In coulometry

In coulometry, current and time are measured, and equation 11.24 or equation 11.25 is used to calculate Q. Equation 11.23 is then used to determine the moles of analyte. To obtain an accurate value for N, therefore, all the current must result in the analyte s oxidation or reduction. In other words, coulometry requires 100% current efficiency (or an accurately measured current efficiency established using a standard), a factor that must be considered in designing a coulometric method of analysis. 

In voltammetry the working electrode s surface area is significantly smaller than that used in coulometry. Consequently, very little analyte undergoes electrolysis, and the analyte s concentration in bulk solution remains essentially unchanged. 

Extraction is widely used in analytical chemistry due to its simplicity, rapidity and ability of using to sepai ate as well as concentration and trace compounds. Extraction is applied to concentrate the detecting components. The combination of extraction and photometry or voltampermetry is known, however it was not used in coulometry. 

In coulometry, one must define exactly the amount of charge that was consumed at the electrode up to the moment when the endpoint signal appeared. In galvanosta-tic experiments (at constant current), the charge is defined as the product of current and the exactly measured time. However, in experiments with currents changing continuously in time, it is more convenient to use special coulometers, which are counters for the quantity of charge passed. Electrochemical coulometers are based on the laws of Faraday with them the volume of gas or mercury liberated, which is proportional to charge, is measured. Electromechanical coulometers are also available. 

In coulometry these exchange membranes are often used to prevent the electrolyte around the counter electrode from entering the titration compartment (see coulometry, Section 3.5). However, with membrane electrodes the ion-exchange activity is confined to the membrane surfaces in direct contact with the solutions on both sides, whilst the internal region must remain impermeable to the solution and its ions, which excludes a diffusion potential nevertheless, the material must facilitate some ionic charge transport internally in order to permit measurement of the total potential across the membrane. The specific way in which all these requirements are fulfilled in practice depends on the type of membrane electrode under consideration. 

In coulometry, one measures the number of coulombs required to convert the analyte specifically and completely by means of direct or indirect electrolysis. 

In coulometry, the analyte is quantitatively electrolyzed and, from the quantity of electricity (in coulombs) consumed in the electrolysis, the amount of analyte is calculated using Faraday s law, where the Faraday constant is 9.6485309 xlO4 C mol-1. Coulometry is classified into controlled-potential (or potentiostatic) coulometry and controlled-current (or galvanostatic) coulometry, based on the methods of electrolysis [19, 20]. 

This integrating circuit is used to give linear and cyclic voltage scans in polarography and voltammetry. It is also used as a coulometer in coulometry. 

In coulometry, the moles of electrons needed for a chemical reaction are measured. In a coulometric (constant current) titration, the time needed for complete reaction measures the number of electrons consumed. Controlled-potential coulometry is more selective than... 

The main problem in evaluating the uncertainty of measurements in coulometry lies in identification of important uncertainty sources and estimation of their contribution (Table 2). With very low instrumental uncertainty, other factors become limiting to the achievable uncertainty, mainly those connected to the chemical processes in the cell and the homogeneity of the material. 

Thin-layer cells have been used with linear sweep, in coulometry and in chronopotentiometry. Here we limit the discussion to linear sweep— descriptions of this and other techniques can be found in Refs. 19-21. 

Amperostatic control is easier than potentiostatic. Therefore they predominated, preferably in coulometry, before the appearance of the potentiostat in 1942. 

In coulometry the stoichiometry of the electrode process should be known and should proceed with 100% current efficiency, and the product of reaction at any other electrode must not interfere with the reaction at the electrode of interest. If there are intermediate reactions, they too must proceed with the desired accuracy. In practice the electrolytic cell is designed to include isolation chambers. Losses of solute through diffusion, through ionic or electrical migration, and simply through bulk transfer must be minimal. Finally, the end point has to be determined by one of the many techniques used in titrations generally, whether coulometric or not. Both indeterminate and determinate end-point errors limit the overall accuracy achieved. Cooper and Quayle critically examined errors in coulometry, and Lewis reviewed coulometric techniques. 

Bulk electrolysis methods are also classified according to purpose. For example, one form of analysis involves determination of the weight of a deposit on the electrode (electrogravimetry). In this case 100% current efficiency is not required, but the substance of interest must be deposited in a pure, known form. In coulometry, the total quantity of electricity required to carry out an exhaustive electrolysis is determined. The quantity of material or number of electrons involved in the electrode reaction can then be determined by Faraday s laws, if the reaction occurred with 100% current efficiency. For electroseparations, electrolysis is used to remove, selectively, constituents from the solution. 

