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Coulometrically generated copper(II) in acetonitrile as an analytical oxidant
0039.9140/80/l IOI-0989SO2.00/0
Tulonto.Vol.27,pp. 989 lo 992 0 Pergamon PressLtd 1980. Printed m Great Britain
COULOMETRICALLY GENERATED COPPER(I1) IN ACETONITRILE AS AN ANALYTICAL OXIDANT B. KRATOCHVIL and I. M. AL-DAHER Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received 25 January 1980. Accepted 6 June 1980)
Summary--Copper(H) was generated at 100% current efficiency from solutions of copper(I) perchlorate in acetonitrile. Micromole quantities of ferrocene and several alkyl-substituted ferrocenes were determined with high precision and accuracy. Hydroquinone and several thiols, on the other hand, could not be determined because of the lack of an acceptor for the protons produced upon oxidation.
was used either as received or after purification by the method of O’Donnell et al. with one modification4 Copper(I) perchlorate was prepared by reaction of cop per metal with copper(I1) perchiorate in acetonitrile.r4 It was recrystallized once from purified acetonitrile and dried under vacuum. Analysis for copper by EDTA titration gave a purity of 99.8% CUCIO~.~CH~CN.‘~ n-Butyl-, amyl-, tert-butyl-, and di-n-butyl derivatives of ferrocene were obtained from Arapahoe Chemicals, and used as received. Ferrocene (Arapahoe Chemicals) was recrystallized twice from heptane and sublimed once.4 Tetramethylbenzidine (Eastman) was recrystallized from acetonitrile. Thiourea (Baker and Adamson) and dodecanethiol (Matheson, Coleman and Bell) were used as received. Thiophenol (Terochem Laboratories) was distilled under vacuum. Hexamethylenetetramine (J. T. Baker) was purified by sublimation under reduced presure. Solutions of approximately 0.02M copper(I) perchlorate were prepared by dissolution of CuC104.4CH3CN in either commercial or purified acetonitrile.
Copper(H) in acetonitrile is a useful reagent for the determination of oxidizable compounds that are insoluble in or react with water.‘-’ 1 Acetonitrile solu-
tions of copper(U) are fairly stable, but require periodic standardization. Ferrocene has been proposed as a primary standard for this purpose because it is soluble in acetonitrile, can be purified readily, is stable on storage, and is oxidized quantitatively to the ferricinium ion by copper(I Constant-current coulometry has several advantages over volumetric titrimetry. Chief among them are the elimination of the need for preparation, storage, and standardization of titrant solutions, and the ability to generate electrochemically exceedingly small quantities of chemically reactive species with accuracy and precision. Accordingly an investigation of the conditions necessary for the quantitative generation of copper(I1) from solutions of copper(I) in acetonitrile was undertaken, with the determination of ferrocene and several alkyl-substituted ferrocenes as sample systems. Previous work had assessed the direct titrations of these compounds with copper( Several alkyl-substituted ferrocene derivatives have been titrated by coulometric oxidation of copper(I) tetrafluoroborate in acetonitrile, with biamperometric end-point detection at an applied potential of 2&50 mV.i’ However, relative standard deviations ranging from 1 to 3% on samples of 6-20 pmole were reported, much larger than would be expected on the basis of the direct potentiometric titrations. Exploratory work on the use of electrogenerated copper(I1) in acetonitrile for the oxidation of several other compounds, including hydroquinone, tetramethylbenzidine, and selected thiols is also described briefly.
