- Email: [email protected]
Potentiometric titrations of metal ions in acetonitrile with polyamine ligands
0039.9140.‘80/1101-0983x)2.00/0
ra1anra. Vol. 27. pp. 983 to 988 0 Pergamon Press Ltd 1980. Printed in Great Britain
POTENTIOMETRIC TITRATIONS OF METAL IONS IN ACETONITRILE WITH POLYAMINE LIGANDS I. M. AL-DAHER*and
B.
KRATOCHVIL
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received 25 January 1980. Accepted 6 June 1980)
Summary-Ethyienediamine,
diethylenetriamine,
and triethylenetetramine
were investigated as titrants
for metal ions in acetonitrile. Copper(B) gave the best titration plots; cobalt(II), manganese(H), nickel(II), iron(II1) and magnesium(I1) also gave results acceptable for analytical determinations. Platinum, silver-silver(I), mercury-mercury(II), carbon, and a copper ion-selective electrode were studied as indicating electrode systems; of these platinum gave the sharpest and largest inflections. The mechanism by which platinum responds as an indicating electrode in these complexation titrations is unclear.
Complexing titrants which have only nitrogen atoms as ligands, such as polyamines, are more selective in their interactions with metal ions than are reagents such as EDTA which possess both nitrogen and oxygen donor atoms. Reilley and Sheldon’ used the selectivity of triethylenetetramine for titrations of metals such as copper, zinc, mercury, and cadmium in the presence of aluminium, bismuth, lead, and the alkaline earths. They emphasized the need when selecting titrants and titration conditions to consider the competitive equilibria of metal ion hydrolysis, hydrogenion contributions from buffers or added acid, and complexing effects by buffers. The potentiometric titration of copper, cadmium and zinc with triethylenetetramine and tetraethylenepentamine with a silver indicating electrode has been investigated by Hulanicki et ~1.~ In aprotic non-aqueous solvents the competing equilibria to be considered in complex formation with metal ions are simpler than in water. This is because neither solvolysis, corresponding to the formation of metal hydroxides in water, nor protonation ot coordination sites on the ligand by protons from the solvent, can occur. The absence of these two processes means that buffers, with the attendant complications of proton contribution and auxiliary complex formation, are unnecessary. In short, there is little need to consider conditional formation constants. The major disadvantage of aprotic solvents as media for complexation titrations is the low solubility
of most metal salts owing to poor solvation of the anions. The most soluble salts are those of large, symmetrical anions of low charge-density such as perchlorate, tetrafluoroborate. and hexafluorophosphate, anions not usually encountered in systems of analytical interest. Common ions such as chloride, nitrate, or sulphate must therefore be exchanged for species such as perchlorate if adequate analytical concentrations of the metal salts are to be obtained. *,Present address: Department of Chemistry, University of Al-Mustasiriyah, Baghdad, Iraq. 983
Despite the inconvenience of this conversion step, it appears worthwhile to assess the scope of metal ion titrations with chelating ligands in aprotic media for possible analytical use. In this study acetonitrile was selected as solvent since it is a poor hydrogen-ion donor or acceptor, is readily available, has a moderately high dielectric constant, is relatively non-toxic, and has a convenient liquid range and low viscosity. As titrants, the polyamines ethylenediamine, diethylenetriamine, and triethylenetetramine were selected because of the solubility of the ligands and their metal complexes in acetonitrile, and because of the stability of some of the ligand-metal complexes.3 Potentiometric titration is preferred so that the stoichiometry and extent of the reactions can be followed. For this purpose both the mercury and silver indicating electrodes used in aqueous complexometric work were investigated. It was found that platinum also functioned as an indicating electrode for several of the titration systems. EXPERIMENTAL
Chemicals
Ethylenediamine (en), diethylenetriamine (dien), triethylenetetramine (trien) and dimethyl sulphoxide (DMSO) (J.T. Baker) were purified by distillation under reduced pressure. Acetonitrile (Matheson, Coleman, and Bell) was dried overnight over calcium hydride and then distilled. The hydrated perchlorate salts of copper( zinc(II), manganese(H), nickel(II), iron(III), cobalt(I1) and chromium(II1) (G. F. Smith Chemical Co.) were converted into the corresponding dimethyl sulphoxide solvates either by the procedure of Selbin, Bull and Holmes4 or of Cotton and Francis.5 All were converted readily and analysed as reported except for the copper(I1) salt, which, when prepared according to the published method,5 precipitated as a pale blue salt that gave an analysis corresponding to the pentasolvate rather than the reported tetrasolvate. Analysis gave C 18.5%. H 4.6%. Cu (iodometric titration) 9.7%; Cu (C10&.5DMSO requires C 18.39%. H 4.63x, Cu 9.73%. As the dimethyl sulphoxide solvates of the perchlorates of magnesium, calcium, and strontium have not been reported previously, details of their preparation are given here. Mg(CIO&.6DMSO. To a saturated solution of anhydrous magnesium perchlorate in dry acetone was added an
I. M. AL-DAHER and B. KRATOCHVIL
984
volume of dimethyl sulphoxide. Upon addition of ethanol a white precipitate formed; this was filtered off, equal
