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Utilization of hyamine as a selective extraction agent in the spectrophotometric determination of cobalt
330
Utilization of hyamine as a selective extraction photometric determination of cobalt
SHORT
agent
COMMUKICATIONS
in the spectro-
In recent years several long-chain aliphatic amines and quaternary ammonium compounds have been usccl to charge-neutralize the anionic cobalt-thiocyanatc complex and thus enhance the percentage of cobalt extracted into organic solvcntsr-8. During the course of a systematic study of the effect of quaternary ammonium salts on the polarographic bchaviour of various metal complexes, it was observed that Hyamine 1622 reacts with cobalt-thiocyanate to form a compound which is insoluble in water but extremely soluble and chemically stable in some organic solvents. The present investigation was undertaken to study the extraction behaviour of the complex and the possible application of hyamine as a selective extraction agent for the cobalt-thiocyanate complex ion.
Absorption curves were recorded with a Beckman DB spectrophotometer. Extinctions were cletermined in matched r.ooo-cm glass cells with a Zeiss model PMQ II spectrophotometer. All PH measurements were made with a Beckman Zeromatic pi meter. The extractions were performed in ordinary scparatory funnels and the organic phase transferred to ro-ml borosilicate glass tubes stopperecl with polyethyk!ne caps. Whenever the organic phase was not completely ckzar, it was centrifuged for 1-2 min before determination of the extinction. All measurements were performed at room temperature, 22 f 2O. Stock solutions of cobalt were prepared by dissolving the appropriate amount of cobalt nitrate in distilled water, and standardized by titration with EDTA using murexide as indicator. Solutions of thiocyanate were prepared by dissolving potassium thiocyanate (driecl at 1209 in distilled water. Hyamine 1G22 (di-isobutylphenoxyethoxyetl~yldimethylbenzylamn~oniun~ chloride monohydrate, M. W. 466.~9 ; Rohrnand Haas Co, Philadelphia, Pa., USA) is very pure and a stock solution was prepared by simply dissolving the product in water. The remaining chemicals were of reagent grade and used without purification.
On acldition of an aqueous solution of Hyamine r622 to a slightly acidic solution of cobalt-rthiocyanate, a blue precipitate is formed which is soluble in several organic solvents (benzene, nitrobenzene, cyclohexane and various halogenated hydrocarbons). The Hyamine reagent is very soluble in halogenated hydrocarbons but practically insoluble in, for instance, benzene 6. However, the cobalt-tbiocyanate-Hyamine complex appears to be most easily soluble in benzene and the complex is stable for several weeks in this solvent. Experiments showed that the complex is quantitatively transferred to a benzene phase in a single extraction whereas the solution must be treated twice with halogenated hydrocarbons in order to obtain complete extraction. The absorption curve of the blue complex in benzene, measured against a blank of pure benzene, is given in Fig. I. Maximum absorption is observed at 326 and 624 nm and a characteristic shoulder is obtained at 590 nm. The same absorption curve and maxima are also obtained when, for instance, chloroform is used as the organic phase. The effect of thiocyanate on the extraction of cobalt was studied by varying the Ann2. CWirn. Acta, 42 (1968) 330-333
SHORT COMMUNICATIONS
331
concentration of potassium thiocyanatc while that of all other components was kept constant. The absorption of the benzene phase measured at 624 nm increases with increasing concentration of thiocyanate in the aqueous phase and reaches a limiting value when the molar concentration of thocyanate is about 150 times that of cobalt. A further increase in the concentration of thiocyanate has no effect on the extinction measured. The experiments were repeated using chloroform as the organic phase. In this case the excess of thiocyanate must be 200-250 times that of cobalt in order to obtain a limiting e.xtinction.
Wovelenght
, nm
Fig. I. Absorption curves of the cobnlt-thiocyanatohynmine of cobalt: 5. IO-~ ilf (curve A) and 3. IO-.) M (curve n).
complex
in-bcnzcnc.