Apparatus
A constant-current coulometer, designed and built in the electronics shop of the Department of Chemistry, Univerprovided current levels of 0.5, 5, or sity of Albertai 20 mA. A Fisher Model 520 Accumet pH-meter was used to follow the potential difference between the indicating electrodes. The titration vessel was a borosilicate H-cell with a 13%ml anode compartment and a 60-ml cathode compartment (Fig. 1). The compartments were separated by an anion-exchange membrane (American Machine and Foundry, AMF A104-EC). i’ Samples were introduced as approximately I-g portions of l-1OmM solutions with a hypodermic syringe through a rubber serum cap. The quantity of sample taken for each titration was obtained by weighing the syringe to the nearest mg on a top-loading balance before and after sample introduction. The potential of the solution in the anode compartment was followed with a platinum wire indicating electrode and a reference electrode consisting of a silver wire immersed in .O.OlM silver perchlorate in acetonitrile in a glass tube with a glass junction’* at one end. The reference compartment was separated from the anode solution by a second longer tube with a fine glass frit. This tube contained O.lM lithium perchlorate in acetonitrile as a bridge solution to prevent ferrocene oxidation by silver ions leaking through the first frit. For each set of titrations about 50 ml of an acetonitrile solution 0.14M in acetic acid and 0.04M in lithium perchlorate were placed in the cathode compartment, and about 70 ml of 0.02M CuCI0,.4CH,CN in the anode
EXPERIMENTAL
Reagents Commercial acetonitrile (Matheson, Coleman and Bell) * Present address: Department of Chemistry, University of Al-Mustasiriyah, Baghdad, Iraq. TAL. 27/l ID--E
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the current was stopped. This first sample was considered as a blank. A second portion of sample solution, weighed to the nearest mg, was then injected into the same solution, deaerated, and copper generated until the same potential was obtained. The time was again recorded and the next sample portion added. Typically 7-9 samples could be titrated successively in a single portion of analyte solution before the potential change at the end-point became too drawn-out to be useful. For the potentiometric titrations an 80-m] cylindrical cell with a Teflon lid was used as a titration vessel. A platinum flag indicating electrode and a silver-O.OlM silver nitrate in acetonitrile reference electrode were used. The reference electrode was separated from the cell solution by the same bridge as described for the coulometer cell. A Metrohm potentiograph E-436 automatic recording titrator delivered t&rant by means of a 5-ml syringe burette at a rate of 0.25 ml/min. Aerial oxidation was minimized by starting each titration immediately upon dissolution of the sample, and by passing argon through the solution during the titration. Magnetic stirring was used.
Coulometer
P
Fig. I. Coulometric cell and electrodes. A, platinum gauze anode; B, platinum wire cathode; C, Ag/O.OlM AgN03 in acetonitrile, refbrence electrode; D, platinum indicating electrode; E, anion-exchange membrane; F, magnetic stirring bar; G, 0.1M lithium perchlorate in acetonitrile; H, glass junction; J, fine frit. compartment. About 0.2 ml of distilled water was added to the solution of copper(I) to prevent the oxidation potential of the copper couple from becoming so high that the ferricinium ion was oxidized further.5 The generating electrodes in both the cathode and anode compartments were spirals of platinum wire. To minimize oxidation by air of the substances being titrated, nitrogen was passed first through a wash bottle containing acetonitrile, then through the cell solution for a few minutes before the titration. During titrations the nitrogen inlet was raised so that the gas passed over the surface of the solution. Procedure
A small portion of sample solution was injected into the anode compartment and the solution deaerated. Then copper(I1) was coulometrically generated at a constant current of 5 mA and the course of the titration was followed potentiometrically. When the potential reached a predetermined value at or near the inflection point of the potential break,
RESULTS AND DISCUSSION The initial work was done with carefully purified acetonitrile. However, it was found that titrations of ferrocene in purified and in commercial acetonitrile gave results that agreed within 0.5 part per thousand (ppt), and so commercial acetronitrile was used in subsequent work. Results of titrations of ferrocene and four substituted ferrocenes with coulometrically generated cop per(II) are given in Table 1. Relative standard deviations ranged from 1 to 4 ppt. The results, which ranged from 99.9 to 100.6% (assuming the ferrocene to be 100% pure), can be considered satisfactory. For the ferrocene derivatives, which were analysed as received, the results indicated purities ranging from 97.3 to 99.9%, with n-butyl- and tert.-butylferrocene appearing to be the purest. Acetylferrocene and benzoylferrocene give too small a change in potential to be determinable with copper( confirming earlier work.’
Table 1. Results of coulometric titratiOnS of ferrocene and ferrocene derivatives with copper(I1) in acetonitrile’
Compound
Potential at end-point, mV
Ferrocene
317 319 320 320 320 320
Ferrocene
246 246 246 239 234 245 244
n-Butylferrocene Amylferrocene tert.-Butylferrocene Di-n-butylferrocene l
No. of runs
Mean purity found, %
In purified acetonitrile 99.86 7 100.06 9 100.0~ 10 100.lo 9 100.1, 9 100.06 8 In commercial acetonitrile 100.0s 7 100.02 8 100.0~ 8 99.66 9 99.1, 8 99.90 8 97.30 9
All titrations done at 5-mA current on samples of l-10 pmole.
