Electrolysis quantities, calculation

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 quantity, Q, of electricity passed through the electrolysis cell is measured in coulombs. It is determined by measuring the current, I, and the time, f, for which the current flows and is calculated from... 

One of the most widely used applications of electrolytic cells is in electrolysis, the decomposition of a compound. Water may be decomposed into hydrogen and oxygen. Aluminum oxide may be electrolyzed to produce aluminum metal. In these situations, several questions may be asked How longw xW it take how much can be produced what current must be used Given any two of these quantities, the third may be calculated. To answer these questions, the balanced half-reaction must be known. Then the following relationships can be applied ... 

In coulometry, the analyte is quantitatively electrolyzed and, from the quantity of electricity (in coulombs) consumed in the electrolysis, the amount of analyte is calculated using Faraday s law, where the Faraday constant is 9.6485309 xlO4 C mol-1. Coulometry is classified into controlled-potential (or potentiostatic) coulometry and controlled-current (or galvanostatic) coulometry, based on the methods of electrolysis [19, 20]. 

Dibenzyl diselenide, (C6H5.CH2)2Se2.—This compound may be obtained in a variety of ways (1) Sodium selenide is heated for several hours under reflux with benzyl chloride in alcoholic solution.2 (2) A good yield of the diselenide results when a solution of selenium in sodium sulphide or sodium hydroxide is shaken on a machine with benzyl chloride.3 (3) Potassium selenosulphate is shaken with the calculated quantity of benzyl chloride in the cold. (4) An alcohol solution of potassium benzyl selenosulphate is treated with an excess of iodine, and the diselenide remaining in solution is precipitated by the addition of water. The precipitate is collected immediately and recrystallised from alcohol, since the presence of iodine appears to aid decomposition, with deposition of selenium, and it is also necessary to wash the crystals with water to remove any traces of hydriodic acid before drying the product.4 (4) An aqueous solution of potassium benzyl selenosulphate is subjected to electrolysis, using a current of 0 25 to 0 5 ampere, the method of operation being similar to that used for di-p-nitrobenzyl diselenide. 

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. 

These empirical laws of electrolysis are critical to corrosion as they allow electrical quantities (charge and current, its time derivative) to be related to mass changes and material loss rates. These laws form the basis for the calculations referenced above concerning the power of electrochemical corrosion measurements to predict corrosion rates. The original experiments of Faraday used only elements, but his ideas have been extended to electrochemical reactions involving compounds and ions. 

The material for the actuator has to be selected on the basis of the titrant to be generated. For the titration of an acid or base, a noble metal electrode is usually adopted. A constant current is often used in the coulometric titration, and the quantity of charge is then calculated by the time of the electrolysis. The analyte can either be an acid or a base. If there are no other interfering redox couples, the titrant generated at the electrodes depends on the direction of the applied current ... 

To calculate quantities of chemicals involved in electrolysis reactions from the quantity of charge that passes, or vice versa... 

Electrolysis of an A compound in a mixture of two RX compounds may be interrupted after a certain quantity of electricity has been passed (e.g., 2F/mol calculated... 

Into the cell is then placed a solution of 15 g. sodium sulfate. 15. g. boric acid and 14 g. of salicylic acid (0.1 mole), just neutralized with the calculated amount of sodium hydroxide. The solution is then diluted to, 175 cc. All the boric acid not dissolve in this quantity of solution, but is kept in suspension by means of rapid mechanical stirring. The cell is placed in a cooling mixture, and when the temperature reaches 15° to 18° the current is turned on. A temperature of 15° to 18° is maintained throughout the experiment. A current of 3 amp. (6 amp. per sq. dm.) is then passed through the solution for a period of 1 hr. 55 min., which is slightly more than the calculated amount (5.4 amp.-hr.) necessary to reduce the salicylic acid to salicylic aldehyde. During the electrolysis 20 g. sodium bisulfite are added at the rate of about 1.5 g. every 10 min. It has been found best not to begin the addition of the sodium bisulfite until the electrolysis has been started about 5 min, since the bisulfite reduced to sulfur when added to soon, or too rapidly thereafter. 

The methods described previously correspond to the partial electrolysis of the analytes in contact with the working microelectrode. By contrast coulometric methods are based upon stoichiometrical relationships (quantitative conversion of the analytes). The concentration of an analyte is calculated from the quantity of electricity, using the equation Q = it. Standardization or calibration curves are not required. This is not therefore a comparative method. 

