous chloride and extraction with ether. The reaction was apparently stopped by the addition of the extractor. During the course of a series of analyses made in this laboratory (4) it became evident that the development of the color depended not only upon the concentration of the reagents but upon the amount of rhenium present. Likewise i t was suspected that the reaction was not stopped by the addition of the extractor but continued in the nonaqueous solution a t a greatly reduced rate. The research herein reported was conducted in an effort to ascertain the magnitude and character of the color changes taking place during the course of the reaction and to establish if possible concentrations and time factors more favorable to analytical applications. The Eastman universal colorimeter, an instrument which has been previously described (6, 6) and used (3) in a research of this type, was used to follow color changes.
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FIGURE1
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Experimental Solutions used were similar t o those of Geilmann, Wrigge, and Weibke ( 2 ) . Potassium perrhenate solutions containing 107
FIGURE3 112
ANALYTICAL EDITION
MARCH 15, 1936
per ml. served as a source of rhenium. The procedure followed throughout was t o run into the sample tube of the colorimeter a known amount of standard perrhenate solution. In a beaker were placed the desired amounts of hydrochloric acid, stannous chloride, potassium thiocyanate, and enough water so that the total volume of this solution and that of the perrhenate would equal 50 ml. The reagent mixture was then quickly mixed with the perrhenate solution and placed in the colorimeter. The time of mixing was taken as the zero time in all cases. The color changes were followed by the -blue wedge. When working with deeply colored solutions it was sometimes found necessary to make slight corrections with other wedges. All concentrations reported are on the weight basis. Figure 1 illustrates the effect of varying the hydrochloric acid concentration in solutions containing 0.40 per cent of stannous chloride, 1.0 per cent of potassium thiocyanate, and 1007 of rhenium. The percentage of acid includes that contained in the stannous chloride solution. In low acid concentrations the color did not reach a maximum until 20 minutes had elapsed. In the neighborhood of 2 per cent the color maximum was reached in 5 minutes, but fading took place over a period of 30 minutes. Figure 2 illustrates the effect of stannous chloride in a system containing 2.0 per cent of hydrochloric acid, 1.0 per cent of potassium thiocyanate, and 1007 of rhenium. Low concentrations of the reagent developed the color slowly, but it is to be noted that a t the end of 35 minutes the color with 0.04 per cent of stannous chloride was almost twice as intense as in a 0.8 per cent solution. In the latter concentration, however, the color maximum was quickly reached and remained practically constant over an appreciable period. Figure 3 illustrates the variation produced by changing the potassium thiocyanate concentration in solutions containing 2.0 per cent of hydrochloric acid, 0.4 per cent of stannous chloride, and 1007 of rhenium. Concentrations above 0.4 per cent produced a maximum color a t the end of 5 minutes, whereas amounts in excess of 1 per cent promoted fading after 15 or 20 minutes. In Figure 4 are represented color concentrations produced by varying amounts of rhenium reduced in solutions containing 2.0 per cent of hydrochloric acid, 0.40 per cent of stannous chloride, and 1 per cent of potassium thiocyanate. Significant is the fading encountered in concentrations of rhenium much above 2007. Under the conditions of the experiment it would appear that 7 minutes is the optimum time interval before extraction. I n the 4007 region there is illustrated a phenomenon which has often been encountered in these laboratories and which may introduce serious error into a determination. Curves A and B represent the behavior of solutions containing exactly the same concentrations of reagents, the difference being that the solution represented by B was shaken vigorously for 1 minute before being placed in the colorimeter. Violent and prolonged agitation does not appear to shift the maximum but does promote more rapid fading of the color. In Figure 5 are plotted colorimeter scale readings against rhenium concentrations. Colors were developed in solutions containing 2.0 per cent of hydrochloric acid, 0.4 per cent of stannous chloride,
113
and 1 per cent of potassium thiocyanate. The time interval before extraction was 7 minutes. The effect of stannic chloride was studied because in actual practice samples are often encountered which contain small amounts of oxidizing agents. These would oxidize the stannous chloride to produce significant concentrations of tetravalent tin. Stannic chloride in concentration# as high as 2 per cent had little or no effect.
Nonaqueous Extractors Butyl acetate, cyclohexanol, and ethyl ether have been useti to extract the rhenium thiocyanate. The mixture of 65 per cent ethyl ether and 35 per cent petroleum ether as used by Malowan (8) for molybdenum extracts rhenium incompletely. Because it had been suspected that nonaqueous solutions of the complex were not stable over a period of hours and that standards for comparison had best be prepared a t the time of the analysis, a study was made of the behavior of these nonaqueous solutions. The color of 1507 of rhenium was developed as described in the preceding paragraph, 7 minutes
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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were allowed, and the extractor was added in three successive portions of 20, 15, and 15 ml. Extracts were then combined and placed in the colorimeter. The entire operation usually required about 12 minutes. Saturated extractors were prepared by shaking the extraction liquid in a separatory funnel just previous to use, with a hydrochloric acid, stannous chloride, and potassium thiocyanate solution of the same type as used in the actual analysis. Figure 6 illustrates the change in color of solution of 1507 of rhenium (as the thiocyanate extract) in butyl acetate which had been shaken with the reagents and in untreated butyl acetate. It is apparent that the saturated extractor is preferable but that in neither case may it be assumed that such solutions constitute satisfactory semi-permanent standards. Figure 7 is interesting in that it apparently makes little or no difference whether ether be saturated or unsaturated with respect to the reagents. However, erratic results are more often obtained in actual practice if the ether is not treated prior to extraction This is in harmony with the findings of Geilmann and co-workers (8). Figure 8 is characteristic of cyclohexanol. The use of cyclohexanol eliminates to a large extent the peculiar results often obtained when ether is used as an extractor but has the decided disadvantage of separating slowly from the aqueous solution.