Relative standard deviation, % 0.29 0.2, 0.2, 0.40 0.1, 0.29 0.30 0.35 0.1s 0.4* 0.31 0.31 0.36
Coulometrically generated copper(B) in acetonitrile Increasing the quantities of ferrocene determined to twice the levels used to obtain the data of Table 1 did not affect the precision or accuracy of the determinations, nor did use of a current level of 20 instead of 5 mA. Potentiometric oxidation
investigation of thiol and hydroquinone
Extension of the coulometric generation of cop per(H) to the determination of several other compounds directly titratable with copper(H) was also investigated. Among the substances studied were hydroquinone, thiourea, I-dodecanethiol, and thiophenol. All gave small, drawn-out breaks unsuitable for analytical use. Acetonitrile is a very weak Bronsted base, and in pure acetonitrile the oxidation of compounds such as hydroquinone and thiophenol is inhibited by the absence of a sufficiently strong base to accept the hydrogen ions produced.* Most Bronsted bases are also Lewis bases of some strength relative to copper( however, and the addition of compounds such as ammonia or pyridine results in undesirable complexation of copper(I1). A number of bases were surveyed in an attempt to find one that could be used as a proton scavenger in acetonitrile; of those investigated hexamethylenetetramine (HMT) showed the most promise. Addition of excess of HMT to solutions of thiophenol before titration with copper resulted in sharp potential changes of several hundred mV at the end-point, but the position of the end-point varied with the amount of HMT present, and tended to be about 2% early. To investigate the reasons for the early end-points, potentiometric titrations with copper(H) of thiophenol, I-dodecanethiol, and hydroquinone (H,Q) in the presence of varying amounts of HMT were performed. Titrations of 2:l mixtures of thiophenol and HMT in O.lM lithium perchlorate with copper(I1) showed two potential breaks. The first break had an inflection of about 80 mV, was smaller than the second break, and appeared at a copper(I1): thiophenol mole ratio of less than 0.5. The second break had a height of about 180 mV and ap peared at a copper(I1): thiophenol mole ratio of about 1 (0.998 and 0.996). A precipitate appeared before the first break was reached and dissolved before the second break started. The precipitate is thought to be the copper(I) salt of the thiol anion. The two breaks, then, are probably caused by formation of the thiol precipitate, followed by dissolution and oxidation of the precipitate according to 4RSH + 2Cu2 + + 4HMT + RSSR + ZCuSR(s) + 4H,MT+ 2CuSR + 2Cu2 + + RSSR. + 4Cu+ When the amount of HMT was increased to a level equivalent to that of thiophenol, the end-point came at a copper( thiophenol mole ratio of 1.15. When HMT was added to the copper(I1) titrant solution in
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1: 1 mole ratio, and the titration was done 2 hr after mixing, a large sharp break of about 640mV was obtained at a copper( thiophenol mole ratio of 0.942. No precipitate appeared in this titration. This is as expected on the basis of the discussion above, for there is no excess of HMT to react with RSH to allow formation of CuSR. The reaction in this case can be written as 2CuHMT2 + + 2RSH + 2Cu+ + RSSR + 2H2MT+ Repeating the last titration 24 hr after mixing of the HMT with copper(I1) gave an end-point corresponding to a copper(I1): thiophenol mole ratio of 1.16, and the potential break was not as sharp. This indicates that there is a slow reaction between copper(I1) and HMT which results in copper(I1) not being available for oxidation of the thiophenol. Solutions of copper(B) in acetonitrile are blue, but turn dark green on addition of HMT. The reaction possibly is formation of a stable copper(I1) complex with HMT. Adam and Piibil report that copper is readily extracted from aqueous solutions (well buffered with HMT) into 1M chloroform solution of phenylacetic acid.lg The chloroform extract is an intense green. There is evidence