ANALYTICAL CHEMISTRY
328 nation of the crucible with other metals, the crucibles were held with silver-wrapped Nichrome tongs and set on a silver sheet. Solution of the fusion was always accomplished by pouring the acid into the crucible-in no case was the crucible set in a beaker. Once the crucible was clean, these precautions helped to produce a low and constant blank. The silver must be removed from the solution before addition of the o-phenanthroline, because a chemical reaction takes place which produces a precipitate. Therefore, no advantage would be obtained by the use of any acid other than hydrochloric acid to dissolve the melt without precipitating any silver salts. Of the methods for removing silver chloride, centrifuging is the easiest and fastest; a clear supernatant liquid is obtained in a few minutes. Centrifuging was the method largely used in this study. After coagulation, the precipitate was easily removed by filtration. If sufficient time were allowed, the precipitate settled ~ J Ygravity alone. With samples high in silica, such as feldspar or silica brick, a precipitate of silicic acid was obtained, but it apparently did not interfere in any way. It was removed along with the silver (ahloride. Buffers of ammonium acetate are used frequently in iron determinations to produce the correct pH, but these reagents cannot be used under the conditions described. The ammonium acetate would produce a heavy gel of silicic acid that would interfcre seriously. The pH of the solution must be adjusted with a glass-electrode p H meter, or by other means that introduce no e\traneous color or interfering ion. Ahcurateresults were obtained with a wide variety of samples, including various silicates, phosphates, and fluorspar. The number of ions interfering with the determination, and the extent of their interference, arc low ( 5 ) . Among ions that might interfere somewhat are nickel, bismuth, molybdate, tungstate, and cyanide. If present a t all, the percentage of these ions T\ ould be very low in the materials for which this procedure was tried and for Jvhich it is recommended. However, any change in hue or any precipitation should serve as a warning for further
study of the material. Silicon carbide, which is apparentiv not wetted very well by the molten flux, could not be completely decomposed a t the temperature used. Single determinations have been made in 15 minutes when all apparatus was ready and when the removal of silver chloride caused no difficulty. The method is easily applied concurrently to a large number of samples. CONC LU SlON
Platinum-rhodium crucibles cannot be used for the precise estimation of iron after a sodium carbonate-borate fusion unless the iron lost to the crucible is recovered by repeated heating and leaching with hot acid. €Ion-ever, excellent results without loss of iron are obtained with a wide variety of materials when the fusions are made in thick-walled cast silver crucibles. ACKNOWLEDGMENT
The author wishes to extend his thanks to Lloyd L. Hall for some of the determinations. LITER.4TURE CITED
(1) Blau, F., Ber., 21, 1077 (1888) (2) Blau, F., Monatsh., 10, 376 (1889). (3) Ibid., 19, 666 (1898). (4) Bowen, N. L., and Schairer, J. F., Am. ,J. Sci., 24, 184 (1932). ( 5 ) Fortune, M’. B., and Melloii. 41.G . , ISD. ENG.CHEJI.,-41.1~
ED.,10, 60 (1938). (6) Gerdeissen, Ber., 22, 245 (1889). (7) Hillebrand, IT.F., and Lundell, G. E. F., “Applied Inorganic bnalvsis.” New York. John Wilev 8: Sons. 1929. (8) Hummil, F. C., and Willard, H. H:, ISD. ENG.CHEM., ANAL. ED.,10, 13 (1938). (9) Saywell, L. G., and Cunningham, B. B., Ibid., 9, 67 (1937). (10) Walden, G. H., Hammett, L. P., and Chapman, R. P., J . Am. Chem. Soc., 53, 3908 (1931). June 1, 1919. W o r k done in cooperation with t h e Tennessee Valley Authority.
RECEIVED
Photometric Determination of Molybdenum by Acetone Reduction of the Thiocyanate ROSCOE ELLIS, JR., AND R . Y. OLSON ICunsus Agricultural Experiment Station, Manhattan, hkn.