Concentration
Solutions containing various amounts of cobalt were analysed following the procedure given below. The extinction plotted against the cobalt concentration showed a straight line in the concentration range 0.5 to 5 -10-J M cobalt in benzene, indicating that Beer’s law is obeyed. The molar extinction coefficient of the complex is 1780. Beer’s law is obeyed also at 326 nm. The molar extinction coefficient at this maximum is 12000. As benzene gives a small absorption at this wavelength, the solutions were extracted twice with chloroform in these experiments. Because the interference from other metal ions is very serious at 326 nm, this more sensitive maximum is of little value in practical analysis and the extinction was measured only at 624 nm in the following experiments. The extraction of the cobalt complex into benzene is complete in a single extraction and independent of PH in the range 1-8. At higher PH values the amount of cobalt extracted decreases rapidly and becomes zero at PH II. A 0.x M acetate buffer of PH 4.6 appeared to be the most suitable. The effect of the hyamine concentration and the stoichiometry of the complex were investigated by the so-called limiting loading methodo. The experiments were performed by varying either the hyamine or the cobalt(I1) concentration while the concentrations of the other components were kept constant. Each solution was extracted with IO ml of benzene and the extinction measured at 624 nm. The results (Fig. 2) indicate a limiting cobalt-to-hyamine ratio of I :2 in the organic phase. At decreases, indicating high hyamine concentrations (above x0-2 M), the extinction that cobalt is not quantitatively extracted. Besides, the benzene phase becomes turbid Anal.
Claim.
Ada,
42 (x968)
330-333
SHORT
332
COMMUNICATIONS
and a clear solution is not obtained even after centrifugation for several minutes. When chloroform is used Sasthe organic phase the turbidity is observed at even lower hyamine concentrations. This effect might bc due to micclle formation of hyamine at high concentrations, but it was not further investigated. In order to avoid these effects it is important to keep the hyamine concentration below 5*10-s M.
0.40 0.30 0.20 0.15
I
0.10
.-E z 0.06 .$ & 0.04 0.03
i
“I.
-6
-4 log
r.Tw
-3
,mole
Fig. 2. Extraction isotlwrms for 0.1 AT acctatc buffcrcd mcclia cxtractctl with x0 ml of bcnzcnc. (A) z.o. lo-0 mole cobalt, 5. x0-3 molt thiocyanntcnnd variousamountsof hyaminc (x =hyaminc); (B) 5.0.10-0 limiting extinction obtained in the prcscncc of more than 4.0. xo- 0 mole byaminc.
mole hyaminc, 5. x0-3 molt thiocynnrrtcand various amounts of cobalt (.x = Cal+) ; liniitin6cxtinction obtainccl in tho prcscncc of more than ~~5.10-0 molt cobalt.
The extraction of cobalt is not affected by the presence of even large excess of anions like chloride, phosphate, sulphate and perchlorate. However, the presence of citrate or very large amounts of acetate (above 0.5 M) causes a large negative error. Cations which do not form complexes with thiocyanate are not extracted and do not interfere. Cl~romium(III) reacts slowly with thiocyanate and only a small fraction (yrobably only the anionic chromium thiocyanate species) is extracted. The nickel complex is easily extracted but this complex as well as the chromium complex shows nearly no absorption at 624 nm. Iron(II1) reacts with thiocyanate and hyamine, and the complex causes significant interference at 624 nm; however, iron(II1) can be effectively masked by adding tbiosulphate and phosphate to the aqueous solution and the extraction of iron avoided by separation of the organic phase as soon as possible after equilibration. A trace of iron-thiocyanate-hyamine complex extracted is easily backwashed by shaking the organic phase with an aqueous solution containing the same masking agents. The red copper(I1) thiocyanate is reduced in the presence of thiosulphate, and the copper(I) thiocyanate formed is not estracted but a small amount of cobalt is coprecipitated. However, if the solution is extracted twice with benzene, a complete extraction of cobalt is obtained even in the presence of a large excess of copper. On the basis of the above results the following procedure is suggested. Procedure The sample containing Anal.
Chim.
0.03-0.3 mg cobalt is transferred
Acta, 42 (1968) 330-333
to a separatory
funnel
SHORT
333
COMBf USICATIOSS
and 5 ml of a.5 M acetate buffer (pH 4.6), 5 ml of I M tbiocyanate and z ml of an aqueous IO/~::oIution of Hyamine x622 are added. The total volume of the aqueous phase shoufd he about 25 ml. Tbc solution is extracted once with 1co.00 ml of benzene and after shaking for 5--x0 min, the organic phase is transferred to the cell and the extinction measured against a blank at 624 nm. If the benzene phase is not perfectly clear, it may be centrifuged for 1-2 min before the measurement. If a large excess of iron, copper or nickel is present, 0.2 Al thiosulphate and 0.2 M phosphate should be added to the aqueous solution, and the benzene phase backwashed with an aqueous solution containing 0.5 A/I thiocyanate, 0.25 N sodium thiosulphate and 0.25 M potassium phosphate.
