Determination of Iron in Tungsten and Tungstic Acid M. L. HOLT AND DONALD SWALHEIM, University of Wisconsin, Madison, Wis.
Buret. A 5-ml. microburet, graduated to 0.02 ml., was used for all titrations. Method of Electrolysis. Pyrex beakers (100-ml.) served as plating cells. Anodes were of platinum 3 X 3 cm. The cathode of electrolytic copper foil, 3 X 3 cm., was so arranged as t o be easily detached from a platinum cathode lead. A cathode current density of 10 to 20 amperes per sq. dm. from a generator through a lamp-bank or from a copper oxide rectifier, was employed. A plating run was timed to about 25 minutes and a new copper foil cathode was always used for each run. The platin bath temperature was kept at about 90" C. Two cells connectef in series made it possible to carry out runs in duplicate when it seemed advisable.
The small amounts of iron in tungsten metal and in tungstic acid have been determined by a method which involves an electrolytic separation of the iron from the bulk of the tungsten as a tungsten-iron alloy, the precipitation of the iron in the alloy as ferric hydroxide, and its quantitative determination by the iodometric method.
Iron by the Iodometric Method
A
VERY small percentage of iron present as an impurity in
.
For reasons of simplicity and convenience the iodometric method was selected for the quantitative determination of the separated ferric hydroxide.
metallic tungsten has a marked effect on the physical properties of the metal-for example, iron decreases the grain size in a sintered tungsten bar and also tends to make ;he metal brittle. Tungsten containing 0.1 per cent of iron is practically unworkable (6),and standard grades of tungsten usually contain only a small amount of iron, in the neighborhood of 0.005 per cent and not exceeding 0.01 per cent (1). Thus a new method for the quantitative determination of the iron impurities present in tungsten and tungstic acid (HzW04 or the anhydride, WOa) should have a definite value. The main difficulty in determining the iron in tungsten is to get a complete separation of the small amounts of iron. I n the usual procedure the iron is precipitated as ferric hydroxide by sodium hydroxide, which leaves the tungsten in solution as sodium tungstate. However, a soluble tungstateiron complex may form and thus leave appreciable amounts of iron in solution. Recently Holt (4) found that a tungsteniron alloy containing 15 t o 35 per cent of iron, depending on conditions, can be electrodeposited from the aqueous alkaline tungsten plating bath suggested by Fink and Jones (2). It was shown that the amount of electrodeposited alloy depended primarily on the amount of iron impurity in the bath and that continued electrolysis removed all the iron from the plating solution. These facts suggested a new method for determining the iron in tungsten or tungstic acid: the preparation of a plating bath from the sample to be analyzed, the electrolytic separation of the iron impurities as a tungsteniron alloy deposited on the cathode, and then a quantitative determination of the iron in the alloy.
The ferric hydroxide, .precipitated with ammonium hydroxide, was washed and then dissolved in about 9 ml. of approximately 4 N hydrochloric acid. It was carefully washed into a 125-ml. ErIenmeyer flask to give a total volume of about 60 ml. Then about 1 gram of solid sodium bicarbonate was added to the solution and the flask was stoppered with a cork having a small vertical outlet slit. The solution was warmed t o about 50" C. and then 0.3 gram of potassium iodide (1ml. of 30 per cent potassium iodide solution) was added. After about 5 minutes the liberated iodine was titrated with sodium thiosulfate solution. The starch indicator was added as the end point was approached as evidenced by the fading of the color of the solution. Grey (8) reports that 0.05 mg. of iron can be accurately determined by this method and the authors found that known solutions of iron could be determined to within 0.02 mg. The use of an inert atmosphere such as carbon dioxide during the titration was found to be very important, although Grey (8) reports that reoxidation of Fe++ by air is negligible and that conditions to avoid this are superfluous. The authors found that known iron solutions gave consistently high results when titrations were carried out in the air. SEPARATION OF IRON TABLEI. ELECTROLYTIC Iron Added
MQ. 2.0 2.0 2.0 1.0 1.0 0.5 0.5 0.2 0.2 0.06 0.05
Materials and Electrolysis Procedure Sodium Carbonate Solution. This solution contained about 300 grams of reagent quality anhydrous sodium carbonate per liter. The solution was allowed t o stand about 12 hours, then warmed and filtered through special filter paper. Sodium Hydroxide Solution. This solution was prepared as used from a saturated solution of reagent sodium hydroxide. Standard Iron Solution. Pure iron wire was dissolved in hydrochloric acid with a little nitric acid and made up to the re uired volume. One milliliter contained 1mg. of iron. hrified Tungstic Acid. Barium tungstate was precipitated from sodium tungstate solution and digested with hydrochloricnitric acid t o give tungstic acid; this was dissolved in ammonium hydroxide and the solution filtered. Tungstic acid was then reprecipitated with hydrochloric-nitric acid, washed by decantation, and carefully dried. Sodium Thiosulfate Solution. A dilute solution (approximately 0.005 N ) was repared from a 0.1 N solution to which a very small amount o?sodium bicarbonate had been added. It was standardized with sodium iodate for each series of titrations. Starch Indicator. This solution was prepared from soluble starch as needed.