washed with ethanol, and dried overnight under vacuum at room temperature. Analysis gave C 20.7%, H 5.2%, Mg (EDTA titration) 3.50%; Mg(ClO& .6DMSO requires C 20.83x, H 5.20x, Mg 3.52%. CU(CIO,)~. 6DMS0. A slurry of 20 g of calcium carbonate in 50 ml of water was treated with 70% perchloric acid in l-ml portions until evolution of carbon dioxide ceased and a pH of I was obtained. Water was removed under vacuum until solid c&urn perchlorate appeared and little liquid remained. The material was dissolved in a minimum amount of acetone, the solution was filtered, an equal volume of dimethyl sulphoxide added, and the mixture shaken. Upon addition of diethyl ether a heavy white precipitate formed. After cooling in ice, the precipitate was filtered off, washed twice with ether, and dried overnight under vacuum at room temperature. Analysis gave C 19.7x, H 4.9x, Ca (EDTA titration) 5.6,x; Ca (ClO& .6DMSO requires C 20.36%. H 5.12x, Ca 5.66%. Sr(C101)2. 6DMSO. This material was prepared in the same way as the calcium salt except that the white precipitate obtained was dissolved in acetone and reprecipitated with ether before drying overnight under vacuum. Analysis gave C 18.5%. H 4.6%; Sr(ClO& .6DMSO requires C l9.08%, H 4.80%. Titration procedure For the complexometric titration studies. approximately 40 mg of each DMSO-solvated metal perchlorate were dissolved in 40 ml of dry acetonitrile and titrated with approximately 0.06M amine in dry acetonitrile. An 80-ml glass cell fitted with a Teflon lid containing openings for the burette tip, electrodes, and nitrogen inlet was used as a titration vessel. The indicating electrode was either mercury-mercury(II) amine, silver-silver ion, or platinum, and the reference electrode was silver-0.01 M silver nitrate in acetonitrile. For the mercury electrode one of the arrangements described by Reilley and Schmidb was employed. One drop of a IO-‘M solution of mercury(IIbn or mercury(IIttrien was added to the solution to be titrated to provide the mercury(I1) ions necessary for the electrode couple. For the silverPsilver ion indicating electrode a silver wire, along with a drop of 0.008M silver nitrate in acetonitrile added to the solution to be titrated, was used, as described by Fritz and Garralda for complexometric titrations in water.’ All titrations were done with the platinum indicating electrode, except where indicated, and on a Metrohm E436 automatic recording titrator at a titrant delivery rate of about 0.25 ml/min. The reference electrode and the solution being titrated were separated by a glass junction’ and a glass frit, with a bridge solution of O.lM lithium perchlorate in acetonitrile between them. Aerial oxidation and water absorption were minimized by starting the titration immediately upon dissolution of the samples, and by carrying out all titrations under a blanket of dry nitrogen.
gen atoms in this system. Therefore the titration system must be kept water-free if useful results are to be obtained. Titrations
with ethylenediamine
Results of titrations of a series of metal ions with en are summarized in Table I. The key features are that cobalt(H) and copper(U) show sharp inflection points at 2: I ratios of en to metal, while manganese(II), nickel(I1) and magnesium(I1) show smaller inflections at 3: I ratios (Fig. I). For copper( a set of titrations in which Cu(CIO,), . 4CH3CN was used as the sample in place of the DMSO solvate gave identical titration curves to those in which DMSO was present, showing that DMSO does not affect the shape of the curves. Calcium(II), strontium(I1). chromium(III), and silver(I) do not yield titration breaks, but zinc gives two, a small sharp break ‘at a ratio of en to zinc of about 1.85:I and a more drawn-out break at a ratio somewhat over 3 : I. Both 2 : I and 3 : I complexes of en with zinc have been reported as existing in water-ethanol mixtures,’ but at high ethanol concentrations, of the order of 80”/,, only the 3: 1 complex is seen. In acetonitrile the behaviour of iron(II1) is similar to that of zinc(II), but the break at the 3: I ratio is sharper. The largest potential change is observed with copper(I1). The 2: I stoichiometry observed here parallels the behaviour of copper(I1) with en in water;‘O~l’ this parallel behaviour is not seen with cobalt(II), which forms with en in water 1: 1, 2: 1, and 3: 1 complexes of roughly equal stability, but shows only a 2: 1 break in acetonitrile. Several titrations of mixtures were investigated to assess the potential selectivity of en as a titrant; the results are summarized in Table I. I
E
RESULTS AND DISCUSSION
Titrations in acetonitrile of hydrated metal salts of the form M(ClO&. xHzO with the polyamine ligands did not give useful end-points, apparently because of formation of metal hydroxides and protonation of the polyamines by water. Ethylenediamine and the other polyamines are quite basic in acetonitrile and can be titrated quantitatively with perchloric acid in dioxan. For example, large potential breaks are observed with a glass electrode at 1: I and 2: I ratios of H+ to en, indicating the appreciable base strength of both nitro-
0
1
1
I
1
2
3
En I Metal Ratio Fig. 1. Relative magnitude of breaks at the inflection points for the potentiometric titration of several metal ions with ethylenediamine in acetonitrile.
Titrations in acetonitrile with polyamine ligands Titrations with dien and trien
2 summarizes the results of titrations of a series of metal ions with dien, and Table 3 with trien. With dien copper(H) shows breaks at ratios of both 1: I and 2: 1, while with trien only a 1: 1 break is seen. In both cases the curves are sharp and stiochiometric. The only other ions of those studied that react stoichiometrically with dien are magnesium(II), cobalt(II), and manganese(II), all of which produce small but well-defined inflections at 2:l ratios (Fig. 2). No metals other than copper gave stoichiometric breaks with trien. The lack of stoichiometry can be attributed in the case of trien to difficulty in selecting reproducible end-points from the drawn-out, asymmetric titration plots, but for dien the non-stoichiometry must be attributed to some other cause since the inflections are generally sharp. The mechanism by which platinum functions as an indicating electrode in these systems has not been established. Measurements of the potentials of a series of solutions of varied copper(K) concentrations in acetonitrile with a platinum indicating electrode and a silver-silver nitrate reference electrode pair were not reproducible, nor were similar measurements on solutions with varied concentrations of trien. Pretreatment of the platinum with nitric acid or acidic iron(I1) sulphate had no effect on its response. In one titration of copper with dien the platinum was replaced by a graphite rod pretreated with molten wax to decrease its porosity;” two sharp breaks of 120 and 300 mV were obtained at stoichiometries of 1: 1 and 2: 1.