that the extractable species is a copper(IIt HMT-phenylacetate complex, formed by way of a copper(IIbHMT-sulphate precursor in the aqueous phase.20 Mel’nichenko and Gyunner2’ observed that copper(B) chloride and HMT do not react in water, but form a dark brown precipitate in methanol, with the composition SCuC12.3HMT. If a complex forms between copper(I1) and HMT in acetonitrile the stability constant is small, for potentiometric titrations of HMT with copper(I1) show no inflection at 1: 1 or 2: 1 HMT-copper(II) ratios. The only other metal-HMT complex reported is that of silver; Job22 and Pawelka23 report the stability constant for the complex in aqueous solution as 3.1 x lo3 and 3.75 x lo3 respectively. Overall, the addition of HMT to either the copper(I1) titrant or to the solution being titrated does not appear useful from an analytical point of view. When l-dodecanethiol in O.lM lithium perchlorate was titrated with a 1: 1 mixture of copper(I1) and HMT, the titration curve showed a potential break of 280 mV at a copper(I1): I-dodecanethiol mole ratio of 1.41. Again the reason for a large amount of copper(I1) being required is probably reaction of a portion of the copper(I1) with HMT. Titrations with copper(I1) of mixtures of hydroquinone and HMT in a ratio of 1:1.96 in O.lM lithium perchlorate showed a potential break of 400 mV at a copper(I1): hydroquinone mole ratio of 1.997. When the amount of HMT was increased to a level equivalent to exactly double the amount of H2Q the stoichiometry and size of break did not change. When the H2Q:HMT ratio was changed to 1:1.06 and 1:1.60 the end-points came at copper:H2Q mole ratios of 1.15 and 1.68 respectively, while the potential breaks became larger and sharper. Increasing the amount of
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B.
KRATOCHWL
HMT to give an H*Q:HMT ratio of 1:2.04 or even 1:4.1 resulted in a positive error at the end-point of about 7% relative, although the quality of the endpoint and the precision of replicate runs was excellent. In summary, it has been shown that copper(H) can be produced with 100% current efficiency from solutions of copper(I) in acetonitrile, and that micromole quantities of ferrocene and some alkyl-substituted ferrocenes can be determined with high precision and accuracy by constant-current coulometric generation of copper( The scope of the coulometric procedure for the determination of other substances appears more limited than does the direct titration. Acknowledgemenrs-Financial support by the Natural Sciences and Engineering Council of Canada and the University of Alberta is gratefully acknowledged. REFERENCES I. B. Kratochvil. Rec. Chem. Prog.. 1966, 27, 262. 2. B. Kratochvil, D. A. Zatko and R. Markuszewski, Anal. Chem., 1966, 38, 770. 3. B. Kratochvil and D. A. Zatko, ibid., 1968, 40, 422. 4. B. Kratochvil and P. F. Quirk, ibid., 1970, 42, 492.
and 1. M.
AL-DAHER
5. P. F. Quirk and B. Kratochvil, ibid., 1970, 42, 535. 6. D. A. Zatko and B. Kratochvil, ibid., 1968, 40, 2120. 7. H. C. Mruthyunjaya and A. R. V. Murthy, Indian J. Chem., 1969, 7, 403. 8. Idem, ibid., 1973, 11, 481. 9. B. C. Verma and S. Kumar, Talanra, 1977, 24, 694. 10. M. P. Sahasrabuddhey and K. K. Verma, ibid., 1976, 23, 725. 11. B. C. Verma and S. Kumar, Microchem. J., 1977, 22, 149. 12. H. L. Kies and H. Ligtenberg, 2. Anal. Chem., 1977, 287, 142. 13. J. F. O’Donnell, J. T. Ayers and C. K. Mann, Anal. Chem., 1965.37, 1161. 14. B. J. Hathaway, D. G. Holah and J. D. Postlethwaite, .I. Chem. Sot., 1961,321s. 15. J. S. Fritz, J. E. Abbink and M. A. Payne, Anal. Chem., 1961.33, 1381. 16. I. M. Al-Daher and B. Kratochvil, ibid., 1979, 51, 480. 17. B. Kratochvil and R. Long, ibid., 1970, 42, 43. 18. N. S. Moe, ibid., 1974, 46, 968. 19, J. Adam and R. Piibil. Talanta. 1972. 19. 1105. 20. F. 1. Miller, Ph. D. Thesis, Gniveriity ‘of Aberdeen, 1973. 21. L. M. Mel’nichenko and E. A. Gyunner, Zh. Neorgnn. Khim., 1966, 11, 529. 22. P. Job, Ann. Chim. (Paris), 1928, 9, 113. 23. F. Pawelka, Z. Elektrochem., 1924, 30, 1980.
