Coulometric determination technique

Coulometric Titrations Controlled-current coulometric methods commonly are called coulometric titrations because of their similarity to conventional titrations. We already have noted, in discussing the controlled-current coulometric determination of Fe +, that the oxidation of Fe + by Ce + is identical to the reaction used in a redox titration. Other similarities between the two techniques also exist. Combining equations 11.23 and 11.24 and solving for the moles of analyte gives... 

THE APPLICATION OF COULOMETRIC BROMINATION TECHNIQUES TO THE DETERMINATION OF UNSATURATED ORGANIC COMPOUNDS... 

Lead has also been determined using the hanging mercury drop technique involving electrolysis of the metal into the mercury followed by anodic stripping to measure the lead concentration (149). Marple and Rogers (150) preferred to use a mercury-plated platinum electrode for the coulometric determination of lead a moderate increase in sensitivity was claimed. 

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

The continuous methods combine sample collection and the measurement technique in one automated process. The measurement methods used for continuous analyzers include conductometric, colorimetric, coulometric, and amperometric techniques for the determination of SO2 collected in a liquid medium (7). Other continuous methods utilize physicochemical techniques for detection of SO2 in a gas stream. These include flame photometric detection (described earlier) and fluorescence spectroscopy (8). Instruments based on all of these principles are available which meet standard performance specifications. 

Two distinctly different coulometric techniques are available (1) coulometric analysis with controlled potential of the working electrode, and (2) coulometric analysis with constant current. In the former method the substance being determined reacts with 100 per cent current efficiency at a working electrode, the potential of which is controlled. The completion of the reaction is indicated by the current decreasing to practically zero, and the quantity of the substance reacted is obtained from the reading of a coulometer in series with the cell or by means of a current-time integrating device. In method (2) a solution of the substance to be determined is electrolysed with constant current until the reaction is completed (as detected by a visual indicator in the solution or by amperometric, potentiometric, or spectrophotometric methods) and the circuit is then opened. The total quantity of electricity passed is derived from the product current (amperes) x time (seconds) the present practice is to include an electronic integrator in the circuit. 

The generation of iodine coulometrically at the anode has an extensive application in the Karl Fischer (KF) technique of water determination. The current... 

The techniques of voltammetry/polarography, atomic absorption, ICP, etc., have in most cases supplanted the coulometric approach for the determination of inorganic analytes. Coulometry and the use of coulometry in food analysis have recently been reviewed [473,476]. 

UV detection, diode-array detector (DAD) and fluorescence have been the detection techniques used, coupled to HPLC for the analysis of OTC. UV detection is set at 355 nm [49-51], 350 nm [40], or at 353 nm [52], Using the diode array detector [49] offers advantages that the target peak can be identified by its retention time and absorption spectrum. Compared to UV detection, fluorescence detection is generally more specific and is less interfered by other compounds in the sample matrix [51]. A HPLC method with electrochemical detection has also been suggested recently. Zhao et al. [53] described HPLC with a coulometric electrode array system for the analysis of OTC, TC, CTC, DC, and methacycline (MC) in ovine milk. An amper-ometric detection coupled with HPLC was developed by Kazemifard and Moore [54] for the determination of tetracyclines in pharmaceutical formulations. 

Oxidative microcoulometry has become a widely accepted technique for the determination of low concentrations of sulfur in petroleum and petroleum products (ASTM D3120). The method involves combustion of the sample in an oxygen-rich atmosphere followed by microcoulometric generation of a triiodide ion to consume the resulting sulfur dioxide. It is intended to distinguish the technique from reductive microcoulometry, which converts sulfur in the sample to hydrogen sulflde that is titrated with coulometrically generated silver ion. 

Ary K, Rona K, Ondi S, Gachalyi B. 1998. High-performance liquid chromatographic method with coulometric detection for the determination of buspirone in human plasma by means of a column-switching technique. J Chromatogr A 797(1-2) 221-226. 

Clement and Paris [17] have devised a pair of methods for the determination of cobalt in steels, especially materials encountered in the nuclear industry. In the first technique, suitable for the analysis of solutions containing 8 to 160 mM cobalt(II), iron(III) is used to oxidize cobalt(II) in a picolinic acid medium, after which the resulting iron(II) is titrated po-tentiometrically with a standard solution of cerium(IV). An alternative procedure, for concentrations of cobalt(II) below 8 mM, involves a constant-current coulometric titration with electrogenerated cerium (IV) to measure the iron(II) that arises from the original reaction between cobalt(II) and iron(III). 

The iodine then reacts with the water that is present. The amount of water titrated is proportional to the total current (according to Faraday s law) used in generating the iodine necessary to react with the water. One mole of iodine reacts quantitatively with 1 mol of water. As a result, 1 mg of water is equivalent to 10.71 C. Based on this principle, the water content of the sample can be determined by the quantity of current that flows during the electrolysis. For this reason, the coulometric method is considered an absolute technique, and no standardization of the reagents is required. 

Coulometry comprises a set of techniques in which the total charge required (not the current, as in potentiometry) to oxidize or reduce the chemical species of interest is measured. The prime virtue of coulometric techniques is that they link the quantity of substance determined directly to the quantity of electrical charge, and thus expensive and often difficult procedures for standardization or calibration can be minimized or eliminated. 

Coulometric techniques involve the determination of the quantity of material electrolyzed from the amount of charge passed through an electrochemical cell during electrolysis. Faraday s law relates the measured charge to the amount of material electrolyzed,... 

Different experimental approaches are possible with the same endpoint detection method. For example, the titration curve can be plotted and the endpoint determined graphically. First and second derivative curves can be plotted or the derivatives obtained electronically. Another approach is to titrate to a predetermined endpoint signal. This technique is very useful with coulometric titrations, and many examples, especially those involving potentiometric endpoint detection, are found in the literature. The most widely applicable way... 

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