985 I
I
1
Table
E
0
1 5iw1 /Metal
2
Ratio
Fig. 2. Relative magnitude of breaks at the inflection point for the potentiometric titration of several metal ions with diethylenetriamine in acetonitrile. However, no useful break of the kind observed with platinum was seen when the graphite electrode was used for the titration of magnesium with dien. A solid-state copper ion-selective electrode (Orion Model 94-29) was also tested. Titrations of copper(I1) with en gave a single inflection of about 450 mV at
Table 1. Summary of titrations of M(ClO,), DMSO salts with en in acetonitrile Species titrated
en:metal ratio at inflection point
Approximate potential change at inflection point, m V
CU*+
2.002, 2.007, 2.00,. 2.000 1.999. 2.00,. 1.999, 2.00, 1.83, 1.86, 1.86 3.00. 3.28. 3. IS 2.999, 3.00,. 3.OO,, . 1.8, 1.8, 1.8. 1.8 2.99,. 3.OOt, 3.OO,, 3.00, -2.5, 2.5, 2.5 3.009, 3.004, 3.000 2.99s. 3.00,. 3.002 2.O33,2.OO,, 1.99,, 2.00, 0.500 l.OOo 2.OO,, 2.006, 2.00,
960 very sharp 240 sharp 80 sharp 140 drawn-out I20 fairly sharp slight inflection 120 fair slight inflection 90 sharp 80 fair to poor 950 very sharp 160 sharp 200 sharp 6OOvery sharp (only Cu2+ titrated)
Cu’+, AgNO, Fe3+. Cr3+
end-point 3-7’4 early for total 2.014 3.00,. 3.003, 2.99s 2.99,. 2.99s
600 sharp 120 fair, ppte (only Cu* + titrated)
Mn2+ Cr3+
3.061,3.065,3.05,
Ni2+,‘Cr3+ Mg’+, Cal’ Sr*+ Mn*‘, CaZ+’
3.0S6 2.83s no inflection no inflection
co*+ ZnZ+ Mn*+ Fe3+ Ni2+ Mg2+ CUz+O Hfb CU’+ 9Ca2+ 1
sr2+, era+
Cu2+, ZnZ+
Tu*+ added as Cu(CIO,), .6CH&N bH+ added as HC104.2H20 (70% aqueous acid).
130 fair (only Fe3+ titrated) 130 drawn-out (only Mn2+ titrated) 190 fairly sharp (only Ni2+ titrated)
986
I.
M. AL-DAHERand B. KRATCICHVIL
Table 2. Summary of titrations of M(ClO.,). . xDMS0 salts with dien in acetonitrile, with platinum as indicating electrode Species titrated
dien :metal ratio at inflection point.
Approximate potential change at inflection point, mV
CU2’
[email protected],, 1.00, 2.00,,. 1.98,, 2.006 2.00,. 2.039. 2.01, 2.12; 1.69. 1.68. 1.68 2.02, 2.03, 2.04 2.00~, 2.00,, 2.000 1.998 1.98, 2.00,. 2.01 I .99,, 2.04, 1.98 2.16, 2.13, 2.14 2.01, for total
330 very sharp 630 very sharp 360 very sharp
Zn” Fe’+ Nil+ Mg” Mn’+ co2+ Ca2+ Ca”. Mg2 +
300 very sharp 220 very sharp 200 sharp 140 very sharp 100 sharp 60 poor, drawn-out 180 fairly sharp (both titrated)
Table 3. Summary of titrations of M(C104),.xDMS0
salts with trien in acetonitrile
Species titrated
trien:metal ratio at inflection point
Approximate potential change at inflection point, mV
cu2+ Zn2+ co2 + Fe3+
1.008, l.OO,, l.OOo 1.07, 1.13, 1.08 1.08.1.07 1.23, 1.44, 1.39 1.66, 1.88. 1.84 1.09. 1.09, 1.09 1.05, 1.07, 1.07
840 very sharp 400 fair break 140 fair break 120 poor break 100 poor, drawn-out 120 poor, drawn-out 6&80 fairly sharp s 100 too drawn-out for use -90 too drawn-out for use
Ni’+ Mn2+ Ca2+ Mg2+
Table 4. Summary of titrations of M(CIO& .xDMSO salts with en in acetonitrile, with Hg/en indicating electrode Species titrated
en:metal ratio at inflection point
Approximate potential change at inflection point, ml/
cu2+ co2 + Mn2+ Ni” Mg2+ Ca2+
1.99*, 1.99,. 1.990, 2.000 2.003. 1.999, 2.00, -3 3.002, 3.00, no inflection no inflection
36&560, very sharp 280 very sharp drawn-out, not useful 130, good drawn-out
Table 5. Summary of titrations of M(ClO,),..xDMSO
salts with en in acetonitrile, with Ag/Ag’ indicating electrode
Species titrated
en:metal ratio at inflection point
Approximate potential change at inflection point, ml/
cu*+
1.998. 2.000, 2.000 1.988, 2.00,, (only silver wire, no Ag’ added) 1.99,. 1.91, I .99s (only silver wire. no Ag’ added) -3 3.00,, 2.99s, 3.003 -3 no inflection
180, sharp
cu2+ co2 +
co*+ Mn2+ Ni’ l Mg” Ca2 +
200. sharp 240, sharp 220. very sharp 120, poor, hard to locate the end-point 150, sharp poor, drawn-out gradual potential change of about 60 mV throughout titration
Titrations in acetonitrile with polyamine ligands
ratios of en to copper(I1) of 2.135, 2.14a. 2.14s, and 2.145 (four consecutive titrations). Although the endpoint could be located reproducibly, it was not stoichiometric. Replacement of the silver-silver nitrate reference electrode with an aqueous saturated calomel electrode in one titration of nickel(I1) with en was also done to test whether trace amounts of silver ion were diffusing into the cell solution and affecting the potential at the end-point. However, results were unchanged from those obtained with the silver reference electrode. Also, addition of silver nitrate to the titration cell in one titration of copper(I1) with en gave a precipitate and a reduction in the size of the end-point break from 1 V to 120 mV. Results of titrations of a series of metal ions with en, either the mercury-en or silver-silver ion system being used as indicating electrode and the silver-silver ion couple as reference electrode, are summarized in Tables 4 and 5. As with the platinum indicating electrode, copper shows the sharpest and largest potential breaks. Cobalt(H) titrations also show sharp inflection points at 2: 1 ratios of en to cobalt(I1) in the case of the mercury electrode, but at less than 2: 1 for the silver indicating electrode. Similar sharp potential breaks, but at a 3: 1 ratio of ligand to metal, are seen with both electrodes for the titration of nickel(I1). With both electrodes manganese(I1) shows drawn-out potential breaks at about 3:l ratios which are not analytically useful. Calcium(I1) does not yield titration breaks with either electrode, while magnesium(H) shows a poor, drawn-out break at about a 3: 1 ratio
