Utilization of Trica pry1met hylammonium Thiocyanate as a Selective Extraction Agent in the Spectrophotometric Determination of Cobalt A. M. WILSON’ and 0. K. McFARLAND Department of Chemistry, Wayne State University, Detroit 2, Mich.
b Tricaprylmethylammonium thiocyanate dissolved in benzene i s a strong liquid ion exchanger which completely extracts the blue cobalt(1l) thiocyairate complex in one equilibration. The buffered extraction solution, pH 8.0, contains sodium citrate, which completely masks the ferric ion interference. An unbuffered scrub solution containing sodium thiocyanate, sodium citrate, and sodium thiosulfate completely removes the cupric and nickelous ion interferences in one equilibration. The described procedure allows cobalt to be determined in the presence of ferric, nickelous, and cupric ions at mole ratios 1000, 480, and 250 times that of cobalt, respectively. Determinations of cobalt in some National Bureau of Standards alloys are presented.
T
HE DETERMINATION of cobalt(I1) ion as its thiocyanate complex was originally proposed by Vogel (12). Later workers (IO, 11) increased the sensitivity of the method by developing the color in lower dielectric constant media such as aqueous ethanol or aqueous acetone. Young (1.4) proposed a more selective method which utilized tartrate or phosphate ions t o mask ferric ion in iron ore samples n-hile extracting the cobalt thiocyanate into a 3 to 1 amyl alcohol-ethyl ether solvent. More recently, several authors ha.i.e described other estraction systems which have been proposed as more selective for cobalt. Strong liquid ion eschangers such as tetraphenylarsonium chloride ( I ) , triphenylmethylarsonium chloride (4, or tetraphenylphosphonium chloride (8) dissolved in water and estracted into an appropriate solvent have been described. An example of a weak liquid ion exchanger, tributylamine (26, I C ) , and an oxonium type liquid ion exchanger, polyethylene glycol (17 ) , have been cited as well. Another interesting method has used the antipyridine cobalt thiocyanate comples (7, 9). All of these extractions have been performed from acidic media and hence have required the use of salts of strong acids to 1 Present address, Department of Chemistry, Emory University, Atlanta 22, Ga.
302
ANALYTICAL CHEMISTRY
mask the ferric ion interference-e.g., fluoride ion. Because strong liquid ion exchangers are not limited in their operation to acidic media, cobalt could be determined more selectively as the thiocyanate in slightly basic solutions where citrate ion can mask the ferric interference more selectively. Previous work (13) had indicated that Hyamine 1622, of the commercially available quaternary ammonium salts investigated at that time, was the most chemically stable quaternary which gave high extractions of cobalt as anionic complexes. This quaternary is limited in its application because it is soluble in only halogenated hydrocarbons. I n preliminary work with chloroform and lJ2-dichloroethane, it was impossible to obtain a dry organic phase by simple centrifugation. Drying over a solid drying agent was impossible because the quaternary-cobalt thiocyanate comples tenaciously adsorbed on the drying agent because of the surfaceactive properties of the quaternary. Our interest shifted to another commercially available quaternary, illiquat 336, which is soluble to greater than 100 grams per 100 grams of benzene, chloroform, and kerosine. Conver3ely, Aliquat 336 is soluble in mater to less than 1 gram per 100 gramq of water: This ensures little emulsification regardless of the ionic strength of the aqueous phase. This was not true in the case of the previously investigated quaternaries which gave high cobalt eutractions. EXPERIMENTAL
Apparatus. All equilibrations were carried out in 40-ml. borosilicate glass test tubes stoppered with polyethylene caps on a Scientific Instruments Universal Rotator a t a room temperature of 23’ + 2’ C. ,411 centrifugations mere performed a t maximum speed in an International Model CL clinical centrifuge equipped with a four place head for 40-ml. test tubes. All spectra were obtained in matched quartz cells with a Cary Model 14 recording spectrophotometer. Absorbances were determined in matched Cores cells with a Beckman Xlodel DU spectrophotometer.
