Anions direct potentiometry

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

The first estimate of the Bk(IV)-Bk(III) potential was made in 1950, only a short time after the discovery of the element. A value of 1.6 V was reported, based on tracer experiments (3). Later, in 1959, a refined value of 1.62 0.01 V was reported for the couple, based on the results of experiments with microgram quantities of berkelium (4). The potential of the Bk(IV)-Bk(III) couple has subsequently been determined by several workers using direct potentiometry (220-224) or indirect methods (218, 225, 226). All of the above-mentioned determinations were performed in media of relatively low complexing capability. The formal potential of the Bk(IV)—Bk(III) couple is significantly shifted to less positive values in media containing anions that strongly complex Bk(IV), such as PCV- and CO32- ions (227). This behavior closely parallels that of the Ce(IV)-Ce(III) couple (228). In fact, the Bk(IV)-Bk(III) couple markedly resembles the Ce(IV)-Ce(III) couple in its oxidation-reduction chemistry. 

Applications Potentiometry finds widespread use for direct and selective measurement of analyte concentrations, mainly in routine analyses, and for endpoint determinations of titrations. Direct potentiometric measurements provide a rapid and convenient method for determining the activity of a variety of cations and anions. The most frequently determined ion in water is the hydrogen ion (pH measurement). Ion chromatography combined with potentiometric detection techniques using ISEs allows the selective quantification of selected analytes, even in complex matrices. The sensitivity of the electrodes allows sub-ppm concentrations to be measured. 

An appropriate ion-specific electrode was found to provide a convenient, precise and relatively inexpensive method for potentiometry of copper(II) ion in copper-complex azo or formazan dyes. Copper(II) ion in copper phthalocyanine dyes can be quantified after anion exchange. Twelve commercial premetallised dyes evaluated using this technique contained copper(II) ion concentrations in the range 0.007 to 0.2%. Thus many copper-complex direct or reactive dyes are likely to contribute low but possibly significant amounts of ionic copper to textile dyeing effluents [52]. 

Potentiometry is the measurement of an electrical potential difference between two electrodes (half-ceUs) in an electrochemical cell (Figure 4-1) when the cell current is zero (galvanic cell). Such a cell consists of two electrodes (electron or metallic conductors) that are connected by an electrolyte solution (ion conductor). An electrode, or half-cell, consists of a single metallic conductor that is in contact with an electrolyte solution. The ion conductors can be composed of one or more phases that are either in direct contact with each other or separated by membranes permeable only to specific cations or anions (see Figure 4-1). One of the electrolyte solutions is the unknown or test solution this solution may be replaced by an appropriate reference solution for calibration purposes. By convention, the cell notation is shown so that the left electrode (Mi,) is the reference electrode the right electrode (Mr) is the indicator (measuring) electrode (see later equation 3). ... 

The most commonly used method for the direct observation of mixtures of ionic species (i.e. anions and cations) and molecular species, at a known pH, is spectrophotometry (ultra-violet or visible). This is described in Chapter 4. Whereas potentiometry enables an ionization constant to be determined in about 20 minutes, spectrophotometry takes at least half a working day. Nevertheless, it is particularly suitable for sparingly soluble substances and also for work at high and low pH values which are beyond the range of the glass electrode. It can be used only for substances which absorb ultra-violet or visible light, and the relevant ionic and molecular species must have different spectra. This method is related to potentiometry in that the spectra are determined in buffers whose pH values are measured by potentionietry. Other spectromctric methods, available for specialized purposes, depend on the same principle. 

As summarised in Table 3.4, some electroanalytical methods have been certified by standardisation bodies for the chemical characterisation of ambient water samples, mostly in the class of inorganic substances. Conductometric detection is used in direct method for ionic constituents and also as chromatographic detector for individual cations and anions. Total, inorganic and organic carbon in water can be also assayed by conductometric detection. Amperometric detection has been certified for dissolved oxygen and cyanide. ISE potentiometry is used for standardised measurements of a set of ions and also for the evaluation of water oxidation-reduction potential. Voltammetric detection is the base for diverse methodologies oriented to the determination of trace elements including the most relevant elemental pollutants. 

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