Solution equilibria polarography

The complexation of Pu(IV) with carbonate ions is investigated by solubility measurements of 238Pu02 in neutral to alkaline solutions containing sodium carbonate and bicarbonate. The total concentration of carbonate ions and pH are varied at the constant ionic strength (I = 1.0), in which the initial pH values are adjusted by altering the ratio of carbonate to bicarbonate ions. The oxidation state of dissolved species in equilibrium solutions are determined by absorption spectrophotometry and differential pulse polarography. The most stable oxidation state of Pu in carbonate solutions is found to be Pu(IV), which is present as hydroxocarbonate or carbonate species. The formation constants of these complexes are calculated on the basis of solubility data which are determined to be a function of two variable parameters the carbonate concentration and pH. The hydrolysis reactions of Pu(IV) in the present experimental system assessed by using the literature data are taken into account for calculation of the carbonate complexation. 

A wide variety of other methods have been used to obtain the concentration of either the metal or ligand in a particular equilibrium solution. These include polarography, spectrophotometry, magnetic resonance, cyclic voltametry and conductivity. Of course, if reliable results are to be obtained, the technique chosen must not significantly perturb the position of equilibrium in the solution. 

On the other hand, for anthraquinone and benzophenone, the ESR results showed that the concentration of the radical anion was almost time-independent, although, in polarography, the height of the first wave increased, consuming the height of the second wave as in Fig. 8.12. This inconsistent result was explained as the establishment of the following disproportionation equilibrium in the solution ... 

In addition to the information that can be obtained from DC polarography as indicated above, polarography can also be used to study solution equilibria of coordination compounds. Thus from measurements of the shift of E1/2 with solution environment, such as pH or concentration of complexing agent, it is possible to ascertain both the stoichiometry and values of the equilibrium constants of the species formed. For example, cis-[Run(bipy)2(H20)2]2+ was found1 to undergo... 

Most of the work on the boric acid-diol reaction during the last twenty years has been done to determine the coordination number of the diol (number of diol molecules) in the complex and to evaluate the equilibrium constant (often called a stability constant) for a number of diol-boric acid reactions. Several techniques have been used to study these questions, including polarimetry (7), optical rotatory dispersion (8), polarography (9), conductivity (3), vapor pressure osmometry (10), and electrochemistry (II, 12, 13). The most frequently studied system has been the electrochemical (pH) titration of boric acid or borax solutions with various diols. 

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

Polarography offers some possibilities for the study of reaction kinetics and mechanisms of homogeneous organic reactions. The main advantages are a rather simple and easily accessible experimental technique, the possibility to work in dilute solutions and limited requirements on the amount of substances studied. The main limitation is that some of the components of the reaction mixture must be polarographically active. But this limitation is not so restrictive as it would appear, because most substances that can be studied spectro photometrically are electro-active as well. For rapid reactions polarography seems to be most useful for a range of second-order rate constants between about 10 -10 sec M, whereas for faster reaetions the specific properties of the electrode, in particular its electrical field and adsorption, can play a role. A certain limitation is that for most systems the equilibrium constant has to be known from independent measurements. 

The complex formation in the Th(TV)-nitrite system was investigated using conductometry, spectrophotometry and polarography in methanol. The composition and equilibrium constants for ternary complexes Th(0Me)2(N02), Th(0Me)2(N02)2 and Th(0Me)2(N02)3 were determined. These data are not relevant for estimates of nitrite complexes in aqueous solution. 

As has emerged from the foregoing discussion, conclusions may be drawn about the donor strength sequence of solvents from the equilibrium stabilities of the resulting solvates. The exact determination of these is very difficult, however, because of the very laborious and complex nature of equilibrium analysis in non-aqueous solutions. Accordingly, in many cases researchers have been satisfied with the qualitative characterization of the stabilities of the solvates formed. Of the possible electroanalytical procedures, polarography appeared the most readily utilizablc for this purpose. 

In principle, the softest adsorbates are liquids, and adsorption at the mercury-solution interface is at the basis of one of the most common analytical technique -polarography. A theory of adsorption on liquid surfaces is facilitated by the fact that liquid are always characterized by an equilibrium configuration and has actually been developed [38], No such a theory is known for less extreme situations, in which the adsorbate does reconstruct partially without attaining an equilibrium configuration. 

Consider on the other hand a similar reaction scheme where all steps are rapid and reversible. The mass transport step will be in equilibrium and the rate of electron transfer will be very rapid, so that a quasi-equilibrium (as was discussed previously) is maintained and step III, the so called chemical step , will also be in equilibrium. The whole process will be a quasi-equilibrium one. This quasi-equilibrium process is the polarographically reversible process. Presently we shall describe two ways by which reversibility can be determined and continue to describe some of the experimental aspects of polarography and voltammetry in stirred solutions, devoting a section to the various uses of the rotating disk electrode. 

Values of Kd can be obtained by electrochemical or spectroscopic techniques. In polarography, the equilibrium between the hydrated and carbonyl form is perturbed and limiting currents governed both by the position of equilibrium (Equation 9) and by the rate of dehydration 13 D.c. polarographic limiting currents thus offer only information about the upper limit of the value of Kd. In linear sweep voltammetry (LSV) the measurement of the current can be carried out over such a short period of time that the equilibrium cannot be reestablished . The current is then proportional to the concentration of the carbonyl form in the bulk of the solutions and can be used for determination of values of Kd, as discussed in detail elsewhere . 

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