Colorimetric Determination of Molybdenum in the Presence of Tungsten Modified Mercaptoacetate Method DUMAS A. OTTERSON and JUDSON
W. GRAAB
lewis Flight Propulsion laboratory, National Advisory Committee for Aeronautics, Cleveland, Ohio
b Methods of eliminating tungsten interference with the colorimetric determination of molybdenum using mercaptoacetic acid were studied. A procedure using citric acid is more effective than several using other reagents. Interferences due to moderate amounts of niobium, tantalum, zirconium, or titanium were also eliminated. This modification extends the colorimetric determination of molybdenum using mercaptoacetic acid to steels and other materials that contain tungsten, niobium, tantalum, zirconium, and titanium.
T
of Kill and Yoe (4) for the colorimetric determination of molybdenum using mercaptoacetic acid has produccd accurate and rapid results at this laboratory on samples of high temperature alloys containing small amounts of molybdenum. except in the presence of tungsten. The molybdenum was precipitated with a-benzoinoxime (3) to separate it from the bulk of the alloy. Any tungsten present in the alloy accompanied thP molybdenum and produced erroneous results. This confirmation of the statement of Rill and Yoe (,$) that tungsten interferes led to a study of ways to eliminate this interference. I n a paper on the spectrophotometric determination of tungsten, Greenberp (1) mentioned a method for the separation and subsequent colorimetric determination of molybdenum using dithiol. The present authors found that citric acid can overcome tungsten interference more efficiently than oxalic acid, tartaric acid, or mannitol. It can also eliminate interference due to moderate amounts of niobium, tantalum, zirconium, and, to a lesser extent, titanium. HE METHOD
APPARATUS AND REAGENTS
Instrument. Adsorbance measurements were made with a Beckman Model B spectrophotometer, using 1.00cm. cells. Ammonium Mercaptoacetate. A 10% (volume) solution of mercaptoacetic acid neutralized with ammonia was used. Buffer Solution. A buffer solution (pH 4) was prepared by mixing 50 ml. 1282
ANALYTICAL CHEMISTRY
of 1X sodium acetate solution with 35 ml. of 1N hydrochloric acid and diluting to 250 ml. A concentrated buffer solution was made by diluting to 100 ml. instead of 250 ml. and was used when necessary to maintain the recommended final volime. hIolybdenum Solution. A 1000p.p.m. molybdenum solution was made by diluting 1.840 grams of ammonium molybdate tetrahydrate to 1 liter. It was standardized by the gravimetric method of Knowles (3) using a-benzoinoxinie. This solution n a s diluted as needed. Tantalum and h’iobium Solutions. Both qolutions were made by dissolving 0.500 gram of the metal (99.9% tantalum or 99.8% niobium) in hydrofluoric acid (nitric acid was added to increase the rate of the solution of niobium). Sulfuric acid was added and the solution was heated (for a t least 5 minutes after the formation of sulfur trioxide fumes). Each solution waq then diluted to 50 ml. nith concentratcd sulfuric acid. Portions were used for the tests without citric acid. For the tests with citric acid, 10 ml. of the sulfuric acid solution were first mixed with a small amount of water containing 0.5 gram of citric acid, then diluted to 100 ml. with aater. Titanium Solution. A solution containing 1000 p.p.m. of titanium was prepared by fusing 0.167 gram of titanium dioxide with potassium pyrosulfate, dissolving the fused mass in 5 ml. of 1 to 1 hydrochloric acid and 0.5 gram of citric acid, and finally diluting t o 100 ml. with water. A titanium solution free from citrate v a s made when needed by dissolving the fused mass in 5 ml. of concentrated sulfuric acid by heating and then diluting to 100 ml. with water. The titanium dioxide was spectrographically examined and found free of molybdenum. Tungsten Solution. A 1000-p.p.m. tungsten solution was made by dissolving 0.224 gram of reagent grade sodium tungstate dihydrate in 250 ml. of water. This salt contained not more than 0.01% molybdenum. Zirconium Solution. A solution containing 1000 p.p.m. of zirconium was prepared by dissolving 0.350 gram of C.P. zirconyl chloride octahydrate in 100 ml. of water. EXPERIMENTAL
The amounts of molybdenum found