Tulonto.Vol.27,pp. 989 lo 992 0 Pergamon PressLtd 1980. Printed m Great Britain
COULOMETRICALLY GENERATED COPPER(I1) IN ACETONITRILE AS AN ANALYTICAL OXIDANT B. KRATOCHVIL and I. M. AL-DAHER Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received 25 January 1980. Accepted 6 June 1980)
Summary--Copper(H) was generated at 100% current efficiency from solutions of copper(I) perchlorate in acetonitrile. Micromole quantities of ferrocene and several alkyl-substituted ferrocenes were determined with high precision and accuracy. Hydroquinone and several thiols, on the other hand, could not be determined because of the lack of an acceptor for the protons produced upon oxidation.
was used either as received or after purification by the method of O’Donnell et al. with one modification4 Copper(I) perchlorate was prepared by reaction of cop per metal with copper(I1) perchiorate in acetonitrile.r4 It was recrystallized once from purified acetonitrile and dried under vacuum. Analysis for copper by EDTA titration gave a purity of 99.8% CUCIO~.~CH~CN.‘~ n-Butyl-, amyl-, tert-butyl-, and di-n-butyl derivatives of ferrocene were obtained from Arapahoe Chemicals, and used as received. Ferrocene (Arapahoe Chemicals) was recrystallized twice from heptane and sublimed once.4 Tetramethylbenzidine (Eastman) was recrystallized from acetonitrile. Thiourea (Baker and Adamson) and dodecanethiol (Matheson, Coleman and Bell) were used as received. Thiophenol (Terochem Laboratories) was distilled under vacuum. Hexamethylenetetramine (J. T. Baker) was purified by sublimation under reduced presure. Solutions of approximately 0.02M copper(I) perchlorate were prepared by dissolution of CuC104.4CH3CN in either commercial or purified acetonitrile.
Copper(H) in acetonitrile is a useful reagent for the determination of oxidizable compounds that are insoluble in or react with water.‘-’ 1 Acetonitrile solu-
tions of copper(U) are fairly stable, but require periodic standardization. Ferrocene has been proposed as a primary standard for this purpose because it is soluble in acetonitrile, can be purified readily, is stable on storage, and is oxidized quantitatively to the ferricinium ion by copper(I Constant-current coulometry has several advantages over volumetric titrimetry. Chief among them are the elimination of the need for preparation, storage, and standardization of titrant solutions, and the ability to generate electrochemically exceedingly small quantities of chemically reactive species with accuracy and precision. Accordingly an investigation of the conditions necessary for the quantitative generation of copper(I1) from solutions of copper(I) in acetonitrile was undertaken, with the determination of ferrocene and several alkyl-substituted ferrocenes as sample systems. Previous work had assessed the direct titrations of these compounds with copper( Several alkyl-substituted ferrocene derivatives have been titrated by coulometric oxidation of copper(I) tetrafluoroborate in acetonitrile, with biamperometric end-point detection at an applied potential of 2&50 mV.i’ However, relative standard deviations ranging from 1 to 3% on samples of 6-20 pmole were reported, much larger than would be expected on the basis of the direct potentiometric titrations. Exploratory work on the use of electrogenerated copper(I1) in acetonitrile for the oxidation of several other compounds, including hydroquinone, tetramethylbenzidine, and selected thiols is also described briefly.
