Quantity of electricity measurement

The apparatus which Hittorf employed consisted essentially of a glass cylinder with a cathode near the top and an anode near the bottom. The anode was chosen to be of the same metal as in the salt used (e.g.. copper in work with copper sulphate solutions) in order to maintain the chemical nature of the electrolyte. The cathode was of platinum, gold or silver on which the metal ions plated out. The upper catholyte solution therefore became more dilute during electrolysis and the lower anolyte more concentrated, so ensuring gravitational stability. After the passage of a known quantity of electricity (measured with a silver coulometer) the upper half of the cell was slid sideways by means of a glass plate and the catholyte solution was analysed. In the next paper (2.) Hittorf analysed the anolyte solution also and introduced middle sections but unfortunately separated the compartments by means of intestinal membranes. He did make it plain, however, that the results should be calculated with respect to the mass of water in the final solution. 

Ampere-Hour Capacity The quantity of electricity measured in Ampere-hours (Ah) which may be delivered by a cell or battery under specified conditions. 

Coupled voltammetric and pH measurements, performed as previously described during the prolonged electrolysis, are necessary to obtain a complete balance between the quantity of electricity measured and that of the compounds consumed or produced. Such information, concerning both C- and 0-containing species, are particularly useful when the electrochemical process is accompanied or followed by significant chemical transformations like hydrolysis, aldolisation, Cannizzaro reaction, enolisation, etc., involving reactants and the intermediate or final products. At room temperature, the chemical transformation of blank"... 

In contrast, the variation of AnQ -Zne- values and of peak oxidation currents significantly depends on both the initial concentrations of OH" ions or sorbitol (Figures 7 and 8). Peak oxidation currents, reach a maximum for Cqh- 3.10" M and suddenly decrease at Cqh- 10-3 m ith the concentration of OH- ions in the bulk of solution (Figure 7). In contrast, AnoH-Z e- s log Cqh- Plots display a minimum value close to 1.2 for Cog- 5.10- but increase sharply for solutions containing more than 0.01 H NaOH. At relatively high Cqh- concentrations (more than 0.01 M) in the bulk, the accumulation of OH ions near the charged surface favors various chemical transformations of polyols or of the intermediate species issued from the electrooxidation process. The consumption of OH" ions is thus increased with respect to the quantity of electricity measured. 

Faraday s Law of electrolysis states that the amount of chemical change, ie, amount dissolved or deposited, produced by an electric current is proportional to the quantity of electricity passed, as measured in coulombs and that the amounts of different materials deposited or dissolved by the same quantity of electricity are proportional to their gram-equivalent weights (GEW) defined as the atomic weight divided by the valence. The weight in grams of material deposited, IF, is given by... 

The quantity of electric charge is measured m coulombs, and the unit of electric current—the number of coulombs per second that go past any point— is the ampere (A), named after French physicist Andre Marie Ampere ... 

The fundamental requirement of a coulometric analysis is that the electrode reaction used for the determination proceeds with 100 per cent efficiency so that the quantity of substance reacted can be expressed by means of Faraday s Law from the measured quantity of electricity (coulombs) passed. The substance being determined may directly undergo reaction at one of the electrodes (primary coulometric analysis), or it may react in solution with another substance generated by an electrode reaction (secondary coulometric analysis). 

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. 

Since a small quantity of electricity can be readily measured with a high degree of accuracy, the method has high sensitivity. Coulometric titrimetry has several important advantages. 

Electrolytic hygrometry in which the quantity of electricity required to electrolyse water absorbed from the atmosphere on to a thin film of desiccant is measured. 

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... 

Measurement of the quantity of electricity used in an electrochemical reaction at constant potential or constant current. 

Coulometric methods of analysis involve measuring the quantity of electricity required to effect a quantitative chemical or electrochemical reaction and are based on Faraday s laws of electrolysis ... 

As a first step, you need information about measurements in electricity. You know that the flow of electrons through an external circuit is called the electric current. It is measured in a unit called the ampere (symbol A), named after the French physicist Andre Ampere (1775-1836). The quantity of electricity, also known as the electric charge, is the product of the current flowing through a circuit and the time for which it flows. The quantity of electricity is measured in a unit called the coulomb (symbol C). This unit is named after another French physicist, Charles Coulomb (1736-1806). The ampere and the coulomb are related, in that one coulomb is the quantity of electricity that flows through a circuit in one second if the current is one ampere. This relationship can be written mathematically. 

Use the measured current and the time for which the current passed to calculate the quantity of electricity used. 

In controlled-potential coulometry, the analyte is electrolyzed quantitatively with 100% current efficiency and the quantity of electricity Q is measured with a coul-ometer ... 

Recently flow coulometry, which uses a column electrode for rapid electrolysis, has become popular [21]. In this method, as shown in Fig. 5.34, the cell has a columnar working electrode that is filled with a carbon fiber or carbon powder and the solution of the supporting electrolyte flows through it. If an analyte is injected from the sample inlet, it enters the column and is quantitatively electrolyzed during its stay in the column. From the peak that appears in the current-time curve, the quantity of electricity is measured to determine the analyte. Because the electrolysis in the column electrode is complete in less than 1 s, this method is convenient for repeated measurements and is often used in coulometric detection in liquid chromatography and flow injection analyses. Besides its use in flow coulometry, the column electrode is very versatile. This versatility can be expanded even more by connecting two (or more) of the column electrodes in series or in parallel. The column electrodes are used in a variety of ways in non-aqueous solutions, as described in Chapter 9. 

Coulometry comprises a set of techniques in which the total charge required (not the current, as in potentiometry) to oxidize or reduce the chemical species of interest is measured. The prime virtue of coulometric techniques is that they link the quantity of substance determined directly to the quantity of electrical charge, and thus expensive and often difficult procedures for standardization or calibration can be minimized or eliminated. 

The quantity of electricity, Q, passed through the electrolysis cell is determined by measuring the current, /, and the time, t, for which the current flows and is calculated from... 

Electrolytic (coukxnetric) hygrometers The quantity of electricity required to carry out a chemical reaction is measured. The principle is based upon Faraday s law of electrolysis. Water is absorbed on to a thin film of dessicant (e.g. P2O5) and electrolysed. The current required for the electrolysis varies according to the amount of water vapour absorbed. The current depends also upon the flowrate. Capable of high precision. Used in the range 1000 to 3000 ppm of water by volume. Somewhat complicated procedure. Recombination of products to water is necessary after electrolysis. Density, pressure and flowrates have to be maintained precisely. Contamination can poison the cell. It is ideal for binary mixtures but is of limited range. Suitable for on-line operation. 

---