Studies in the polarographic (D.C and A.C) behaviour of amino acid complexes of vanadium

Abstract

As already pointed out the chief techniques employed in the present work are the classical d.c. polarography and its variation the a.c. polarography. It is the complementary use of these two techniques that has been helpful in unravelling the complicated nature of the various types of polarographic waves obtained in the vanadium system, and to draw conclusions regarding the nature of the electrode process. In this chapter a comparative account of the important aspects of the behaviour of the vanadium system is given. 1. Pre-waves. One of the interesting aspects in the study of the polarographic behaviour of vanadium (V) and vanadium (IV) in acid solutions (pH < 2.5) is the practical identity of the latter 2/3 portion of the wave in the reduction of vanadium (V) with the reduction wave of vanadium (IV). It is usually found that the height of the first wave is half the height of the second wave, leading to the reduction process of vanadium (V) to vanadium (II) via +4 state. However, in buffer solutions of pH greater than 5 it is found that the height of the first wave is more than half the height of the second wave. When this happens the limiting current of the first wave is not entirely diffusion controlled and the increase in height can be attributed to a pre-wave due to the adsorption of vanadium (II) produced at the electrode surface. In some cases the pre-wave appears quite distinct while in other cases there is superposition of the above process with vanadium (V) to vanadium (IV) step. (Fig. 2) 2. Fresh and aged test solutions of vanadium (V). (i) Between pH 3 and 7. When the pH is increased beyond pH 3, the test solutions are deep yellow in colour. The colour remains stable for many days. The polarograms obtained in absence of amino acids are highly ill-defined and contain a number of split waves very close to each other. Hence it has not been possible to make detailed investigations in this pH range. (ii) Above pH 7. A factor in the polarographic studies of vanadium (V) is the formation of deep yellow coloured solutions which decolourize on aging. Fast polarograms taken in buffer solutions and also in presence of ligands show a number of split waves. In these solutions it is not possible to identify waves corresponding to the reduction of vanadium (V) to vanadium (IV) and vanadium (IV) to vanadium (II). (Figs. 1 and 4). However, all these test solutions have one feature in common i.e. the presence of a polarographic wave at ca -0.4 V, which decreases in height as the colour of the solution goes on fading. In fully decolourized solutions this wave is completely absent. Reproducible polarograms can only be obtained in decolourized solutions. The fast polarograms taken with coloured solutions have been used to understand qualitatively the nature of the changes occurring with time. It has been suggested in this thesis (p. 89) that polynuclear decavanadate species of vanadium (V) are responsible for the deep yellow colour. The polynuclear species being unstable in alkaline solutions get converted into colourless mononuclear species, which form complexes with the ligands. All interpretation is done only with the polarograms obtained in decolourized solutions. Similar colour changes have been observed by other workers. 3. Electrode processes occurring in vanadium systems. Vanadium (V) in acid solutions is reduced to vanadium (II) via +4 state. Further reduction of vanadium (II) to vanadium (metal) does not occur in the usual polarographic range since the standard potential of the reaction V²? to V is -1.075 to -0.3 V vs S.C.E. The diffusion current plateau of the first wave and the full second wave are usually observed in perchloric acid solutions (of pH < 3). In alkaline solutions in which the amino acid anion is the ligand, and where the hydroxyl ion acts as the competing ligand, the vanadium (V) to vanadium (IV) step is not clearly seen under all conditions. The behaviour of vanadium (V) depends upon the composition of the base solution. In general it is found that high concentrations of the ligand and low concentrations of the hydroxyl ion are favourable for the formation of a wide bend at the foot of the wave (for e.g., Fig. 6 curves 1 and 2; Fig. 13 curves 18, 19 and 20; Fig. 26 curves 7 and 8; Fig. 31 curve G). Under favourable conditions the bend extends to 1/3 the height of the total wave. The view taken in this thesis is that when this happens, the bend portion could be taken to represent reduction of vanadium (V) to vanadium (IV). When the bend is well formed and occupies 1/3 the portion of the wave, the latter 2/3 portion closely resembles the polarograms obtained with vanadium (IV) under similar conditions. (For e.g., Fig. 6 curve 1 and Fig. 9 curve 13c; Fig. 13 curve 18 and Fig. 16 curve 18c; Fig. 31 curve G and Fig. 33 curve 6c). When the ligand concentration is low and