1261
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3 and Dobkina ( I ) report that cadmium reacts with diethyldithiocarbamate a t p H 1.5 to 9.0; the same authors (2) list 18 elenielits extractable as diethyldithiocarbamate complexes a t pH 3, and comment that bismuth, lead, and nickel can be extracted from very acid solutions. Nevertheless, the results presented here indicate that in extractions a t low pH, considerable decomposition of diethyldithiocarbamate can occur rapidly. Extractions should therefore be performed without delay and in the presence of a large excess of reagent to offset this decomposition. Of particular moment, however, is the fact that data for pHcutrwtability curves, of the type obtained by Irving and Williams ( 6 ) , may be seriously affected by reagent decomposition, since the required excess of reagent a t equilibrium would not be maintained. Complementary data obtained by treatment of metaldiethyldithiocarbamate complexes a t low pH would he similarly affected since the equilibrium
MeDen
+ 2Hf
hIe+r
+ 2HUe
would be influenced by decomposition of the reagent released frorn the complex. Data presented by the author ( 7 ) indicates that the diethyldithiocarbamate complexes of copper and cobalt are more unstable in the presence of dilute hydrochloric acid t h a n thr corresponding complexes of (ti-p-iiaphth?-lthiocarbazone
even though the diethyldithiocarbamate complexes may in fact be more stable a t higher p H values. See also Cholak, Hubbard, and Burkey (3). The mechanism of the decomposition is not clear although diethyldithiocarbamate can be transformed into tetraethylthiuram disulfide by mild oxidation ( 6 ) . The latter compound would not chelate with metals. ACKNOWLEDGMENT
Thanks are due to C. S Piprr, who suggested the undertnking of this work. LITERATURE CITED
(1) Chernikhov, IT, A , , a n d Dohkina, R . M., Zauodska2/a Lab., 15,90G
.----,
(1949).
(2) Ibtd., p. 1143. (3) Cholak, J., Hubbard, D. iM., and Rurkey, 11. E., IND. EXG. CHEM.,ANAL.ED.,15, 759 (1943). (4) Drabkin. D. L.. J . Assoc. Offic. Agr. Chemists, 22, 320 (1932). 15) Eaton, J. L. (to Sharples Chemicals, Inc.), U. 8.Patent 2,464,i99 (March 22, 1949). (6) Irving, H., and Wiliiains, R. J. P., J . Chem. Soc., 1949, 1511. (7) Nartin, A. E., unpublished data. RECEIVED for review March 13, 19.53. Accepted May 7, 1 9 3 .
Ultraviolet Absorption Spectrum of Molybdenum Thiocyanate Complex G. E. MARICLE
4YD
D. F. BOLTZ, W a y n e University, Detroit, Mich.
r THE colorimetric determination of molybdenum utilizing a
1 molybdenum thiocyanate complex has been extensively in-
vestigated (9-5,7-II, IS, 16). The spectrophotometric methods based on this complex have been carried out in the visible region a t approximately 460 mp (9, 12, 14). In a systematic study of the ultraviolet absorption spectra of inorganic complex ions, the authors have investigated this thiocyanate complex and found a characteristic ultraviolet absorbancy mamnum suitable for the determination of small amounts of molybdenum.
certain variables on the maximum absorbancy in the ultraviolet region.
A selected volume of a standard molybdate solution was mixed with 2 ml. of 1 to 1 sulfuric acid, 1 ml. of hydrazine sulfate solution, and 10 ml. of distilled water. This mixture was heated in a water bath for 15 minutes at 90" to 95" C. and then cooled to room temperature. Twenty milliliters of the potassium thiocyanate reagent were added to the cooled solution. Exactly 5 minutes after addition
.