All p H measurements and adjustments were made with a Leeds & Northrup Model 7664 p H meter equipped with Leeds & Northrup microelectrodes. The DH meter was calibrated with a standard phosphate buffer DH 7.01. Reagents. Aliquat 336 is t h e registered trade mark of a commercial variety of tricaprylmethylammonium chloride produced by the General Mills Chemical Division. The manufacturer states t h a t the R groups, in the formula R8CH3?;Cl, are a mixture of Cg and Clo straight carbon chains with the Cs predominating. The Aliquat 336, as supplied, is a light yellow viscous liquid with a slight alcohol odor. I t s minimum assay in the active ingredient is 88% with an average molecular weight of 442 grams. Thus, a 5% by weight solution of Aliquat 336 is approximately a 0.1M solution. Because this concentration of the reagent gave quantitative extractions of cobalt and mas easily pipetted, it was used throughout the studies conducted. To convert the Aliquat 336 into its thiocyanate form, a liter of 5% Aliquat 336reagent benzene solution was equilibrated three times with equal volumes of 1 . O X aqueous sodium thiocyanate solutions in a 3-liter separatorp funnel. It was assumed that the chloride ion was completely exchanged by the thiocyanate ion, although this was not determined. The residual imbibed aqueous phase was removed from the organic phase by centrifugation in 40-ml. test tubes for 5 minutes, followed by careful decantation. The 5% Aliquat 336thiocyanate in benzene reagent \!-ill be henceforth referred to as the “extractant.” The extractant was stored in glass-stoppered brown bottles until dispensed for use. A small sample of the extractant was stored in a clear, glass-stoppered flask, unprotected from normal laboratory illumination, for a month. This solution evidenced no signs of decomposition after this period and gave a spectrum identical to that of the freshly prepared, centrifuged extractant us. reagent benzene (cf. curve 1 of Figure 1). A stock 0.0200-11 cobalt sulfate solution was made from G. F. Smith Chemical Co. “iron and nickel-free” reagent cobalt sulfate. All other chemicals were reagent grade or better and used without further purification. It was necessary to backwash the
organic phase to remove traces of copper and nickel interferences. These backwash solutions contain selective masking agents for copper and nickel, as do the aqueous phases from which cobalt is extracted. Because these backwash solutions were not always of the same composition as the aqueous extraction solution, they will be termed “scrub” solutions and the procedure referred to as “scrubbing.” The evaluation of the composition of these various extraction and scrub solutions will be presented in the results and discussion sections. Procedure. Previous studies ( I S ) had shoivn that equilibrium is reached within 5 minutes of contacting a t 60 inversions per minute on a unit similar to the Universal Rotator with liquid ion eschangers of this type. Unless noted otherwise, all contacts were carried out for 15 minutes a t 60 r.p.m. to ensure establishment of equilibrium. A 5-minute centrifugation is adequate to clarify the phases. I n all studies the volumes of the extractant and aqueous phase mere equal. In preliminary studies which evaluated the estraction conditions 5.00-ml. volumes of each phase were used. In the interference studies and in the cobalt determinations in the NBS standard alloys, the initial extraction was from 10.00 ml. of aqueous phase into exactly 10 ml. of extractant phase. il 5-in1. aliquot of the organic phase was removed and contacted with exactly 5 ml. of the scrub solution for the interval noted. DISCUSSION AND RESULTS
Effect of Sodium Thiocyanate on Cobalt Extractions. To check the completeness of the cobalt extraction, the per cent cobalt extraction was determined by extracting a cobalt-60 tracer a t a carrier cobalt sulfate concentration of 4 X 10-4M with varying sodium thiocyanate into the extractant phase. The solutions were unbuffered and the p H was experimentally found to be 8.5 =t0.5. The percentage cobalt extraction was 99.98% or greater from 0.0 to 3.0N sodium thiocyanate. It is to be expected that the extraction would be complete a t an appreciable thiocyanate ion concentration of the aqueous phase, but not a t zero thiocyanate ion concentration in the aqueous phase. This result indicates that the partition coefficient for the extractable cobalt complex is very large. If one assumes that the extractable species is the tetrathiocyanatocobaltate(I1) ion, one can propose the following mechanism for the complete scavenging of cobalt from the aqueous phase by this liquid ion exchanger: CO+’ S04-’-t 4[R&H&SC?j]o [(R~CH~N)~CO(SCN)~
1.50
W 0
1.00
z 4
m K
0 Y)
m
a
0.50
0.0c WAVE LENGTH, my
Figure 1.