in the synthetic samples by the various procedures were compared. The synthetic samples were prepared by mixing suitable aliquots of the various ions. Recommended Procedure. To a n aliquot of t h e sample in a 25-ml. Tolunietric flask, add 5 ml. of 2.5% citric acid. Keutralize most of t h e acid present n i t h either dilute sodium hydroxide or saturated sodium acetate. T h e latter must be used in t h e presence of a n easily hydrolyzed substance such as niobium. Adjust t h e pH of t h e solution t o approximately 4 n i t h I S sodium acetate. A p H indic4;rting test paper n i t h a color change new 4, such as pHydrion paper A or B (Micro Essential Laboratory, Brooklyn, S . Y,), may be used. Add 5 ml. of the buffer solution, 5 nil. of 2% potassium chlorate, and 1 ml. of 10% ( T olume) mercaptoacetate reagent. Dilute the solution to volume and meaiure the absorbance a t 365 mp nithin 30 minutes. Analysis of National Bureau of Standards Samples. Difficulty was experienced in the colorimetric molybdenum determination of National Bureau of Standards steel samples after separating molybdenum by t h e a-benzoinoxime precipitation of Knonles ( 3 ) . -4 modification of the method was developed and used on several high temperature alloys with little difficulty and consistently good results ( 2 ) . T n o rational Bureau of Standards samples (steel No. 132) were augmented by addition of sodium tungstatc and analyzed by this procedure. Results (Table I) indicated that molybdenum may be determined with an error of approximately 1% using the citric acid procedure in the presence of large amounts of tungsten Analysis of aliquots of the same samples by the method of Will and Yoe producrd high results. Details of the modified procedure aro as follows: n’eigh out a sample that contains 1 to 20 mg. of molybdenum. T o dissolve, add 20 ml. of 1 to 5 sulfuric acid and warm. If necessary, add nitric acid or hydrochloric acid t o complete the solution, and heat to incipient fumes of
sulfuric acid to remove the nitric or hydrochloric acid. After cooling, transfer to a volumetric flask and dilute to volume. Pipet a suitable aliquot into a 100-ml. beaker the same day and dilute t o 35 ml. Add enough ferrous ammonium sulfate to reduce vanadic and chromic ions and sufficient 1 to 5 sulfuric acid to make the sulfuric acid content at least 6YG (volume). Cool to 5' to 10" C. in an ice bath. Add, while stirring, 2 ml. of freshly prepared 2y0 cu-benzoinoxime reagent, then 5 drops of bromine water, and finally 1 ml. more of the a-benzoinoxime reagent. Allow t o stand 10 minutes. Filter, using vacuum, through a long-stemmed, medium-porosity sintered-glass Buchner funnel with a 20-mm. diameter filter disk covered n i t h about 1/4 inch of 90-mesh granulated Alundum. Rinse well with freshly prepared cold wash solution containing 2.5 ml. of reagent and 1 ml. of sulfuric acid per 100 ml. T o dissolve the molybdenum precipitate, rinse the precipitation beaker with 3 ml. of 57c sodium hydroxide. Transfer t o the filter, being careful to bring the sides of the filter into contact with the sodium hydroxide. Stir the contents of the Buchner funnel well to assure complete solution of molybdenum. Filter into a clean 25-ml. volumetric flask, and wash with a small portion of water. Repeat with 2 ml. of 5% sodium hydroxide. Finally, wash well with water, and dilute to volume. Pipet a suitable aliquot into a 25-ml. volumetric flask and continue as with the recommended procedure. Clean the funnel for subsequent work b y rinsing first with acetone, then -with sodium hydroxide, water, and finally dilute sulfuric acid.
colored complex. Slight deviations mTere observed in the cases of the highest citrate concentration. Tungsten interfered less in the presence of 5000 p.p.m. citric acid than with 1000 p.p.m. (Table 11) when the mercaptoacetate reagent concentration was 0.4%. Effect of Mercaptoacetate Concentration. A thorough study of t h e effects of t h e mercaptoacetate concentration was beyond t h e scope of this paper. However, t h e calibration curves (Figure 1) included two curves in which t h e mercaptoacetate concentration was 1.6% (volume) as well a s five curves with 0.4% (volume)
mercaptoacetate. Of t h e two calibration curves made using 1.6% (volume) mercaptoacetate reagent, one contained no citric acid and the other 5000 p.p.m. of citric acid. These curves are virtually identical. The slopes are less than that of the curve made using 0.4y0 (volume) mercaptoacetic acid and no citric acid, but greater than that of the curve made using 0.4% (volume) mercaptoacetate and 5000 p.p.m. citric acid. The first observation agrees with that of R i l l and Yoe, who state that higher concentrations of mercaptoacetate cause the color intensity of the molybdenum colored complex to decrease. The second observation sugr
Table I.