APPARATUS A Y D SOLUTIOYS
The absorbancy measurements were made with a Beckman hlodel DU spectrophotometer equipped with an ultraviolet accessory set and 1.000-cm. silica cells. The reference cells contained a reagent blank solution of isobutyl alcohol saturated with ammonium thiocyanate. A standard molybdate solution was prepared by dissolving 0.504 gram of pure sodium molybdate in 1 liter of redistilled water containing 5 ml. of sulfuric acid. One milliliter of this solution contained 0.2 mg. of molybdenum A potassium thiocyanate solution was prepared by dissolving 50 grams of potassium thiocyanate i n redistilled water and diluting to 1 liter. -4 1% hydrazine sulfate solution was piepaled by dissolving 1 gram of hydrazine sulfate in waim redistilled water. .4ftei cooling, the solution was diluted to 100 ml. The isobutyl-ammonium thiocyanate solution used as the estractant for the molybdenum thiocyanate complex was prepared by adding 2 grams of ammonium thiocyanate to 50 ml. of freshly distilled isobutyl alcohol. The solution wab stiired thoroughly and after standing the supernatant solution was used for the extraction. n-Butyl alcohol van be used instead of isobutyl alcohol. CHEMICAL BASIS
The treatment of an acidic solution of molybdate ions with an excess of potassium thiocyanate results in the formation of quinquivalent molybdenum thiocyanate complex, presumably lIo(SCN)s ( 1 , 6 ) . This complex has a yellow to orange-red color and possesses characteristic absorbanry maxima in the ultraviolet and visible regions. The following procedure was used in studying the effect of
I
e 3
1262
ANALYTICAL CHEMISTRY constant for any 15- to 20-minute period. It is recommended that all absorbancy measurements be made 15 minutes after the addition of the thiocyanate. Effect of Diverse Ions. The effect of certain diverse ions was studied in solutions containing 4 p.p.m. of molybdenum. One thousand parts per million of the following ions did not interfere: tungstate, phosphate, nitrate, acetate, cadmium, zinc, potassium, sodium, and magnesium. An error of less than 2.5% was considered negligible. Table I lists those ions which xere found to cause interferences. DISCUSSION
Extraction of the molybdenum thiocyanate complex rrith isobutyl alcohol is advisable for maximum development of absorbancy. The lower dielectric constant of the isobutyl alcohol undoubtedly accounts for the decrease in the dissociation of the thiocyanate complex. Hydrazine sulfate is used as a reducing agent because chlorostannous acid interferes with absorbancy measurements taken in the ultraviolet region. The interferences due t o ferric, ferrous, titanyl, and vanadate ions limit the usefulness of the ultraviolet method unless these ions are removed by preliminary treatment. Eleven solutions (4 p.p.m. of molybdenum) gave a mean absorbancy value of 0.539 with a standard deviation of 0.0033.
RIM.
of the thiocyanate the complex was extracted with 15 to 17 ml. of isobutyl alcohol. The extraction was repeated twice and the total volume adjusted to 50 ml. The absorbancy was measured a t a definite time after the addition of the thiocyanate. SOLUTION VARIABLES
*
Table I.