Spectra of Aliquat 336 thiocyanate, cobalt, and interfering ions
.... .. ------ - .- .-
-0- 0-A - A -
5% Aliquat 336 thiocyanate-benzene, “extractant” vs. benzene 4.00 X lO-‘M Cod in extractant vc. extractant 4.0 X 1 O-5M Fe+S in extractant vc. extractant 9.6 X 1 O-SM N i * in extractant vs. extractant 9.6 X 1 0-3M’Ni+2 in extractant vc. extractant 2.0 X 10% Cu+’ in extractant VI. extractant (7.4 X 10-3)”~Cr’3 in extractant vs. extractant (7.4 X lO-3)bM Cr+3in extractant VI. extractant
-a-O-
-0-0-
See discussion for completeness of chromium extraction and extraction conditions in general. Aqueous sample digested 1 hour at 95’ C. before extraction.
aqueous phase. This result would indicate that additional thiocyanate ions in the aqueous phase are superfluous for the complete extraction of cobalt. However, since high concentrations of competitive masking agents are necessary for a practical separation of cobalt from its interferences, these studies mere not pursued and it \Tas decided always to fix the thiocyanate ion concentration a t 1 . O N to assure complete extraction of cobalt in one equilibration. Spectra of Aliquat 336 Thiocyanate, Cobalt, and Interfering Ions. The spectra of the “extractant” us. benzene and of cobaltous, ferric, cupric, chromic, and nickelous ions as their thiocyanate complex species in the extractant media us. the extractant blank are shown in Figure 1. The gradual onset of absorption of the Aliquat 336 thiocyanate a t less than 500 mp is probably a function of the quaternary ammonium ion itself or some stable, nonstrippable impurity produced by the manufacturing process, [(R~CH~N)ZSO~I~ because a 5% Aliquat 336 chloridebenzene solution run against benzene where the subscript “0” indicates the gave an almost identical curve. At species in the organic phase and the less than 350 mp, it is known that thioabsence of a subscript indicates the
+
10
+
*
cyanate ion itself starts to absorb strongly. This extractant yields a smoothly compensated reference line, when run against itself from 750 to 300 mp. .4t 300 mp, a sharp inflection upward results as the slit opens rapidly from 0.3 to 3.0 mm. All metal ions were extracted from 1. O M sodium thiocyanate solutions. Cobalt was extracted a t the natural pH of this solution. This solution also contained 0.1M perchloric acid for all the other metal ions, Before equilibration with the extractant, all of these aqueous solutions were highly colored, except that of cobalt. After extraction, only the chromic aqueous solutions were still highly colored. This indicates that the removal of all the metal ions but chromic was complete. Since chromic ion is known to form six distinct inert complexes with thiocyanate ion (@, this is not unexpected. Presumably only the anionic chromium(II1) thiocyanate species, which are still a small fraction of the total after 1 hour a t 95’ C., are extractable. The maxima of the spectra of cupric, chromic, and nickelous ions, as would be expected, have been considerably shifted to longer wavelengths and are as much VOL 35, NO. 3, MARCH 1963
303
Table 1.
Effect of p H on Cobalt Extraction
Buffer Ammonia-H~SO4 Tris-H&04 AbsorbAbsorbpH ance PH ance 7.42 .
0.693 .. . ~
.
0.696 0.697 0.693 0.484 0.251
8.00 8.63 8.90 9.50 11.50
7.00
0.678
8.05 8.40 8.60 8.80 9.10
0.675 0.652 0.570 0.356
0.Sii
as 100 times as intense as spectra observed in aqueous media (6). Even Crouthamel and Johnson (3) did not observe maxima as intense or a t wavelengths as long for these ions in 3.0 to 3.5M ammonium thiocyanate dissolved in a 60% acetone-water media. The only indication that a third maximum might appear for a nickelous thiocyanate complex is the observation of an ill-defined shoulder by Hartmann and Schllifer (6) a t 280 mp with a molar absorptivity of approximately 5 X 100. The maxima of the nickelous thio-
Table
Ion.
