Determination of Molybdenum in Titanium Alloys by Precipitation from Homogeneous Solution Using Thioacetamide WILLIAM N. McNERNEY and WILLIAM F. WAGNER Department o f Chemistry, University o f Kentucky, Lexington, Ky.
Thioacetamide, Fisher Scientific Co., lot No. 752848. &Tartaric acid, Rastiimn Kodak Co., white label. Hydrofluoric acid, 48%. Central Scientific Co., reagent. grade. Hydrogen peroxide. 30%, Fisher Scientific Co., C.P. grade. Sulfuric acid, specific gravity 1.84, D u Pont, reagent grade. Titanium tetrachloride, Fisher Scientific Co., technical grade. Standard molybdenum solution (1 ml. = 0.0050 gram of molybdenum). Dissolve 7.500 grams of molybdenum trioxide in a small amount of 15% sodium hydroxide and dilute to 1 lit,er.
b Molybdenum in titanium alloys can b e determined b y precipitation of molybdenum sulfide with thioacetamide. The method obviates the use of a rapid stream of hydrogen sulfide and a long digestion period. A dense precipitate is farmed which is easily filterable without the use of paper Pulp.
A
for determining molybdenum in titanium alloys by precipitating it with gaseous hydrogen sulfide was proposed by Norwitz and Codell (8). Excellent r.esults were obtained, but it was necessary to carry out the precipitation without heating to pi-event the hydrolysis of titanium, and the molybdenum sulfide had to digest overnight before filtration. Flaschka and Jacobljevich (1) reported the successful precipitation of molybdenum sulfide by the hydrolysis of thioacetamide. Certain differences from the gaseous hydrogen sulfide procedures have been recognized and need further study (4). Precipitation of molybdenum sulfide from homogeneous solution by thioacetamide gives a more dense, easily filtered precipitate and the procedure requires less time. METHOD
EXPERIMENTAL
REAGENTS
AIolybdie anhydride, Nercli & Co., Inc., reagent grade. W
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* ', ,
-.-
,
..
~
1 mm.
Figure I. Molybdenum sulfide precipitated b y hydrogen sulfide
rigure L. moiyoaenum suinae precipitated b y hydrolysis of thioacetamide
'
Effect of Pressure. Some investigators (1,2,5)have recommended the use of pressure flasks for the precipitation by hydrogen sulfide and thioacetamide, while others ( 3 )have found them unnecessary. Experiments in which a five- t o tenfold excess of solid thioacetamide was added immediately before hydrolysis gave consistent results a t atmospheric pressure. Effect of Acidity. The effect of varying the acidity on the precipitation was studied by changing the concentration of the sulfuric acid in the solution. The results closely paralleled those of Normitz and Codell (8). An optimum concentration of approximate] y 1N sulfuric acid vas selected. Filtratioin and Ignition. The nse of filter pape:r pulp (1, S, 6) was discon:r the first few deterniinatinued aft< tions. Pr ecipitation from homogeneous solut,iongave a dense, coagulated precipitate t h a t settled readilv. and i t was washt:d easily from thk'flasks. Figure 1 81lows a nhotomicroeranh of im sulfide precipike'd by ydrogen sulfide. Figure 2 ybdenum sulfide formed by vsis of thioaeehamide. The :les in Figure 2 are compact aggregates that filter as effectively as larger cry;stalline precipitates. Consequently iorous porcelain filter crncibles were utsed for filtration and ignition of the precipitates.
Test of Separation. Eight aliquots of the standard molybdenum solution containing 0.0500 gram of molybdenum (equivalent to 5.00% molybdenum in a 1-gram sample of alloy) were analyzed hy the hydrolysis of thioacetamide in approximately IN sulfuric acid in a bath of boiling water for 60 minutes. Results of the eight determinations gave an average of 0.0501 gram of molybdenum with a standard deviation of 0.00025 gram. Four determinations of molybdenum in solutions containing 0.0500 gram of tungsten complexed by the addition of 5 grams of tartaric acid gave an average of 0.0502 gram of molybdenum m.ith a standard deviation of 0.00021 gram. No hydrolysis of titanium or attack on glassware were experienced upon heating the solution to hydrolyze the thioacetamide. Two determinations of molybdennm in the presence of 1 gram of titanium were made. The titanium was added as the tetrachloride to the molybdenum solution in a platinum dish. 5 ml. of 487" hvdrofluoric acid \,-ere added, the solu&bas diluted to about 60 ml., and 2 ml. of 30% hydrogen peroxide nere added. After being heated to eliminate the yellow titanium-peroxide complex, the solution was diluted to approximately 175 ml. and the molybdenum r a s determined by the regular procedure. The average of the t a o determinations was 0.0500 gram of molpbdenum. VOL. 29, NO. 8. AUGUST 1957
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1177
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PROCEDURE
Place a 1-gram sample of alloy in a large platinum dish. Add 10 ml. of water and 2 ml. of concentrated sulfuric acid. Slowly add 5 ml. of hydrofluoric acid. After the sample has dissolved, dilute it to about 60 ml., and add 2 nil. uf 30% hydrogen peroxide. The titanium-peroxide complex should impart a golden-yellow color to the solution. Heat slowly on a hot plate until the solution becomes colorless. Maintain the volume of solution a t approximately 60 nil. at all times during the heating period by the addition of mater. Cool, dilute to about 120 ml., and transfer to a flask. If tungsten is present, add 30 ml. of a solution containing 5 grams of tartaric acid. Dilute to a final volume of about 155 nil., and add 1 gram of thioacetamide. After the thioacetamide is dissolved. heat the flask to the temperature of boiling water and maintain that temperature for 60 minutes. Cool to room temperature. at which time the precipitate should settle. The supernatant solution should be clear. Filter through a porous porcelain filtering crucible (Coors KO. 679-3) which previously has been brought to constant tveight at 650" C. K a s h the precipitate with a solution containing 2 nil. of sulfuric acid and 1 gram of tartaric acid diqsolred in 100 ml. of solution. The
Table I.
