Coulometer electronic

During coulometry at constant potential, the total amount of charge (g) is obtained by integration of the current (7) - time (0 curve or g can be determined directly by using a coulometer (electronic integrator). In principle, the end point 1 = 0, i.e., when the concentration of the species under study becomes zero, can be reached only at infinite time, however, in practice the electrolysis is stopped when the current has decayed to a few percent of the initial values. The change of I and g as a function of time at a constant potential >> e. for a stirred solutions and for an uncomplicated electrolysis, is as follows ... 

It is apparent (Fig. 1.21) that at potentials removed from the equilibrium potential see equation 1.30) the rate of charge transfer of (a) silver cations from the metal to the solution (anodic reaction), (b) silver aquo cations from the solution to the metal (cathodic reaction) and (c) electrons through the metallic circuit from anode to cathode, are equal, so that any one may be used to evaluate the rates of the others. The rate is most conveniently determined from the rate of transfer of electrons in the metallic circuit (the current 1) by means of an ammeter, and if / is maintained constant it can eilso be used to eveduate the extent. A more precise method of determining the quantity of charge transferred is the coulometer, in which the extent of a single well-defined reaction is determined accurately, e.g. by the quantity of metal electrodeposited, by the volume of gas evolved, etc. The reaction Ag (aq.) -t- e = Ag is utilised in the silver coulometer, and provides one of the most accurate methods of determining the extent of charge transfer. 

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. 

Originally, the number of coulombs passed was determined by including a coulometer in the circuit, e.g. a silver, an iodine or a hydrogen-oxygen coulometer. The amount of chemical change taking place in the coulometer can be ascertained, and from this result the number of coulombs passed can be calculated, but with modern equipment an electronic integrator is used to measure the quantity of electricity passed. 

The way in which these alternatives with their particular measuring characteristics are carried out can be best described by (1) controlled-potential coulometry and (2) coulometric titration (controlled-current coulometry). Both methods require an accurate measurement of the number of coulombs consumed, for which the following instrumental possibilities are available (a) chemical coulometers, (b) electrochemical coulometers and (c) electronic coulometers. 

The electronic coulometers allow extremely accurate determinations even of small current or voltage effects there may be some, although low, noise interference, but with today s computerization this and other background signals (e.g., residual current) can be easily eliminated. 

The author has used a home built electronic coulometer (Dr. H. Luftmann, Univer-sitat Munster). 

Overall, the process requires the consumption of two electrons and two protons. The structure and acidity of effective proton donors vary from mineral to carbon acids often, a simple dialkyl malonate is effective. It is easy to monitor current consumption using a simple, commercially available coulometer [3,4]. 

A meter placed in the circuit (see Figure 5.1) tells us the charge (or current) that flows through the electrodes. Following from equation (5.3), though, we see that in fact an ammeter (or coulometer) also tells us how much charge (or current) has flowed through the solution of the cell. In other words, the meter tells us how many electrons have been consumed by electrode reactions in solution. We see... 

Copper was removed from solution by making the hanging mercury-drop electrode (HMDE) sufficiently cathodic, thereby reducing Cu to form Cu. The electrons required for reduction are registered by a coulometer or ammeter in the circuit as charge or current, respectively. When the ammeter read-out says zero (or at least when the coulometer read-out shows that the overall charge passed is constant at a very small level and has stopped increasing), then it is assumed that exhaustive electrolysis (or deposition ) is complete, i.e. we say the solution is exhausted . 

Reduction of analyte occurs at the cathode (on the right-hand side of the cell). Once formed, however, the reduced form of the analyte couple diffuses across the cell - it may also be swept along by the stirred solution - and/or be re-oxidized again at the anode. Clearly, a single molecule of analyte could be oxidized and reduced many times, thus leading to an artificially high charge at the coulometer. For this reason, the two halves of the coulometry cell should be separated if possible, e.g. with a semipermeable membrane or frit, or we should ensure that the product of electron transfer should be a solid, i.e. it is immobilized as soon as it is formed. 

