Solvent Extraction Studies of Chromium(ll1) with Tri-n-octylamine B. E. McClellan,' M. K. Meredith,* Ray Parmelee, and J. P. Beck3 Department of Chemistry, Murray State University, Murray, Ky. 42071
Chromium(II1) has been reported to be non-extractable as a n ion association complex with high molecular weight amines ( I ) . This is probably due to the slowness with which the coordinated water molecules are replaced by negative ions. Stability constant data show that chromium(II1) should form an anionic com,plex with thiocyanate which might be extractable as an ion association complex with high molecular weight amines ( 2 ) .This study examines the extraction characteristics of such a system and attempts to separate other transition metal ions from chromium(111). EXPERIMENTAL Apparatus. A Tracerlab Model P-20D scintillation detector connected to a Model SC-71 scaler was used to count the gammaemitting isotopes in both the organic and the aqueous phases. An Eberbach water bath shaker was used for agitation of the samples. Reagents. Chrorniurn(1U) Solution. A 30 gram/l. solution of chromium nitrate was prepared from Matheson Coleman and Bell reagent grade material. Enough chromium-51 (Nuclear Science and Engineering Corporation) in the plus three oxidation state was added to give approximately 10,OOO counts per minute per milliliter of the aqueous solution. Potassium Thiocyanate. A saturated solution of the reagent grade material (Matheson Coleman and Bell) was prepared in deionized water. Tri-n-octylarnine.A 5% solution of the liquid (Eastman Organic Chemicals) was prepared in reagent grade carbon tetrachloride. Procedure. Five milliliters of an aqueous phase consisting of chromium(II1) a t a concentration of.4 x 10-4M (20 ppm), plus enough chromium-51 to give a count of approximately 10,000 cpm/ml, and thiocyanate at a concentration of 4.75M was allowed to stand at 25 "C and 35 "C in a constant temperature water bath for periods ranging from 5 minutes to 2 hours. Following the standing period, the aqueous phase was made 0.1M in hydrochloric acid and extracted with 0.25M tri-n-octylamine in carbon tetrachloride by agitation in a 125-ml separatory funnel for 2 minutes on a mechanical shaker. The phases were allowed to separate, and were counted for gamma activity to obtain the per cent extraction of the chromium(II1). In order to study the effect of the thiocyanate concentration on the extractability of chromium(III), a chromium(II1) solution of 4 X 10-4M. spiked with chromium-51, was allowed to stand a t 50 "C for 2 hours in a water bath in the presence of various concentrations of thiocyanate ranging from 0 to 9.5M. Following the standing period, an aliquot of the aqueous phase was made 0.1M in HC1 and extracted with 0.25M tri-n-octylamine in CCll by shaking for 2 minutes. The phases were allowed to separate and were counted. A study of the effect of the tri-n-octylamine concentration on the per cent extraction of chromium(II1) was made. The chromium(II1) concentration, plus chromium-51 was again 4 x lO-4M and the thiocyanate concentration was 4.7531.1. The aqueous solutions were allowed to stand at 50 "C for 2 hours prior to making 0.1M in HCl and extracting with tri-n-octylamine in CC14 in concentrations from 0.001 to 1.OM. The phases were allowed to separate and were counted. Author to whom inquiries should be addressed. Present address, Seagram's Distillery, Louisville, Ky. 40201. Present address, Dow-Corning, Elizabethtown Plant, Elizabethtown, Ky. 42701. ( 1 ) B. E. McClellan and V . M. Benson, Anal. Chem., 36, 1985 (1964). (2) Henry Freiser and Quintus Fernando, "Ionic Equilibria in Analytical Chemistry,"John Wiley and Sons, New York, N . Y . , 1963, p 31 1.
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The effect of acidity on the extraction of chromium(II1) with tri-n-octylamine. was studied using conditions similar to the previous experiments. The hydrochloric acid concentration was varied from 0.01 to 2.OM. A study of the effect of the chromium(II1) concentration on the per cent extraction of the ion with the tri-n-octylamine system was made. The optimum extraction conditions described above were used and the chromium(II1) concentration was varied from 0.01 to 20 ppm. Studies of the extraction characteristics of other transition metal ions such as Co(II), Zn(II), Fe(III), Mn(II), Cd(II), and Hg(I1) were also made. The metal ion concentration, plus radioactive tracer, was in the range of 10-4M. The thiocyanate, trin-octylamine, and hydrogen ion concentrations were varied in order to determine the optimum conditions. The mole ratios of amine to metal were also determined for each ion. Extractions were performed at 25 "C to determine the effectiveness of the method for the separation of Co(II), Zn(II), Fe(III), and Hg(I1) fiom chromium(II1). The [CNS-] was lM, the [TOA] was O.lM, the [H+] was 0.1M, and the metal ion concentrations were all 1 X 10-4M. The shaking time was 2 minutes. Gammaemitting isotopes were used as tracers to determine the distribution of all ions.