In the electrolysis of an aqueous AgN03 solution, 0.67 g of Ag is deposited after a certain period of time, (a) Write the half-reaction for the reduction of Ag". (b) What is the probable oxidation half-reac-tion (c) Calculate the quantity of electricity used, in coulombs. 

The theory for the different reaction schemes involves ordinary (rather than partial) differential equations, because the electrolyzed solution is assumed to be essentially homogeneous (see Section 11.3.1). The concentrations are functions of t during the bulk electrolysis, but not of x. The measured responses in coulometry are the i-t curves and the apparent number of electrons app consumed per molecule of electroactive compound. From the quantity of electricity passed during the electrolysis Q t), app can be calculated as... 

Electrolysis of solutions can be used for electrodeposition of a trace metal on an electrode. The selectivity and efficiency which would be present for electrolytic deposition of macro amounts of ions at a controlled potratial is not present, however, for trace amounts. The activity of trace amounts of the species is an unknown quantity even if the concentration is known, since the activity coefficient is dependent upon the behavior of the mixed electrolyte system. Moreover, the concentration of the tracer in solution may not be known accurately since there is always the possibility of some loss through adsorption, complex formation with impurities, etc. Nevertheless, despite these uncertainties it has been found that the Nemst equation can be used, with some caution, for calculating the conditions necessary for electrolytic deposition of trace metals. 

Potassium peroxidisulfate separates as a crystalline precipitate after the first 10 minutes. At the end of the electrolysis, filter the precipitate on a small sintered glass filter or a Gooch crucible, wash with alcohol and ether, and dry. The yield is about 5 grams. From the yield, calculate the current efficiency, that is, the observed yield divided by the maximuni theoretical yield for the quantity of electricity passed. The current eflSciency should be about 50 per cent. 

Some of the first methods of measuring quantities of electricity involved the use of chemical coulometers. To do this, an electrolytic cell is placed in seri with the sample electrolysis cell so that the same current passes through both. A typical coulometer cell consists of a platinum crucible containing a silver-nitrate solution and a silver anode. Silver metal is deposited on the preweighed platinum crucible and the latter reweighed to determine the amount of electricity passed Q is calculated from Equation 4.10. 

Acquiring a suitable moderator looked more difficult. The German scientists favored heavy water, but Germany had no extraction plant of its own. Harteck calculated at the beginning of the year that a coal-fired installation would require 100,000 tons of coal for each ton of heavy water produced, an impossibility in wartime. The only source of heavy water in quantity in the world was an electrochemical plant built into a sheer 1,500-foot granite bluff beside a powerful waterfall at Vemork, near Rjukan, ninety miles west of Oslo in southern Norway. Norsk Hydro-Elektrisk Kvaelstofaktieselskab produced the rare liquid as a byproduct of hydrogen electrolysis for synthetic ammonia production. 

As reported in the previous study (1), in an electrolysis cell using an active catalyst electrode, carbon dioxide was generated when alcohol was added in the electrolyte. For the addition of ethylene glycol in the electrolyte, assuming the following chemical reaction, the quantity of carbon dioxide can be calculated. 

The quantity of substances formed during electrolysis can be calculated by considering the number of electrons involved in the redox reaction and the amount of electrical charge that passes into the cell. The amoimt of electrical charge is measured in coulombs and is related to the magnitude ofthe current and the time it flows (1 C = 1 A-s). 

Figure 19.20 shows the steps involved in calculating the quantities of substances produced in electrolysis. Let us illustrate the approach by considering molten CaCla in an electrolytic cell. Suppose a current of 0.452 A is passed through the cell for 1.50 h. How much product will be formed at the anode and at the cathode In solving electrolysis problems of this type, the first step is to determine which species will be oxidized at the anode and which species will be reduced at the cathode. Here the choice is straightforward because we only have Ca and Cr ions in molten CaClj. Thus, we write the half- and overall cell reactions as... 

To provide a feel for the quantities involved in a chlor-alkali plant and to allow us to illustrate the size of some of the key equipment, we adopt here a reference plant based on the use of membrane cells and calculate an approximate electrolysis area mass balance. The characteristics of this plant serve as reference material for the specific examples used in later chapters. We defer energy considerations to those later chapters, but we list here all the relevant parameters. 

A hygroscopic mixture of trichloroacetic acid (FM 163.39) and dichloroacetic acid (FM 128.94) containing an unknown quantity of water weighed 0.721 g. On controlled potential electrolysis, 224 C passed at —0.90 V, and 758 C were required to complete the electrolysis at — 1.65 V. Calculate the weight percent of each acid in the mixture. 

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