A photometric method for the determination of small amounts of molybdenum in solutions is presented. 4 s in previous methods, molybdenum is determined by the yellow-amber color of its thiocyanate. The use of acetone as a reducing agent increases the sensitivity and eliminates the rapid fading of the color complex encountered when other reducing agents are used.
T
HE yellow-amber color developed by molybdenum, a thio-
cyanate salt, and a, reducing agent has been used for a number of years for the colorimetric determination of small amounts of molybdenum. Stannous chloride has been the most common reducing agent used. Hurd and Allen ( 2 )made a study of solvent extracting solutions, concentrations of reagents, and other variables involved, and determined the conditions which allowed the maximum color development and minimized the rate of fading of the color complex. Grimaldi and Wells ( 1 ) dispensed with solvent extracting solutions and developed the color in a water-acetone solution using stannous chloride as a reducing agent. This method stabilized the color somewhat. I n trying to determine the amounts of molybdenum in soils, the above methods were used by the authors and attempts were made
to evaluate the color with an Evelyn photometer. Rapid color fading was found to occur n?th each method. Although it was possible to standardize the procedure somewhat by establishing a fixed period of time before the readings were taken, it was thought highly desirable to find a reducing agent that would give color stability over a long period of time. It was felt that color stability would certainly improve the accuracy and convenience of the determination. Therefore, a method was developed in which acetone is used directly as a reducing agent and by which color stability is obtained for a period of 48 hours. The sensitivity of the determination is also increased over the two previous methods. REAGENTS
Potassium thiocyanate (water solution). Dissolve 10 grams of potassium thiocyanate in 100 ml. of distilled water.
329
V O L U M E 22, NO. 2, F E B R U A R Y 1 9 5 0 Table I. Transmittancies Obtained with Different Concentrations of Molybdenuni and Various Reducing Agents J I o l y b d e n u m Concentration, Parts per Million 1 5 10 20
Reducing Agent
7 Acetone Stannous chloride Camphor NaHSOs NaHS03 Metal 1 Arnino-2-naphthol-4-si1lfonic acid Na0S03 iYa&Os Hydroxylamine hydrochloride
+ +,
%
9
78.0
20.5 35.0
7.0 19.0
99.0
73.0
71.0
88.0 76.8
7i:o
68:3
7 2 . .5
71.0
97.0
97.0
68.5 92.0
69.5
+
0
5
% 1.0 8.0 58.0 76.0 56.0
67.3
88.0
Acetone, reagent grade. Hydrochloric acid, specific gravity 1.18 to 1.19. Standard molybdenum solution for preparing calibration curve (1000 p.p.m. of molybdenum). Prepare a stock solution containing 1000 p.p.m. of molybdenum by dissolving 1.8401 grams of dry ammonium molybdate [ (SH!)8.\
[email protected] in water and diluting to 1 liter. Prepare working standards by quantitative dilution of the stock solution. PROCEDURE
Evaporate or dilute the molybdenum solution to be analyzed until 20 ml. of the solution contain from 5 to 1000 micrograms of molybdenum. Place 20 ml. of the solution in a 50-ml. volumetric flask and add enough hydrochloric acid or other nonoxidizing acid to make the final 50 ml. of solution 1.2 to 2 N in respeci to acid. When using water solutions of molybdenum, 7 ml. of hydrochloric acid are sufficient. Add 3 ml. of 10% potassium thiocyanate solution and 15 ml. of acetone, and place the sample in a water bath a t 60" to i o " C. for a t least 20 minutes. When the molybdenum concentration of the solution is in the range given above, loss of acetone during heating is not of any importance even when the heating continues as long as 2 or 3 hours. Cool, dilute to volume. and determine transmittancy in a filter photometer or spectrophotometer at 420 mp. Det.ermine parts per million of molybdenum in the solution by reference t o a standard calibration curve prepared from known standard molybdenum concentr:itions.
