Electrode contamination

The auxiliary electrode, which is normally a mercury pool, must be positioned in a compartment separate from the working electrode. Such a separation compromises the desired symmetric disposition of the electrodes. Normally, the compartments of a macroelectrolysis cell are separated by sintered glass frits, such that the catholyte and the anolyte are not mixed. In fact, if the working electrode is involved, for example, in a cathodic process, the auxiliary electrode will act as an anode. This implies that the auxiliary electrode will produce oxidized material (by anodic decomposition of the solvent itself, of the supporting electrolyte, of mercury-contaminated products or of electroactive residues diffused at the auxiliary electrode) that may subsequently be reduced at the working electrode, contaminating and falsifying the primary process. 

Basically, the calomel electrode consists of mercury, mercurous chloride (calomel), and chloride ion. The concentration of potassium chloride is 0.1 M in an aqueous-organic solvent (50 50) of the same nature as that contained in the solution to be investigated. The junction with the test solution is realized either with a capillary or a porous stone. When the capillary is used, a small hydrostatic pressure is maintained inside it in order to avoid any electrode contamination by the test solution. In the main part of our investigation, the porous stone junction was used. Moreover, the calomel electrode is thermostatted at 20°C, and temperature variations of this electrode giving appreciable emf variations involve uncertainty on the pon determination on the order of 0.2-0.3 poH unit/ 10°C. 

Depending on the samples being titrated, electrode contamination may be an issue. The electrodes may become coated when substances such as oils and sugars are titrated. This results in delayed endpoint detection and an overtitration. A dark brown color on the electrodes indicates that they should be cleaned. The coatings can usually be removed by polar organic solvents or by cleaning the platinum physically. 

Of the other established methods one can be less certain. Electrochemical methods, other than ion selective electrodes, seem to be practised more in academic laboratories than industrial, and are prone to fundamental problems relating to electrode contamination and chemical interferences. Nuclear methods would seem to have reached their apogee (if one classifies radio immuno assay as biological rather than nuclear) and the same seems to be true of thermal methods. This is not to say that these methods will not continue to be used and to be important. It is a comment that despite the missionary work of numerous adherents of these methods, one notes in the large industrial laboratories much more application of and enthusiasm for spectroscopy, chromatography and biological methods. 

The electrochemical detector in the form described above is extremely sensitive but suffers from a number of drawbacks. Firstly, the mobile phase must be extremely pure and in particular free of oxygen and metal ions. A more serious problem arises, however, from the adsorption of the oxidation or reduction products on the surface of the working electrode. The consequent electrode contamination requires that the electrode system must be frequently calibrated to ensure accurate quantitative analysis. Ultimately, the detector must be dissembled and cleaned, usually by a mechanical abrasion procedure. Much effort has been put into reducing this contamination problem but, although diminished, the problem has not been completely eliminated particularly in the amperometric form of operation. Due to potentially low sensing volume the detector is very suitable for use with small bore columns. 

Classical methods for analysis of manganese have been the periodate method in air, and the permanganate method in water (Saric 1986). Nowadays, among the solid-state analytical methods available, neutron activation analysis (NAA) is the most reliable to determine manganese in biological and environmental materials. This method of choice combines both high specificity, sensitivity and reproducibility for very low concentrations of manganese, whereas X-ray fluorescence (XRF) spectroscopy showed standardization problems and arc/ spark emission spectroscopy suffered from electrode contamination (Chiswell and Johnson 1994). 

Of the solid state analysis methods, namely, neutron activation analysis (NAA), X-ray fluorescence spectroscopy (XRF), and arc/spark emission spectroscopy, only NAA has found wide application for manganese analysis of biological samples. Although Birks et al. [102] claim high sensitivity for XRF analysis of manganese in freeze-dried samples, there are problems of standardization of the technique at low manganese concentrations, while solid emission spectroscopy suffers markedly from electrode contamination. On the other hand, NAA has both a high specificity and sensitivity... 

Measurement of differential capacitance. Differential capacitances vary with potential, but they can be measured provided the amplitude of the applied a.c. potential E used for measurements does not exceed a few milivolts ([1], p. 29). Differential capacitances are independent of E provided E is small enough. The measurements of the differential capacitance can be erroneous because of contamination by traces of strongly adsorbed organic impurities in the electrolyte. Gra-hame introduced the systematic use of the dropping mercury electrode and was able, in this way, to considerably minimize electrode contamination. Adsorption of impurity traces is generally a slow process because of diffusion control, and frequent renewal of the mercury drop provides a clean surface [21-24]. 

Amperometric 10-12 10= Cannot be used with gradient elution. Detects only ionic solutes. Excellent sensitivity, selective but problems with electrode contamination. 

Titanium.—Ti, a.n. 22 a.w. 46-1. An obscure micro-constituent of plants and higher animals. Fish, including mackerel, carp, herring, whiting, have values from 0-3 mg. Ti to 0-9 mg. per kg. fresh tissue, Mammalian liver has an average value of 0-6 mg. per kg. The metal has been detected spectroscopically in human lung tissue and blood ash. Webb has shown that serious errors may arise from the use of carbon electrodes contaminated by traces of titanium which are vmmaskcd by the alkalies in tissue ash. For this reason, many claims as to the distribution of titanium require confirmation. 

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