Iodine coulometer

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. 

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).

Apart from the coulometers described, there are also instruments in which products of electrolyses are determined by titration. The iodine coulometer is based upon the anodic liberation of iodine from solutions of potassium iodide, the iodine being determined by titration with thiosulphate. Besides current efficiency also energy efficiency /) is very important in technical practice. This is the ratio of theoretically required quantity of energy Wt to the quantity of energy Wt actually consumed for the electrochemical preparation of a given product (expressed as a percentage) ... 

To separate sulfur-containing organophosphate insecticides, a column temperature of 180° C. and a nitrogen carrier gas flow of 39 ml. per minute were used. The coulometer was operated at maximum sensitivity (512 ohms) at a damping position of 4 and a bias of +100. An iodine-iodine ion cell was used. 

The reliability of this value of the faraday has been confirmed by measurements with the iodine coulometer designed by Washburn and Bates, and employed by Bates and Vinal. The apparatus is shown in Fig. 4 it consists of two vertical tubes, containing the anode (A) and cathode (C) of platinum-iridium foil, joined by a V-shaped portion. 

The faraday, calculated from the work on the iodine coulometer, is thus 96,514 coulombs compared with 96,494 coulombs from the silver coulometer the agreement is within the limits of accuracy of the known atomic weights of silver and iodine. In view of the small difference between the two values of the faraday given above, the mean figure 96,500 coulombs is probably best for general use. 

The results obtained at the cathode in the iodine coulometer show that Faraday s laws hold for the reduction of iodine to iodide ions the laws apply, in fact, to all types of electrolytic reduction occurring at the cathode, e.g., reduction of ferric to ferrous ions, ferricyanide to ferro-cyanide, quinone to hydroquinone, etc. The laws are applicable similarly to the reverse process of electrolytic oxidation at the anode. The equivalent weight in these cases is based, of course, on the nature of the oxidation-reduction process. 

Calculate the amount of iodine that w ould be liberated by a quantity of electricity which sets free 34.0 cc. of gas, at S.T.P., in an electrolytic gas coulometer. 

Tests of Faraday s Law Tinder Varying Conditions. We have already seen that, if disturbing effects are taken into account, Faraday s law applies to all electrochemical reactions which have been carefully studied. The tests so far mentioned, however, have all been made at ordinary temperature, under atmospheric pressure, and in aqueous solutions. A number of researches have been carried out to find out whether variations in the nature of the solvent, or variations in the physical conditions, such as temperature and pressure, have any influence on the constant in Faraday s law. No real variation in the constant has yet been observed. There are, to be sure, many apparent deviations from the law, such as that observed with the copper coulometcr, which gives a deposit at the cathode which is lighter than the computed value. In this case, as has been seen, the cause of the discrepancy has been found to be the occurrence of a disturbing reaction, In every similar case a simple explanation of the apparent deviation has been readily found. The comparison of the iodine, and of the copper coulometer, with the silver coulometer, as has been described in previous sections, affords precise evidence, for these reactions at least, that Faraday s law is indc-... 

The Iodine Coulometer. The iodine coulometer, as developed by Washburn and Bates,7 has a precision equal to that of the silver coulom-... 

The equilibrium, I2 + I = Ia, exists in these solutions, but. this does not affect the stoichiometrical relations. After the electrolysis is completed, a delivery tube is connected to D, and the anode and cathode portions of the electrolytes are drawn over into separate flasks. The two portions are then titrated for iodine with arsenious acid solution which lias been standardized against carefully purified iodine. By comparison with the silver coulometer, Bates and Vinal8 found the electrochemical equivalent of iodine to be 0.00131505 gram per coulomb, leading to a value of the faraday of 96,514. The accuracy of the experimental work may be judged from the results for the different experiments given in Table I. 

An air sample, polluted with SO2, is passed through a continuous coulometric cell which automatically maintains a small concentration of I2 by electrogenerating it from acidic potassium iodide. The SO2 is oxidized to SO3 by the iodine. If the air sample flow rate is 5 1/min, and the coulometer averaged an output of 1.40 mA to maintain the I2 concentration for 10 min, what is the concentration... 

A galvanic cell-type (Hersch cell) coulometer consists of a platinum (Pt) cathode, KBr (or KI) electrolyte (stripping solution), and active carbon anode. When air containing ozone is bubbled into the solution, the bromine (or iodine) liberated by the ozone is reduced at the Pt cathode and hydroxide ion discharges at the carbon anode ... 

The quantity of electricity passed through a cell may be determined by measuring the current as a function of time, and determining the area under the current-time curve. A calibrated galvanometer with a short response time is used. Alternatively a chemical coulometer is commonly employed. This consists of an electrolytic cell in series with the experimental cell, the same amount of electricity therefore passing through both the chemical reaction at the cathode or anode (or both) of the coulometer must occur with 100% current efficiency, and should be easily and accurately estimated. The deposition of silver at the cathode of a silver coulometer (q.v.), the dissolution of silver from a silver anode (see Faraday constant) and the reaction 2e + l2 2r of the iodine coulometer (q.v.) all satisfy these conditions, and another common and convenient device is the copper coulometer (q.v.). 

When the quantity of electricity is small, a colorimetric method of estimation will be more sensitive. This has been applied to the iodine formed in an iodine coulometer, and to silver dissolved from a silver anode (estimated as silver dithizonate). Volumetric analysis has also been applied for instance, when water is electrolysed, the reaction in the cathode compartment is e + H20- iH2 + 0H", and the alkali may... 

This very accurate coulometer makes use of the reaction e + 2l2 I. The cathode and anode compartments contain Pt electrodes, and are separated by a column of 10% KI solution a standardised solution of I2 in KI is introduced to cover the cathode. During electrolysis, iodine is formed at the anode and dissolves in the KI solution, while iodine is reduced to iodide at the cathode. After the experiment both the loss and gain can be determined volumetrically. 

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