Instability of Diethyldithiocatbamic Acid at low pH A. E. MARTIN Commonwealth Scientific and Industrial Research Organization, Division of Soils, Plant and Soils Laboratory, Brisbane, Australia work in this laboratory on the extraction of copper as a complex of diethyldithiocarbamate with chloroform from citrate buffers a t p H values of 1 to 6 showed irregular and low recoveries of added metal. Three separate possibilities could account for the facts: low pH inhibits complex formation between metal and ligand in the aqueous phase; preferential extraction of reagent into the solvent layer takes place before appreciable chelation can occur; or inactivation or decomposition of the reagent occurs prior to chelation. These hypotheses are not necessarily mutually exclusive, and it was recognized that combinations of possibilities could take place. The following results were obtained in an effort to explain the observed facts. ECENT
Table 11. Effect of pH on Partition of Excess Diethyldithiocarbamate between Citrate Buffers and Chloroform
EXPERIMENTAL RESULTS
Materials and Methods. A411liquid reagents, including distilled water, were purified by redistillation from borosilicate glass. Citric acid was obtained relatively free from copper by recrystallization of the A.R.-grade salt. The buffer solutions employed contained citric acid (0.048 M) and sulfuric acid (2 ml. per 100 ml.) to simulate conditions obtaining after a wet digestion procedure. The pH value of each buffer was adjusted with 10 N ammonium hydroxide using the glass electrode and checked accurately a t 26” C. immediately before extraction of the copper complex. No allowance was made for salt error in these relatively concentrated solutions. The reagent solution was obtained by dissolving 26.8 mg. of Merck’s “pro analysi” sodium diethyldithiocarbamate in water, filtering, and diluting to 100 ml. Influence of pH on Extraction of Copper-Diethyldithiocarbamate Complex. Fifty micrograms of copper were extracted from 100 ml. of citrate-sulfuric acid buffer and 2 ml. of diethyldithiocarbamate reagent-Le., 100% theoretical excess of reagentby shaking for 1 minute with 20 ml. of chloroform. A portion of the separated solvent layer was removed from each funnel, and its light absorption was measured photometrically. The copper concentration was interpolated from data obtained from a standard curve. Table I shows the results obtained.
Table I.
Table I1 summarizes the results from this series and demonstrates that some diethyldithiocarbamate was extracted along with the copper-diethyldithiocarbamate complex in the pre jious. series. The amount extracted, however, is somewhat irregular and in general is inversely proportional to the hydrogen ion concentration of the buffer. If low recoveries of copper a t high acidity are due only to an unfavorable partition of reagent between water and solvent, a reverse state of affairs would be expected. It thus appears that considerable decomposition of reagent occurs in addition to solvent extraction.
Relation Between pH and Extractability of Copper with Diethyldithiocarbamate
pH of Buffer
% Extraction
1.05 2.00 3.04 4.05 5.00 6.02
30 55 57
82 98 101
It should be noted that the metal was added prior to the addition of reagent, to ensure that decomposition of the latter (if it occurred) was minimal. Extraction of the complex increased somewhat irregularly but was clearly dependent on pH, reaching the maximum value a t p H 5 to 6. Extraction of Reagent into Solvent. Ten milliliters of each of the chloroform extracts from the previous series were pipetted into 50-ml. volumes of citrate-sulfate buffer a t pH 9.0 containing 607 of copper. Each mixture was shaken for 1 minute to allow excess reagent (which had been extracted into the chloroform in the previous series) to react with the added copper. Absorptiometric measurements were made on a portion of each solvent layer, and the excess over the previous srries \vas calculated in terms of reagent.
DiethyldithioExtracted, carbamate y
PH, of Original Buffer 1.05
3.2
Trace Trace
2.00 3.04 4.05
17.1 16.6 31.0
5.00
6.02
Table 111. Effect of Time on Stability of Diethyldithiocarbamate Original p H of Buffer 1.05
i
6,021
I
Reaction Time, Min.
Copper Recovered, %
30
0
60
61.6 0 0
0 30 60
98.0 96.4 64.0
Stability of Reagent with Time. After 4 hours, the original citrate buffers (pH 1 to 6) were adjusted to p H 9 by addition of ammonia. Shaking with 607 of copper and 10 ml. of chloroform then gave no color in the organic phase, thus indicating that no reagent was left in the aqueous phase after this period of time. The relation between extractability of copper with time was therefore investigated a t two pH values. Fifty-milliliter portions of citratesulfuric acid buffer were placed in separating funnels containing 20 ml. of chloroform, and 2 ml. of diethyldithiocarbamate solution were added to each. At standard time intervals, sufficient 10 AVammonia was added to reach pH 9.0. After addition of ammonia, 5 0 copper ~ were then added, and the complev \vas extracted into the solvent laver by shaking for 1 minute, Table I11 shows the recovery of copper. Even a t zero time the decomposition of the reagent is appreciable a t pH 1.05: a t pH 6.02 decomposition is slight at first but increases rapidly after 30 minutes. The recovery of copper at pH 1.05 in this series was twice that recorded in Table I. This indicates that although the copper complex may be formed fairly readily a t this low pH value, both metal chelate and excess reagent were rapidly decomposed during the 1-minute shaking period used in the first series, in spite of the attempt to stabilize the reagrnt by adding the metal solution first. DISCUSSION
The instability of diethyldithiocarbamate a t low pH is not of great practical importance in the routine determination of copper since the complex is normally formed in ammoniacal solution a t about p H 9.5. Drabkin (4)reports that color intensity is unaffected in the pH range 5.7 to 9.2. On the other hand Chernikhov
1260
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, Wayne 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