the hydroxyl concentration is high two or three closely placed split waves are observed. It is likely in these cases that vanadium (IV) is forming at least two complexes in sluggish equilibrium. The vanadium (V) to vanadium (IV) wave is superimposed by vanadium (IV) to vanadium (II) wave formed by one of the complexes. In DL-valine system in buffer solutions of pH 10.56 and higher anion concentrations only one wave corresponding to the reduction of vanadium (V) to vanadium (II) is obtained. The polarographic behaviour of vanadium (IV) system is very interesting. In alkaline solutions with all the amino acids employed test solutions in general are either pale pink or golden yellow in colour. The pink coloured solutions invariably give polarograms with two cathodic split waves which are as is, at different concentrations of the ligand. In the case of golden yellow coloured solutions split waves are obtained the relative heights of which usually vary with the composition of the base solution. Pink coloured solutions are obtained when the anion concentration is high as compared to hydroxyl ion concentration. On the basis of a.c. polarographic behaviour it is concluded that in pink coloured solutions of vanadium (IV) reduction occurs via +3 state, while in golden yellow coloured solutions, which consist of different complex species in sluggish equilibrium, the reduction proceeds directly to the +2 state. In some base solutions for e.g., 0.1 M total glycine and pH ca 9.35 and 0.1 M DL-valine and pH ca 8.56 (Fig. 6 curve 13 and Fig. 9 curve 3c; and Fig. 13 curve 5 and Fig. 27 curve 11c) no portion of vanadium (V) polarograms bears any resemblance to the vanadium (IV) polarograms. In such cases it is obvious that vanadium (IV) complex produced at the electrode surface is different from that in the bulk. Oxidation wave of vanadium (IV) to vanadium (V) has been noticed in base solutions of pH above 7 with all the amino acids employed. The reduction wave of vanadium (III) in all base solutions corresponds to reduction to vanadium (II). Oxidation wave however corresponds to vanadium (IV) in ?-alanine, ?-alanine and DL-valine systems and to vanadium (V) via vanadium (IV) in glycine and L-asparagine systems. In glycine, ?-alanine and L-asparagine systems in buffer experiments where reduction of vanadium (IV) to vanadium (II) via +3 state occurs, it is found that part of the polarograms of vanadium (III) closely resembles the latter half of the vanadium (IV) polarogram. It is rather interesting that in the anion solutions of ?-alanine and L-asparagine, the vanadium (III) wave completely spans the entire voltage range of vanadium (IV) polarogram. In the glycine system, however, the vanadium wave spans part of the initial portion of the vanadium (IV) wave. In the case of L-asparagine system it appears that several vanadium (III) complexes are produced in bulk while only one vanadium (III) complex appears to have been formed at the electrode surface. (Figs. 34 and 35). The a.c. polarographic behaviour has helped to elucidate the reduction mechanism of vanadium (IV) system and thus has been of very great help in understanding the nature of the electrode processes occurring in the vanadium system. In all buffer experiments (except in the ?-alanine system) it is found that the d.c. polarograms of vanadium (IV) show 1–1 cathodic split waves irrespective of the presence or absence of a third split wave. The a.c. polarograms obtained under similar conditions show a peak at potential corresponding to the second cathodic split wave except in solutions of pH ca 9.8 in glycine and ?-alanine systems (where a.c. waves are not observed possibly due to the irreversibility of the process). In solutions containing 0.1 M total anion in all the amino acid systems studied two or three a.c. waves, the relative heights of which depend upon the ligand concentration and pH are obtained. In such solutions the d.c. polarograms show cathodic split waves due to different complexes in sluggish equilibrium. Another way in which the a.c. polarogram has been helpful is in understanding whether the d.c. polarogram is fully formed or not. This is amply illustrated in the a.c. polarograms obtained in the amino acid anion solutions. The incomplete formation of the second half of the a.c. wave and the appearance of the a.c. peak at potentials near about the diffusion current region indicates that part of the d.c. polarographic wave lies beyond the final current rise. A.C. polarograms have also been helpful in finding out whether there is any adsorption of the depolarizer or the reduction product. The a.c. polarograms taken in ?-alanine system exhibit a dip on the falling part of the a.c. wave (i.e. below the baseline) indicating that the vanadium (II) complex formed at the electrode surface is adsorbed. 4. Reversibility of the electrode reaction. Although the composite wave method is the best method for deciding the reversibility of an electrode process it is the usual practice to decide the reversibility of reactions on the basis of the values of E?