Ni + I
x
Cr + a
Mn +a
c1-
c10,-
MOOi-' w0,-3
SO,-'
HPOi-'
vo,-'
2.00 x 2.0 x 5.0 x 5.0 x 5.0 x 9.6 x 9.6 x 9.6 x 9.6 x 9.6 x 9.6 x 1.5 1.5 x 1 5 x 1.0 x 8.0 x 1.0 1.0 x 1.0 x 4.0 x 2 x 5 x 2.5 x
10-4 10-1 lo-*
lo-* lo-* lo-* 10-3
lo-* lo-* lo-* lo-* 10-I lo-*
lo-'
10-1 10-1
10-1 10 1 10-1 10-1
10-3 10-2
Effect of Diverse Ions
Absorbance' without scrub
Concentration, M
co+z Fe +I c u +2
II.
cyanate complex observed here a t 715, 425, and 348 mp have the following respective molar absorptivities: 4.4 X 10, 2.9 X loz, and 3.9 X los. As can be seen from their large ratios one to the other, it is hard to represent graphically all three a t one concentration level. The 348-mp peak will be used later to evaluate scrubbing conditions for complete removal of the nickel interference. The broad spectra of ferric, cupric, nickelous, and chromic (if heated for prolonged periods in the presence of thiocyanate ion) will interfere with a determination of cobalt when extracted from aqueous thiocyanate solution by this extractant, if 625 mp is used (cf. Figure 1). Because the interference due to nickelous ion would be even more serious a t the more sensitive cobalt maximum, 325 mp, cobalt was determined a t the 625-mp wavelength. From the approximate molar absorptivity of ferric, cupric, and nickelous ions at 625 mp, it was calculated that a 1.0% positive relative error in the cobalt determination would be made a t the following. respective ion-to-cobalt molar ratios, if successful masking and/or scrubbing conditions could not be
0.345 f 0.003" 0.345 -2.4 -2.4 0.365f 0.410 0.406 0.414 0.458f 0.407 0.405 0.285 0.332 0.336i 0.344 0.345 0.344 0.345 0.343 0.346 0.350 0.288 0.342
Relative error,
%
f0.9 hO.0
+600
+600 +6.0
$18.8 +17.6 +19.9 +32.8 +18.0
+17.4 -17.4 -3.8 -2.6 -0.3 10.0 -0.3 fO.0 -0.6 f0.3 $1.5 -16.5 -0.9
AbsorbanceC after ucrub
Relative error,
...
... ...
0.343 0.346. 0.353' 0.357 0.354 0.355' 0.370' 0.349 0.341* 0,312' 0,341' 0.3450
... ... ...
%
-0 6 +0.3 +2.3 $3.5 +2.6 f2.9 +7.2 hO.0 -1.2 -9.6 -1.2
10.0
*..
...
... ...
...
... ...
0.287'
-*ii. 8
.
.
I
... ,..
...