tative procedures before reduction. Precipitation by raising the p H of the solution with ammonium hydroxide was found to be a satisfactory method for removing the iron. If the iron concentration is less than 100 p.p.m., the determination can be carried out without its removal by extending the period of heating during reduction to approximately 1 hour. However, because a longer period of heating is required, it is recommended that the removal of iron be made a standard procedure for all determinations. The iron should be removed from the solution before the acid, thiocyanate, and acetone are added. Inasmuch as the molybdenum concentration of solutions may often be increased by evaporation and acetone reduction provides a highly sensitive method of determination, it is not recommended that the colored complex be extracted with ether or other solvents unless it is impossible to evaporate the solution sufficiently. DISCUSSION AND RESULTS
Table I compares the light transmittancies obtained with different molybdenum concentrations using various reducing agents. All the reducing agents were tried a t several concentrations. Sodium bisulfite, sodium bisulfite and Metol, hydroxylamine hydrochloride, 1 amino-2-naphthol-Csulfonic acid plus sodium sulfite plus sodium persulfate, and camphor gave color stability but failed to give sufficient sensitivity for the determination of small amounts of molybdenum. Acetone was the only reducing agent tested that gave both color stability and sensitivity.
If i t is present in high concentrations, ferric iron interferes with the molybdenum determination. If ferric iron is left in solution, a fine precipitate is formed when the reduction process is carried out. If its concentration in the final solution exceeds 100 p.p.m., the iron should be removed from the solution by standard quanti2.0
-
18.
-
1.6
-
1.4
-
L
0
20
30
40
50
so
1 I M E I N MINUTES
Figure 2. Rate of Fading of RIolybdenum Color Complex with Different Reduction >\lethods Solutions contain 5 p.p.m. mol) bderium
-IC m
o.oov
10
L
5 10 15 M O L Y B D E N U M C O N C E N T R A T I O N I N PARTS P E R M I L L I O N
20
Figure 1. Intensity of Molybdenum Color Complex Developed by Different Reducing Agents
-1comparison of the intensity of the color complcx developd by different reducing agents is shown in Figure 1, where optical density is plotted against parts per million of molybdenum, The aretone reduction method increases the intensity of the color complex, thus allowing a more precise determination of small amounts of molybdenum-2 p.p.m. molybdenum, for example, produce:l an optical density of 0.20 using stannous chloride, 0.23 using the Grimaldi-Wells method, and 0.32 using acetone. The increase in color intensity is even greater a t the higher concentrations of molybdenum. A comparison of the rate of fading of the color complex using the three reduction methods is shown in Figure 2, where changes in transmittancy are plotted against time in minutes. All comparisons were made using 5 p.p.m. of molybdenum. Although a significant amount of fading occurred in 5 minutes and the color intensity was greatly diminished in 1 hour when stannous chloride was used as the reducing agent, acetone reduction produced an unchanging color. The fading of the color complex using stannous chloride is more rapid a t higher concentrations; however,
330
ANALYTICAL CHEMISTRY
Table 11. Effect of Time of Heating at 70" C. on Development of Color Complex in Solutions Containing 3 P.P.M. of Molybdenum Time of Heating, Minutes 10
Light Transmittancy,
70
20 5 20.5 20 3 20.3 20 .5 20.3
20
30 40 50
60
rapid fading is still encountered at concentrations of 1 p,p.m. of molybdenum. The acetone reduction method has given color stability a t concentrations up to 20 p.p.m. of molybdenum for a period of 48 hours. As in previous methods, the acidity of the solution in Tvhich the molybdenum determination is carried out must be carefully controlled when acetone is used as the reducing agent. I t was found that the final solution should be betneen 1.2 and 2 S in respect to hydrochloric acid or other nonoxidizing acids in order to obtain accurate results. This is essentially the same as determined by Hurd and Allen ( 2 )using stannous chloride. Iron is the only interfering element that has been encountered by the authors in their experimental work. However, i t is assumed that elements interfering with the determination of mo-
lybdenum as the thiocyanate by other reducing methods n ill also interfere with the acetone reduction. Grimaldi and Wells ( I ) have devised a method for eliminating the interference of tungsten and vanadium. They have also found that phosphates interfere with the color reaction when tungsten is present in appreciable amounts and that large amounts of nitrates cause excessive color fading. Table I1 compares transmittancy values obtained after various periods of heating during the reduction process. ,Inexamination of the table seems to indicate that heating for 10 minutes is sufficient for color development. I t is recommended that heating be continued for 20 minutes to ensure full color development. This allows for the reduction of small amounts of ferric iron, and also allows for a range of temperature> fiom GO" to 70" C. during the period of heating. The temperature of the solutions a t the time of reading may vary from 15' to 40' C. without affecting the transmittancy values. Considerable variations in room temperature are, therefore, allo\vable. LITER4TURE CITED
(1) Grinialdi, F. S.,and Wells. K. C., ISD. EXG.CHEM.,A s ~ r , ED., . 15, 315 (1943). (2) Hurd, L. C., and Allen, H. O., Ibid.,7, 396 (1935). R E C E I V E .luguet D 1 , 1949. Contribution 411, Department of Agronomy, Kansas Agricultural Experiment Station
Estimation of Solubility of Solids in Liquid Ammonia and Liquid Sulfur Dioxide GEORGE W. WATT, WILLIAM A. JENKINS, AND CECIL V. ROBERTSON C'niversity of Texas, Austin, Tex.