Apparatus
A constant-current coulometer, designed and built in the electronics shop of the Department of Chemistry, Univerprovided current levels of 0.5, 5, or sity of Albertai 20 mA. A Fisher Model 520 Accumet pH-meter was used to follow the potential difference between the indicating electrodes. The titration vessel was a borosilicate H-cell with a 13%ml anode compartment and a 60-ml cathode compartment (Fig. 1). The compartments were separated by an anion-exchange membrane (American Machine and Foundry, AMF A104-EC). i’ Samples were introduced as approximately I-g portions of l-1OmM solutions with a hypodermic syringe through a rubber serum cap. The quantity of sample taken for each titration was obtained by weighing the syringe to the nearest mg on a top-loading balance before and after sample introduction. The potential of the solution in the anode compartment was followed with a platinum wire indicating electrode and a reference electrode consisting of a silver wire immersed in .O.OlM silver perchlorate in acetonitrile in a glass tube with a glass junction’* at one end. The reference compartment was separated from the anode solution by a second longer tube with a fine glass frit. This tube contained O.lM lithium perchlorate in acetonitrile as a bridge solution to prevent ferrocene oxidation by silver ions leaking through the first frit. For each set of titrations about 50 ml of an acetonitrile solution 0.14M in acetic acid and 0.04M in lithium perchlorate were placed in the cathode compartment, and about 70 ml of 0.02M CuCI0,.4CH,CN in the anode
EXPERIMENTAL
Reagents Commercial acetonitrile (Matheson, Coleman and Bell) * Present address: Department of Chemistry, University of Al-Mustasiriyah, Baghdad, Iraq. TAL. 27/l ID--E
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the current was stopped. This first sample was considered as a blank. A second portion of sample solution, weighed to the nearest mg, was then injected into the same solution, deaerated, and copper generated until the same potential was obtained. The time was again recorded and the next sample portion added. Typically 7-9 samples could be titrated successively in a single portion of analyte solution before the potential change at the end-point became too drawn-out to be useful. For the potentiometric titrations an 80-m] cylindrical cell with a Teflon lid was used as a titration vessel. A platinum flag indicating electrode and a silver-O.OlM silver nitrate in acetonitrile reference electrode were used. The reference electrode was separated from the cell solution by the same bridge as described for the coulometer cell. A Metrohm potentiograph E-436 automatic recording titrator delivered t&rant by means of a 5-ml syringe burette at a rate of 0.25 ml/min. Aerial oxidation was minimized by starting each titration immediately upon dissolution of the sample, and by passing argon through the solution during the titration. Magnetic stirring was used.
Coulometer
P
Fig. I. Coulometric cell and electrodes. A, platinum gauze anode; B, platinum wire cathode; C, Ag/O.OlM AgN03 in acetonitrile, refbrence electrode; D, platinum indicating electrode; E, anion-exchange membrane; F, magnetic stirring bar; G, 0.1M lithium perchlorate in acetonitrile; H, glass junction; J, fine frit. compartment. About 0.2 ml of distilled water was added to the solution of copper(I) to prevent the oxidation potential of the copper couple from becoming so high that the ferricinium ion was oxidized further.5 The generating electrodes in both the cathode and anode compartments were spirals of platinum wire. To minimize oxidation by air of the substances being titrated, nitrogen was passed first through a wash bottle containing acetonitrile, then through the cell solution for a few minutes before the titration. During titrations the nitrogen inlet was raised so that the gas passed over the surface of the solution. Procedure
A small portion of sample solution was injected into the anode compartment and the solution deaerated. Then copper(I1) was coulometrically generated at a constant current of 5 mA and the course of the titration was followed potentiometrically. When the potential reached a predetermined value at or near the inflection point of the potential break,
RESULTS AND DISCUSSION The initial work was done with carefully purified acetonitrile. However, it was found that titrations of ferrocene in purified and in commercial acetonitrile gave results that agreed within 0.5 part per thousand (ppt), and so commercial acetronitrile was used in subsequent work. Results of titrations of ferrocene and four substituted ferrocenes with coulometrically generated cop per(II) are given in Table 1. Relative standard deviations ranged from 1 to 4 ppt. The results, which ranged from 99.9 to 100.6% (assuming the ferrocene to be 100% pure), can be considered satisfactory. For the ferrocene derivatives, which were analysed as received, the results indicated purities ranging from 97.3 to 99.9%, with n-butyl- and tert.-butylferrocene appearing to be the purest. Acetylferrocene and benzoylferrocene give too small a change in potential to be determinable with copper( confirming earlier work.’
Table 1. Results of coulometric titratiOnS of ferrocene and ferrocene derivatives with copper(I1) in acetonitrile’
Compound
Potential at end-point, mV
Ferrocene
317 319 320 320 320 320
Ferrocene
246 246 246 239 234 245 244
n-Butylferrocene Amylferrocene tert.-Butylferrocene Di-n-butylferrocene l
No. of runs
Mean purity found, %
In purified acetonitrile 99.86 7 100.06 9 100.0~ 10 100.lo 9 100.1, 9 100.06 8 In commercial acetonitrile 100.0s 7 100.02 8 100.0~ 8 99.66 9 99.1, 8 99.90 8 97.30 9
All titrations done at 5-mA current on samples of l-10 pmole.