Allol
A B
c
J>
Analysis
of Titanium Alloys
Spectrographic Analyses, yo W A1 Ce Sd 0 62 0 83 0 00 0 00 0 88 6 06 0 00 0 00 0 00 1 62 0 00 1 00 1 90 1 98 2 10 0 00 0 00 0 61
?\Io 3 11 2 88
tartaric acid may be omitted if tungsten is absent. Ignite the crucible and precipitate a t 650" C. for 60 to 90 minutes. Cool and weigh as molybdenum trioxide. being careful not to expose the crucible to air currents that might displace the powdery precipitate.
Thioacetamide Procedure ?\Io, No. of Std av. cc detns. dev., 5 3 11 6 0 02 2 88 8 0 02 1 00 5 0 05 1 08 6 0 04
partment of Mining and Metallurgy for the titanium alloys used in this research and 0. F. Edwards for the photomicrographs. LITERATURE CITED
(1) Flaschka, H., Jacobljevich. H d n a l . Chim. Acta 4, 482 (1950). (2) JIcSerney, W. N., "Study of the Quantitative Precipitation of AIolybdenum by Thioacetamide," h1.S thesis, University of Kentucky. 1955. (3) Xorwitz, G., Codell, hI ASAL. CHEII 2 5 , 1438 (1953). ( 4 ) Sxift, E. H., Butler, E. A , Zhzrl . 28, 146 (1956). ( 5 ) Yagoda, H., Fales, H. A , J . . i t t i . Chem. Soc. 58, 1494 (1936). RECEIVEDfor review December 13, 1956. Accepted March 15, 1057. ~
RESULTS
Results obtained for the determinntion of molybdenum in four experimental titanium alloys supplied by the Department of Mining and Metallurgy are shown in Table I. The spectrographic analyses for coniparison were performed by a conimercial laboratory. ACKNOWLEDGMENT
The authors nisli to thank J. P. Hanimond and R. E. Swift a t the De-
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Constant Cur rent Potentiometric Determination of Manganese CALVIN
0.HUBER
and IRVING SHAlN
Department of Chemistry, University
of Wisconsin, Madison, Wis.
,The technique of constant current potentiometry was applied to two commonly used methods for the determination of manganese. In the Volhard method a significant improvement in accuracy of end point detecIn the Linganetion is achieved. Karplus method, the titration was as precise and accurate as with the classical potentiometric end point.
A
THE more frequently encountered volumetric methods for the determination of manganese are the Volhard determination ( I O ) , and the Lingane-Karplus method ( 7 ) . The Volhard method involves the oxidation of manganous ion to manganese dioxide with standard permanganate. Although it is possible to obtain quantitative results by using the Fischer (4) modification of the Volhard method, visual end point detection is difficult owing to the JIOXG
1 178
ANALYTICAL CHEMISlRY
presence of brown manganese dioxide formed during the titration. No potentiometric end point method has been reported, although Brann and Clapp (1) investigated the use of a n aniperometric method with one polarized electrode us. a silver-silver chloride electrode. Talsky (9) used a portable spectroscope to detect the first excess permanganate. The Lingane-Karplus method involves the oxidation of manganous ion to manganese(II1) by permanganate in the presence of a complexing agent, sodium pyrophosphate. Although the end point cannot be detected visually owing to the pink color of the manganese(II1) pyrophosphate complex, excellent results are obtained by following the titration potentiometrically. Both the end point potential and the size of the break at the end point are dependent on pH. Under the best conditions (pH = 6), the potential break is under 300 mv. Thus the titration curve should be plot-
ted for best results. If the titration conditions are reproduced carefully, subsrquent determinations may be made merely by titrating to the predetermined end point potential. Goffart, Nichel, and Pitance ( 6 ) applied a n amperometric method to the detection of this end point, and DUYckaerts ( 3 ) used potentiometry with one polarized electrode. The constant current potentiometric method involves the measurement of the difference in potential between two platinum wire electrodes which are polarized with a constant current. This technique n-as introduced by Dutoit ( 2 ) and was interpreted in terms of voltammetry by Reilley, Cooke, and Furman (8). The mpthod by which the end point is detected can be explained by reference to Figure 1. These experimental current-voltage curves were obtained in solutions containing the same electrolytes a t the same p H as would be present