A convenient and rapid method for the estimation of the solubility of solids in liquefied gases is described, and data therehj- obtained are compared with earlier results obtained by other methods. Solubility data at 25" C. for sodium chloride and rubidium chloride in liquid ammonia, and sodium iodide, potassium bromide, and twelve alkaline earth halides in liquid sulfur dioxide are given.
C
ONSIDERSBLE effort has been expended in the develop-
ment of methods for the determination of the solubility of solids in liquefied gases in general and liquid ammonia in particular (1, 5-5, 8-12). All the methods thus far employed are characterized by more or less elaborate equipment and involved manipulative procedures designed to provide highly accurate rcsults. In connection with certain work in progress in these laboratories there has arisen the need for a simple and rapid method for the estimation of the solubility of a \vide variety of solid compounds in solvents such as liquid ammonia and liquid sulfur d i o d e . For these purposes i t is usually sufficient to know the relative order of magnitude of the solubilities. Accordingly, attention has been directed toward the development of a method characterized by its simplicity and rapidity of application. EXPERIMENTAL
Materials. Commercial liquid ammonia was dried and dispensed as described by Johnson and Fernelius (7). Sulfur dioxide from a commercial cylinder was dried by passing the gas through concentrated sulfuric acid, then through phosphorus pentoxide. The gas \vas condensed in a trap cooled with dry ice and acetone,
and subsequently distilled into the tulles used in the solubility determinations. -411solids used in solubility determinations were prepared and/ or purified, dried, and analyzed ( 11 ) before use. Procedure. A weighed sample of the solid (approximately 0.1 gram) is introduced into one end of a glass filter tube about 18 em. in length, with a n inside diameter of 5 to 10 mm., and with a fritted-glass disk (porosity C or D, 2 ) a t the mid-point. This end of the tube is sealed and cooled, and the tube and its contents are weighed. The tube is flushed out with the anhydrous gas, after which the open end of the tube is attached to the source of anhydrous gas. The end of the tube containing the solid is immersed in a dry ice-acetone bath at a temperature of approximately - 7 5 " C. and a suitable quantity of solvent (0.8 to 1.0 gram) is condensed on the solid sample. The open end of the tube is then sealed off under conditions that permit determination of the weight of the glass removed in making the seal. (This is done by use of a weighed glass rod, determination of the weight of rod plus glass removed, and getting the weight of the latter by difference. The net weight loss involved in this procedure has been found not to exceed 0.8 mg. and is usually of the order of 0.4 mg.) The sealed tube is allo\\-ed to warm to room temperature, weighed, and agitated in a thermostat (25.0' * 0. I ") for 48 hours. The tube is removed from the thermostat, inverted, and centrifuged at 2000 r.p.m. for from 3 to 5 minutes, thus effecting a separation of the saturated solution and the excess undissolved solid. The end containing the saturated solution is cooled to