Table 6. Summary of titrations of M(ClO&.xDMSO
with the silver indicating electrode, but does not show an inflection with the mercury electrode. Tables 6 and 7 summarize the results of titrations of a series of ions with trien, with the mercury-trien or silver-silver ion indicating electrode and a silversilver ion reference electrode. The only system studied that gave a break at a clean, stoichiometric 1: 1 ratio was magnesium(I1) with the mercury-trien indicating electrode. All the other systems showed breaks at ratios of more than 1: 1 even though some of the inflections were fairly large or sharp. The silver-silver ion indicating electrode did not provide satisfactory end-points for any of the metalion titrations. All the systems investigated showed drawn-out, poor breaks at ratios of more than 1: 1, an exception being manganese(II), which showed fair through non-stoichiometric single inflections. Titration of aqueous solutions of manganese(I1) perchlorate or tetraphenylborate with an ethanolic solution of trien has been reported by Chiswell’3 to yield a precipitate of [Mn,trien,]X,. nH,O. A 1: 1 Mn(II)-trien salt was reported to be obtained upon mixing anhydrous manganese perchlorate and trien in equimolar amounts in ethanol and allowing the solution to crystallize over a period of weeks. These results indicate that end-points at ratios of both I:1 and greater than 1: 1 might be expected for titrations of manganese(I1) and perhaps of other metals with trien. This is seen in the manganese(IIttrien titration using a mercury-mercury(I1) trien indicating electrode; two small breaks are observed (Table 6), the
salts with trien in acetonitrile, with Hg/trien indicating electrode
Species titrated
trien : metal ratio at inflection point
Approximate potential change at inflection- point, mV
CU2+ co2+ Mn2’
1.1, 1.1, 1.1 1.4, 1.3, 1.4 l.O9a, 1.089, 1.09,. 1.09, 1.06~, 1.054 1.29, 1.29, l.lO,, 1.09,, 1.1I3 with a second break, drawn-out, at less than 2: 1 ratio l.Ooo, l.oO,, l.OOo, 1.006, 1.02, w 1.2
360 (1st) asymmetric 180 (2nd) asymmetric 240 sharp 120 (1st), good 80 (2nd), fair 90, good
NiZf Mg2 + Ca2 +
Table 7. Summary of titrations of M(ClO.,),.xDMSO
150, good drawn-out, not calculable
salts with trien in acetonitrile, with Ag/Ag+ indicating electrode
Species titrated
trien:metal ratio at inflection point
Approximate potential change at inflection point, mV
cu2 +
-1 c 1.4 1.21, w 1.1 w 1.1, 1.1, 1.1 l.lOa, 1.08,. 1.25,,, l.O& -1
200 (1st). poor 120 (2nd), drawn-out 120, poor 100, poor 800, poor 180, fair drawn-out
co2+ Mn2+ Ni2 + Mg2 + Ca2+
981
988
1.
M. AL-DAHERand B. KRAT~CHVIL
first at trien to manganese ratios of about 1.05: 1 and the second at ratios of about 1.29: 1. Only single breaks. however, are seen with the platinum and silver-silver ion indicating electrodes, at about 1.05: I and 1.14:1 ratios. In summary, a range of metal-ligand titrations can be performed in an aprotic solvent such as acetonitrile. Potentiometric titrations are possible in several cases through use of a platinum indicating electrode, which was found to be superior to the other indicating electrodes investigated, including mercury, silver, carbon, and the copper ion-selective electrode. The mechanism by which platinum functions as an indicator is not clear. In general the results, though of experimental interest, do not indicate that acetonitrile offers any analytical advantage over water as a solvent for metal-ligand titrations. Acknowledyemertts-Financial assistance by the National Research Council of Canada and the University of Alberta are gratefully acknowledged. This paper is dedicated to Professor Harvey Diehl on the occasion of his seventieth birthday.
REFERENCES C. N. Reilley and M. V. Sheldon, Talanta, 1958, 1, 127; Chemist-Analyst,
1957,
46, 59.
2. A. Hulanicki, M. Trojanowicz, and J. Domaska, Talanta, 1973, 20, I 117. Constants of 3. L. G. Silltn and A. E. Martell, Stability Meral Ion Comolexes. SDecial Publication No. 17. Chemical Society, London, 1964. 4. J. Selbin, W. E. Bull and L. H. Holmes, Jr., J. Inorg. Nucl. Chem., 1961, 16, 219. 5. F. A. Cotton and R. Francis, J. Am. Chem. Sot., 1960, 82, 2986. 6. C. N. Reilley and ri. W. Schmid, Anal. Chem.. 1958.30, 947. 7. J. S. Fritz and B. B. Garralda, ibid., 1964, 36, 737. 8. N. S. Moe, ibid., 1974, 46, 968. 9. E. P. Koptenko and P. K. Migal, Zh. Neorgan. Khim., 1975.20. 1818. Inorganic IO. F. A. Cotton and G. Wilkinson, Advanced Chemistry, 2nd Ed., p. 905 Wiley-Interscience, New York, 1966. II. R. M. Smith and A. E. Martell, Critical Stability Constants, Vol. 2, Plenum. New York, 1975. Electro12. D. T. Sawyer and J. L. Roberts, Experimental chemistry for Chemists, p. 69. Wiley, New York, 1974. 13. B. Chiswell, I!lorg. Chim. Acta, 1975, 12, 195.
ra1anra. Vol. 27. pp. 983 to 988 0 Pergamon Press Ltd 1980. Printed in Great Britain
POTENTIOMETRIC TITRATIONS OF METAL IONS IN ACETONITRILE WITH POLYAMINE LIGANDS I. M. AL-DAHER*and
B.
