May 15, 1943
ANALYTICAL EDITION
This method need not be limited t o the determination of formaldehyde liberated by the action of periodate On serine* It can probably be applied t o the determination of formaldehyde in other types of samples; as long as the compound can be isolated by distillation, reasonably free from interfering substances, i t can be estimated polarographically.
315
Literature Cited (1) Barnes, E. C., and Speicher, H.W., J. I d . Hyo., 24, 10 (1942). (2) Boyd. M. J., and Logan, M. A., J. BWZ. chm., 146,279 (1942). (3) Jahods, F. G., CoZZection Czechosloo. Chem. Commun., 7, 415 (1935). (4) Shinn, L.A,, and Nicolet, B. H., J.Biol. Chem., 138,91 (1941). (5) Winkel, A.,and Proske, G., Ber., 69,693 (1936).
Determination of Small Amounts of Molybdenum in Tungsten and Molybdenum Ores F. S. GRIMALDI AND R. C. WELLS, Geological Survey, u.
A rapid method has been developed for the determination of small amounts of molybdenum in tungsten and molybdenum ores. After removing iron and other major constituents the molybdenum thiocyanate color is developed in water-acetone solutions, using ammonium citrate to eliminate the interference of tungsten. Comparison is made by titrating a blank with a standard molybdenum solution. Aliquots are adjusted to deal with amounts of molybdenum ranging from 0.01 to 1.30 mg.
of the calcium tungstate ores froin contact-metaM O Smorphic T deposits in the western states contain a t least a trace of molybdenum, and in many such ores as much as 0.5 per cent of molybdenum niay be present. As concentrates from such ores are subject to a penalty, the determination of molybdenum is important. I n its search for new supplies of the different strategic metals the U. S. Geological Survey has examined various samples of possible molybdenum content t h a t n-ere on the border line betm-een a little and no molybdenum. For such work a quick b u t accurate method was needed. One method developed for this purpose is based on fluorescence ( 1 ) ; the other depends on chemical analysis and colorimetric estimation of the molybdenum. T h e amber color developed b y molybdenum, thiocyanate, and a reducing agent has long been used as a sensitive test for molybdenum, the colored complex usually being concentrated b y extraction with organic solvents such as ether, butyl acetate, or cyclohexanol, before colorimetric estimation. Ether, the most common solvent employed, has several objections, as noted by Sandell (6). The authors have found that even a few milligrams of tungsten or vanadium cause high results with the ether extraction alone, unless tartrate or citrate is present, Hurd and Allen ( 3 ) studied the conditions for maximum stability of color, using various solvents, and stated that “butyl acetate, although a n excellent solvent for the thiocyanate, should be used with care. The color produced b y a given concentration of molybdenum appears to depend to no slight extent upon the manner in which the solvent has been saturated with the reagents.” They reported t h a t cyclohexanol is difficult to handle, separates slowly from aqueous solution, and may develop a turbidity.
s. Department of the Interior, Washington, D. C.
The writers dispense with extractions altogether, but employ a n acetone-mater solution, as acetone appears to have a stabilizing action and prevents fluctuations of the color with time. The interference of tungsten is eliminated by the use of amnionium citrate. The essential features of the procedure are that the molybdenum thiocyanate color is developed in a water-acetone solution and the comparison is made b y titrating a blank containing the necessary reagents v i t h a standard molybdenum solution.
Reagents STASSOUSCHLORIDE(10 PER CEXT). Dissolve 20 grams of stannous chloride dihydrate in 100 ml. of concentrated hydrochloric acid and 100 nil. of water. A M ~ I O S I CTHIOCYASSTE ~I (10 PERCEST). Dissolve 20 grams in 200 ml. of water. A~IIIOSITXCITRATE, reagent grade. ACETOSE, reagent grade, STAXDARD MOLYBDEXUM SOLUTIOX.1 ml. = 0.1 mg. of molybdenum trioxide. Dissolve 0.1 gram of pure molybdenum trioxide in a little sodium hydroxide, neutralize to litmus with hydrochloric acid, and make up to 1 liter in a volumetric flask. As the standard degenerates with time, it is necessary to make up a fresh solution every month.
