Spectrophotometric Determination of Molybdenum by the Dithiooxamide Method Dale A. Williams,’ Ira J. Holcomb,* and D. F. Boltz
Department of Chemistry, Wayne State University, Detroit, Mich. 48202
A systematic stucly of sulfur-containing ligands as chromogenic reagents has resulted in the development of a new spectrophotometric method for the determination of molybdenum. Dithiooxamide, also commonly called rubeanic acid, has been founcl to give a blue color with molybdenum(VI) that serves as the basis of this new method. Dithiooxamide (DTOA) has been used in the spectrophotometric determination of copper (1-7), ruthenium (8, 9 ) , cobalt, nickel, palladium, and silver (9). The utilization of dithiooxamide as a reagent for the determination of traces of metals has been reviewed by Ray and Xavier ( 1 0 ) . Although molybdate has been reported to interfere in the spectrophotometric determination of palladium by the dithiooxamide method ( 9 ) , the only reference to the formation of a blue color by a molybdenum and dithiooxamide reaction was found :in a technical bulletin ( 1 1 ) .However, no information was given concerning the conditions required for color formation. This paper reports the optimum conditions necessary for the development of the blue molybdenum-dithiooxamide color and the results of a spectrophotometric study of this colored system.
EXPERIMENTAL Apparatus. Absorbance measurements were made in 1.000-cm and 5.000-cm cells with a Cary Model 14 spectrophotometer. Reagents. Standard MoLybdenum(VZ) Solution. Dissolve 6.509 g of sodium molybdate dihydrate, Na2MoOu2H20, in distilled water and dilute to 500 nil. This solution contains 5.00 mg of molybdenum per ml. Prepare dilute standard solutions containing 3 to 450 pg of molybdenum per ml by transferring 0.3 to 45 ml of the 5 mg/ml standard solution to 500-ml volumetric flasks and diluting to volume. Store standard molybdenum(V1) solution in polyethylene bottles. Dithiooxamide Solution. Transfer 2.500 g of dithiooxamide to a 500-ml volumetric flask m d dilute to volume with absolute ethanol. Diuerse Ion Solutions. Prepare all diverse ion solutions from reagent grade chemicals dissolved in distilled water, usually so that 1 ml of solution contains 5.0 or 10.0 mg of diverse ion. All other chemicals used in this study were reagent grade. Recommended General Procedure. Transfer 5.00 ml of an aqueous sample containing 15 to 2000 pg of molybdenum(V1) to a 50-ml volumetric flask. Add 15.0 ml of 2-propanol and 2.0 ml of N,N’-dimethylformamide. Cautiously add 8.0 ml of concentrated sulfuric acid to the flask from a fast draining 10-ml measuring pipet while gently swirling the flask. Add immediately 7.0 ml of the 0.5% (w/v) dithiooxamide reagent solution. Allow the solution to equilibrate and then cool to ambient temperature which takes about one hour. Dilute to volume with 2-propanol. Measure the absorbance in 1.000- or 5.000-cm cells a t 600 nm using a reagent blank solution in the reference cell. Prepare the reagent blank solution using the recomm.ended general procedure except, substitute 5 ml of distilled water for the sample solution.
RESULTS AND DISCUSSION Absorption Spectra. The visible absorption spectra for the blue molybdenum-dithiooxamide complex are shown in Figure 1. The absorbance maximum is at 600 nm. A more sensitive but irreproducible ultraviolet absorbance Present address, Department of Chemistry, Oral Roberts University, Tulsa, Okla. Present address, Parke-Davis and Co., Detroit, Mich.
