From an analytical point of view, it is obvious that KMR measurements of metal chelates permit quantitative estimates of certain metal ion mixturesLe., Cd-Pb mixtures-and of isotope content-Le., Pb“7 in lead. This procedure also forms the basis for isotope exchange studies-i.e., Pb”7Y Pb+2 --* PbY Pb”7+2. The kinetic study of chelate exchange reactions (PbaiY* PbY is PbY* PbmiY) by normal mixing methods is also possible by employing deuterated chelates, Y *; the S M R technique has the advantage of continuous measurement of the course
+
+
+
+
of reaction, thus avoiding the usual timeconsuming separations and their possible effect on the amount of exchange and the use of radioactive isotopes. One of the chief disadvantages of the NMR technique is the concentration level which must be employed. LITERATURE CITED
(1) Gutowsky, H. S., J . Chem. Phys. 37, 2196 (1962). \ - I
(2) Kula, R. J., Sawyer, D. T., Chan, 8. I., Finley, C. M., J . Am. Chem. SOC. 85,2930 (1963). (3) Pople, J. A,, Schneider, W. G., Bernstein, H. J., “High Resolution Nuclear
Magnetic Resonance,” McGraw-Hill, New York, 1959. (4)Saunders, bi., Yamada, M., J . Am. Chem. SOC.85, 1882 (1963). ( 5 ) Singer, J., Sudrneier, J. L., Reilley, C. N., unpublished work. (6) Weakliem, H. A., Hoard, J. L., J. Am. Chem. SOC.81, 549 (1959). RECEIVED for review December 26, 1963. .4ccepted March 9, 1964. Research SUPported in part by National Institutes of Health Grant RG-8349. One of the authors (R. J. D.) gratefully acknowledges the help of a Kational Science Foundation Coo erative Graduate Fellowship. Presentel at the Southeastern Regional ACS Meeting, Xovember 15, 1963.
Colorimetric Detection and Spectrophotometric Determination of Vanadium Using a Specific Reaction P. L. SARMA Department of Chemistry, University of North Dakota, Grand Forks, N. D.
b In the presence of nonreducing acids, small quantities of vanadium(V) give a yellow color, large quantities an orange color. It i s a specific test for vanadium(V). The color was most stable and intense with concentrated sulfuric acid than with many other acids. Using a spot technique and one drop of a sodium metavanadate solution, a minimum of 10 p.p.m. of vanadium(V) was detected with a few drops of concentrated sulfuric acid. Absorption by a mixture of concentrated sulfuric acid and sodium metavonadate solutions followed Beer’s law between 450 and 540 mp. An optimum concentration range for estimating vanadium(V) by this method will depend upon the wavelength and the amount of acid used.
D
OF VANADIUM by redox titrimetry (6) involves a prior separation of vanadium from other oxidizable or reducible materials. Likewise, detection and determination of vanadium using hydrogen peroxide (14)) diphenylamine (7), phosphotungstic acid ( I @ , strychnine (4, benzohydroxamic acid (18), 2,6-pyridinedicarboxylic acid (9), benzoylphenylhydroxylamine ( I I ) , and many other colorimetric reagents use nonspecific reactions of vanadium. Simple vanadate ions have a tendency to undergo condensation reactions, especially in the presence of a nonreducing acid, forming polyvanadic acids (IO). Such a condensation reaction is generally accompanied by deepening of color. The mono-, di-, and tetravanadic acids are colorless; ETERMINATION
1076
ANALYTICAL CHEMISTRY
but the pentavanadic acid is orangeyellow, and the octsvanadic acid is brown-red. Also, the vanadates show a tendency to form complexes with acids forming heteropoly acids. Both these types of reactions are characteristic of vanadium@), but the composition and stability of the products are dependent upon the nature and the amount of acid inducing these reactions. Therefore, an investigation was begun in 1962 utilizing these specific reactions in the of detection and determination vanadium. Independently, Mittal and Mehrotra (8) worked on a method of estimating vanadium(V) with acetic, succinic, malonic, benzoic, and phthalic acids. However, the results of the present investigation showed that, because of smaller pK,‘s, the color intensities and stabilities were better with some inorganic acids than with most, or perhaps any, organic acid. EXPERIMENTAL
Apparatus. Beckman Model D B Spectrophotometer, Beckman Laboratory Potentiometric Recorder, and 0.998-cm. silica cells. Procedure. About 2.4 rams of sodium metavanadate, r\la#03, were dissolved in about one liter of water, filtered through a highly retentive quantitative filter paper, and the vanadium concentration was determined by titrating it against reagent grade ferrous ethylenediammonium sulfate, Fe[C2H4(NH&! (S0&.4H20, using sodium diphenylamne sulfonate as the indicator (2). This solution contained 0.975 mg. of vanadium per milliliter. -411 quantitative measurements were made with dilutions from this solution. At lower concentrations of vanadium, sodium metavanadate solutions pro-