Standard Samples
(Per cent molybdenum found using indicated variation of method) mnn ---P.P.M. 5000 No CA CA and P.P.M. NBS Steel and 0.4y0 O.4y0 CA and Sample AIR R1R l.6%MR 132. 7.8 7.02 6.97 7.8 6.96 7.4 7.03 132, 13% Wb 8.03 7.02 7.03 6070 Wb 12.2 7.03 7.06 153c 8 61 8 39 8 57 8 35 C4, citric acid; MR, mercaptoacetate reagent. a Recommended values for steel 132 are 7.07% h l o and 6.297, W. Tungsten percentage augmented by addition of aliquot of NasWOa solution prior to a-benxoinoxime separation. c Recommended values for steel 153 are 8.397, Mo and 1.58y0W.
3 4
5
Citric Acid.
400
Mercaptoacetate Reaaent.
P.P.M.
1
0
2
1,000 5,000
3 4
10;000
5
25,000 0 5,000
6
7
Table II.
300 7 / 2 5 ml
Figure 1 . Effect of various concentrations of citric acid and mercaptoacetate reagent on absorbance curves
RESULTS
Comparison of Complexing Agents. lIolybdenuni was determined in samples containing 500 y of tungsten and a negligible amount of molybdenum (0 05 y) by the mercaptoacetate method of Will and EToe fd), modified by adding 1000 p.p.ni. of the various possible coniplexing substances prior to neutralization: citric acid, oxalic acid, tartaric acid, and mannitol. Only vrith citric acid \%asthe tungsten mercaptoacetic acid complex formation negligible. K i t h tartaric acid, oxalic. acid, and mannitol the absorbance at 365 mu \\:is equivalent to 5, 60, and 68 y of niolybdmum, respectively. K i t h no additiveq, the absorbance n :IS equivalent to 66 y of molybdenum. Effect of Citric Acid Concentration. A number of standard niolybdcnuni curves (Figure 1) n e r e made using t h e recommended procedure and others n ith citric acid concentrations of 0, 1000, 5000, 10,000, and 25.000 p.p.m. T h e molybdenum color pioduced b y a given concentration of molybdenum decreased n ith increasing citric acid For citric acid concentrations u p t o 10,000 p p.m., Reer'q lair was obcyc~l I n - tlic n i o l ~bdcnuin
IO0 200 MOLYBDENUM
0
vol: %. 0.4
0.4 0.4 0.4 0.4 1.6
1.6
Elimination of Selected Interferences
Metals Present in 25 M1. Colored Solution, -/ W 500 W 500, 1\10 199 w 1000 w 2000 IV 2000, blo 99
Molybdenum Found, y,in Presence of 5000 p.p.m. CA 1000 p.p.m. CA N o CA and 0.40/, MR and 1.6Yc MR 66, 65, 66 0, 0 2, 2 200, 198
S b 5000 Nb 5000, 110 99 Ta 5000 T a 5000, Mo 99
6, 4
2a, 2a) 2a 99a, 99a, 99a
10, 10, 10
390b
@, @, Oc lOOc, 9ic
400b
2, 2
2c, 2c 101c, 98c, l0lC 2c, 2c 100c, 99c, l0OC
200
T a 5000, 110 199 200, 200 Zr 5000 880*,250* 0 Zr 5000, 110 99 Zr 5000, h10 199 200, 194, 195, 199 Zr 10000, 110 199 196, 184, 184 Ti 200 oc, o c Ti 200, 110 199 200 Ti 1000 24 10, 10, 10 Ti 1000 8a, 80 Ti 1000 24 Ti 1000. 110 199 200d 0 5000 p.p.m. citric acid and 0.4mc mercaptoacetate. Large amount of precipitate. c Blank correction applied. d 1000 p.p.m, oxalic acid and 0.4% mercaptoacetate.
0, 0, 0, 0 97, 99, 100, 98
lC,
10, 5, 10
VOL. 30, NO. 7, JULY 1958
1283
gests a competition for the molybdenum by the citric acid to form a colorless complex and b y the mercaptoacetate reagent to form the colored complex. This aspect was not studied further because the tungsten interference was more pronounced in the presence of the higher mercaptoacetate concentration. I n fact, tungsten interfered less with 1000 p.p.m. citric acid and 0.4% mercaptoacetate than rT-ith 5000 p.p.ni. citric acid and 1.6% mercaptoacetate (Tables I and 11). Other Ions. T h e interferences due t o z i r c o ~ u m ,niobium, tantalum, a n d titanium were also briefly studied because these components of high temperature alloys, in addition t o tungsten, are those most a p t t o remain with t h e molybdenum after t h e abenzoinoxime separation. Siobium, tungsten, and tantalum are known t o contaminate the molybdenum precipi-
tate of a-benzoinoxime ( 2 ) . Zirconium and titanium were thought capable of doing so because of their tendency to hydrolyze. As much as 5000 y of tantalum, niobium, or zirconium causes negligible or no interference using the recommended procedure (Table 11). Only 200 y of titanium could be tolerated under these conditions. There is no evidence that a larger amount of citric acid could eliminate the interference of a greater amount of titanium. On the other hand, oxalic acid appears to be a better complexing agent for titanium than citric acid. The interference of these ions appears to be due, for the most part, to the formation of a precipitate upon neutralization of solutions containing them. Even in the presence of citric acid, a precipitate might be formed. A simple method of evaluating the degree of interference was found in treating an aliquot of the
sample in the same way as that used for the development of the molybdenum color, but using no mercaptoacetate reagent. I n the few cases where this interference was not completely eliminated, blank corrections, determined for each sample, were used. The maximum blank was equivalent to 3 y of molybdenum. LITERATURE CITED
(1) Greenberg, Paul, ANAL. CHEM. 29, 896-8 (1957).