*
maximum was observed a t 263 nm. The blue molybdenumDTOA complex is attributed to a charge transfer band. This band would involve the electronic transition of nonbonding electrons in dithiooxamide and the vacant d orbitals of the molybdenum(V1) ion (12). Effect of Variables. Molybdenum Concentration. Conformity to Beer’s law was observed for the Mo(V1)-DTOA complex for 0 to 10 ppm and 0 to 22 ppm of molybdenum when 5.00- and 1.00-cm cells, respectively, were used. The optimum concentration ranges on the basis of Ringbon; plots (13, 14) were 2 to 8 ppm for 5.00-cm cells and 6-20 ppm for 1.00-cm cells. The molar absorptivity of the complex a t 600 nm is 2.2 X lo3 1. mole-’ cm-I and the detection limit based on Kaiser’s concept ( 1 5 ) is 0.02 kg/ml. Reagent Concentration. A large excess of dithiooxamide was necessary for maximum color development. In a series of tests using 30 ppm of molybdenum and 1to 10 ml of the reagent solution, 7.0 ml of the 0.5% (w/v) dithiooxamide solution gave the best results. The unique chemistry of this complexation reaction necessitates the large excess of reagent. The dithiooxamide reagent is stable for over one month. Catalyst. Because the molybdenum-dithiooxamide reaction does not occur at ambient temperature and requires heat to develop the characteristic blue color in sulfuric acid, a search for a reaction catalyst was made. The effect of N,N’-dimethylformamide, dimethyl sulfoxide, pyridine, propylene carbonate, and diethylamine was studied. It was concluded that N,N’-dimethylformamide (DMF) is the best catalyst for the reaction under the specified experimental conditions. Two ml of DMF gave a 13% increase in the absorbance of the blue Mo(V1)-DTOA complex. Larger amounts (-6 ml) of DMF cause partial decomposition of the blue complex.
o L
950
I
500
I I 550 600 WIVELENGTH. nm
650
0
Figure 1. Absorption spectra of Mo-DTOA complex (5-cm cells, reagent blank solution in reference cell) (1) 1.2 ppm of Mo, (2) 2.4 ppm of Mo, (3) 3.8 ppm of Mo
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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Table I. Effect of Heating on Mo-DTOA Complexa Minutes in water bath
Concd sulfuric acid, nil
Table 11. Effect of Diverse Ions on 15 ppm of Mo(V1)
Absorbance a t 600 nm'
PLrmlsi,ble Amount
10 15
20 30 40 0 5 10
15
0.378 0.451 0.438 0.405 0.312 0.452 0.425 0.388 0.362
Temperature of bath = 90°C. Ma = 20 ppm.
Solvent. The effect of changing water-miscible alcohols was studied. The only alcohol which gave maximum development of the blue color and stability of color was 2-propanol. The amount of 2-propanol initially added to the aqueous sample was varied between 20 and 40% in 2% increments. The optimum amounts of 2-propanol is 30% which corresponds to the use of 15 ml of 2-propanol in the recommended procedure. Temperature. The Mo(V1)-DTOA color forming reaction requires heat but is sensitive to fluctuations in temperature and heating time. Therefore, internal heating generated by the solvation of the sulfuric acid was compared with external heating using a constant temperature water bath. The internal heating method was investigated using 1.0 to 10.0 ml of concentrated sulfuric acid corresponding to final sulfuric acid concentrations of 0.36 to 3.60M. The use of 8.0 ml of concentrated sulfuric acid in the internal heating method gave maximum color development as shown in Table I. Acid catalysis is suggested by the fact that in the absence of sulfuric acid, external heating at 90 O C failed to develop the blue Mo(V1)-DTOA complex. Time. The stability of the Mo(V1)-DTOA complex was studied over a 22-hour period and the decrease in absorbance of 0.006 unit corresponded to a 1.3% relative error. Water Concentration. The amount of water in the reaction medium was critical. The percentage of water was varied between 10 and 20% corresponding to a 5.0- to 10.0-ml sample aliquot. It was found that 5 to 6 ml of aqueous solution gave optimum results in the determination of 20 ppm of molybdenum. Lower results are obtained as the concentration of water is increased. Sequence of Reagent Addition. Because molybdenum(VI) does not form an insoluble precipitate with dithiooxamide, the order of addition of concentrated sulfuric acid and the dithiooxamide solution is inconsequential and should give similar results, a fact proved experimentally. However, since some other metal ions form precipitates with DTOA under slightly acidic or basic conditions ( 2 ) , the sulfuric acid should be added before the dithiooxamide reagent if other metal ions are present in the sample solution. Diverse Ions. An investigation of the effect of diverse ions was made using 15 ppm of molybdenum and the 1.000-cm cell. The tolerance limit was considered to be that amount giving a deviation less than three standard deviations. The results of this study are shown in Table 11. In concentrations of 200 ppm, cadmium, chromium(III), magnesium, manganese(II), zinc, acetate, oxalate, tartarate, citrate, chloride, perchlorate, phosphate, thiocyanate, and EDTA did not interfere. Other ions which did not interfere a t lower concentrations were sodium (130 ppm), ammonium (100 ppm), zirconyl (50 ppm), and osmium(VIII) (75 ppm). 2026
Ion
Add'd ah
Au(II1)
HAuCli Ca(C2H302)2 CO(C104)* CU(C10~)2 Fe*(SO: )3 Ni(C104)* HZPtC16 Na2W04 3 Na2HAs0, -
Ca(I1) Co(I1) Cu(I1) Fe(II1) Ni(I1) Pt(1V) WWI)
adddd
Relati\ i
oror,
amount,
P W
20
-1 2
2
200 2
-18
10 0
2 50 2 10
-34 4 7
+6
-36 -4 -6 pptn.