RESULTS AND DISCUSSION A graph of per cent extraction of chromium(II1) as the thiocyanate complex with tri-n-octylamine in carbon tetrachloride us. time at 25 "C and 35 "C is shown in Figure 1. This plot reflects the slowness with which coordinated water molecules are replaced with thiocyanate anions to form the negative species such as C ~ ( H Z O ) ~ ( C N S ) ~ - . Heating greatly facilitates the removal of the coordinated water. Only about 42% extraction of chromium(II1) occurred after 60-hour standing at 25 "C as compared to the much more rapid and complete formation of the extractable species a t 35 "C. The complex. probably Cr(H20)2(CNS)4-! was found to be immediately extractable as the ion association complex with tri-n-octylamine following its slow formation. A plot of per cent extraction of chromium(II1) us. thiocyanate concentration is shown in Figure 2. At these particular conditions, about 90% extraction occurs above 5M thiocyanate. The high concentration of thiocyanate required reflects the moderate stability of anion complexes such as Cr(H20)2(CNS)4- (log k l k 2 k 3 h 4 = 1.7) (2). The equation for the distribution ratio of ion association complexes with high molecular weight amines takes the form D = K* [R3N]nrp.nif all other variables such as thiocyanate and acidity are held constant ( I ) . Therefore, log D = n log [RsN] + log K* and a plot of log D us. log [RBN]should yield a straight line with a slope of n where n is the number of amines per chromium in the complex, Figure 3 shows such a plot where the slope of the straight line is unity. This indicates a complex with the probable formula R ~ N + H C ~ ( H Z ~ ) Z ( C Nas S ) ~the extracted species. A plot of per cent extraction of chromium(II1) us. hydrochloric acid concentration (Figure 4) shows an unexpectedly high dependence on the [HCl] of the aqueous phase. A maximum of 97% extraction of chromium(II1) occurred at an [HCl] of 0.07M from a thiocyanate medium of 4.75M using 0.25M amine solution in cc14. The chro-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
IO
8ot J a c Y
0
40W
w 1.0
a
-1
w &
M
0
40
80
60 TIMElmin)
100
I20
Figure 1. Extraction of chromium(ll1) as the thiocyanate complex with tri-n-octylamine A . 35 "C, 6. 25 "C [Cr3+] = 4 X
I
0.I
[CNS-] = 4.75,[Amine] = 0.25
0
-I 0
-2.0
LOGlR3N)
Figure 3. Plot of log D YS. log [R3N] for the extraction of chromium(I I I ) with tri-n-octylamine [Cr3+) = 4 X lo-',
1
2
3
4 THIOCYANATE
5
6
7
8
3
1
0
MOLARITY
Figure 2. Extraction of chromium(1I I ) with tri-n-octylamine at various concentrations of thiocyanate [Cr3+] = 4 X
[H+]
[CNS-] = 4.75,[ H + ] = 0.1
= 0.1,[Amine] = 0.25
2o
mium(II1) solution was allowed to stand in a constant temperature water bath a t 50 "C for 1 hour prior to the 2-minUte extraction period. If the solution is allowed to stand a t room temperature following the 1-hour heating period, for any appreciable length of time prior to extraction, a decomposition of the thiocyanate complex occurs. The solution, however, may remain a t room temperature for up to 1 hour without any appreciable decomposition. After standing for 60 hours, the per cent extraction decreased from 94% to 67%. It is essential that the hydrochloric acid be added just prior to extraction with tri-n-octylamine solution, as a precipitate (probably sulfur) results on heating thiocyanate in the presence of hydrochloric acid. The acid, however, is necessary to form the amine hydrochloride from the free base. For this reason, a reflux method such as that suggested by Morrison and Freiser for the extraction of chromium(II1) with acetylacetone is ineffective (3). The per cent extraction of chromium(II1) with tri-noctylamine was independent of the chromium(II1) concentration in the range studied. In the chromium(II1) concentration range of 0.01 to 20 ppm (4 X lO-4M), the per cent extraction values varied from 95.8 to 96.1%. This is in agreement with previous findings as the per cent extraction normally does not vary with the metal ion concentra(3) George Morrison and Henry Freiser, "Solvent Extraction in Analytical Chemistry,"John Wiley and Sons, New York, N . Y . . 1957,p 201.