Cobalt is completely extracted only in the presence of a large excess of thiocyanate indicating that only the species Co(SCN)$ - reacts with byamine, As the mole ratio of cobalt to hyamine in the organic phase is I :2, the reaction is probably: Co(SCN)4"-
+2&N++(Co(SCN).@LN)n)orn
where I&N denotes Hyamine x622. Hyamine xCr22 appears to be a very suitable reagent for the extraction of the cobalt-thiocyanate comples. The commercial product is very pure and gives no absorption in the visible region. Other long-chain amines and quaternary ammonium compounds tested, like Hyamine 23S9 (Rohm and Harrs) and various Arquads (Armour Hess Chemicals Ltd., Leeds), are all slightly coloured and hard to purify. The suggested method with Hyamine 1622 and benzene as the organic ph‘ase is simpler and faster than earlier mcthodss~s and there is less interference from large amounts of chromium, copper and nickel. Following the described procedure, cobalt may be determined in the presence of chromium, nickel, copper and iron at mole ratios of 600, 600, 450 and 250, respectively. However, the tricapryln~ethylan~n~onium-thiocyanate method6 appears to be advantageous when the excess of iron escecds 250 times that of cobalt. NILS GUNJ )ERSEN EINAR JACOWSEN
I hf.Zrcctlm,Angem Charta., 68 (Ig$i)436.
Anal. Chem., 158 (rg.57) 358. AnaIysC. 84 (1959) X77. Y. NAKATSUKASA, Rnul. CJritn. Ada, 27 (rgG2) 376 5 A, M. WILSON AND 0. K. MCFARLAND, A~2al.CJmr2., 35 (1963) 302. G A. R. SELMER-OLSEN, Amd. Chir22.Acfn, 31 (rgG.}) 33. 7 H. MATSUO, S. CHAKI AND S. HARA, Ja#mr Analyst, x4 (rgG5) g35. 8 H. WATANABE hNI) Ii. ~KATSUKA, Amal. China. A&r, 38 (1967) 547. g Ii. A. ALLIN, J. Am. CJtem. sm., 80 (~958) 4133. 2
hf. %XEGI.ER,
i?.
3 -4. w. r\SHDROOK, 4 M. FUJIXOTO AND
(Received
March Gth, x968) Anal.
Cl&t. Actu, 42 (x968)
330-333
Utilization of hyamine as a selective extraction photometric determination of cobalt
SHORT
agent
COMMUKICATIONS
in the spectro-
In recent years several long-chain aliphatic amines and quaternary ammonium compounds have been usccl to charge-neutralize the anionic cobalt-thiocyanatc complex and thus enhance the percentage of cobalt extracted into organic solvcntsr-8. During the course of a systematic study of the effect of quaternary ammonium salts on the polarographic bchaviour of various metal complexes, it was observed that Hyamine 1622 reacts with cobalt-thiocyanate to form a compound which is insoluble in water but extremely soluble and chemically stable in some organic solvents. The present investigation was undertaken to study the extraction behaviour of the complex and the possible application of hyamine as a selective extraction agent for the cobalt-thiocyanate complex ion.
Absorption curves were recorded with a Beckman DB spectrophotometer. Extinctions were cletermined in matched r.ooo-cm glass cells with a Zeiss model PMQ II spectrophotometer. All PH measurements were made with a Beckman Zeromatic pi meter. The extractions were performed in ordinary scparatory funnels and the organic phase transferred to ro-ml borosilicate glass tubes stopperecl with polyethyk!ne caps. Whenever the organic phase was not completely ckzar, it was centrifuged for 1-2 min before determination of the extinction. All measurements were performed at room temperature, 22 f 2O. Stock solutions of cobalt were prepared by dissolving the appropriate amount of cobalt nitrate in distilled water, and standardized by titration with EDTA using murexide as indicator. Solutions of thiocyanate were prepared by dissolving potassium thiocyanate (driecl at 1209 in distilled water. Hyamine 1G22 (di-isobutylphenoxyethoxyetl~yldimethylbenzylamn~oniun~ chloride monohydrate, M. W. 466.~9 ; Rohrnand Haas Co, Philadelphia, Pa., USA) is very pure and a stock solution was prepared by simply dissolving the product in water. The remaining chemicals were of reagent grade and used without purification.
