Coulometer

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. 

Apparatus. The source of current is a potentiostat which is used in conjunction with a reference electrode (commonly a saturated calomel electrode) to control the potential of the working electrode. The circuit will be essentially that shown in Fig. 12.2(a) but with the addition of the integrator or of a coulometer. 

Correlation coefficient critical values (T) 842 Costs of equipment 11 Coulomb 504, 529 Coulometer 531... 

An instantaneous detector is related functionally to an integrating detector as an ammeter is to a coulometer. Most detectors can function in either manner, though not necessarily with comparable efficiency or... 

Thomas Edison was faced with the problem of measuring the electricity that each of his customers had used. His first solution was to use a zinc coulometer, an electrolytic cell in which the quantity of electricity is determined by measuring the mass of zinc deposited. Only some of the current used by the customer passed through the coulometer. (a) What mass of zinc would be deposited in 1 month (of 31 days) if... 

C19-0097. Electrochemistry can be used to measure electrical current in a silver coulometer, in which a silver cathode is immersed in a solution containing Ag" " ions. The cathode is weighed before and after passage of current. A silver cathode initially has a mass of 10.77 g, and its mass increases to 12.89 g after current has flowed for 15.0 minutes. Compute the quantity of charge in coulombs and the current in amperes. 

C19-0098. When Thomas Edison first sold electricity, he used zinc coulometers to measure charge... 

C19-0099. In a silver coulometer (see Problem ), both electrodes are silver metal. Draw a molecular picture that illustrates the reactions that occur during operation of this coulometer. 

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. 

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. 

In contrast with the other chemical coulometers, which are fairly accurate especially with high Q values, but cumbersome, the coulometric coulometer is rapid and sensitive, especially with low Q values. 

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. 

Although the E 636 allows Karl Fischer water determinations, as any other titration, a separate microprocessor-controlled 658 KF processor has been developed, and there is also the microprocessor-controlled 652 KF coulometer (see pp. 221-222). 

A variant of the Karl-Fischer water determination was described [40], By heating the drug substance, the contained water was transferred into a titration cell by a carrier gas. The automated system consisted of an oven sample processor and a coulometer. 

Moisture has been determined by Karl Fisher moisture titration at 300°C (model Metrohm 756 KF Coulometer with 707 KF oven, available from Brinkmann Instruments, Inc. of Westbury, NY, USA). 

Fig. 1.97.1. Schema of the Coulometer MeBzelle DL 36 for measurement of residual moisture content (RM) after Karl Fischer. In the titration cell (1) iodine is electrolytically produced (3) from an iodine-containing analyt (2). Water in the titration cell reacts with the iodine. When the water is used up, a small excess of iodine is produced, which is detected by special electrodes, which leads to iodine production being stopped. The amount of water in the cell can be calculated from the reading of the coulometer, and the amount of electrical charge needed. The solids are introduced into the cell either by a lock, or the water is desorbed in an oven and carried by a gas stream into the cell. 10 pg in a sample can be detected with an accuracy of reading of 0.1 pg (KF Coulometer DL36, Mettler-Toledo AG, CH-8603 Schwerzenbach, Switzerland).

Ethyl lactate was purchased from Aldrich Chemical Company, Inc., and used without further purification. The water content was 0.8 mg/mL by Karl Fisher titration (Metrohm model 684 KF coulometer). 

Faraday s laws of electrolysis form the basis of quantitative coulomet-ric analysis. They are ... 

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]. 

If we were to place a zero-resistance meter between the two electrodes, we could monitor the amount of charge that flows. Such a meter would be called an ammeter if it measured the current, or a coulometer if it measured the charge. (In practice, most modem meters are multi-function devices and can measure both, changing from one function to another at the flick of a switch.)... 

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... 

Figure 5.1 Schematic representation of a cell during discharge, showing a meter reading the charge or current passed. A coulometer placed in series will measure the charge passed, while an ammeter in series will measure the current passed.

Coulometry carried out with a proper coulometer is usually highly accurate. We need to note, however, that it is an extremely common experimental practice to obtain Q as the integral of current 1 with time t An even more approximate method is to draw a graph of current (as y ) against time (as x ) and then ascertain the area under the curve. If the current is constant, then charge is obtained as Q = l xt This latter... 

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 . 

Can we just take the coulometer reading when determining the overall amount of copper(ii) in solution ... 

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. 

Volhard titration analychem Determination of the halogen content of a solution by titration with a standard thiocyanate solution. fol,hart tT tra-shan voltameter See coulometer. val tam-ad-ar ... 

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