/?–E?/?. For a one or two electron reversible process the expected values of E?/?–E?/? are of the order 0.058 and 0.029 V respectively. In the present work the values of E?/?–E?/? of cathodic waves of vanadium (IV) in the glycine, ?-alanine, DL-valine and L-asparagine systems (in buffer experiments and anion solutions) vary between 0.065 and 0.10 V. However, no a.c. waves are obtained. Although one might regard 0.09 V as representing an irreversible process it is rather difficult to expect that when the values of E?/?–E?/? are of the order of 0.070 V there should be no a.c. wave. In the present work conclusions regarding reversibility of vanadium (IV) to vanadium (III), vanadium (III) to vanadium (II) and vanadium (IV) to vanadium (II) reductions are also based on the a.c. polarographic behaviour. The results presented in this thesis further indicate that great caution should be exercised in deciding about the reversibility mainly on the basis of the values of E?/?–E?/?. It is usually recognized that the value of ? is near about 0.5 for electrochemical reactions, whether reversible or irreversible. When ? is 0.5, the cathodic reduction wave and the anodic oxidation wave would be symmetrical. Kalousek and Tocksfeein have theoretically calculated reduction and oxidation curves for various values of ? ranging from zero to unity both for reversible and irreversible reactions. It is found that when ? deviates from 0.5, the reduction and the oxidation waves are not symmetrical. In the expression for E?/?–E?/? namely E?/?–E?/? = 0.059/? if ? = 0.5, the value of E?/?–E?/? for an irreversible one electron reduction would be 0.116 V. If a reduction process is shown to be irreversible say by a.c. method, a value of 0.070 V for E?/?–E?/? can only be interpreted as due to the fact that the value of ? for that process is near unity. The varying values of E?/?–E?/? under different experimental conditions indicates that the value of ? depends upon the experimental conditions since all these processes have been shown to be irreversible by the a.c. polarographic method. Another way of interpreting E?/?–E?/? is by assuming the formation of irreversibly oxidized polynuclear complexes. It has been shown by Lax and Wang that when dinuclear complexes are reduced reversibly giving dinuclear reduction products the value of E?/?–E?/? is 0.030 V for a one electron process and 0.015 V for a two electron process. The view taken in this thesis is that the formation of polynuclear complexes at the low concentration of the depolarizer used is not quite feasible. Further as pointed out earlier polynuclear vanadium (V) species in alkaline solutions give rise to mononuclear species. (p. 89) 5. Nature of complexes. In the present thesis complexes of vanadium (V), (IV) and (III) employing glycine, ?-alanine, ?-alanine, DL-valine and L-asparagine have been investigated. The structure of these amino acids is as follows: H?N?–CH–COO?|R where R is H in glycine, CH? in ?-alanine, C?H? in DL-valine and CH?CONH? in L-asparagine. NH?–CH?–CH?–COO? (?-alanine) The amino acids exist in the following three forms in equilibrium with each other, the relative concentrations depending upon pH of the solution: H?N?–CH–COOH H?N?–CH–COO? H?N–CH–COO?R In acid solutions zwitterion is the ligand while in the alkaline solutions it is the anion. The work reported in this thesis indicates that vanadium (IV) in alkaline solutions of amino acids under various experimental conditions either gives pink or golden Yellow coloured solutions indicating complex formation. The characteristics of the polarographic waves (anodic and cathodic) are different from those of similar pH in absence of amino acids. In the case of vanadium (V) since the test solutions are all deep yellow in colour when freshly prepared it is not possible to conclude regarding complex formation on the basis of colour. Since the characteristics of the polarograms are different from those in absence of amino acids it can be concluded there is complex formation. In the case of vanadium (III), the half-wave potentials noticed in alkaline solutions of amino acids are more negative than the standard +3/+2 potential of the V³?–V²? couple. Further, except in the ?-alanine system, in all other systems, in buffer experiments, the second split wave in the reduction of vanadium (IV) can be identified with the reduction wave of vanadium (III) under similar conditions, which also supports complex formation in the case of vanadium (III). It is not possible to analyze the polarographic data in these systems to obtain the formulae of the complexes on account of the complicated behaviour of the systems. The following are the factors which are responsible for the data not being amenable for analysis: 1. Most of the polarograms consist of split waves which are close to each other. 