Electrical coulometer

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. 

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... 

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).

COULOMETER. Also known as cnulombmeier, a device for the measurement of electric currem. Originally developed (1916) by the U.S. National Bureau of Standards, the silver coulometer consists of a small platinum vessel, acting as the cathode, into which a pure silver anode is immersed. An aqueous solution of silver nitrate (15% AgNO<. wl) of very high purity is used as the electrolyte, In use. both the quantity of silver deposited and the lime are carefully noted. These measurements permit a calculation of the average currcnl strength. 

Apparatuses called coulometers are used for the measurement of the quantity of electricity. These are in principle electrolytic cells in which a given electrochemical process is allowed to proceed under exactly defined conditions it must be ascertained that the process in question proceeds in one direction only thus guaranteeing 100 p. c. current efficiency of electrolysis. From the amounts of products obtained which must be suitably determined, the quantity of electricity which passed through the cell can be calculated by applying Faraday s law. 

One of the most accurate instrument for the measurement of quantities of electricity is the silver coulometer. A solution of purest silver nitrate in distilled water (20 to 40 parts AgN03 to 100 parts H20) is electrolyzed in a platinum crucible which serves as the cathode. An anode of pure silver rod is partly immersed into the solution and enclosed by a ceramic diaphragm so that mechanically separated anode slime cannot sink to the bottom of the crucible. Current density should not exceed 0,02 amp. per sq. cm. on the cathode and 0,2 amp. per sq. cm. on the anode. The level of liquid within the diaphragm should be somewhat lower than in the platinum crucible. When the electrolysis is finished the platinum crucible is washed with pure distilled water, dried and weighed. From the weight increase the quantity of electricity (in coulombs) passed through the solution is then calculated. 

If minute quantities of electricity are to be measured the electrolytic gas coulometer is recommended in this case a 15 per cent solution of sodium hydroxide is used as electrolyte while the electrodes are of platinum or nickel. Both the hydrogen and the oxygen escaping from the closed electrolytic cell... 

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 ... 

The silver nitrate is purified by repeated crystallization from acidified solutions, followed by fusion. The purity of the salt is proved by the absence of the so-called volume effect, the weight of silver deposited by a given quantity of electricity being independent of the volume of liquid in the coulometer this means that no extraneous impurities are included in the deposit. The solution of silver nitrate employed for the actual measurements should contain between 10 and 20 g. of the salt in 100 cc. it should be neutral or slightly acid to methyl red indicator, after removal of the silver by neutral potassium chloride, both at the beginning and end of the electrolysis. The anode should be of pure silver with an area as large as the apparatus permits the current density at the anode should not exceed 0.2 amp. per sq. cm. To prevent the anode slime... 

The electrolytic gas coulometer is useful for the approximate measurement of small quantities of electricity the total volume of hydrogen and oxygen liberated in the electrolysis of an aqueous solution of sulfuric acid or of sodium, potassium or barium hydroxide can be measured, and from this the quantity of electricity passed can be estimated. If the electrolyte is dilute acid it is necessary to employ platinum electrodes,... 

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. 

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. 

In following the movement of the boundary, no matter how it is formed, use is made of the difference in the refractive indices of the indicator and experimental solutions if the boundary is to be clearly visible, this difference should be appreciable. If the distance (1) moved in a given time and the area of cross section (a) of the tube are measured, and the equivalent concentration (c) of the experimental solution is known, it is only necessary to determine the number of coulombs (Q) passed for the transference number to be calculated by equation (13). The quantity of electricity passing during the course of a moving boundary experiment is generally too small to be measured accurately in a coulometer. It is the practice, therefore, to employ a current of known strength for a measured period of time the constancy of the current can be ensured by means of automatic devices which make use of the properties of vacuum tubes. 

In organic solvents and according to the cell configuration, the ohmic drop may be prohibitive and the potentiostat must have a high output voltage (50 V or more). The integration of the current with a coulometer enables one to verify Faraday s law and to calculate the electric yield For A + ne- —> B... 

The amount, or quantity, of electricity passing through a circuit is measured in coulombs, which are, in turn, the product of the amperes and time in seconds.2 At an international electrical conference in London (1908) the ampere was adopted as one of the fundamental units. The ampere is defined as the steady current which, when passed through a solution of silver nitrate in water, under definite conditions to be described later, deposits silver at the rate of 0.00111800 gram per second. This value of the ampere is one-tenth of the c.g.s. (electromagnetic) unit within a few parts in one hundred thousand. The instrument used for measuring current in terms of deposited silver is called a silver coulometer or a silver voltameter.8... 

Coulometers. As already mentioned, any instrument designed to measure a quantity of electricity by a determination of its electrochemical effect is called a coulometer. There have been many types of coulometers proposed, but only those which possess exceptional accuracy, or convenience in practical use, will be discussed. 

Example 4. The transport number during electrodialysis (dynamic state transport number) may be measured using a similar two-compartment cell with reversible electrodes such as silver-silver chloride electrodes (Figure 4.6). The transport number is calculated from the transported ions and the amount of electricity passed through the membrane, which is measured with a coulometer. To eliminate the effect on the transport number of electrolytes diffusing through the membrane, it is desirable that a solution of the same concentration be used on both sides of the membrane. When there is a concentration difference between the two sides, the transport number is affected by diffusion of electrolytes through the membrane. 

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