MOLVBDCNUM
Molybdenum Concentration. The ultraviolet absorption spectra for various concentrations of molybdenum were determined and conformity to Beer’s law was found a t 320 m r for concentrations from 0 to 7 p.p.m. of molybdenum. The optimum concentration range is 1 to 6 p.p.m. of molybdenum. A good absorbancy maximum occurs a t 320 mp with a reagent blank solution in the reference cell, as shown in Figure 1. Figure 1 also shows a comparison of the absorbancy maxima in the ultraviolet and visible regions. The ratio of the absorbancy index a t 320 m r to the absorbancy index a t 460 mp is about 1.4, indicating a greater sensitivity for the ultraviolet method (Figure 2). Acidity. The effect of the sulfuric acid concentration was determined using 4 p.p.m. of molybdenum, 1 ml. of hydrazine sulfate, and 20 ml. of potassium thiocyanate in a total volume of 50 ml. About 2 ml. of 1 to 1 sulfuric acid gives the most reproducible results. A study of other acids indicated that sulfuric acid gives the best results and that hydrochloric acid should be avoided. Thiocyanate Concentration. The effect of various concentrations of thiocyanate was determined using 4 p.p.m. of molybdenum, 1 ml. of hydrazine sulfate, and 2 ml. of 1 to 1 sulfuric acid in a total volume of 50 ml. I t was found that a slight increase in absorbancy results with an increase in concentration of thiocyanate. Twenty milliliters of 5% potassium thiocyanate per 50 ml. of solution were found to give a satisfactory concentration. Stability. The effect of allowing the solution to stand for varying lengths of time after the addition of thiocyanate and before extraction was determined. It was found that the absorbancy values were not affected by allowing the solution to stand for 5 to 20 minutes before extraction. Absorbancy readings in this case were taken 28 minutes after the addition of the thiocyanate. The stability of the absorption system was determined for a 2bhour period. It was found that the absorbancy increases gradually over this period, although the absorbancy was fairly
Interfering Diverse Ions Amount Added.
Ions
Added as sac1 cuso4 Cr(C104)a NaClO: KzCrz07 FeSO, FeNHI(SO4) z KMnOd MnSOp KzTiO(Cz0:)z NaaVOa
P.P.M. 100
10 100 I1000
50 200 1 200 100 5 5
Error,
Permissible Amount, P.P.N.
2.5 7.5 1.0 27.0 1.0
100
%
40.0
6.0 36.0 16.0 35.0 52.0
100 2 100 50 0
0 0
10 0 0
The ultraviolet spectrophotometric method using the thiocyanate method is more sensitive than the ultraviolet method using the peroxymolybdic acid complex (16). LITERATURE CITED
(1) Babko, A. K., J . Gen. Chem. (C.S.S.R.), 17, 642 (1947). (2) Barshad, I., ANAL.C H E M . , 21, 1148 (1949). (3) Cunningham, T. R., and Homner, H. L., IND.EXG.CmM., ANAL.ELI.,3, 106 (1931). 14) Ellis. R.. and Olsen. R. V.. AXAL.CHEM..22. 328 (1950). (5) Grimaldi, F. S.,and Wells, H. C , IND.ENG.CHEM., AI~AL. ED., 15, 315 (1943). (6) Hiskey, C. F., and AIeloche, V. W’ , J . Am. Chem. Soc., 62, 1565, 1819 (1940). (7) Hoffman, J. I., and Lundell, G. E. F., J . Research -Vatl. Bur. Standards, 23, 497 (1939). ( 8 ) Hurd, L. C., and Allen, H. O., IND.ENG. CHEM.,ANAL.ED., 7, 396 (1935). (9) Kapron, M.,and Hehman, P. L.. I b i d . , 17, 572 (1945). (10) Nichols, AI. L., and Rogers, L. H., Ibid., 16, 137 (1944). (11) Parks, R. Q., Hood, S. L., Hurwitz, C., and Ellis, G. H., I b i d . , 15, 532 (1943). (12) Sandell, E . B., “Colorimetric Determination of Traces of Metals,” 2nd ed.. p. 455, New Tork, Interscience Publishers, Inc., 1950. (13) Sandell, E. B., IND.ENG.CHEM.,ANAL.ED.,8, 336 (1936). (14) Snell, E’. D., and Snell, C. T., “Colorimetric Methods of Analysis,” Vol. I I , 2 n d ed., p. 479, New York, D. Van Nostrand and Co., 1949. (15) Telep, G., and Boltz, D. F., AKAL.CHEM.,22, 1030 (1950). (16) Ward, F. N., Ibid., 23, 788 (1951). RECEIVED for review January 2 , 1953. Accepted May 15, 1953. Presented before the Division of Analytical Chemistry a t the l2lst Meeting of the AMERICAN CHEMICAL SOCIETY, Buffalo, N. Y.