Iron Found Mg. 1.98 1.97 1.99 1.02 0.99 0.54 0.53 0.20 0.21 0.06 0.07
Purified Tungstic Acid and Added Iron I n order to check the completeness of the separation of iron from tungstic acid, known amounts of standard iron solution were added to a purified tungstic acid plating bath and then separated by electrodeposition as tungsten-iron alloy. The plating bath was prepared by dissolving 8 grams of tungstic acid in 75 ml. of the sodium carbonate solution. This was electrolyzed once to ensure complete removal of the last traces of iron, then the known amount of iron was added while the solution was being stirred. Small amounts of added iron do not precipitate as the hydroxide from this alkaline plating bath, presumably because of complex formation. The iron was then 254
ANALYTICAL EDITIOK
MAY 15, 1939 TABLE11. IRON IN TUNGSTIC ACID Brand of HzwO4
Trial
Iron Found NaOH method Electrolytic method
%
% A
1
2 3 1 2 3 1 2 3
B
C D
1
E F Q
20-gram sample.
2 3 1 2 3 1 2 3
0.0045 0.0043 0.0045 0.0009 0.0007 0.0008 0.0011 0 I0009 0.0009 0,0041 0.0043 0.0041 0.0011 0.0011 0.0010 0.0057 0.0060 0.0058
0.0047 0.0050 0.0048 0.0023Q 0.0025" 0.0023" 0.0016 0.00155 0.0017Q 0.0046 0,0047 0.0047 0.0014a 0.0014'
....
0.0066 0.0069 0.0070
removed from the hot bath as tungsten-iron alloy by 3 or 4 successive plating runs. A thin alloy film on the copper cathode seems to interfere with continued deposition on that cathode, so successive runs with new copper cathodes are necessary to remove all the iron. That the iron was completely removed from the bath by such a procedure was proved by an additional plating run with a new copper cathode. No cathode deposit was obtained then, but if as little as 0.05 mg. of iron was added to such a bath it gave on electrolysis a weighable cathode deposit and even smaller amounts of iron had a definite cathode effect. The copper cathodes plated with the small amount of tungsten-iron alloy were put in a 100-ml. beaker, covered with about 30 ml. of water, and then dissolved in 8 ml. of hydrochloricnitric acid. An additional 10 ml. of hydrochloric-nitric acid was then added and the solution was carefully eva orated to about 10 ml. t o ensure fairly complete precipitation the tungsten as tungstic acid and then diluted before filtration. The iron was precipitated from the filtrate by ammonium hydroxide with gentle warming and a short period of standing to ensure coagulation. Filtration, washing, redissolving, double precipitation, and careful washing removed all traces of copper. The iron was then determined by the iodometric method. Care was taken to remove all the copper, because according t o Grey (3) even traces interfere with the method for determining iron. Separate runs showed that the ferric hydroxide was free of copper. Also the copper foil used gave negative tests for iron.
OF
Typical results obtained on the analysis of a number of known solutions are given in Table I, and show that the electrolytic separation of known amounts of iron from the plating bath is very satisfactory.
Iron in Commercial Tungstic Acid Six representative brands of technical and c. P. tungstic acid were chosen for analysis. To check the value of this new method all samples were analyzed by the usual sodium hydroxide separation and by the electrolytic separation. Tengram samples were used for analysis unless otherwise indicated and all per cents are calculated on the basis of iron in tungstic acid. In the sodium hydroxide method the sample was dissolved in a sodium hydroxide solution which contained about 0.6 gram in excess of that required to form sodium tungstate, and then it was made up to a volume of about 125 ml. This was warmed to assist solution and to coagulate the ferric hydroxide. After standing about 12 hours it was filtered and the ferric hydroxide was washed, redissolved, reprecipitated, and then determined iodometrically. In the electrolytic method the sample was used to prepare a carbonate plating bath-10 grams of tungstic acid dissolved in about 90 ml. of the sodium carbonate solution. The bath was heated to about 90" C . and then electrolyzed in a series of 3 or 4 successive plating runs using new copper cathodes for each run. The small amount (1to 6 mg.) of tungsten-iron alloy on the copper cathodes was then treated according t o the procedure described for the known solutions. Typical results showing the amounts of iron found by the two methods in various samples of tungstic acid are included i n Table 11.