Cations added as chloride or sulfate. Anions added aa sodium salta. Absorbance of 2.00 X l o - W Co+P mearmred at 625 mp with slit width of 0.04 mm. after extraction for 15 minutes from standard extraction solution; except where noted. Absorbance of 2.0 X 1 O - W Co+l measured at 625 mp with slit width of 0.04 mm. after scrubbing for 15 minutes with the standard extraction eolution which also contained 0.90M Na&Oo; except where noted. Cobalt standard determined on 6 replicates with uncertainty expressed aa standard deviation. ' Scrubbed with unbuffered 1.5M Na&08, pH -7.5. f Extracted from standard extraction solution which also contained 0.90M N&ISIOI. 8 Scrubbed with unbuffered 0.20M NaSCN; 0.70M Naacit.; 0.90M Nr&Os, pH -6.9. * Scrubbed with unbuffered 0.20M NaSCN; 0.70 Nar cit.; pH -6.9. Absorbance after 30 minutes contact from standard extraction solution. Aqueous solution heated for 1 hour at 95' C., cooled, and extracted. b
304
ANALYTICAL CHEMISTRY
found: 3.1 X 3.1 X lO-l, and 1.2. Effect of pH and Masking Agents on Cobalt Extraction. Because the three carboxalate groups of citrate ion are completely ionized a t p H 8.0 or greater, two buffer systems were investigated; ammonia-sulfuric acid and tris(hydroxymethy1)aminomethane, hereafter called "tris,"-suIfuric acid. For comparison a statistical study was made on the absorbance of 4.00 X lO-'M cobalt extracted from an unbuffered 1.OM sodium thiocyanate solution into the standard extractant. Seven determinations gave an average absorbance of 0.686 with a standard deviation of h0.003. The results of the pH effect with the two buffer systems a t constant base concentrations of 0.10M are summarized in Table I. At low pH values, the ammonia buffer gave consistently high results while the tris buffer gave consistently low results. Within two standard deviations, the extraction of cobalt did not decrease significantly until greater than pH 8.90 and 8.40 in the case of the ammonia and tris buffers, respectively. Since the maximal buffering capacity of the ammonia and tris buffer occurs a t p H 9.25 and 8.00, respectively, tris was chosen to buffer the aqueous extraction solution a t pH 8.0 f 0.2 in all further studies. A study of the effect of the total concentration of the tris buffer from 0.02 to 0.64M resulted in no significant increase or decrease in the extraction of cobalt for this concentration range. A total concentration of 0.10121was chosen as a convenient value. The effect of two masking agents, sodium citrate and sodium thiosulfate, was studied. Using the same other extraction conditions as outlined previously, the concentration of sodium citrate was varied from 0.028 to 1.0-11 and sodium thiosulfate was varied from 0.060 t o 0.8451, both without significant effect. Because it was anticipated that a sodium thiosulfate scrub might aid in removing any residual cupric or nickelous ion from the organic phase, several studies were made on the effect of a 1.5M sodium thiosulfate scrub on the retention of cobalt. A 5-ml. aliquot of the organic phase was removed and scrubbed with an equal volume of 1.5M sodium thiosulfate for periods up to 1hour. The absorbanceoftheorganic phases which were subjected to the scrub was identical to the corresponding organic phases which were not subjected to the scrub. At this point it was decided to make up a standard stock extraction solution for ease of manipulation in succeeding studies. A stable solution of the following concentrations could be made; 1.50M sodium thiocyanate, 1.OO.M sodium citrate, and 0.15Mtris-sulfuric acid
buffer, pH 8.0. Seven milliliters of this stock solution was diluted to 10 ml. t o yield the desired concentration of its various constituents. Beer's Law Study. Varying concentrations of cobalt were extracted from the standard extraction solution into the standard extractant. Beer's law is obeyed in the cobalt concentration range from 2.00 t o 100 X 10-6M. The study contained 20 determinations in this conccntration range and yielded a molar absorptivity of 1.70 + 0.03 X The uncertainty is expressed as standard deviation. Effects of Diverse Ions on Cobalt Extractions. The effects of five cations and seven anions which might interfere with the determination of cobalt are summarized in Table 11. The cations were added as the chloride or sulfate. The anions were added as the sodium salts. The procedure followed was to add 7 ml. of the standard extraction solution, sodium thiosulfate where noted, the interfering ion, the standard aliquot of cobalt, check the pH and adjust where necessary, bring the volume to 10 ml., pipet EIexactly 10 ml. of standard extractant, and contact for 15 minutes. Ferric, manganous, chloride, molybdate, perchlorate, phosphate, sulfate, and