KRATOCHVIL
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received 25 January 1980. Accepted 6 June 1980)
Summary-Ethyienediamine,
diethylenetriamine,
and triethylenetetramine
were investigated as titrants
for metal ions in acetonitrile. Copper(B) gave the best titration plots; cobalt(II), manganese(H), nickel(II), iron(II1) and magnesium(I1) also gave results acceptable for analytical determinations. Platinum, silver-silver(I), mercury-mercury(II), carbon, and a copper ion-selective electrode were studied as indicating electrode systems; of these platinum gave the sharpest and largest inflections. The mechanism by which platinum responds as an indicating electrode in these complexation titrations is unclear.
Complexing titrants which have only nitrogen atoms as ligands, such as polyamines, are more selective in their interactions with metal ions than are reagents such as EDTA which possess both nitrogen and oxygen donor atoms. Reilley and Sheldon’ used the selectivity of triethylenetetramine for titrations of metals such as copper, zinc, mercury, and cadmium in the presence of aluminium, bismuth, lead, and the alkaline earths. They emphasized the need when selecting titrants and titration conditions to consider the competitive equilibria of metal ion hydrolysis, hydrogenion contributions from buffers or added acid, and complexing effects by buffers. The potentiometric titration of copper, cadmium and zinc with triethylenetetramine and tetraethylenepentamine with a silver indicating electrode has been investigated by Hulanicki et ~1.~ In aprotic non-aqueous solvents the competing equilibria to be considered in complex formation with metal ions are simpler than in water. This is because neither solvolysis, corresponding to the formation of metal hydroxides in water, nor protonation ot coordination sites on the ligand by protons from the solvent, can occur. The absence of these two processes means that buffers, with the attendant complications of proton contribution and auxiliary complex formation, are unnecessary. In short, there is little need to consider conditional formation constants. The major disadvantage of aprotic solvents as media for complexation titrations is the low solubility
of most metal salts owing to poor solvation of the anions. The most soluble salts are those of large, symmetrical anions of low charge-density such as perchlorate, tetrafluoroborate. and hexafluorophosphate, anions not usually encountered in systems of analytical interest. Common ions such as chloride, nitrate, or sulphate must therefore be exchanged for species such as perchlorate if adequate analytical concentrations of the metal salts are to be obtained. *,Present address: Department of Chemistry, University of Al-Mustasiriyah, Baghdad, Iraq. 983
Despite the inconvenience of this conversion step, it appears worthwhile to assess the scope of metal ion titrations with chelating ligands in aprotic media for possible analytical use. In this study acetonitrile was selected as solvent since it is a poor hydrogen-ion donor or acceptor, is readily available, has a moderately high dielectric constant, is relatively non-toxic, and has a convenient liquid range and low viscosity. As titrants, the polyamines ethylenediamine, diethylenetriamine, and triethylenetetramine were selected because of the solubility of the ligands and their metal complexes in acetonitrile, and because of the stability of some of the ligand-metal complexes.3 Potentiometric titration is preferred so that the stoichiometry and extent of the reactions can be followed. For this purpose both the mercury and silver indicating electrodes used in aqueous complexometric work were investigated. It was found that platinum also functioned as an indicating electrode for several of the titration systems. EXPERIMENTAL
Chemicals
Ethylenediamine (en), diethylenetriamine (dien), triethylenetetramine (trien) and dimethyl sulphoxide (DMSO) (J.T. Baker) were purified by distillation under reduced pressure. Acetonitrile (Matheson, Coleman, and Bell) was dried overnight over calcium hydride and then distilled. The hydrated perchlorate salts of copper( zinc(II), manganese(H), nickel(II), iron(III), cobalt(I1) and chromium(II1) (G. F. Smith Chemical Co.) were converted into the corresponding dimethyl sulphoxide solvates either by the procedure of Selbin, Bull and Holmes4 or of Cotton and Francis.5 All were converted readily and analysed as reported except for the copper(I1) salt, which, when prepared according to the published method,5 precipitated as a pale blue salt that gave an analysis corresponding to the pentasolvate rather than the reported tetrasolvate. Analysis gave C 18.5%. H 4.6%. Cu (iodometric titration) 9.7%; Cu (C10&.5DMSO requires C 18.39%. H 4.63x, Cu 9.73%. As the dimethyl sulphoxide solvates of the perchlorates of magnesium, calcium, and strontium have not been reported previously, details of their preparation are given here. Mg(CIO&.6DMSO. To a saturated solution of anhydrous magnesium perchlorate in dry acetone was added an
I. M. AL-DAHER and B. KRATOCHVIL
984
volume of dimethyl sulphoxide. Upon addition of ethanol a white precipitate formed; this was filtered off, equal