On acldition of an aqueous solution of Hyamine r622 to a slightly acidic solution of cobalt-rthiocyanate, a blue precipitate is formed which is soluble in several organic solvents (benzene, nitrobenzene, cyclohexane and various halogenated hydrocarbons). The Hyamine reagent is very soluble in halogenated hydrocarbons but practically insoluble in, for instance, benzene 6. However, the cobalt-tbiocyanate-Hyamine complex appears to be most easily soluble in benzene and the complex is stable for several weeks in this solvent. Experiments showed that the complex is quantitatively transferred to a benzene phase in a single extraction whereas the solution must be treated twice with halogenated hydrocarbons in order to obtain complete extraction. The absorption curve of the blue complex in benzene, measured against a blank of pure benzene, is given in Fig. I. Maximum absorption is observed at 326 and 624 nm and a characteristic shoulder is obtained at 590 nm. The same absorption curve and maxima are also obtained when, for instance, chloroform is used as the organic phase. The effect of thiocyanate on the extraction of cobalt was studied by varying the Ann2. CWirn. Acta, 42 (1968) 330-333
SHORT COMMUNICATIONS
331
concentration of potassium thiocyanatc while that of all other components was kept constant. The absorption of the benzene phase measured at 624 nm increases with increasing concentration of thiocyanate in the aqueous phase and reaches a limiting value when the molar concentration of thocyanate is about 150 times that of cobalt. A further increase in the concentration of thiocyanate has no effect on the extinction measured. The experiments were repeated using chloroform as the organic phase. In this case the excess of thiocyanate must be 200-250 times that of cobalt in order to obtain a limiting e.xtinction.
Wovelenght
, nm
Fig. I. Absorption curves of the cobnlt-thiocyanatohynmine of cobalt: 5. IO-~ ilf (curve A) and 3. IO-.) M (curve n).
complex
in-bcnzcnc.
Concentration
Solutions containing various amounts of cobalt were analysed following the procedure given below. The extinction plotted against the cobalt concentration showed a straight line in the concentration range 0.5 to 5 -10-J M cobalt in benzene, indicating that Beer’s law is obeyed. The molar extinction coefficient of the complex is 1780. Beer’s law is obeyed also at 326 nm. The molar extinction coefficient at this maximum is 12000. As benzene gives a small absorption at this wavelength, the solutions were extracted twice with chloroform in these experiments. Because the interference from other metal ions is very serious at 326 nm, this more sensitive maximum is of little value in practical analysis and the extinction was measured only at 624 nm in the following experiments. The extraction of the cobalt complex into benzene is complete in a single extraction and independent of PH in the range 1-8. At higher PH values the amount of cobalt extracted decreases rapidly and becomes zero at PH II. A 0.x M acetate buffer of PH 4.6 appeared to be the most suitable. The effect of the hyamine concentration and the stoichiometry of the complex were investigated by the so-called limiting loading methodo. The experiments were performed by varying either the hyamine or the cobalt(I1) concentration while the concentrations of the other components were kept constant. Each solution was extracted with IO ml of benzene and the extinction measured at 624 nm. The results (Fig. 2) indicate a limiting cobalt-to-hyamine ratio of I :2 in the organic phase. At decreases, indicating high hyamine concentrations (above x0-2 M), the extinction that cobalt is not quantitatively extracted. Besides, the benzene phase becomes turbid Anal.
Claim.
Ada,
42 (x968)
330-333
SHORT
332
COMMUNICATIONS
and a clear solution is not obtained even after centrifugation for several minutes. When chloroform is used Sasthe organic phase the turbidity is observed at even lower hyamine concentrations. This effect might bc due to micclle formation of hyamine at high concentrations, but it was not further investigated. In order to avoid these effects it is important to keep the hyamine concentration below 5*10-s M.
0.40 0.30 0.20 0.15
I
0.10
.-E z 0.06 .$ & 0.04 0.03
i
“I.
-6
-4 log
r.Tw
-3
,mole
Fig. 2. Extraction isotlwrms for 0.1 AT acctatc buffcrcd mcclia cxtractctl with x0 ml of bcnzcnc. (A) z.o. lo-0 mole cobalt, 5. x0-3 molt thiocyanntcnnd variousamountsof hyaminc (x =hyaminc); (B) 5.0.10-0 limiting extinction obtained in the prcscncc of more than 4.0. xo- 0 mole byaminc.
mole hyaminc, 5. x0-3 molt thiocynnrrtcand various amounts of cobalt (.x = Cal+) ; liniitin6cxtinction obtainccl in tho prcscncc of more than ~~5.10-0 molt cobalt.