duced a yellow color with sulfuric, hydrochloric, nitric, phosphoric, iodic, perchloric, periodic, molybdic, formic, acetic, trichloroacetic, ethylenediamine tetraacetic, sulfamic, sulfanilic, and sulfosalicylic acids; but the color faded away partially or completely through hydrolysis. Citric, oxalic, and tartaric acids also gave a yellow color; but on standing they reduced vanadium, producing the blue color of vanadium(1V). Ascorbic acid produced a transient greenish-blue color a t the moment of mixing. Thus, it provided a sensitive and specific test ( I S ) for the detection of vanadium@). An increase in the amount of a nonreducing acid decreased hydrolysis and increased the color intensity. Because the water content is small, concentrated sulfuric and glacial acetic acids were investigated as reagents for the detection and determination of vanadium(V). With a few hundred micrograms of vanadium(V) in one milliliter of a sodium metavanadate solution, small quantities of glacial acetic acid produced only a light yellow color and larger quantities gave an orange color. However, a large excess of this acid precipitated vanadium if the solution contained milligram quantities of vanadium per milliliter. For example, 5.0 ml. of a solution containing 5 mg. of vanadium(V) per milliliter produced an orange color with 10 ml. of glacial acetic acid. It changed to a yellow colored solution almost immediately, to a yellow colored turbidity after about 20 minutes, and to an orange colored precipitate in about 45 minutes. An increase in the amount of glacial acetic acid caused a more rapid precipitation of the heteropoly acids. No amount of sulfuric acid produced any turbidity or precipitate even on prolonged standing. A.R. grade ammonium metavanadate
to the sample until the pH was about 8.5. The hydroxides of these metals where then filtered off. Permanganate was separated from vanadium(V) by adding methanol to the near-boiling filtrate and then filtering. Vanadium (V) appeared in the alkaline filtrate without any reduction. Bromates produced bromine with concentrated sulfuric acid, but its interference was eliminated by boiling off bromine.
,wrt
,"Io-1
I
YAMUYY. I I Y m I .
RESULTS
'WAVELENliTH,
mu
Figure 1. Visible atlsorption spectra of vanadium(V) in the presence of sulfuric and acetic acilds 5.0 ml. of water and 5.0 ml. o f sodium metReference water avanadate (0.5 ing. V/ml.l. 8. 5.0 ml. of glacial aceti,: acid and 5.0 ml. of sodium metavanadate (0.5 mg. V/ml.). Reference 9M acetic acid C 5.0 ml. o f concentrated sulfuric acid and 5.0 ml. of sodium metavanodtrte (0.5 mg. V/ml.). Reference 9M sulfuric acid D. 5.0 ml. of concentrated sulfuric acid and 5.0 ml. of sodium metavanocate (5 mg. V/ml.). Reference 9M sulfuric acid A.
and commercial gradea of sodium tetravanadate, NaZV4011,sodium orthovanadate, h-a3V04,and sodium pyrovanadate, NadVzO,, gave a yellow or an orange color with nonreducirlg acids, but the blue color of vanadiurn(1V) sulfate was unaffected by nonreducing and nonoxidizing acids. In the spot test detection, identification limits were determined by adding four or five drops of concentrated sulfuric or glacial acetic acid to one drop of a sodium metavanadate solution. The test was performed also with acetic acid vapors. One or two drops of glacial acetic acid were placed in a spot plate depression adjacent to that containing the test sample. Within the identification limit, a yellow color was produced by the viipors of the acid. Interfering colors of cerium(IV), cobalt, copper, and nickel ions were removed by adding Eodium hydroxide
WIVELFNCTII, "la
Figure 2. Ultraviolet absorption spectrum of vanadium(V) in the presence of sulfuric acid. Reference water. 9M sulfuric acid 25.00 ml. o f sodium motavanadote (0.0202 mg. V/ml.) and 25.00 ml. ol:water C. 25.00 ml. of sodium mctavonadate (0.0202 mg. V/ml.) and 25.0 f 0.5 ml. of concentrated A. B.
sulfuric acid
The absorption wavelengths and the absorptivities of the yellow solutions were determined by mixing sodium metavanadate solutions and sulfuric acid in volumetric flasks, Figure 1. Transmittances were read or recorded after cooling the mixtures to room temperature, Figures 1 to 5. Because the stability and the color intensity were higher with sulfuric than with
Figure 4. Determination of optimum vanadium concentrations for estimating vanadium(V) with concentrated sulfuric acid A. 25.00 ml. of sodium metavanadate and 75.0 f 0.5 ml. o f concentrated sulfuric acid a t 450 Reference 14M sulfuric acid 8. 25.00 ml. of sodium metavonadate and 25.0 f 0.5 ml. of concentrated sulfuric acid a t 450 mg. Reference 9M sulfuric acid C. 25.00 ml. of sodium metavonadate and 75.0 ?= 0.5 ml. of concentrated sulfuric acid a t 51 0 mp. Reference 14M sulfuric acid D. 25.00 ml. o f sodium'metavanadateand 25.0 f 0.5 ml. of concentrated sulfuric acid a t 51 0 mp. Reference 9M sulfuric acid mp.