2. Most of the polarographic waves are irreversible or quasi-reversible in character and the half-wave potentials practically remain constant at various concentrations of the ligand (under various experimental conditions). 3. The composition of the complexes is very sensitive to pH ratio. In presence of high concentrations of the ligand and comparatively low concentration of the hydroxyl ion, the mechanism of reduction differs from that in which the concentration of the hydroxyl ion is relatively higher. Tentatively suggested on the basis of data presented in this thesis. The following formulae of the complexes have been tentatively suggested: V(OH)???(Gly)? V(OH)?(Gly) V(OH)?(Gly)? V(OH)(Gly)?¹ V(OH)(Val) (in the bulk) V(OH)(Gly)?? V(OH)???(Val)? (species accepting electrons at the electrode surface) V(OH)(Asp) (in the bulk) V(OH)(Asp)? (species accepting electrons at the electrode surface) 6. Polarograms with pure amino acids. One of the convenient methods for estimating vanadium is to have vanadium in the +4 state. Very well-formed single reduction waves are obtained in base solutions containing pure amino acids and 0.005% gelatin. (Glycine Fig. 8 curves 12–16; Fig. 15 curves 8–12). It is interesting that up to 7 mM concentration of the depolarizer the diffusion current constants are practically the same. 7. Diffusion current constants of vanadium (V) and vanadium (IV). It is rather surprising that the diffusion current constant of vanadium (V) usually falls off with increase in the concentration of the depolarizer or the ligand. In the case of vanadium (IV), however, this trend is not marked. The diffusion current constants observed are quite low for one electron oxidation wave. In comparison, the values of the diffusion current constant for the reduction wave are not as low as for a two electron reduction process. However, the values are slightly lower than what could be expected for a two electron reduction process. D.C.C. varies between 2.0 and 3.0 as compared to 3.8 usually noticed for a two electron process. 8. Gradation in the properties of the amino acid complexes. As already pointed out, it has not been possible on the basis of polarographic data to obtain the formulae or the stability constants of the vanadium complexes. Work carried out in this laboratory on cadmium–amino acid anion complexes indicates that the stability constants decrease in the order: glycine, ?-alanine, DL-valine and ?-alanine. The stability constants of L-asparagine complexes are only slightly lower than those of ?-alanine and slightly higher than those of DL-valine complexes. Similar trend has also been noticed in the case of lead–amino acid anion complexes. In the present work the above order of the ligands is only used for qualitatively assessing the extent of the competing action of the hydroxyl ion. It is rather difficult to compare the nature of the polarograms under the same conditions of pH, ligand concentrations etc. due to the fact that the pK value of the amino acids vary. The conclusions arrived at are only on a sort of qualitative argument. ?-alanine complexes are concluded as being weaker than glycine complexes by the nature of the wide bend at the foot of vanadium (V) polarograms (p. 191) or by the appearance of a third split wave following the first two split waves which are as 1:1 (p. 199). In the case of the ?-alanine complexes it has not been possible to obtain 1:1 split waves in vanadium (IV) polarograms. In this thesis the formation of 1:1 split wave representing reduction via +3 state is attributed to the formation of a complex containing lesser number of hydroxyl groups than a complex which gets reduced straight to the +2 stage. It is therefore clear that the competing action of the hydroxyl group in ?-alanine system is greater than in glycine or ?-alanine systems. By a comparison of the polarograms of vanadium (V) in 0.1 M total DL-valine taken at pH 8.39, 8.86 and 9.31 with those of glycine and ?-alanine it has been concluded that the competing action of the hydroxyl group is greater in DL-valine than in glycine or ?-alanine (p. 235 and 236). That the competing action of the hydroxyl group is more in the case of L-asparagine as compared to ?-alanine is indicated by the polarograms of vanadium (V) obtained in buffer solution of pH 8.8 and anion concentration 0.1 M. (Fig. 14 curve 10; Fig. 32 curve 1). In L-asparagine the wide bend at the foot of the wave has practically disappeared whereas in the ?-alanine system it is very well formed. The formation of the wide bend is associated with vanadium (V) complex having less number of hydroxyl groups. The present work has indicated that though the stability constants of vanadium with various amino acid anions could not be determined, it has been possible on the basis of a comparison of the polarographic behaviour to conclude that the order of decreasing stability is as follows: glycine > ?-alanine > DL-valine ? L-asparagine > ?-alanine.