washed with ethanol, and dried overnight under vacuum at room temperature. Analysis gave C 20.7%, H 5.2%, Mg (EDTA titration) 3.50%; Mg(ClO& .6DMSO requires C 20.83x, H 5.20x, Mg 3.52%. CU(CIO,)~. 6DMS0. A slurry of 20 g of calcium carbonate in 50 ml of water was treated with 70% perchloric acid in l-ml portions until evolution of carbon dioxide ceased and a pH of I was obtained. Water was removed under vacuum until solid c&urn perchlorate appeared and little liquid remained. The material was dissolved in a minimum amount of acetone, the solution was filtered, an equal volume of dimethyl sulphoxide added, and the mixture shaken. Upon addition of diethyl ether a heavy white precipitate formed. After cooling in ice, the precipitate was filtered off, washed twice with ether, and dried overnight under vacuum at room temperature. Analysis gave C 19.7x, H 4.9x, Ca (EDTA titration) 5.6,x; Ca (ClO& .6DMSO requires C 20.36%. H 5.12x, Ca 5.66%. Sr(C101)2. 6DMSO. This material was prepared in the same way as the calcium salt except that the white precipitate obtained was dissolved in acetone and reprecipitated with ether before drying overnight under vacuum. Analysis gave C 18.5%. H 4.6%; Sr(ClO& .6DMSO requires C l9.08%, H 4.80%. Titration procedure For the complexometric titration studies. approximately 40 mg of each DMSO-solvated metal perchlorate were dissolved in 40 ml of dry acetonitrile and titrated with approximately 0.06M amine in dry acetonitrile. An 80-ml glass cell fitted with a Teflon lid containing openings for the burette tip, electrodes, and nitrogen inlet was used as a titration vessel. The indicating electrode was either mercury-mercury(II) amine, silver-silver ion, or platinum, and the reference electrode was silver-0.01 M silver nitrate in acetonitrile. For the mercury electrode one of the arrangements described by Reilley and Schmidb was employed. One drop of a IO-‘M solution of mercury(IIbn or mercury(IIttrien was added to the solution to be titrated to provide the mercury(I1) ions necessary for the electrode couple. For the silverPsilver ion indicating electrode a silver wire, along with a drop of 0.008M silver nitrate in acetonitrile added to the solution to be titrated, was used, as described by Fritz and Garralda for complexometric titrations in water.’ All titrations were done with the platinum indicating electrode, except where indicated, and on a Metrohm E436 automatic recording titrator at a titrant delivery rate of about 0.25 ml/min. The reference electrode and the solution being titrated were separated by a glass junction’ and a glass frit, with a bridge solution of O.lM lithium perchlorate in acetonitrile between them. Aerial oxidation and water absorption were minimized by starting the titration immediately upon dissolution of the samples, and by carrying out all titrations under a blanket of dry nitrogen.
gen atoms in this system. Therefore the titration system must be kept water-free if useful results are to be obtained. Titrations
with ethylenediamine
Results of titrations of a series of metal ions with en are summarized in Table I. The key features are that cobalt(H) and copper(U) show sharp inflection points at 2: I ratios of en to metal, while manganese(II), nickel(I1) and magnesium(I1) show smaller inflections at 3: I ratios (Fig. I). For copper( a set of titrations in which Cu(CIO,), . 4CH3CN was used as the sample in place of the DMSO solvate gave identical titration curves to those in which DMSO was present, showing that DMSO does not affect the shape of the curves. Calcium(II), strontium(I1). chromium(III), and silver(I) do not yield titration breaks, but zinc gives two, a small sharp break ‘at a ratio of en to zinc of about 1.85:I and a more drawn-out break at a ratio somewhat over 3 : I. Both 2 : I and 3 : I complexes of en with zinc have been reported as existing in water-ethanol mixtures,’ but at high ethanol concentrations, of the order of 80”/,, only the 3: 1 complex is seen. In acetonitrile the behaviour of iron(II1) is similar to that of zinc(II), but the break at the 3: I ratio is sharper. The largest potential change is observed with copper(I1). The 2: I stoichiometry observed here parallels the behaviour of copper(I1) with en in water;‘O~l’ this parallel behaviour is not seen with cobalt(II), which forms with en in water 1: 1, 2: 1, and 3: 1 complexes of roughly equal stability, but shows only a 2: 1 break in acetonitrile. Several titrations of mixtures were investigated to assess the potential selectivity of en as a titrant; the results are summarized in Table I. I
E
RESULTS AND DISCUSSION
Titrations in acetonitrile of hydrated metal salts of the form M(ClO&. xHzO with the polyamine ligands did not give useful end-points, apparently because of formation of metal hydroxides and protonation of the polyamines by water. Ethylenediamine and the other polyamines are quite basic in acetonitrile and can be titrated quantitatively with perchloric acid in dioxan. For example, large potential breaks are observed with a glass electrode at 1: I and 2: I ratios of H+ to en, indicating the appreciable base strength of both nitro-
0
1
1
I
1
2
3
En I Metal Ratio Fig. 1. Relative magnitude of breaks at the inflection points for the potentiometric titration of several metal ions with ethylenediamine in acetonitrile.
Titrations in acetonitrile with polyamine ligands Titrations with dien and trien
2 summarizes the results of titrations of a series of metal ions with dien, and Table 3 with trien. With dien copper(H) shows breaks at ratios of both 1: I and 2: 1, while with trien only a 1: 1 break is seen. In both cases the curves are sharp and stiochiometric. The only other ions of those studied that react stoichiometrically with dien are magnesium(II), cobalt(II), and manganese(II), all of which produce small but well-defined inflections at 2:l ratios (Fig. 2). No metals other than copper gave stoichiometric breaks with trien. The lack of stoichiometry can be attributed in the case of trien to difficulty in selecting reproducible end-points from the drawn-out, asymmetric titration plots, but for dien the non-stoichiometry must be attributed to some other cause since the inflections are generally sharp. The mechanism by which platinum functions as an indicating electrode in these systems has not been established. Measurements of the potentials of a series of solutions of varied copper(K) concentrations in acetonitrile with a platinum indicating electrode and a silver-silver nitrate reference electrode pair were not reproducible, nor were similar measurements on solutions with varied concentrations of trien. Pretreatment of the platinum with nitric acid or acidic iron(I1) sulphate had no effect on its response. In one titration of copper with dien the platinum was replaced by a graphite rod pretreated with molten wax to decrease its porosity;” two sharp breaks of 120 and 300 mV were obtained at stoichiometries of 1: 1 and 2: 1.
