Metals, 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 metal ions such as Al3 + which are readily hydrolysed can present problems in aqueous solution, but these can often be overcome by the use of non-aqueous solvents. Typical non-aqueous solvents, with appropriate supporting electrolytes shown in parentheses, include acetic acid (CH3C02Na), acetonitrile (LiC104), dimethylformamide (tetrabutyl-ammonium perchlorate), methanol (KCN or KOH), and pyridine (tetraethyl-ammonium perchlorate), In these media a platinum micro-electrode is employed in place of the dropping mercury electrode. 

Lead and thallium are elements whose polarographic determination causes no trouble. Their half-wave potentials are very favourably situated in the potential range of various electrolytes. Therefore the polarographic waves of these metals are easily measurable. The determination of lead and thallium was often used in toxicology and industrial hygiene (see [3]). 

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. 

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). 

Zereini F, Alt F, Messerschmidt J, von Bohlen A, Liebl K, Piittmann W (2004) Concentration and distribution of platinum group elements (Pt, Pd, Rh) in aiibome particulate matter in Frankfurt am Main, Germany. Env Sci Technol 38 1686-1692 Zereini F, Alt F, Messerschmidt J, Wiseman C, Feldmann 1, von Bohlen A, Muller J, Liebl K, Piittmann W (2005) Concentration and particle size distribution of toad-specific heavy metals in urban airborne particulate matter. Env Sci Technol 39 2983-2989 Zhao Z, Preiser H (1986) Differential pulse polarographic determination of trace levels of platinum. Anal Chem 58 1498-1501... 

In order to avoid this difficulty, researchers sought a reference ion whose polarographic half-wave potential does not depend (or scarcely so) on the solvent, and the rubidium ion proved suitable [PI 47]. With the aim of characterizing the stability sequence of the solvates, therefore, Gutmann [Gu 68] compared the halfwave potentials, relative to the rubidium ion, of the alkali and alkaline earth metal ions, determined in various solvents. The stability series thus obtained corresponded not only to the sequence of the donicities of the solvents but, as expected, also to the decreasing stability sequence of the complexes formed by the metal ions in the given solvents. 

In addition to carbon and the noble metals, the principle material used for the working electrode is mercury, especially for capillary and film electrodes, Because of the high overvoltage of hydrogen on mercury, the latter electrodes ensure a relatively wide potential range for voltammetric/ polarographic determinations in various supporting electrolyte solutions. If capillary electrodes are used, the electrode surface can be renewed easily and reproducibly as with the DME, the SMDE and the HMDE. 

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. 

For a long time the dropping mercury electrode was the basic electrode used in polarography. However, with time, new problems needed for their solution new electrode materials, as mercury cannot be used in the anodic range. This was the case for the direct polarographic determination of some anions, for some organic compounds, and for noble metal and rare-earth metal ions. The mercury electrode... 

The polarographic studies of a large number of metal species in aqueous media give ill-defined waves due to the occurrence of maxima, hydrolytic reactions and catalytic hydrogen waves, whereas such investigations in nonaqueous solvents do not present these problems. - In addition the lower oxidation states of metal ions can be studied in nonaqueous media. The polarographic determinations of half-wave potentials of metal ions in different solvents indicate a measure of relative strengths of the metal ion solvent bonds. ... 

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 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. 

Faradaic rectification polarographic studies have been carried out for a mixture containing several metal ions together and also for individual inorganic depolarizers so as to explore the applicability and limitations of the method and to determine kinetic parameters for some of them. For comparison, some of the dc and ac polarograms have also been recorded simultaneously. In the following, the details of the experimental technique used will be described and the potentiality of the technique in qualitative and quantitative analysis will be examined. The applicability of the method in the... 

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. 

Bond et al. [791 ] studied strategies for trace metal determination in seawater by ASV using a computerised multi-time domain measurement method. A microcomputer-based system allowed the reliability of the determination of trace amounts of metals to be estimated. Peak height, width, and potential were measured as a function of time and concentration to construct the database. Measurements were made with a potentiostat polarographic analyser connected to the microcomputer and a hanging drop mercury electrode. The presence of surfactants, which presented a matrix problem, was detected via time domain dependent results and nonlinearity of the calibration. A decision to pretreat the samples could then be made. In the presence of surfactants, neither a direct calibration mode nor a linear standard addition method yielded precise data. Alternative ways to eliminate the interferences based either on theoretical considerations or destruction of the matrix needed to be considered. 

For an irreversible reduction the half-wave potential is determined not only by the standard electrode potential but also by the polarographic overvoltage. For a simple electrode process the metal ion-solvent interaction is mainly responsible for the polarographic overvoltage and hence E[ j of such irreversible reductions may also be considered as a function of the solvation 119f... 

A perfect prototype of an ideally cation-permselective interface is a cathode upon which the cations of a dissolved salt are reduced. Experimental polarization curves measured on metal electrodes fit the theory very closely. Since in dimensional units the limiting current is proportional to the bulk concentration, the polarization measurements on electrodes may serve for determining the former. This is the essence of the electrochemical analytical method named polarography. (For the theory of polarographical methods see [28]—[30].)... 

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