Complexation polarographic determination

Marin, D. Mendicuti, F. Polarographic Determination of Composition and Thermodynamic Stability Constant of a Complex Metal Ion, /. Chem. Educ. 1988, 65, 916-918. 

The polarographic determination of manganese, iron, cobalt and nickel is not so often used as that of the above mentioned elements. Probably, competition with other analytical methods is the main reason. However, the polarography of these metals has been thoroughly investigated and it is frequently used for studies of equilibrium constants of their complexes. 

During the anodic polarization of a dropping mercury electrode the chloride, bromide, iodide, cyanide, thiocyanide and sulphide ions form slightly soluble salts or stable complexes with mercury. In the presence of these substances so-called anodic oxidation waves appear on polaro-graphic curves. The polarographic determination of chloride has received most attention. In many cases the dilution of the sample with 0.1 N H2SO4 is satisfactory and the solution can be polarographed directly [3]. 

There is evidence that interactions between Co " and DOM are significant in humic-rich environments such as pore waters (Elderfield and Hepworth, 1975 Nissenbaum and Swaine, 1976) and fresh waters (Barsdate and Mat-son, 1967 Means et al., 1977). Mantoura (1976) used a multi-metal, multiligand equilibrium speciation model and calculated that <10% of Co is bound to lake-water fulvic acid. The kinetically non-labile nature of Co complexes with humic compounds (MacCarthy and O Cinneide, 1974) has been used as a basis for a polarographic determination of metal complexa-tion capacity of natural waters (Hanck and Dillard, 1977). 

Direct polarographic analysis in biological fluids is rare due to the interferences from surface active agents. Further proteins themselves can show a complex electroactive character. Thus various separation steps are usually carried out prior to the polarographic determination. These steps can minimise the interference and can also be used to enhance the selectivity of the method. Many common separation steps are used in combination with polarographic analysis. 

Table 9. Determination of Cu in wastewater samples by polarography and atomic absorption. Polarographic determination anodic stripping voltammetry = determination of ionic and weakly complexed Cu species. Atomic absorption = determination of Cutot ...

The author has developed a method for the determination of traces of Cr(VI) in Cr(III)-complex dyes, down to 0.1 ppm Cr(VI) based on differential pulse polarographic determination of the Cr(VI) after preconcentration of the Cr(VI) traces from the aqueous Cr-complex dyestuff solution by anion-exchange resin (AG1-X4) the adsorption and elution are made in opposite direction, thus minimising elution volume, contact time (oxidation of resin), and band spreading (cf.). The setup used is shown in the Fig.5. 

It is known, that the polarographic determination of Co " " can be improved significantly by adding dimethyl glyoxime (DMG) to the solutioif. The adsorption of the Co -DMG complex at the mercury electrode enhances the analytical signal. But the reduction of the complex in 0.1 M NH4CI occurs at nearly the same potential as the Zn " "-reduction. An... 

A group of polarographic determinations that can be classified both with the determinations based upon transformation into an active compound, and with methods in which the concentration change of an active compound (reacting with the substance to be determined) is followed, are methods which are based upon the formation of complex compounds between inorganic ions and the organic compound to be determined. 

The formation of the Eu-XO complex may be seen from the following experimental findings The color of XO solution changes from orange to red upon addition of Eu " ", and its absorption peak at 430 nm moves to 563 nm. The composition of the complex was determined by Job s method of continuous variation using both spectrophotometric and polarographic data to be Eu " XO = 1 1. The degree of dissociation a = 0.134 (Inczedy, 1976) and its apparent stability constant... 

This investigation suggests that ethylenediamine tartrate may be a useful reagent for the polarographic determination of a variety of metal ions. A similarity to ethylenediaminetetraacetic acid complexes, also used in polarography [2], is to be expected. 

In the presence of many metal ions, diorthohydroxyazo dyes exhibit two polarographic reduction waves, the first due to free dye and the second to metal-dye complex. Highly sensitive analytical methods based on this principle have been developed for example, Ni or Fe may be determined in the presence of an excess of aluminum thank to thiazolylazo derivatives (563). 

Stability constants of metal complexes of 9-hydroxy-4//-pyrido[l,2-n]pyrimidin-4-one [Ni(II), Co(II), Zn(II), and Cd(II)] were determined by potentiometric and polarographic investigations (93JCC283). The distribution coefficient of risperidone (11) in H20- -octanol at pH 7.4 (log D — 2.04) was determined by an RP-HPLC method (01JMC2490). 

The macrocyclic hexamine [18]aneN6 was further found to recognize catechol, catecholamines and biologically relevant compounds (see Chart II)64). It interacts with all of these donor compounds in neutral pH solutions to form 1 1 complexes, which were determined polarographically. The stability constants pL are summarized in Table 6. 

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

In the application of the polarographic method of analysis to steel a serious difficulty arises owing to the reduction of iron(III) ions at or near zero potential in many base electrolytes. One method of surmounting the difficulty is to reduce iron(III) to iron(II) with hydrazinium chloride in a hydrochloric acid medium. The current near zero potential is eliminated, but that due to the reduction of iron(II) ions at about - 1.4 volts vs S.C.E. still occurs. Other metals (including copper and lead) which are reduced at potentials less negative than this can then be determined without interference from the iron. Alternatively, the Fe3 + to Fe2+ reduction step may be shifted to more negative potentials by complex ion formation. 

Mohamed [63] investigated the complexation behavior of amodiaquine and primaquine with Cu(II) by a polarographic method. The reduction process at dropping mercury electrode in aqueous medium is reversible and diffusion controlled, giving well-defined peaks. The cathodic shift in the peak potential (Ep) with increasing ligand concentrations and the trend of the plot of EVl versus log Cx indicate complex formation, probably more than one complex species. The composition and stability constants of the simple complexes formed were determined. The logarithmic stability constants are log Bi = 3.56 log B2 = 3.38, and log B3 = 3.32 [Cu(II)-primaquine at 25 °C]. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

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