There have been numerous applications of controlled-potential coulometry to analysis. Many electrodeposition reactions that are the basis of electrogravimetric determinations can be employed in coulometry as well. However, some electrogravimetric determinations can be used when the electrode reactions occur with less than 100% current efficiency, for example, the plating of tin on a solid electrode. Coulometric determinations can, of course, also be based on electrode reactions in which soluble products or gases are formed (e.g., reduction of Fe(III) to Fe(II), oxidation of 1 to I2, oxidation of N2H4 to N2, reduction of aromatic nitro compounds). Many reviews concerned with controlled-potential coulometric analysis have appeared (1, 20-22) some typical applications are given in Table 11.3.2. 

The theory for the different reaction schemes involves ordinary (rather than partial) differential equations, because the electrolyzed solution is assumed to be essentially homogeneous (see Section 11.3.1). The concentrations are functions of t during the bulk electrolysis, but not of x. The measured responses in coulometry are the i-t curves and the apparent number of electrons app consumed per molecule of electroactive compound. From the quantity of electricity passed during the electrolysis Q t), app can be calculated as... 

The diagnostic criteria for the EC case in coulometry are (a) a current that does not decay to background and (b) a continually increasing (with app values that are larger than expected). Further details about EC reactions in coulometry are available (100-102). 

We will now briefly consider the results of theoretical treatments for other cases of coupled chemical reactions in coulometry. Detailed reviews have appeared (103, 104). 

A number of other reaction schemes in coulometry have been treated (103, 104). The diagnostic criteria for various reaction mechanisms are given in Table 12.7.1 (103). 

Let us begin by pointing out the basic differences between voltammetry and the two types of electrochemical methods that we discussed in earlier chapters. V oltammetry is based on the measurement of tlie current that develops in anelecirochomical cell under conditions where concentration polarization exists. Recall from Section 22H-2 that a polarized electrode is one to which we have applied a voltage In excess of that predicted by the Nernst equation to cause oxidation or reduction to occur. In contrast, poientiomeirlc measurements are made at currents that approach zero and where polarization is absent. Voltammetry differs from coulometry in that, with coulometry, measures are lakcn to minimize or compensate for the effects of concentration polarization. Furthermore, in voltammetry there is minimal consumption of analyte, whereas in coulometry esse ntially all of the analyte is converted to another Slate. 

In coulometry, the analyte in the sample volume is exhausted completely. This distinguishes the method from amperometry or voltammetry where the level of current is measured, which is controlled by the concentration through its influence on the rate of... 

In the conductivity, potentiometric, and voltam-metric measurements the response is correlated to concentration or activity of the analyte usually by using calibration curves. In coulometry, however, the charge measured gives directly the amount of substance and therefore no calibration is needed. However, in coulometry the sample is consumed in the measurements and the problem is that the method requires 100% current efficiency to be reliable. Conductimetry and potentiometry are sample nonconsuming methods. In voltammetry, only an insignificant amount of the sample is consumed and therefore the measurement can be repeated. Only in voltammetric stripping methods of very low concentrations of the analyte the amount consumed at the electrode reaction has to be considered if repeated measurements are to be done. 

In coulometry, electrons participating in a chemical reaction are counted to learn how much analyte reacted. A constant current of I amperes (= / C/s) flowing for t seconds provides an electric charge of = It ... 

In coulometry, we measure the total number of electrons (= current x time) that flow during a chemical reaction. 

The essential difference between voltammetric and other potentiodynamic techniques, such as constant current coulometry, is that in voltammetry an electrode with a small surface area (< 10mm ) is used to monitor the current produced by the species in solution reacting at this electrode in response to the potential applied. Because the electrode used in voltammetry is so small, the amount of material reacting at the electrode can be ignored. This is in contrast to the case in coulometry where large area electrodes are used so that all of a species in the cell may be oxidized or reduced. 

Electrochemical detection of ascorbic acid is based on the oxidation of ascorbic acid to DHA (Fig. 2). Amperometry and coulometry are the measurements of current at a constant electrode potential. The main difference between these two measurements is the amount of analyte oxidized in the detector in amperometry the oxidation and current are limited in coulometry, the analyte is totally oxidized. The structure of an amperometric detector is usually a flow-by cell, whereas in coulometry a porous flow-through cell is used. In coulometry, a higher amount of analyte is allowed in contact with the electrode surface and sensitivity increases. Working with electrochemical detector, the components of mobile phase must allow for distinct separation of ascorbic acid and be conductive to carry the charge of the analyte. However, the mobile phase must not yield too high background signal. 

---