Relative standard deviation, % 0.29 0.2, 0.2, 0.40 0.1, 0.29 0.30 0.35 0.1s 0.4* 0.31 0.31 0.36
Coulometrically generated copper(B) in acetonitrile Increasing the quantities of ferrocene determined to twice the levels used to obtain the data of Table 1 did not affect the precision or accuracy of the determinations, nor did use of a current level of 20 instead of 5 mA. Potentiometric oxidation
investigation of thiol and hydroquinone
Extension of the coulometric generation of cop per(H) to the determination of several other compounds directly titratable with copper(H) was also investigated. Among the substances studied were hydroquinone, thiourea, I-dodecanethiol, and thiophenol. All gave small, drawn-out breaks unsuitable for analytical use. Acetonitrile is a very weak Bronsted base, and in pure acetonitrile the oxidation of compounds such as hydroquinone and thiophenol is inhibited by the absence of a sufficiently strong base to accept the hydrogen ions produced.* Most Bronsted bases are also Lewis bases of some strength relative to copper( however, and the addition of compounds such as ammonia or pyridine results in undesirable complexation of copper(I1). A number of bases were surveyed in an attempt to find one that could be used as a proton scavenger in acetonitrile; of those investigated hexamethylenetetramine (HMT) showed the most promise. Addition of excess of HMT to solutions of thiophenol before titration with copper resulted in sharp potential changes of several hundred mV at the end-point, but the position of the end-point varied with the amount of HMT present, and tended to be about 2% early. To investigate the reasons for the early end-points, potentiometric titrations with copper(H) of thiophenol, I-dodecanethiol, and hydroquinone (H,Q) in the presence of varying amounts of HMT were performed. Titrations of 2:l mixtures of thiophenol and HMT in O.lM lithium perchlorate with copper(I1) showed two potential breaks. The first break had an inflection of about 80 mV, was smaller than the second break, and appeared at a copper(I1): thiophenol mole ratio of less than 0.5. The second break had a height of about 180 mV and ap peared at a copper(I1): thiophenol mole ratio of about 1 (0.998 and 0.996). A precipitate appeared before the first break was reached and dissolved before the second break started. The precipitate is thought to be the copper(I) salt of the thiol anion. The two breaks, then, are probably caused by formation of the thiol precipitate, followed by dissolution and oxidation of the precipitate according to 4RSH + 2Cu2 + + 4HMT + RSSR + ZCuSR(s) + 4H,MT+ 2CuSR + 2Cu2 + + RSSR. + 4Cu+ When the amount of HMT was increased to a level equivalent to that of thiophenol, the end-point came at a copper( thiophenol mole ratio of 1.15. When HMT was added to the copper(I1) titrant solution in
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1: 1 mole ratio, and the titration was done 2 hr after mixing, a large sharp break of about 640mV was obtained at a copper( thiophenol mole ratio of 0.942. No precipitate appeared in this titration. This is as expected on the basis of the discussion above, for there is no excess of HMT to react with RSH to allow formation of CuSR. The reaction in this case can be written as 2CuHMT2 + + 2RSH + 2Cu+ + RSSR + 2H2MT+ Repeating the last titration 24 hr after mixing of the HMT with copper(I1) gave an end-point corresponding to a copper(I1): thiophenol mole ratio of 1.16, and the potential break was not as sharp. This indicates that there is a slow reaction between copper(I1) and HMT which results in copper(I1) not being available for oxidation of the thiophenol. Solutions of copper(B) in acetonitrile are blue, but turn dark green on addition of HMT. The reaction possibly is formation of a stable copper(I1) complex with HMT. Adam and Piibil report that copper is readily extracted from aqueous solutions (well buffered with HMT) into 1M chloroform solution of phenylacetic acid.lg The chloroform extract is an intense green. There is evidence