a
0 0 0
5
200 100 200' 0 ~ ~ 0 ~ 20 4 2 0 KN03 N0325 -2 7 0 NHJV03 v03a Causes less than 3.0% relative error using 15.0 ppm molybdenum.
Copper(II), nickel(II), cobalt(II), and platinum(1V) interfered. These ions give red, purple, orange-red, and red complexes with DTOA under the conditions used. The interference of iron(II1) indicates ion interaction inasmuch as 5 ppm of iron(II1) plus dithiooxamide exhibitis no color or absorbance a t 600 nm. Arsenate is believed to form an insoluble heteropoly molybdate compound with the molybdate. Ions which form insoluble sulfates, e.g., lead(I1) and barium(II), interfere. Precision. A precision study of a series of ten solutions containing 4.00 ppm of molybdenum, for the inherently more sensitive method using 5.00-cm cells, gave a mean absorbance, standard deviation, ana relative standard deviation of 0.453, 0.003, and 0.66%, respectively. For a serie3 of ten solutions containing 15.0 ppm of molybdenum and using 1-cm cells, the mean absorbance, standard deviation, and relative standard deviation were 0.344, 0.003, and 0.87%, respectively. All spectrophotometric measurements were made a t 600 nm. Composition of Molybdenum(V1)-Dithiooxamide Complex. The composition of the blue molybdenum(V1)dithiooxamide complex was studied by the mole ratio method. A 2.09 X 10-4M molybdenum(V1) solution and a 4.10 X 10-2M dithiooxamide solution were used in this study; the final molybdenum concentration was 20 ppm. Figure 2 indicates that a 6:l DTOA to Mo(V1) complex is formed. This is a high ratio inasmuch as dithiooxamide has four sites of basicity per molecule. The chemistry of this system is apparently similar to that reported previously for the copper(I1)-dithiooxamide complex in sulfuric acid (7). Several observations indicate that an unusual type of complex has been formed. First, to obtain complete color development of the 20 pprn of molybdenum(V1) level when stoichiemetric or slightly larger amounts of dithiooxamide are present, it was necessary to use additional sulfuric acid. Second, an approximately 1:l greenish yellow complex with an absorbance maximum of 415 nm was observed for low concentrations of dithiooxamide. The absorbance of the greenish yellow complex does not decrease with an increase in the blue dithiooxamide complex of molybdenum, but rather tha absorbance maximum is lost in the spectrum of the blue complex after about 15 minutes. Finally, the catalytic effect of polar compounds such as N,N'-dimethylformamide indicates the stabilization of a charged reaction intermediate. A consideration of these observations indicates that an association reaction involving dithiooxamide has occurred and the associated species complexes the molybdenum ion to give the characteristic blue complex. The following equations illustrate the nature of the proposed
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
05t-
I
/
stoichiometric point (16). Inasmuch as small experimental errors produce very large changes in the calculated Kf value, especially when the DTOA concentration is a sixth power expression, only the magnitude of the conditional formation constants can be determined. Two sets of data gave a mean overall conditional formation constant of 8 X loz3which indicates a rather stable complex is formed. The molar absorptivity of the molybdenum-dithiooxamide complex (t = 2.2 X lo3) is larger than the corresponding value for peroxymolybdic acid ( e = 9.6 X lo2) but lower than the absorptivities for the molybdenum-l-pyrrolidinecarbodithioate ( E = 8.5 X lo3) and the molybdenum-carminic acid complexes (e = 1.4 X lo4) which have been studied previously (17-19).