Solution of Sample With substances like scheelite that contain up to 40 per cent of tungstic oxide and possibly as much as 1 per cent of molybdenum oxide, fuse in platinum 0.5 gram of the sample with 2.5 grams of sodium carbonate and about 0.05 gram of sodium nitrate ( 2 ) . Begin the heating a t a low temperature and gradually increase the heat until a homogeneous melt is obtained. Extract the cooled melt with water containing a drop of alcohol to reduce manganate, add a little paper pulp, filter, and wash with warm 1 per cent sodium carbonate solution. Make just acid with hydrochloric acid. If any chromate is present, reduce it with a little sulfurous acid. Evaporate to about 25 ml. on the steam bath to remove carbon dioxide. At this point silica may separate out. Without removing silica neutralize with 10 per cent sodium hydroxide and add 10 ml. in excess, add a little paper pulp, and digest on the bath about 10 minutes. Filter hot and wash with dilute sodium hydroxide. Add phenolphthalein to the filtrate, make it barely acid with hydrochloric acid, and transfer it to a 100-ml. volumetric flask. The solution is now ready for the estimation of molybdenum. With molybdenite ores, wulfenite ores, and ores high in silica, in which tungsten is lorn, an acid attack of the sample is preferable. The usual practice (6) involves treatment with nitric and sulfuric acids and eventual extraction of the molybdenum in a sulfuric acid solution. On a 0.5-gram sample. 2 ml. of sulfuric acid are sufficient.
Colorimetric Estimation of Molybdenum Take a 15-ml. aliquot of the neutral molybdenum solution in a 100-ml. beaker; if a larger or smaller aliquot is taken, adjust volume to 15 ml. Add to the solution a t room temperature 1.5
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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grams of ammonium citrate and stir until it is dissolved. Now add 5 ml. of 10 per cent ammonium thiocyanate and 25 ml. of acetone, cool, add dropwise by pipet exactly 7 ml. of stannous chloride solution, and stir. Accurate measurement is important, as the acidity of the solution is controlled by the quantity of stannous chloride added. This develops the amber color of the molybdenum thiocyanate. To a blank in another 100-ml. beaker add 15 ml. of water, 1.5 grams of ammonium citrate, 5 ml. of ammonium thiocyanate, and 25 ml. of acetone. Add by pipet exactly 7 ml. of stannous chloride solution. Titrate this blank to a color match by adding standard molybdenum solution from a buret. Occasionally the unknown will precipitate silica on the addition of stannous chloride, especially if the addition is too rapid. If this occurs, filter off the silica at this point before matching the color.
TABLE 11. INTERFERENCE OF IRON 7 -
No.
WOs Gram 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.1 0.1 0.3 0.3 0.3
1 2 3 4 5 6 7 8
9 10 11 12
-
(Sample matched immediately) Taken MoOa Fe as FeCla MoOa Found Gram 0.02 0.07 0.02 0.07 0.02 0.03 0.05 0.05 0.10 0.0001 0.0002 0.0010
Error
Mo.
Mo.
Mo.
0.03 0.03 0.10 0.10 0.30 0.40 0.40 0.40 0.40 0.50 0.50 0.50
0.08 0.09 0.19 0.20 0.52 0.65 0.65 0.63 0.64 0.55 0.60 0.75
+0.05 +0.06 +0.09 +0.10 +0.22 C0.25 +0.25 +0.23 4-0.24 +0.05 +O.lO +0.25
Results with Known Mixtures In the experimental work accurately measured quantities of solutions of known titer, containing the sodium salts of the elements in question, were transferred to 100-ml. beakers. The solutions were adjusted to 15 ml. and made just acid before the addition of the reagents used for the development of the color. Estimations were made by titrating blanks with standard molybdenum solution. Table I lists the results obtained in the presence of tungsten. The color is stable in the presence of tungsten for at least 3 hours. ~~
OF MOLYBDEXUM IN THE PRESENCE OF TABLEI. ESTIMATION TENGSTEK
No.