The extraction of cobalt is not affected by the presence of even large excess of anions like chloride, phosphate, sulphate and perchlorate. However, the presence of citrate or very large amounts of acetate (above 0.5 M) causes a large negative error. Cations which do not form complexes with thiocyanate are not extracted and do not interfere. Cl~romium(III) reacts slowly with thiocyanate and only a small fraction (yrobably only the anionic chromium thiocyanate species) is extracted. The nickel complex is easily extracted but this complex as well as the chromium complex shows nearly no absorption at 624 nm. Iron(II1) reacts with thiocyanate and hyamine, and the complex causes significant interference at 624 nm; however, iron(II1) can be effectively masked by adding tbiosulphate and phosphate to the aqueous solution and the extraction of iron avoided by separation of the organic phase as soon as possible after equilibration. A trace of iron-thiocyanate-hyamine complex extracted is easily backwashed by shaking the organic phase with an aqueous solution containing the same masking agents. The red copper(I1) thiocyanate is reduced in the presence of thiosulphate, and the copper(I) thiocyanate formed is not estracted but a small amount of cobalt is coprecipitated. However, if the solution is extracted twice with benzene, a complete extraction of cobalt is obtained even in the presence of a large excess of copper. On the basis of the above results the following procedure is suggested. Procedure The sample containing Anal.
Chim.
0.03-0.3 mg cobalt is transferred
Acta, 42 (1968) 330-333
to a separatory
funnel
SHORT
333
COMBf USICATIOSS
and 5 ml of a.5 M acetate buffer (pH 4.6), 5 ml of I M tbiocyanate and z ml of an aqueous IO/~::oIution of Hyamine x622 are added. The total volume of the aqueous phase shoufd he about 25 ml. Tbc solution is extracted once with 1co.00 ml of benzene and after shaking for 5--x0 min, the organic phase is transferred to the cell and the extinction measured against a blank at 624 nm. If the benzene phase is not perfectly clear, it may be centrifuged for 1-2 min before the measurement. If a large excess of iron, copper or nickel is present, 0.2 Al thiosulphate and 0.2 M phosphate should be added to the aqueous solution, and the benzene phase backwashed with an aqueous solution containing 0.5 A/I thiocyanate, 0.25 N sodium thiosulphate and 0.25 M potassium phosphate.
Cobalt is completely extracted only in the presence of a large excess of thiocyanate indicating that only the species Co(SCN)$ - reacts with byamine, As the mole ratio of cobalt to hyamine in the organic phase is I :2, the reaction is probably: Co(SCN)4"-
+2&N++(Co(SCN).@LN)n)orn
where I&N denotes Hyamine x622. Hyamine xCr22 appears to be a very suitable reagent for the extraction of the cobalt-thiocyanate comples. The commercial product is very pure and gives no absorption in the visible region. Other long-chain amines and quaternary ammonium compounds tested, like Hyamine 23S9 (Rohm and Harrs) and various Arquads (Armour Hess Chemicals Ltd., Leeds), are all slightly coloured and hard to purify. The suggested method with Hyamine 1622 and benzene as the organic ph‘ase is simpler and faster than earlier mcthodss~s and there is less interference from large amounts of chromium, copper and nickel. Following the described procedure, cobalt may be determined in the presence of chromium, nickel, copper and iron at mole ratios of 600, 600, 450 and 250, respectively. However, the tricapryln~ethylan~n~onium-thiocyanate method6 appears to be advantageous when the excess of iron escecds 250 times that of cobalt. NILS GUNJ )ERSEN EINAR JACOWSEN
I hf.Zrcctlm,Angem Charta., 68 (Ig$i)436.
Anal. Chem., 158 (rg.57) 358. AnaIysC. 84 (1959) X77. Y. NAKATSUKASA, Rnul. CJritn. Ada, 27 (rgG2) 376 5 A, M. WILSON AND 0. K. MCFARLAND, A~2al.CJmr2., 35 (1963) 302. G A. R. SELMER-OLSEN, Amd. Chir22.Acfn, 31 (rgG.}) 33. 7 H. MATSUO, S. CHAKI AND S. HARA, Ja#mr Analyst, x4 (rgG5) g35. 8 H. WATANABE hNI) Ii. ~KATSUKA, Amal. China. A&r, 38 (1967) 547. g Ii. A. ALLIN, J. Am. CJtem. sm., 80 (~958) 4133. 2
hf. %XEGI.ER,
i?.
3 -4. w. r\SHDROOK, 4 M. FUJIXOTO AND
(Received
March Gth, x968) Anal.
Cl&t. Actu, 42 (x968)
330-333