DISCUSSION
I 0
20
e0
60
00
100
I20
140
TIME AFTER MIXING, min.
Figure 3. Color stability of polyvanadic acids a t 460 mp. Reference water 25.0 f 0.5 ml. of Concentrated sulfuric acid and 25.00 ml. of sodium metavanadate (0.5 mg. V/ml.). B. 25.0 ml. of glacial acetic acid and 25.00 ml. of sodium metavonadate (0.5 mg. V/ml.)
A.
acetic acid (Figure 3 E), absorptivities were determined only with concentrated sulfuric acid. Even with this acid, the color intensity decreased sharply immediately after mixing (Figure 3 A). Therefore, all transmittance readings were taken 30 minutes after mixing. Using these yo T readings, % absorptances (100 -% 2') were calculated and plotted against logarithms of vanadium concentrations (Figurea 4 and 5 ) . The concentrations corresponding to the greatest slopes of these curves gave ( I ) the optimum vanadium concentrations for the determination of vanadium(V). These concentrations are shown in Table I. Within these optimum concentration ranges, absorbances were found to vary linearly with concentrations. The slopes of these linear graphs gave the absorptivities (Table I). Because the molecular identities were unknona, the molar absorptivities of these absorbing species could not be calculated.
Using only one drop of a test solution, sulfuric and glacial acetic acids detected as little as 10 and 100 p.p.m. of vanadium(V), respectively. With acetic acid vapors the identification limit was 16 p.p.m. Acetic anhydride and glacial acetic acid showed about the same sensitivity. The appearance of a yellow or an orange color on addition of a nonreducing acid to an inorganic material is a specific test for vanadium(V). It appeared more selective than the wellknown test for nickel with dimethyl-
iX,o-'
IX,..~
IXl0-I
1
V A N A W Y , m+./ml
Figure 5. Determination of optimum vanadium concentrations for estimating vanadium(V) with concentrated sulfuric acid 25.90 ml. of sodium metavanadate ond 75.0 i 0.5 ml. of concentrated sulfuric acid at 480 mp. Reference 14M sulfuric acid 8. 25.00 ml. of sodium metavanadate and 25.0 f 0.5 ml. of concentrated sulfuric acid a t 480 mp. Reference 9M sulfuric acid C. 25.00 ml. of sodium metovanadate and 75.0 f 0.5 ml. of concentrated sulfuric acid a t 540 mp. Reference 14M sulfuric acid D. 25.00 ml. of sodium metovanodote and 25.0 + 0.5 ml. of concentrated sulfuric acid at 540 mp. Reference 9M sulfuric acid A.
VOL. 36, NO. 6, MAY 1964
1077
Table
I.
Wavelength,
a
at 450 mp, and 260 to 900 p.p.m. a t 480 mp. Mixing of 25.00 ml. of the
Absorptivities and Optimum Concentrations for Determining Vanadium(V) with Concentrated Sulfuric Acid
ml Absorptivities, mg.-cm.
mp
a
b
0
b
450 480 510
16 4.8
5.7 2.3 0.6
0.01-0.07 0.04-0.2 0.2 -0.8
0.13-0.45
1.0
unknown with 75.0 ml. of concentrated sulfuric acid should permit determination of 40 to 280 p.p.m. of vanadium a t 450 mp and 160 to 800 p.p.m. a t 480 mp. Dilute sulfuric acid would be more convenient to use than concentrated sulfuric acid, but trace quantities of vanadium(V) cannot be determined with 9M sulfuric acid (Table 11).
Optimum vanadium concentration in the mixture, mg./ml. 0.04-0.2 .. .- ..c
25.00 ml. of sodium metavanadate and 75.0 f 0.5 ml. of concentrated sulfuric acid.
* 25.00 ml. of sodium metavanadate and 25.0 f 0.5 ml. of concentrated sulfuric acid. Insufficient data to determine the optimum concentrations.
CONCLUSION
Table 11.