985 I
I
1
Table
E
0
1 5iw1 /Metal
2
Ratio
Fig. 2. Relative magnitude of breaks at the inflection point for the potentiometric titration of several metal ions with diethylenetriamine in acetonitrile. However, no useful break of the kind observed with platinum was seen when the graphite electrode was used for the titration of magnesium with dien. A solid-state copper ion-selective electrode (Orion Model 94-29) was also tested. Titrations of copper(I1) with en gave a single inflection of about 450 mV at
Table 1. Summary of titrations of M(ClO,), DMSO salts with en in acetonitrile Species titrated
en:metal ratio at inflection point
Approximate potential change at inflection point, m V
CU*+
2.002, 2.007, 2.00,. 2.000 1.999. 2.00,. 1.999, 2.00, 1.83, 1.86, 1.86 3.00. 3.28. 3. IS 2.999, 3.00,. 3.OO,, . 1.8, 1.8, 1.8. 1.8 2.99,. 3.OOt, 3.OO,, 3.00, -2.5, 2.5, 2.5 3.009, 3.004, 3.000 2.99s. 3.00,. 3.002 2.O33,2.OO,, 1.99,, 2.00, 0.500 l.OOo 2.OO,, 2.006, 2.00,
960 very sharp 240 sharp 80 sharp 140 drawn-out I20 fairly sharp slight inflection 120 fair slight inflection 90 sharp 80 fair to poor 950 very sharp 160 sharp 200 sharp 6OOvery sharp (only Cu2+ titrated)
Cu’+, AgNO, Fe3+. Cr3+
end-point 3-7’4 early for total 2.014 3.00,. 3.003, 2.99s 2.99,. 2.99s
600 sharp 120 fair, ppte (only Cu* + titrated)
Mn2+ Cr3+
3.061,3.065,3.05,
Ni2+,‘Cr3+ Mg’+, Cal’ Sr*+ Mn*‘, CaZ+’
3.0S6 2.83s no inflection no inflection
co*+ ZnZ+ Mn*+ Fe3+ Ni2+ Mg2+ CUz+O Hfb CU’+ 9Ca2+ 1
sr2+, era+
Cu2+, ZnZ+
Tu*+ added as Cu(CIO,), .6CH&N bH+ added as HC104.2H20 (70% aqueous acid).
130 fair (only Fe3+ titrated) 130 drawn-out (only Mn2+ titrated) 190 fairly sharp (only Ni2+ titrated)
986
I.
M. AL-DAHERand B. KRATCICHVIL
Table 2. Summary of titrations of M(ClO.,). . xDMS0 salts with dien in acetonitrile, with platinum as indicating electrode Species titrated
dien :metal ratio at inflection point.
Approximate potential change at inflection point, mV
CU2’
[email protected],, 1.00, 2.00,,. 1.98,, 2.006 2.00,. 2.039. 2.01, 2.12; 1.69. 1.68. 1.68 2.02, 2.03, 2.04 2.00~, 2.00,, 2.000 1.998 1.98, 2.00,. 2.01 I .99,, 2.04, 1.98 2.16, 2.13, 2.14 2.01, for total
330 very sharp 630 very sharp 360 very sharp
Zn” Fe’+ Nil+ Mg” Mn’+ co2+ Ca2+ Ca”. Mg2 +
300 very sharp 220 very sharp 200 sharp 140 very sharp 100 sharp 60 poor, drawn-out 180 fairly sharp (both titrated)
Table 3. Summary of titrations of M(C104),.xDMS0
salts with trien in acetonitrile
Species titrated
trien:metal ratio at inflection point
Approximate potential change at inflection point, mV
cu2+ Zn2+ co2 + Fe3+
1.008, l.OO,, l.OOo 1.07, 1.13, 1.08 1.08.1.07 1.23, 1.44, 1.39 1.66, 1.88. 1.84 1.09. 1.09, 1.09 1.05, 1.07, 1.07
840 very sharp 400 fair break 140 fair break 120 poor break 100 poor, drawn-out 120 poor, drawn-out 6&80 fairly sharp s 100 too drawn-out for use -90 too drawn-out for use
Ni’+ Mn2+ Ca2+ Mg2+
Table 4. Summary of titrations of M(CIO& .xDMSO salts with en in acetonitrile, with Hg/en indicating electrode Species titrated
en:metal ratio at inflection point
Approximate potential change at inflection point, ml/
cu2+ co2 + Mn2+ Ni” Mg2+ Ca2+
1.99*, 1.99,. 1.990, 2.000 2.003. 1.999, 2.00, -3 3.002, 3.00, no inflection no inflection
36&560, very sharp 280 very sharp drawn-out, not useful 130, good drawn-out
Table 5. Summary of titrations of M(ClO,),..xDMSO
salts with en in acetonitrile, with Ag/Ag’ indicating electrode
Species titrated
en:metal ratio at inflection point
Approximate potential change at inflection point, ml/
cu*+
1.998. 2.000, 2.000 1.988, 2.00,, (only silver wire, no Ag’ added) 1.99,. 1.91, I .99s (only silver wire. no Ag’ added) -3 3.00,, 2.99s, 3.003 -3 no inflection
180, sharp
cu2+ co2 +
co*+ Mn2+ Ni’ l Mg” Ca2 +
200. sharp 240, sharp 220. very sharp 120, poor, hard to locate the end-point 150, sharp poor, drawn-out gradual potential change of about 60 mV throughout titration
Titrations in acetonitrile with polyamine ligands
ratios of en to copper(I1) of 2.135, 2.14a. 2.14s, and 2.145 (four consecutive titrations). Although the endpoint could be located reproducibly, it was not stoichiometric. Replacement of the silver-silver nitrate reference electrode with an aqueous saturated calomel electrode in one titration of nickel(I1) with en was also done to test whether trace amounts of silver ion were diffusing into the cell solution and affecting the potential at the end-point. However, results were unchanged from those obtained with the silver reference electrode. Also, addition of silver nitrate to the titration cell in one titration of copper(I1) with en gave a precipitate and a reduction in the size of the end-point break from 1 V to 120 mV. Results of titrations of a series of metal ions with en, either the mercury-en or silver-silver ion system being used as indicating electrode and the silver-silver ion couple as reference electrode, are summarized in Tables 4 and 5. As with the platinum indicating electrode, copper shows the sharpest and largest potential breaks. Cobalt(H) titrations also show sharp inflection points at 2: 1 ratios of en to cobalt(I1) in the case of the mercury electrode, but at less than 2: 1 for the silver indicating electrode. Similar sharp potential breaks, but at a 3: 1 ratio of ligand to metal, are seen with both electrodes for the titration of nickel(I1). With both electrodes manganese(I1) shows drawn-out potential breaks at about 3:l ratios which are not analytically useful. Calcium(I1) does not yield titration breaks with either electrode, while magnesium(H) shows a poor, drawn-out break at about a 3: 1 ratio