(2) Hillebrand, W.F., Lundell, G. E. F., Brisht. H. K.. Hoffman. J. I.. “.4&likd Inorganic Analysis,” p: 310, Wiley, New York, 1953. (3) &ox-les, X. B., Bur. Standards J . Research 9, 1 (1932). (4) Kill, Fritz, 111, Yoe, J. J., ANAL. CHEM.25, 1363-6 (1953). RECEIVEDfor review July 17, 1957. rlccepted January 31, 1958.
Anomalous Copper Results with the Use of Porcelain Crucibles HARRY ZEITLIN, MICHAEL M. FRODYMA, and GEORGE IKEDA Department o f Chemistry, University o f Hawaii, Honolulu 14, Hawaii
b A blue ash and anomalous results for copper in tuna meat, obtained following dry-ashing of samples in porcelain crucibles, instigated a reinvestigation into use of this type of crucible in the procedure. Samples were dry-ashed in porcelain, silica, and platinum crucibles and copper was determined by the carbamate reagent according to ‘the AOAC method. Erratic results were obtained only when porcelain crucibles were used. Blue ash and erratic results are attributed to the action of a basic flux on copper present within the crucible. Satisfactory results for copper content of tuna meat were obtained by wet-ashing of samples, followed by determination of copper by a Venenate-carbamate method.
I
of work connected with the cause of an off-color condition in yellowfin tuna that often appears on precooking prior to canning, it was necessary to ash samples of cooked off-color tuna meat in porcelain crucibles. The ash obtained was copper blue. When portions of the ash were tested with diethyldithiocarbamate reagent, a golden-brown color was produced, confirming the presence of copper. A quantitative study of the copper content of normal and offN THE COURSE
1284
ANALYTICAL CHEMISTRY
color tuna meat was undertaken, to ascertain whether this trace metal might be a color producer contributing to off-color in tuna flesh in the form of copper proteins or as an ionic copper catalyst. Inasmuch as the study was stimulated by the appearance of a blue ash, i t appeared logical to continue in the investigation of the dry-ashing procedure ( I ) recommended by the Association of Official Agricultural Chemists for the determination of copper. I n excess of 70 determinations for copper in tuna meat from several sources were thus carried out in porcelain crucibles by various n-orkers adhering scrupulously to identical time schedules, reagents, and methods. -4 blue ash was frequently but not invariably obtained. The results were variable to a n extreme degree, with poor precision, and were, on the average, much higher than mould be expected on the basis of previous studies on copper in marine fish (6). S o conclusions, moreover, could be drawn on the role of copper as a color producer in tuna flesh. Consideration of all the factors involved as possible causes for the puzzling results led the authors to suspect the porcelain crucibles. T o check this possibility, it was decided to repeat and expand the previous work. A choice section of a loin of cooked tuna meat
v a s selected as the sole source of samples for the analyses. The dry-ashing was carried out in various types of crucibles coninionly available, such as porcelain (old and new), silica, and platinum; and the determinations were repeated by a sensitive wet-ashing procedure. I n this work the term “old crucible” refers to one previously used in the analysis of meat samples, whereas a “new crucible” is one removed from an unopened carton and never before used. The crucibles are of a type commonly used in the United States. APPARATUS A N D REAGENTS
All glassware used was treated with cleaning solution, followed by three washings with copper-free distilled water. Porcelain and silica crucibles n ere washed with detergent and boiled for 2 hours in concentrated nitrichydrochloric acids (1 t o 1). The cooled crucibles were washed with t a p water, distilled m,ter, and finally three times with copper-free distilled water. The dried crucibles were then heated to constant weight in a muffle furnace a t TOO” to 800” C. Platinum crucibles ryere scrubbed with scouring sand, washed with detergent, and treated with warm concentrated hydrochloric acid. They n-ere then washed with t a p water, distilled water, and three times with copper-free distilled water. They were finally heated to constant weight in a muffle furnace a t 700” to 800” C.