7 8 9 10 11 12 13 14
-Taken-
RI 003
woa
MoOa
Found
Gram
Mo. 0.06 0.20 0.50 0.80 0.03
hfQ. 0.06 0.19 0.51 0.82 0.03 0.06 0.11 0.39 0.72 1.02 1.43 0.03 0.39 0.69
None None None None 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.40 0.40 0.40
0.06 0.10 0.40
0.70 1.00
1.50 0.03 0.40 0.70
TABLE111. INTERFERENCE OF IRON WITH TIME
NO.
-TakenWOi
Fe
1 2 3
Gram 0.3 0.3 0.3
Gram 0.07 0.07
MoOi
Ma. 0.30 0.40 0.50
0.07
MOO] Found Matched Matched immediafter 15 ately minutea Mo. Mo. 0.55 0.65 0.68 0.89 0.77 1.10
color. Arsenic, antimony, sodium, and ammonium salts, calcium, aluminum, and silica do not interfere. Nitrates tend to cause a fading of color with time. Preferably the final solution should contain less than 0.3 gram of nitrate.
Determination of Molybdenum in Synthetic Scheelite K. J. Murata of this laboratory prepared pure synthetic members of the scheelite-powellite mineral series by the method of Michel ( d ) , and these were analyzed for molybdenum. The ground samples were attacked by 10 ml. of hydrochloric acid and 2 drops of nitric acid, evaporated to dryness, and taken up with 1.5 grams of ammonium citrate and 4 drops of ammonium hydroxide. The color was then developed and compared. The results, as indicated in Table IV, showed good agreement with the synthetics.
Interference Iron causes a serious positive error and must be removed completely. The error is not so much a function of the quantity of iron present as of the time elapsed before the sample is matched. Table I1 illustrates this interference of iron when the solution is matched immediately. Table I11 shows the interference as a function of time. Iron as well as many other interfering bases is, however, removed at the start by the carbonate fusion. Small or moderate amounts of iron are further removed with sodium hydroxide as described in the procedure. Excess ammonium hydroxide may be substituted for sodium hydroxide but the digestion is best conducted in a stoppered Erlenmeyer flask. I n four experiments, 0.05,0.30, 1.00, and 50.0 mg. of iron in the presence of 0.3 gram of tungstic oxide were completely removed by this method. Phosphate causes no interference when tungsten is absent. With tungsten present the color fades and turns blue-green with time. If matched within 10 minutes, 20 mg. of phosphorus pentoxide in the final aliquot with 0.3 gram of tungstic oxide show negligible interference. K i t h 0.1 gram of tungstic oxide, 80 mg. of phosphorus pentoxide cause little trouble during this 10-minute interval. The interference of phosphate may be minimized by using more ammonium citrate. Large amounts of vanadium interfere because of the bluegreen color of vanadium in reduced solutions. Amounts of vanadium up to 2 mg. of vanadium trioxide in the final aliquot, however, do not appreciably affect the molybdenum
TABLEIv. DETERMIN.4TION O F MOLYBDENUM I N SYNTHETIC SCHEELITE Preparation No. 4 26 28 6
27 29 7 31 5
MOOSin Synthetic Scheelite
MoOa Found
%
%
0.07 0.22 0.25 0.29 0.36 0.36 0.51 0.72 0.79
0.07 0.23 0.24 0.31 0.34 0.34 0.50 0.68
0.74
Acknowledgment The authors wish to thank K. J. Murata for kindly supplying samples of synthetic scheelite of known molybdenum content.
Literature Cited (1) Eng. Mining J., 143,65 (1942). (2) Hillebrand, W. F., U.S. Geol. Survey, Bull. 700, 185 (1919). (3) Hurd, L. C., and Allen, H. O., IND. ENQ. CHEM.,ANAL.ED., 7, 396 (1935). (4) Michel, L., BUZZ.soc. franc.mineral.,2, 142 (1879) ; 17, 612 (1894). (5) Sandell, E. B., IND.ENQ.CHEW,ANAL.E D . , 8, 336 (1936). (6) Scott, W. W., “Standard Methods of Chemical Analysis”, 6th ed., Vol. I, p. 597, New York, D. Van Nostrand Co., 1936. PuBLxsHmD by permission of the Director, U. 8. Geological Survey.
