Extraction of Metal Thiocyanate Complexes with Tributyl Phosphate Copper(l1) Thiocyanate L A B E N M. MELNICK' and HENRY FREISER University o f fittsburgh, Pittsburgh 73, f a .
To determine the optimum volume of tributyl phosphate needed, a series of extract'ions was carried out' in which the volume of tributyl phosphate n w varied from 5 to 30 ml. in 5-ml. increments. Tnent>--five-milliliter aliquots of the copper solut,ion were taken, the ratio of thiocyanate to copper being constant a t 25 to 1. The p H values of the samples were 3.20 =k 0.05, After extraction, copper was determined in the raffinate. T o find the affect of acidity on the extraction of copper, samples were run in the pH range 1.46 to 5.13 with the ionic strength constant. Twenty-five-milliliter aliquots of 0.0001285M copper were taken, and the p H values were adjusted by the addition of dilute sodium hydroxide. The thiocyanate to copper ratio was maintained conr-tant a t 20 to 1. Extractions were made with 25 ml. of tributyl phosphate, and the copper remaining in aqueous solution was determined. The effect of copper concentration on the ext'raction of copper was studied next. Twent,y-five-milliliter aliquots of two copper solutions, 0.001285.11 and 0.00514O.U copper, were taken. Thiocyanate was added to each aliquot so that the ratio of thiocyanat,e t o copper for each group of samplcs ranged from 5 to 25 to 1 in increments of 5 . The p H values of the solutiors were adjusted to 3.20 f 0.05 ivith dilute sodium hydroxide, the final volume of the samples being approximately 45 ml. Ext,ractions ivere made with 25 ml. of tributyl pho.3phate. Copper was then determined in the raffinate. Extractions of copper were nclst made a t four different temperatures to find the tcrnperature coefficient of extraction. Extractions were made using 25-ml. aliquots of 0.001285M copper solution with the thiocyanate to copper ratio maintained constant at 25 to 1. The pH values of the samples were 3.20 i.0.05, and the volume of tributyl phosphate for each extraction was 25 ml. The extraction temperatures, measured after the phases wcre mixed for 30 seconds, were varied from 10' to 55' C. Copper was determined in the raffinate, and the per cent copper extracted vas calculated.
The study of the extraction of metal thiocyanate complexes with tributyl phosphate has been extended to include effects of temperature, pH, thiocyanate to copper ratio, and concentration of copper on the extraction of copper. Spectral transmittance data were obtained for copper thiocyanate in tributyl phosphate. Under the proper conditions copper(I1) thiocyanate can be extracted completely from aqueous solution.
T
RIBLJTYL phosphate has previously been found to be a useful solvent for extracting iron(II1) thiocyanate (1, 6). Preliminary results indicated that copper(I1) thiocyanate could also be extracted with tributyl phosphate. Although copper(I1) is reduced by thiocyanate in basic solution ( 4 ) ,reduction in acid solution does not normally occur (9). The extraction of copper( 11) thiocyanate from acid solution n-as studied with regard to its analytical potentialities. REAGENTS AND APPARATUS
Unless otherwise stated, all reagent,? are C . P . or reagent grade. .411 p H measurements were made with a Beckman p H meter. Ammonium thiocyanate. Stock solutions were standardized with silver nitrate. Tributyl phosphate obtained from Commercial Solvents Corp., and used without further purification Stock copper solutions prepared with pure copper. Calibrated weights and buret. Beckmin DU spectrophotometer. EXPERIMENTAL PROCEDURE
RESULTS AND DISCUSSION
The extraction of copper( 11)thiocyanate from aqueous solution The smallest ratio of thiocyanate to copper to give maximum is affected by ratio of thiocyanate to copper, volume of tributyl extraction is between 23 and 24 to 1 (see Figure 1). Assuming phosphate, copper concentration, and temperature. After each that the compound extracted is [Cu(SCN)*],, the theoretical extraction, copper in the raffinate was determined colorimetrically as the diethvldithiocarbamate (5). Each raffinate was diluted optimum ratio of thiocyanate to copper should be 2 to 1. Hencp, t o 100 ml. in a volumetric flask. .4n appropriate aliquot was placed in a sec&d 100-nil. volumetric flask, and diluted t o about 50 ml. T h e following additions were made, the solution being mixed after each addition: 5 ml. of 20% citric acid, concentrated ammonia added dropwise until the solution was basic to litmu., 2 ml. of 1% gum arabic, and 10 ml. of 0.1% sodium diethyldithiocarbamate. After diluting to the mark, spectrophotometric measurement !vas made a t t h e absorption maximum of 445 mp. I n order t o find the thiocyanate to copper ratio a t which maximum extraction occurs, ammonium thiocyanate was added in varving amounts t o 25-ml. aliquots of 0.00128531 copper sulfate solution so that the mole ratio 0.001ZB5 M COPPER of thiocyanate to copper varied from 5 to 1 to 25 to 1. The pH values of the samples pH.3.201 001 were adjusted to 3.20 i 0.05 with dilute TEMP. AT SEPARATIW * 26.0'20 5.C s o d i u m h y d r o x i d e , the total volume of each solution being approximately 45 ml. Copper(I1) thiocyanate was extracted from 0.001W M WPRR each solution with 25 ml. of tributyl phosDH: 1.90 A 0.05, w s 26 phate, and the raffinate was collected as preTLMR AT ~LPUIATION ar'toi'c. viously described (6). The temperature rise on shaking the aqueous and organic phases was noted, and the p H of the raffinate was 0 5 (0 Ib 20 ?h 30 THIOCYUlATE TO CCQPEll R A W measured. Copper in the raffinate was then TOP VOLUME (MLI determined spectrophotometrically. Figure 1. Extraction of copper as Figure 2. Extraction of copper a function of ratio of thiocyanate as a function of volume of tri1 Present address, U. S. Steel Corp., Pittsburgh
i 6'
i
J
13, Pa.