LITERATURE CITED
Flgure 2. Mole-ratio plot for molybdenum(V1)-dithiooxamide complex
reactions involved in the development of the colored system. S
s
II
H,N-C-C
s
/ \
s
II
$-C-NH, NH
+
a+ MO
s
I1
-C- t4,N-C-C
1
s
c
/ \ / \ I1
F
"2
An estimation of the formation constant of the MoDTOA complex was made using Ayres' method which involves using the curvature of the mole-ratio plot near the
(1)E. J. Center and R. M. Maclntosh, lnd. Eng. Chern., Anal. Ed., 17, 239 (1945). (2)H. H. Willard, R. E. Mosher, and A. J. Boyle, Anal. Chem.. 21, 598 (1949). (3)P. W. West and M. Compere, Anal. Chem., 21, 628 (1949). (4)A. Lemoine. Anal. Chlm. Acta, 8,528 (1952). (5)D. S.McCann, P. Burcar, and A. J. Boyle. Anal. Chem., 32, 547 (1960). (6)A. Paul, Anal. Chem., 35, 2119 (1963). (7)D. A. Williams and D. F. B o k , Anal. Lett, 8, 103 (1975). (8)G.H. Ayres and F. Young, Anal. Chem., 22, 1281 (1950). (9)J. Xavier and P. Ray, J. lndian Chern. SOC.,35, 432 (1958). (10)P. Ray and J. Xavier, J. lndian Chern. SOC.,38, 535 (1961). (11) Mallinckrodt Chemical Works Bulletin, St. Louis, Mo.. "Dithiooxamide and Its N,M-Disubstituted Derivatives" (1959). (12) B. Person and J. Sandstrom, Acta Chern. Scand., 18, 1059 (1964). (13) A. Ringbom. Z.Anal. Chem., 115, 332 (1938). (14)G.H. Ayres, Anal. Chem., 21, 652 (1949). (15)H. Kaiser, Anal. Chern., 42, (4),26A (1970). (16)G. H. Ayres, "Quantitative Chemical Analysis," 2nd ed., Harper and Row, Publishers, New York, N.Y., 1968,p 474. (17)G.Telep and D. F. Boltz., Anal. Chern., 22, 1030 (1950). (18)M. B. Kalt and D. F. B o k , Anal. Chern., 40, 1086 (1968). (19)A. Lee and D. F. Boltz., Microchern. J., 17,380 (1972).
RECEIVEDfor review January 31, 1975. Accepted June 26, 1975.
Similarity Measures for the Classification of Binary Infrared Data H. 6. Woodruff, S. R. Lowry, G. L. Ritter, and T.
L. lsenhour
Department of Chemistry, University of North Carolina, Chapel Hill, N.C. 275 14
One of the most straightforward methods of identifying an unknown compound from its spectrum is to compare it to each member of a file of known spectra. If the unknown corresponds to a spectrum in the file, an exact match is possible and the unknown is identified. If, however, the unknown is not in the file, some measure of similarity is required such that those spectra in the file most similar to the unknown are found during the search. The compounds corresponding to these spectra should reveal useful structural information about the unknown even though there may be no exact match. This process is the basis for the nearest neighbor (NN) decision rule. The NN decision rule has been a well-known classification technique for a number of years (1-5). More recently, the NN approach has been applied to several chemical problems (6-9). The data set is subdivided into a number of chemical classes. The unknown is compared to each member of the dat.a set and is predicted to belong to the same class as its nearest neighbor (i.e., the most similar
data set member). Two measures of similarity, the Hamming distance and the Tanimoto similarity measure, are discussed in this paper as means of classifying binary infrared spectra. DATA SET The data set used in this study is the same as the one employed in previous work (10). There are thirteen mutually exclusive classes, each containing 200 spectra selected from the ASTM file of 91,875 binary infrared spectra (made accessible a t the Triangle Universities Computation Center in North Carolina by the R. J. Reynolds Tobacco Company). Each spectrum consists of 139 dimensions representing 0.1-pm intervals from 2.0-15.9 pm. The only atoms present are C, H, 0, and N, with the carbon content ranging from C1-15. Computations were performed on the Triangle Universities Computation Center IBM 3701165 teleprocessing with the University of North Carolina Computation Center IBM 360175 and on Raytheon 706 and 704 computers using FORTRAN IV and PL/I programs.
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