Table 6. Summary of titrations of M(ClO&.xDMSO
with the silver indicating electrode, but does not show an inflection with the mercury electrode. Tables 6 and 7 summarize the results of titrations of a series of ions with trien, with the mercury-trien or silver-silver ion indicating electrode and a silversilver ion reference electrode. The only system studied that gave a break at a clean, stoichiometric 1: 1 ratio was magnesium(I1) with the mercury-trien indicating electrode. All the other systems showed breaks at ratios of more than 1: 1 even though some of the inflections were fairly large or sharp. The silver-silver ion indicating electrode did not provide satisfactory end-points for any of the metalion titrations. All the systems investigated showed drawn-out, poor breaks at ratios of more than 1: 1, an exception being manganese(II), which showed fair through non-stoichiometric single inflections. Titration of aqueous solutions of manganese(I1) perchlorate or tetraphenylborate with an ethanolic solution of trien has been reported by Chiswell’3 to yield a precipitate of [Mn,trien,]X,. nH,O. A 1: 1 Mn(II)-trien salt was reported to be obtained upon mixing anhydrous manganese perchlorate and trien in equimolar amounts in ethanol and allowing the solution to crystallize over a period of weeks. These results indicate that end-points at ratios of both I:1 and greater than 1: 1 might be expected for titrations of manganese(I1) and perhaps of other metals with trien. This is seen in the manganese(IIttrien titration using a mercury-mercury(I1) trien indicating electrode; two small breaks are observed (Table 6), the
salts with trien in acetonitrile, with Hg/trien indicating electrode
Species titrated
trien : metal ratio at inflection point
Approximate potential change at inflection- point, mV
CU2+ co2+ Mn2’
1.1, 1.1, 1.1 1.4, 1.3, 1.4 l.O9a, 1.089, 1.09,. 1.09, 1.06~, 1.054 1.29, 1.29, l.lO,, 1.09,, 1.1I3 with a second break, drawn-out, at less than 2: 1 ratio l.Ooo, l.oO,, l.OOo, 1.006, 1.02, w 1.2
360 (1st) asymmetric 180 (2nd) asymmetric 240 sharp 120 (1st), good 80 (2nd), fair 90, good
NiZf Mg2 + Ca2 +
Table 7. Summary of titrations of M(ClO.,),.xDMSO
150, good drawn-out, not calculable
salts with trien in acetonitrile, with Ag/Ag+ indicating electrode
Species titrated
trien:metal ratio at inflection point
Approximate potential change at inflection point, mV
cu2 +
-1 c 1.4 1.21, w 1.1 w 1.1, 1.1, 1.1 l.lOa, 1.08,. 1.25,,, l.O& -1
200 (1st). poor 120 (2nd), drawn-out 120, poor 100, poor 800, poor 180, fair drawn-out
co2+ Mn2+ Ni2 + Mg2 + Ca2+
981
988
1.
M. AL-DAHERand B. KRAT~CHVIL
first at trien to manganese ratios of about 1.05: 1 and the second at ratios of about 1.29: 1. Only single breaks. however, are seen with the platinum and silver-silver ion indicating electrodes, at about 1.05: I and 1.14:1 ratios. In summary, a range of metal-ligand titrations can be performed in an aprotic solvent such as acetonitrile. Potentiometric titrations are possible in several cases through use of a platinum indicating electrode, which was found to be superior to the other indicating electrodes investigated, including mercury, silver, carbon, and the copper ion-selective electrode. The mechanism by which platinum functions as an indicator is not clear. In general the results, though of experimental interest, do not indicate that acetonitrile offers any analytical advantage over water as a solvent for metal-ligand titrations. Acknowledyemertts-Financial assistance by the National Research Council of Canada and the University of Alberta are gratefully acknowledged. This paper is dedicated to Professor Harvey Diehl on the occasion of his seventieth birthday.
REFERENCES C. N. Reilley and M. V. Sheldon, Talanta, 1958, 1, 127; Chemist-Analyst,
1957,
46, 59.
2. A. Hulanicki, M. Trojanowicz, and J. Domaska, Talanta, 1973, 20, I 117. Constants of 3. L. G. Silltn and A. E. Martell, Stability Meral Ion Comolexes. SDecial Publication No. 17. Chemical Society, London, 1964. 4. J. Selbin, W. E. Bull and L. H. Holmes, Jr., J. Inorg. Nucl. Chem., 1961, 16, 219. 5. F. A. Cotton and R. Francis, J. Am. Chem. Sot., 1960, 82, 2986. 6. C. N. Reilley and ri. W. Schmid, Anal. Chem.. 1958.30, 947. 7. J. S. Fritz and B. B. Garralda, ibid., 1964, 36, 737. 8. N. S. Moe, ibid., 1974, 46, 968. 9. E. P. Koptenko and P. K. Migal, Zh. Neorgan. Khim., 1975.20. 1818. Inorganic IO. F. A. Cotton and G. Wilkinson, Advanced Chemistry, 2nd Ed., p. 905 Wiley-Interscience, New York, 1966. II. R. M. Smith and A. E. Martell, Critical Stability Constants, Vol. 2, Plenum. New York, 1975. Electro12. D. T. Sawyer and J. L. Roberts, Experimental chemistry for Chemists, p. 69. Wiley, New York, 1974. 13. B. Chiswell, I!lorg. Chim. Acta, 1975, 12, 195.