tungstate at the levels studied did not interfere. Higher levels were not examined because these were in many cases 100 times the level to be found in the NBS alloys to be studied. Vanadate interferes seriously a t a 250 to 1 molar ratio to cobalt apparently by forming a cobalt complex, since the negative relative error is constant with contact times up t o half an hour. Since the maximum molar ratio of this element to cobalt in the standard alloys to be studied would be 0.28, and since the interference was not significant at a molar ratio of 12.5, no further studies were made. Although the error due to cupric ion is lowered a hundred-fold by the presence of 0.90M sodium thiosulfate in the original extraction solution, the error due to the nickelous ion is increased. Also the elimination of the error due to both of these ions is not complete when the organic phase is subjected t o a scrub with 1.5M sodium thiosulfate. The cupric interference, up to a molar ratio of 500, is completely removed by scrubbing with solutions containing a t least 0.90M sodium thiosulfate if sodium thiosulfate is not present during the original equilibration. The nickelous interference persisted even after scrubbing with solutions containing citrate ion in the presence of 1.OM sodium thiocyanate, and sodium thiosulfate scrubs were not effective in removing this interference. Since the nickel thiocyanate complex was prob-
ably being prevented from being scrubbed out by the high thiocyanate ion concentration in scrub solutions which also contained citrate ion, a study was made of the effect of lowering the thiocyanate ion concentration in scrub solutions containing citrate. Only 24% of the initially extracted cobalt remained after a scrub with 1.4M sodium citrate if no sodium thiocyanate was in the scrub solution. Using the characteristic spectrum of the nickel thiocyanate complex in the extractant and its molar absorptivities, it was found that the extractant contained 1.0 X 10-4X nickel when equilibrated with a 9.6 X 10-zM solution of nickelous ion in the standard extraction solution. The nickel concentration was lowered to 5 X 10-6M by scrubbing with an unbuffered 0.20~11 sodium thiocyanate, 0.7051 sodium citrate, 0.9OJ1 sodium thiosulfate scrub solution. Table I1 shows that this scrub condition completely removes the nickel interference and apparently retains the cobalt concentration intact. This scrub solution was used where necessary in the analysis of the NBS standard alloys. Chromic ion yields a serious negative interference at a molar ratio of 750 and is only partially removed by contacting for half an hour. At a molar ratio of 75, this interference can be removed, or compensated for, by contacting for half an hour. Digesting the sample for one hour a t 95' C. appears t o be beneficial. This digested sample when scrubbed with the standard scrub solution produced an error-free result. These results can best be explained by postulating a hetero-nuclear complex of chromium and cobalt with thiocyanate ion which is completely extracted but lowers the absorbance of cobalt thiocyanate a t 625 mp. With small amounts of this complex, the absorbance of the cobalt thiocyanate complex can be returned to its original value by additional extraction time or via scrubbing. This investigation was not pursued because the maximum chromium t o cobalt molar ratio to be found in the standard alloys to be studied was only 30.5. Results of Cobalt Determinations in Some NBS Alloys. To demonstrate that cobalt can be selectively determined in the presence of all these individually studied interferences, some NBS alloys were examined which contained molar ratios which approached the levels studied individually. Two sample preparation conditions were used on samples which contained greater than 8% cobalt. Samples of 90 to 250 mg. were dissolved in aqua regia and warmed until effervescence ceased. At this point one sample was directly taken to dryness and then taken up in 1.OM
Table 111. Summary of Analysis of Some National Bureau of Standards Alloys Found, Sample,O NBS No. wt. (mg.j yo CO' 41.7 0.5 168 128.15b 41.6 & 0 . 3 175.60c 20.4 f 0 . 6 1187 93.605 21.5 f 0 . 2 139.2OC 8.91 rt 0.04 153a 254.75* 8.81 f 0.07 199.95O 0.0843 115 106.55 0.0845 122.10 0,0844 123.15 0.0850 128.45 0,0854 133.95 0.0847f Average = 0 0005 0.0809 142,95d 0.0760f 218.95d 0.0742' 226.70a 0,0756f 267.OOd Sample weights rounded off t o nearest 0.05 mg. Numbers 168, 1187, and 153a dissolved in aqua regia and either: b Taken t o drvness and diluted to 100 ml. with 1.OM HCl. c Two ml. of HzSOI were added, fumed t o dryness, and diluted to lo0 ml. mth 0.9M HZSOI,and suitable aliquots taken for ana11/sis. Samales of No. 115 were weighed" directly