that the extractable species is a copper(IIt HMT-phenylacetate complex, formed by way of a copper(IIbHMT-sulphate precursor in the aqueous phase.20 Mel’nichenko and Gyunner2’ observed that copper(B) chloride and HMT do not react in water, but form a dark brown precipitate in methanol, with the composition SCuC12.3HMT. If a complex forms between copper(I1) and HMT in acetonitrile the stability constant is small, for potentiometric titrations of HMT with copper(I1) show no inflection at 1: 1 or 2: 1 HMT-copper(II) ratios. The only other metal-HMT complex reported is that of silver; Job22 and Pawelka23 report the stability constant for the complex in aqueous solution as 3.1 x lo3 and 3.75 x lo3 respectively. Overall, the addition of HMT to either the copper(I1) titrant or to the solution being titrated does not appear useful from an analytical point of view. When l-dodecanethiol in O.lM lithium perchlorate was titrated with a 1: 1 mixture of copper(I1) and HMT, the titration curve showed a potential break of 280 mV at a copper(I1): I-dodecanethiol mole ratio of 1.41. Again the reason for a large amount of copper(I1) being required is probably reaction of a portion of the copper(I1) with HMT. Titrations with copper(I1) of mixtures of hydroquinone and HMT in a ratio of 1:1.96 in O.lM lithium perchlorate showed a potential break of 400 mV at a copper(I1): hydroquinone mole ratio of 1.997. When the amount of HMT was increased to a level equivalent to exactly double the amount of H2Q the stoichiometry and size of break did not change. When the H2Q:HMT ratio was changed to 1:1.06 and 1:1.60 the end-points came at copper:H2Q mole ratios of 1.15 and 1.68 respectively, while the potential breaks became larger and sharper. Increasing the amount of
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B.
KRATOCHWL
HMT to give an H*Q:HMT ratio of 1:2.04 or even 1:4.1 resulted in a positive error at the end-point of about 7% relative, although the quality of the endpoint and the precision of replicate runs was excellent. In summary, it has been shown that copper(H) can be produced with 100% current efficiency from solutions of copper(I) in acetonitrile, and that micromole quantities of ferrocene and some alkyl-substituted ferrocenes can be determined with high precision and accuracy by constant-current coulometric generation of copper( The scope of the coulometric procedure for the determination of other substances appears more limited than does the direct titration. Acknowledgemenrs-Financial support by the Natural Sciences and Engineering Council of Canada and the University of Alberta is gratefully acknowledged. REFERENCES I. B. Kratochvil. Rec. Chem. Prog.. 1966, 27, 262. 2. B. Kratochvil, D. A. Zatko and R. Markuszewski, Anal. Chem., 1966, 38, 770. 3. B. Kratochvil and D. A. Zatko, ibid., 1968, 40, 422. 4. B. Kratochvil and P. F. Quirk, ibid., 1970, 42, 492.
and 1. M.
AL-DAHER
5. P. F. Quirk and B. Kratochvil, ibid., 1970, 42, 535. 6. D. A. Zatko and B. Kratochvil, ibid., 1968, 40, 2120. 7. H. C. Mruthyunjaya and A. R. V. Murthy, Indian J. Chem., 1969, 7, 403. 8. Idem, ibid., 1973, 11, 481. 9. B. C. Verma and S. Kumar, Talanra, 1977, 24, 694. 10. M. P. Sahasrabuddhey and K. K. Verma, ibid., 1976, 23, 725. 11. B. C. Verma and S. Kumar, Microchem. J., 1977, 22, 149. 12. H. L. Kies and H. Ligtenberg, 2. Anal. Chem., 1977, 287, 142. 13. J. F. O’Donnell, J. T. Ayers and C. K. Mann, Anal. Chem., 1965.37, 1161. 14. B. J. Hathaway, D. G. Holah and J. D. Postlethwaite, .I. Chem. Sot., 1961,321s. 15. J. S. Fritz, J. E. Abbink and M. A. Payne, Anal. Chem., 1961.33, 1381. 16. I. M. Al-Daher and B. Kratochvil, ibid., 1979, 51, 480. 17. B. Kratochvil and R. Long, ibid., 1970, 42, 43. 18. N. S. Moe, ibid., 1974, 46, 968. 19, J. Adam and R. Piibil. Talanta. 1972. 19. 1105. 20. F. 1. Miller, Ph. D. Thesis, Gniveriity ‘of Aberdeen, 1973. 21. L. M. Mel’nichenko and E. A. Gyunner, Zh. Neorgnn. Khim., 1966, 11, 529. 22. P. Job, Ann. Chim. (Paris), 1928, 9, 113. 23. F. Pawelka, Z. Elektrochem., 1924, 30, 1980.