Coulometer mercury

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. 

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 . 

The mercury coulometer has been employed chiefly for the measurement of quantities of electricity for commercial purposes, e.g., in electricity meters. The form of apparatus used is shown in Fig. 5 the anode consists of an annular ring of mercury A) surrounding the carbon cathode (C) the electrolyte is a solution of mercuric iodide in potassium iodide. The mercury liberated at the cathode falls off, under the influence of gravity, and is collected in the graduated tube Z). From the height of the mercury in this tube the quantity of electricity passed may be read off directly. When the tube has become filled with mercury the apparatus is inverted and the mercury flows back to the reservoir J . In actual practice a definite fraction only of the current to be measured is shunted through the meter, so that the life of the latter is prolonged. The accuracy of the mercury electricity meter is said to be within 1 to 2 per cent. 

The second type of cell is a mercury pool type. A mercury cathode is particularly useful for separating easily reduced elements as a preliminary step in an analysis. l or example, copper, nickel, cobalt, silver, and cadmium are readily separated from ions such as aluminum, titanium, the alkali metals, and phosphates. The precipitated elements dissolve in the mercury little hydrogen evolution occurs even at high applied potentials because of large overvoltage effects. A coulomet-ric cell such as that shown in Figure 24-5b is also useful for coulometric determination of metal ions and certain types of organic compounds as well. 

A constant-potential coulometric determination of copper is being done using a mercury-pool cathode and a water coulometer. A volume of 32.14 ml of hydrogen-oxygen mixture is obtained. The temperature of the gas is 24.0°C and the barometric pressure in the room is 752.0 mm of mercury. The water vapor pressure above the 0.1 M sodium sulfate solution in the coulometer is as follows ... 

UV-spectra were measured with a Perkin-Elmer Lambda 5 spectrophotometer and NMR-spectra with a Bruker WN-250 spectrometer. Macroscale electrolyses were performed in a U-shaped cell with a stirred mercury pool under N2- The potential was controlled by a Jaissle potentiostat. Model 5000T-B, and the progress of the reaction followed by a coulometer developed and built by the electronic workshop of the University of Konstanz. A silver/silver chloride electrode (SSE) was used as a reference. 

Bismuth may be deposited on platinum electrodes from a number of different electrolytes. (See Table 3.) Lingane (24) also separated bismuth from other metals having reduction potentials differing by less than 0.2 V using a stirred mercury-pool cathode at —0.35 V vs. SCE. His work is noteworthy for its direct use of coulometers to... 

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