to copper
462
butyl phosphate
V O L U M E 2 7 , NO. 3, M A R C H 1 9 5 5
463
L
t8 4 0
05
1.0
1.5
2.0
2.5
3.0
4.0
35
45
bo
PH
Figure 3.
Influence of pH on extraction of copper
the bracketed quantities referring to activities. Then the association number, n, would be equal to the slope of a plot of the logarithm of the tributyl phosphate copper activity versus the logarithm of the aqueous copper activity. From calculations using the above equation it was found that the association number varied from 5 to 17 depending on the thiocyanate to coppcr ratio. However, the following should be taken into consideration: Each curve consisted of only two points. Molarities were used instead of activities. The ionic strength for the
,
centration differed from the ionic strength of the samples a t the other copper concentration
m
P &Q-
i i
fl..
P
O.WIz.5 Y (yI.
COPPER PH.3.2OfOos
*.
25
"a,
0.0012M Y COPPER O.OO$Iu) Y C O P K R PH * ) . l o t 0 . 0 6 AT SEPARATION I 26.0't O.5.C.
Figure 4. Effect of copper concentration on extraction of copper O I
I
one may conclude that the effect of tributj-l phosphate extraction of copper(I1) thioc>-anate upon the formation of this complex is far less significant than it seemed to be upon the analogous iron(II1) case ( 6 ) . After the aqueous and organic phases had been mixed, the temperature was about 1 higher than before mixing. Only a 30-second mixing period was needed to establish equilibrium. Samples mixed for 3 minutes x i t h tributyl phosphate showed no improvement in separation. -41~0,20 ml. of tributyl phosphate were sufficient to obtain good extractions for the samples and conditions involved in this work (see Figure 2). There were no discernible volume changes of the phases upon equilibration. Figure 3 shows that the extraction of copper, a t constant ionic strength, is independent of pH over the range 2.58 to 5.13 Below a pH of 2.58 there is a slight decrease in extraction of copper. The pH independence of extraction is to be expected since thiocyanic acid is a strong acid, From Figure 4 it may be seen that the fraction of copper extracted a t a given thiocyanate to copper ratio increases with increasing copper concentration. This same anomaly was found on extraction of iron(II1) chloride from hydrochloric acid solution ( 2 ) . This was attributed to the formation of a tetramer ( 2 ) and also to a self-salting-out phenomenon (7). If a polymer of copper(11) thiocyanate is extracted, then the distribution of copper may be expressed by
( C U ) ~ T B=P (CUPa9
or
F u r t h e r s t u d y would therefore be necessary to determine whether the dependence of extraction of copper on copper concentration is due to the ex-
log (Cu), TBP = log K
+ n log (Cu),,
\
32n5
.
10'6p w m 1 .
10.2" C., extraction again was complete. The tem-
cyanate in tributyl phosphate (see Figure 6) showed this compound to have an absorption maximum at 382.5 mp and a molecular extinction coefficient of 620. For comparison, the molecular extinction coefficients for other colored copper compleres are 61.2 for t.he copper ammonia complex ( 5 ) , 95,500 for copper dithizonat'e in carbon tetrachloride ( 8 ) . and 9100 for copper diethyldithiocarbamat'e in aqueous solution. The spectrophotometric determination of copper(I1) thiocj-anate i n tributyl phosphate Lvould therefore be a relatively insensitive method. LITERATURE CITED
(1) rlren, >I., and Freiser, H., Anal. Chim. A d a , 6 , 412 (1'3521. (2) Dodson. R. W., Forney, G. J., and Swift, E. 11.. J . Ani. C h o n . Soc., 58, 2573 (1936). (3) Jewsbury, A , Analgst, 78, 3G3 (1953). (4) Latimer, W. AI., "Oxidation States of the Elements and Their Potentials in -4queous Solution," Prentice-Hall, Sew York, 1938.
(5) hlehlig, J. P., IXD. EXG.CHEX, ASAL.ED.,13, 533 (1911). (6) hIelnick, L. AI., Freiser, IT., and Beeghly, H. F., AXAL.CHEM., 25, 856 (1953). (7) Sachtrieb, N. H., and Fryxell, R. E., J . Am. Chem. Soc., 70,
3552 (1948). (8) Sandell, E. B., "Colorimetric Determination of Traces of Metals,'' 2nd ed., p. 302, Interscience, Sew York, 1950. M.S. thesis, University of Pittsburgh, 1945. (9) Spakomki, a4.,
RECEIVED for review July 16, 1954. Accepted October 25, 1954. Presented before the Pittsburgh Conference o n Analytical Chemistry and Applied Spectroscopy, March 1953. Tttken in part from the doctoral thesis of L. M . Melnick. Contribution KO.936 from t h e Department of Chemistry, University of Pittsburgh.