t 1.0
0
2 0
(HCII
Figure 4. Extraction of chromium(ll1) as the thiocyanate complex with tri-n-octylamineat various concentrations of HCI [Cr3+] = 4 X
0
[CNS-] = 4.75,[Amine] = 0.25
10
20
30 Tyn*
Figure5. Plot of log [Cr(lli)]t,o/[Cr(lll)]f [Cr3+] = 4 X 25 "C
50
40
60
(hOllSl
VS.
time
[CNS-] = 4.75,[Amine] = 0.25.[H + ] = 0.07,t =
tion as long as the ligand/metal ion ratio is high enough to supply a t least a stoichiometric amount of ligand. A plot of log [ C r + 3 ] f = ~ / [ C r + $us. ] f time is shown in Figure 5. The initial chromium(II1) concentration was 4 x
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, F E B R U A R Y 1974
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Table I. O p t i m u m Conditions f o r t h e Extraction of Selected Transition Metals Metal ion
lM'1
x x x x x 1x 1x
Cr(II1) Co(I1)
4
1 1 1 5
Zn(I1)
Fe(II1) Mn(I1) Cd(I1) HgW)
10-4 10-4
lo-'
10-4 10-4 10-4 10-4
[CNS -1
4.75M 0.90M 0.90M 1.50M 6 .OOM 3 .OOM 0.10M
10-4M, the thiocyanate concentration 4.75M, the trioctylamine concentration 0.25M, the [ H + ] 0.07M, and the temperature was 25 "C. The straight line plot obtained indicates a first-order reaction in chromium(II1). A mechanism similar to that proposed by various authors ( I , 4, 5) is postulated for the ion association system as follows:
Step 2 is the slow process, with step 3 and the distribution of the extractable complex from aqueous to organic phase being rapid. This is evidenced by the fact that the extraction of chromium(II1) complex is very rapid once it has formed in the preliminary heating and standing process. Some possible competing reactions are:
H+
+
2CNS-
+=
H(CNS),-,,,,
(4)
Good e t . al. (6) proposed a competing reaction analogous to step 6 for the extraction of iron(II1) from a chloride system with high molecular weight amines. The green color of chromium(II1) darkens to a deep violet when heated with excess thiocyanate. If the pH is held constant, Beer's law is obeyed between 0.01 and 0.20 mg/ml of chromium a t a wavelength of 568 nm. The method, however, has little to recommend it as the molar absorptivity is low (180). ( 4 ) M. L. Good and S.E. Bryan, J . Inorg. Nucl. Chem., 2 0 , 140 (1961). (5) J. Y . Ellenburg, G. W. Leddicotte, and F. L. Moore, Anal. Chem., 2 6 , 1045 (1954) (6) M. L. Good. S. E. Bryan, F. F. Holland, and G. J. Maus, Jr., J . Inorg. Nucl. Chem., 25, 1 167 (1963)
308
W'1
U.'OAl
0.25M 0.10M 0.10M 0.05M 0.30M 0.10M 0.25M
0.07M 0.15M 0.30M 0.075M 0.30M 0.075M 0.075M
Max. yo E
97 .O% 99.8% 99.9% 98.5% 70 . O % 74.6% 99 .O%
Mole ratio
1:l 2:l 2:l 1:1,2:1,3:1 .. . 1:l 2:l
Chromium(V1) as Cr042- or C r ~ 0 7 ~was - found to be immediately extractable with tri-n-octylamine without the addition of thiocyanate as the chromium is already in the anionic form. Although no detailed study of this system has been made a t this time, the extracted species would be expected to be [R3N+H]2 Cr042- or [R3N+H]2 et al. have reported a similar extraction C r 2 0 ~ ~ Fasolo -. of chromium(VI) with tribenzylamine ( 7 ) . Separation of many transition metals from chromium(II1) is possible using this extraction system by virtue of the rapid formation of the anionic thiocyanate complexes of most metals as compared to the relatively slow formation of the C r ( H 2 0 ) ~(CNS)4- complex. Table I shows the conditions determined for extraction of selected transition metal ions with TOA. Since Co(II), Zn(II), Fe(III), and Hg(I1) all show quantitative extraction a t much lower CNS- concentration than does chromium(III), this alone allows the separation of these ions from chromium(II1). Also, these ions extract very rapidly a t rcmm temperature as opposed to the very slow formation of the extractable complex of chromium(II1). Mn(1I) and Cd(I1) cannot be completely extracted with TOA. Therefore, considering the fact that the other transition metals studied were extractable from lower thiocyanate concentrations than that required for chromium(II1) and that the chromium(II1)-thiocyanate complex is quite slow in forming, it is easy to separate these other transition metals from chromium(II1). Several extractions were performed using mixtures of 1 x l O - 4 M Cr(III), Co(II), Zn(II), Fe(III), and Hg(I1) under the conditions described in the experimental section. Very little (