Here, m and M are the amount and the molar mass of the analyte. The coul-ometer is usually an electronic one that integrates the current during the electrolysis, although chemical coulometers, e.g. a silver coulometer and a gas coulometer, can also be used. In this method, the deposition of the analyte is not a necessary process. All substances that are electrolyzed with 100% current efficiency can be... 

Commercial coulometers deliver electrons with an accuracy of —0.1%. With extreme care, the Faraday constant has been measured to within several parts per million by coulom-... 

In a typical procedure, the main compartment in Figure 17-28 is filled with anode solution and the coulometric generator is filled with cathode solution that may contain reagents designed to be reduced at the cathode. Current is run until moisture in the main compartment is consumed, as indicated by the end-point detection system described after the Example. An unknown is injected through the septum and the coulometer is run again until moisture has been consumed. Two moles of electrons correspond to 1 mol of H20 if the I2 H20 stoichiometry is 1 1. 

For halogenated compounds, combustion gives C02, H20, N2, and HX (X = halogen). The HX is trapped in aqueous solution and titrated with Ag+ ions in a coulometer (Section 17-3). This instrument counts the electrons produced (one electron for each Ag+) during complete reaction with HX. 

Coulometer — (previously coulombmeter, or also voltameter) A coulometer is an instrument to measure charge, i.e., to perform -> coulometry. Richards and Heimrod [i] suggested in 1902 the name coulometer to replace the previously used term voltameter . Modern coulometers perform electronic integration of - current over time. However, the first coulometers utilized - Faraday s law, e.g., by weighing the amount of silver that has been deposited on a silver electrode in a silver electrolyte solution the charge could be calculated (silver... 

Coulometers, like the balance, are basic instruments for absolute analysis and they are still used as the most reliable and precise instruments for the analysis of absolute standards. Coulometers are frequently used in elucidating electrochemical reactions because they allow determining the number of transferred electrons when the molar amount of electrolyzed compound is known (-> Faraday s law). When the charge is measured as a function of time, the technique is called chrono-coulometry. See also coulometric titration. 

The mean number of electrons, n, associated with the overall oxidation process, during each electrolysis, is calculated from the analytical results and from the experimental value of the quantity of electricity, Qex, read on a coulometer during the experiment ... 

PotentiostatS and Coulometers For controlled-potential coulometry, we use a potentiostat similar in de.sign to that shown in Figure 22-8. Generally, however, the potentiostat is automated and equipped with a computer or an electronic current integrator that gives the charge in coulombs necessary to complete the reaction, as shown in Figure 22-12. 

The sample was dissolved in 100 mL of methanol after electrolysis for 30 min, the reaction was judged complete. An electronic coulometer in series with the cell indicated that the reduction required 31.23 C. Calculate the percentage of C6H5NO2 in the sample. 

Coulometer A device that permits measurement of the quantity of charge. Electronic coulometers evaluate the integral of the cur-rent/time curve chemical coulometers are based on the extent of reaction in an auxiliary cell. 

Physical Measurements. For the electrolyses, a Wenking potentiostat model 70TS1 and a Koslow Scientific coulometer model 541 were used. Voltammetry with wax-impregnated graphite and rotating platinum electrodes was performed as described elsewhere (7, 8). IR and electronic spectra were measured on Perkin-Elmer 225 and Cary 14 instruments. X-band ESR spectra were recorded at room temperature on a JEOL MES-3X spectrometer. Phosphorus-31 NMR spectra were recorded in the pulse mode on a Varian XL-100 instrument at 40.5 MHz using a deuterium lock, or on a Bruker HFX-90 instrument at 36.43 MHz using a fluorine lock. 

Because the current is not constant during the potentiostatic operation, it has to be integrated during the experiment for calculating the charge transfer and the current efficiency. Coulometers or electronic integrators are commercially available. If a computer data acquisition system is used, the current integration is possible by software. 

Coulometry. Faraday s laws of electrolysis, enunciated in 183 form the basis of coulometric techniques. By the beginning of the present century the silver coulometer had been shown to provide an accurate means for the measurement of quantities of electricity. An excellent survey of various chemical and other coulometers is available ( ). The electronic digital coulometer, first described in 1962 ( ), was a major practical advance. 

---