VOL. 8, NO. 2
As a result of the study of the development of color in the Geilmann reaction it is recommended that when applied quantitatively to amounts of rhenium under 5007, the concentration of hydrochloric acid be held a t 2.0 per cent, potassium thiocyanate a t 0.4 per cent, and stannous chloride a t 0.2 per cent. Seven minutes should in general elapse before extraction, and the solution should be shaken no more than is necessary to produce uniformity. Behavior of nonaqueous extractors has been studied and data are presented which indicate that such solutions do not constitute satisfactory permanent standards. Literature Cited (1) Druce, Rec. trav. chim., 54,334 (1935). (2) Geilmann, Wrigge, and Weibke, Z . anorg. allgem. chem., 208, 217 (1932). ENQ.CHB~M., Anal. Ed., 4,236 (1932). (3) Hurd and Chambers, IND. (4) Hurd and Reynolds, unpublished research, University of Wiaconsin, 1933. (5) Jones, Am. Dyestu$Reptr., 13, 121 (1924). (6) Jones, J . Optical Soc. Am., 4,421 (1920). (7) Maag and McCollum, IND. ENO.CHEM.,17,524 (1925). (8) Malowan, Z . anorg. Chem., 108,73 (1919). (9) Noddack, “Das Rhenium,” Leipaig, Leopold Voss, 1933. RXICEI~XID August 10, 1935. Based upon the senior thesis of Bernard J. Babler, University of Wisconsin, 1936.
Separation of Stannic Oxide from Various Oxides by Ignition with Ammonium Iodide Application to Analytical Purification of Ignited Stannic Oxide EARLE R. CALEY AND M. GILBERT BURFORD,’ Frick Chemical Laboratory, Princeton University, Princeton, N. J.
V
OLATILIZATION of arsenic, antimony, or tin in the form of their chlorides from various compounds by gentle ignition with dry ammonium chloride was apparently first recommended as an analytical procedure by Rose ( 5 ) . Rammelsberg (4)employed repeated ignition with ammonium chloride in a method for the approximate separation of stannic oxide from tungstic oxide. The conversion of stannic oxide into volatile stannic chloride in this way is, according to the authors’ attempts, an impractical analytical procedure since an excessive number of successive ignitions are usually required. For example, in one experiment a sample of ignited stannic oxide weighing 0.5635 gram left a residue of 0.0127 gram of oxide after five successive ignitions with excess quantities of ammonium chloride. On the other hand it was found that ammonium iodide decomposes even highly ignited stannic oxide with great readiness, so much so that the amount of stannic oxide usually encountered in an analytical precipitate can be volatilized as iodide by a single ignition with ten times its weight of this ammonium salt. Since, a t the temperature required for this volatilization, most of the other oxides likely to be found with tin oxide either are not attacked or are converted into relatively nonvolatile iodides which may be changed quantitatively back to the oxides, ignition with ammonium iodide can be used as a general method for the separation of stannic oxide from these others. A t the time the present work was done it was thought that the analytical application of ammonium iodide in this manner was entirely new, but there was later located a single passing statement 1 Present
N. Y.
address, Department of Chemistry, Cornell University, Ithaca,
by Moser (3) to the effect that metastannic acid could be separated from silicic acid by ignition with ammonium iodide. This present paper contains the results of experiments on the action of dry ammonium iodide a t various temperatures on stannic oxide, ferric oxide, cupric oxide, lead oxide, nickel oxide, zinc oxide, antimony trioxide, tungstic oxide, and silicon dioxide, special attention being paid to the conditions for quantitative separations. As an example of the practical value of this general method there is given a rapid, convenient procedure for the determination of the exact stannic oxide content of ignited impure tin oxide precipitates, such as those obtained from the nitric acid treatment of common nonferrous alloys.
Materials and Apparatus J. T. Baker’s reagent ammonium iodide was found to be sufficiently pure, particularly in regard to a low content of nonvolatile matter, to be used without further purification. One lot was found to leave no residue on ignition, while another contained an average of only 0.02 per cent nonvolatile matter. Though it is desirable for such analytical work to have a reagent that leaves no weighable residue on ignition, it was found that satisfactory results could be obtained even with ammonium iodide of lower purity by allowing for a blank correction according to the purity of the chemical and the amount taken for an ignition. The various oxides were all prepared from pure chemicals by conventional methods. Ordinary porcelain crucibles were used in the ignition experiments, and, while in a few cases a gas burner was used as