Absorption of Visible Radiations by Vanadium(V) in the Presence of Sulfuric Acid
Vanadium concentration in the mixture,a mg./ml. 0.0608 0.101
450 mp
0.808
86.0
0.405
a
99.0 98.2 92.9
% Transmittance 480 mp 510 mp 99.5 99.5 99.1 99.5 97.5 99.0 95.1 98.5
540 mp 99.6 99.6 99.6 99.5
25.00 ml. of sodium metavanadate and 25.0 f 0.5 ml. of 9M sulfuric acid.
Generally, colorimetric analyses are subject to many interferences. I n the determination of vanadium(V) with sulfuric acid, interferences are caused only by a few elements whose sulfates and oxides are insoluble. Inert coloring materials interfere with any colorimetric detection and determination. Because the interference is minimum, the determination of vanadium with sulfuric acid is expected to be one of the best known colorimetric methods of analysis. ACKNOWLEDGMENT
glyoxime. While dimethylglyoxime gives a red color or precipitate with nickel and iron(I1) and other colors with copper, cobalt, and chromium(VI), no cation or anion was found to produce a yellow or an orange color on addition of a nonreducing acid. Even niobium and tantalum, which resemble vanadium in most of their chemical properties, failed to give this test. Nonreducing acids gave a white precipitate of niobium pentoxide hydrate with potassium niobate but gave no color or precipitate with tantalum potassium fluoride. As with any colorimetric detection and determination, presence of colored materials interfered with this test, but these interferences were eliminated by procedures described under Experimental. Chroniium(V1) should not make detection and determination impossible, since it can be separated from vanadium(V) as oxinate (12) or as lead chromate (17). The yellow solutions formed by mixing sodium metavanadate solutions and concentrated sulfuric or glacial acetic acid absorbed near the ultraviolet end of the visible spectrum (Figure 1 B and 1 C). As the concentration of vanadium was increased, the color changed from yellow to orange, and the absorption began a t a longer wavelength (Figure 1 C and 1 D). Within certain concentration ranges (Table I), the yellow solutions formed by mixing concentrated sulfuric acid and sodium metavanadate solutions absorbed 450, 480, and 510 mp radiations according to Beer’s law. Therefore, within these ranges, sulfuric acid appeared to offer a specific method for the determination of 1078
ANALYTICAL CHEMISTRY
vanadium(V). If an unknown containing vanadium(V) was treated to remove the coloring materials, nearly neutralized, mixed with a suitable amount of concentrated sulfuric acid, and the absorbance measured a t a wavelength shown in Table I, milligrams of vanadium per milliliter of the mixed solution should be obtained by dividing the measured absorbance by the appropriate absorptivity (Table I). If vanadium in the unknown existed in another oxidation state, concentrated nitric acid can oxidize it to 99% vanadium(V) (6). A complete oxidation is possible with the method of Willard and Fenwick (16). .Inother method is to boil with 70% perchloric acid, when chromium and vanadium are completely oxidized to chromic and vanadic acid but manganese is left unoxidized (17‘). Also, an alkaline oxidizing fusion changes all vanadium compounds to vanadium(V) ( 3 ) . Interfering colored organic materials present in the unknown should also be destroyed during these oxidation processes. A spectrophotometric determination of vanadium with sulfuric acid would be better at 450 and 480 mp than a t 510 or 540 mp (Table I). At longer wavelengths, the optimum concentration ranges and the absorptivities were smaller. Also, an increase in the amount of concentrated sulfuric acid increased the sensitivity but reduced the suitable concentration range. Table I indicates that by mixing equal volumes of concentrated sulfuric acid and a vanadium(V) unknown, 80 to 400 p.p.m. of vanadium in the original sample can be determined accurately
The author thanks Marvin S. Davis for drawing the graphs. LITERATURE CITED
(1) Apes, G . H., ANAL. C ~ M21, . 652
(1949). (2) Charlot, G., BCzier, D., “Quantitative Inorganic Analysis;” p. 623 Wiley, New York 1957. (3) Dum. H. E.. “dncvclooedia of Chemical T&hnoldgy,” b01.- 14, p. 595, R. E. Kirk, D. F. Othmer, eds., Interscience Encyclopedia, New York, 1955. (4) Gregory, A. W., Chem. News 100, 221. (5) Hillebrand, W. F., Lundell, G. E. F., “Applied Inorganic Analysis,” 2nd ed., p. 452-60, Wiley, New York, 1953. (6rKelle G. L., Wiley, J. A., Bohn, R. T., %right, W. C., Ind. Eng. Chem. 11, 632 (1919). (7) Meaurio, V. L., Andes SOC.Quim. Argentina 5 , 185 (1917). (8) Mittal, R. K., Mehrotra, R. C., 2. Anal. Chem. 196, 92 (1963). (9) Pearse, G. A., ANAL. CHEW. 34, ’
.