255
In these analyses of tungstic acid the amount of iron found by the electrolytic separation was in all cases greater than that found by the sodium hydroxide method. This difference in the results supports the assumption that the formation of a soluble tungstate-iron complex may prevent the complete separation of iron from tungstic acid by sodium hydroxide. In similar later separations of iron with sodium hydroxide, the alkaline tungstate filtrate when used as a plating bath gave on electrolysis a weighable cathode deposit of irontungsten alloy. TABLE111. IRON IN TUNGSTEN METAL Brand
Sample
Trial
Tungsten Used Grams
Iron Found
I
A
1 2 3 1 2 3 1 2 3 1 2 1 2
2.9113 4.7646 2.4986 3.0813 3.6570 4.3806 3.1553 2.6362 3.3400 4.9511 5.5951 3.5296 3.7627
0.0162 0.0160 0.0164 0.0106 0.0073 0.0079 0.0079 0.0076 0.0072 0.0047 0.0054 0.0030 0 0032
I1
I11
%
I
Iron in Metallic Tungsten The analysis of tungsten metal required a special procedure for putting the sample in solution. Tungsten dissolves readily in alkali when used as the anode during an electrolysis a t moderate current densities (6), and the plating bath was thus prepared by using the tungsten (strip, rod, or wire) to be analyzed as the anode and copper foil as the cathode with the usual sodium carbonate solution as the electrolyte. With moderate current densities the tungsten dissolves according to its chemical equivalent (W") to give a clear solution. If the current density is too high the resulting bath contains small amounts of dark substance, presumably a lower oxide which dissolves readily to give a clear solution when the bath is heated. Weighing the anode before and after electrolysis gave the amount of tungsten in the solution-i. e., the size of the sample to be analyzed. The iron was then separated from the bath as tungsten-iron alloy by 3 or 4 regular plating runs using platinum anodes and copper foil cathodes, the first of which was the cathode used during the anodic solution of the tungsten. The tungsteniron alloy was analyzed by the usual method. Results of a number of analyses are given in Table 111. These results are not entirely satisfactory, but the lack of uniformity in the distribution of the iron throughout the samples may explain this variation. This is supported by the fact that excellent results were obtained when tungsten TABLEIV. IRON IN POWDERED TUNGSTEN METAL 0.0038 N
Trid 1 2 3 4
NazS?03
Reauired M1. 1.06 1.03 1.04 1.03 Av. 1.04
I r o n Found
Variation from Average
%
%
0.0045 (2) 0.0043 (8) 0.0044 (2) 0.0043 (8) 0.0044 (2)
-to. -0.00004 00010 .....
-0.00004
powder was used. For analysis of the powder, 5-gram samples were first oxidized to tungsten trioxide by careful heating in the air and then dissolved in the sodium carbonate solution to give the usual plating bath. The iron was then removed electrolytically and the resulting alloy analyzed in the regular manner. The results of the analysis of a c. P. tungsten powder (&gram samples) are given in Table IV.
256
INDUSTRIAL AND ENGINEERING CHEMISTRY
Conclusions The data presented show that the electrolytic method described is entirely satisfactory for the determination of iron in tungstic acid and powdered tungsten metal. The method is new in that it offers a means of removing the iron completely from the bulk of the tungsten compound in the form of a tungsten-iron alloy, which on the average contains about 20 per cent of iron. Thus instead of having to make a difficult and perhaps incomplete chemical separation of a few thousandths per cent of iron, it is necessary only to analyze the small amount of alloy thus obtained. Since the iron is finally separated as ferric hydroxide, other ordinary impurities of tungstic acid should not interfere. The results given indicate that iron in tungsten or tungstic acid can bedetermined to within about oeo2 mg* by this electrolytic separation. The iodometric method for the
VOL. 11, NO. 5
quantitative determination of these small amounts of iron was found to be very good and was chosen because it is relatively simple and requires very little experience. However, it is the alloy separation of the iron which is the basis of the method and any other preferred procedure, volumetric or colorimetric, could be used for the final determination of the iron.
Literature Cited (1) Driggs, F. H.,private communication. (2) Fink, C. G., and Jones, F. L., Trans. Electrochem. SOC.,59,461
(1931). (3) Grey, E. C.,J. Chem. SOC.,1929,35. (4)Holt, M.L., Trans. Electrochem. Soc., 66,453(1934). (5) Smithell, “Tungsten”, p. 17, New York, D. Van Nostrand Co., .nnn
1Yi)O.
(6) Thompson, M. de Kay, and Rice, C. W., Trans. Electrochem. SOC., 67,71 (1935).
Determination of Carotene in Silage An Improved Method D. M. HEGSTED, J. W. PORTER, AND W. H. PETERSON University of Wisconsin, Madison, Wis.
A
LTHOUGH the old Willstatter-Stoll (I??) method for the determination of carotene has been modified and improved in recent years, all modifications (2, 7, 9) depend upon pigment distribution between petroleum ether and alcohol (90 per cent methyl or 85 per cent ethyl) for the separation of carotene and Xanthophylls. This procedure has not proved reliable when applied to silage. Kane and Wiseman (4)found spectrographic evidence for pigments other than carotene in the petroleum ether solution of A. I. V. (acidtreated) silage. Peterson et al. (6,6)showed that the carotene content of some A. I. V. silages was higher than that of the forage used in their preparation, and suggested that this might be due to pigments developed in the silage. Other workers (3, 22) have also found abnormally high carotene values for A. I. V. silages. Subsequently Quackenbush, Steenbock, and Peterson (8) found that thecarotene fraction obtained from A. I. V. alfalfa silages by the petroleum ether-ethyl alcohol procedure contained three pigments other than carotene. These pigments were separated from carotene by the use of magnesium oxide chromatograms and were found to be biologically inactive. Since these pigments amounted to as much as 40 per cent of the so-called carotene, the figures for such silages obtained by the petroleum ether-alcohol procedure are obviously too high. I n an attempt to find a method, other than the tedious chromatographic procedure which would separate the pigments, several solvents were used in place of 85 per cent ethyl alcohol. As a check on the effectiveness of the solvent, the carotene and noncarotene contents of the fractions were carefully determined by chromatographic separation. Clausen and McCoord ( I ) had used diacetone for the separation of the carotenoid pigments in blood and this solvent proved satisfactory for the separation of those in silage. While absolute separation of pigments with such similar solubilities is probably impossible, nearly quantitative results were obtained.
Procedure A 500- t o 1000-gram sample of fresh silage is well mixed and ground. Twenty-five grams are weighed into a flask and 200 cc. of alcohol are added immediately. The mixture is refluxed for 40 minutes and the alcoholic solution decanted. The residue is extracted again by refluxing 40 minutes with another 200-cc. portion of alcohol and the solution decanted. To the combined alcoholic extracts 40 cc. of 20 per cent alcoholic potassium hydroxide are added, and the solution is shaken and allowed to stand overnight. An alternative procedure to the alcohol extraction is to reflux the sample with alcoholic potassium hydroxide. The determination may then be run immediately. In either case the total volume of the alcoholic extracts must be measured as a basis for the calculation*of the carotene content. To remove any carotene not extracted by the alcohol 80 cc. of Skellysolve Benzine, b. p. 65 to 75 are added to the residue and heated just to the boiling point. The flask is then set aside t o cool. Aliquots equivalent to equal amounts of silage are taken from both the supernatant Skellysolve and the alcoholic extracts. Usually 5 cc. of the Skellysolveextract are placed in a separatory funnel with 2 cc. of 20 per cent alcoholic potassium hydroxide and shaken. Twenty-five cubic centimeters of the alcoholic extract are then added, followed by 15 cc. of Skellysolve and 7 or 8 cc. of water, and the mixture is well shaken. After complete separation of the two layers, the alcoholic solution is drawn off and re-extracted with two more 15-cc. portions of Skellysolve. Three extractions are sufficient to remove all the carotene, although the Skellysolve is still yellow with xanthophylls. The combined Skellysolve extracts in a separatory funnel are washed free of alkali with four 15-cc. portions of water. The noncarotene pigments are then removed by extracting the Skellysolve solution with four 10-cc. portions of a diacetone solution consisting of 100 volumes of diacetone (acetone-free, from Commercial Solvents Corp., Terra Haute, Ind.) and 6 volumes of water. The mixture is shaken vigorously after each addition of diacetone solution, and, after standing until complete separation of the phases, the lower layer is drawn off and discarded. The carotene solution is now washed twice with water to remove diacetone, made to 50-cc. volume, and read upon the spectrophotometer. O,
