Bromine coulometric generation

The most important oxidants in glacial acetic acid as solvent are lead tetraacetate, which oxidizes mercaptans RSH to disulfide RSSR cerium(rv), a strong oxidant, though its reactions tend to be slow iodobenzene dichlorideand bromine, which may be obtained conveniently by coulometric generation. 

A number of instrument manufacturers offer automatic coulometric titrators, most of which employ a potentiometric end point. Some of these instruments are multipurpose and can be used for the determination of a variety of species. Others are designed for a single type of analysis. Examples of the latter are chloride titrators, in which silver ion is generated coulometrically sulfur dioxide monitors, where anodically generated bromine oxidizes the analyte to sulfate ions carbon dioxide monitors, in which the gas, absorbed in monoethanolamine, is titrated with coulometrically generated base and water titrators, in which Karl Fischer reagent (see Section 20C-5) is generated electrolytically. 

Some typical important industrial applications of coulometry include the continuous monitoring of mercaptan concentration in the materials used in rubber manufacture. The sample continuously reacts with bromine, which is reduced to bromide. A third electrode measures the potential of B12 vs. Br and, based on the measurement, automatically regulates the coulometric generation of the bromine. Coulometry is used in commercial instruments for the continuous analysis and process control of the production of chlorinated hydrocarbons. The chlorinated hydrocarbons are passed through a hot furnace, which converts the organic chloride to HCl. The latter is dissolved in water and the Cl titrated with Ag" ". The Ag" " is generated by coulometry from a sUver electrode, Ag°. It is necessary for the sample flow rate to be constant at all times. Integration of the coulometric current needed to oxidize the silver to silver ion results in a measurement of the Cl concentration. 

An advantage of the technique is the use of an electrical standard to replace chemical standards and the problems associated with their preparation and stability. The coulometric titration also permits the generation of reagents such as copper(I) or bromine, which are difficult to employ as standard solution, or others such as silver(II) or chlorine, which are virtually impossible to use in any other way. A disadvantage of the coulometric titration is its lack of specificity. 

Reference to Table 4.1 indicates that olefins can be determined by the electrochemical generation in situ of halogens. Bromine is effective for both olefins and sulfur compounds and is the basis for an automatic coulometric titrator for continuous analysis of petroleum streams.17 The basic principle of this instrument is a potentiometric sensing system that monitors bromine concentration in a continuously introduced sample stream. The bromine in the solution reacts with the sample components and causes a decrease in the concentration of bromine. When this decrease is sensed by the potentiometric detection electrodes, the electrolysis current producing bromine adjusts itself to maintain the bromine concentration. Because the sample is introduced at a constant rate, the electrolysis current becomes directly proportional to the concentration of the sample component. Thus, the instrument records the electrolysis current as concentration of sample component and provides a continuous monitor for olefins or sulfur in petroleum streams. 

Coulometric titration of SOj, H2S In a cell containing Br-ZBrj in an acidic medium, elec-trolytically generated bromine is titrated with ambient levels of either SOj or HjS. The voltage perturbation seen by the analyzer as a result of the change in the redox potential when SO2 (or H2S) is introduced into the cell will be proportional to the SO2 concentration in the sample stream. 

Oxidation-Reduction Titrations Table 24-2 indicates the variety of reagents that can be generated coulometrically and the analyses to which they have been applied. Electrogenerated bromine is... 

The use of dual intermediates was introduced into coulometric titrimetry by Buck and Swift (35) in 1952. They developed an improved method for the titrimetric bromination of organic compoxinds such as aniline that react slowly. Their method involved the electrical generation of an excess of bromine, the excess being back-titrated with electrically generated Cu(I). This constitutes another method of overcoming a sluggish end point. A dual-intermediate titrator with modern electronic instrximentation has been described recently (36). ... 

There have since been many similar instruments, largely dedicated to measurement of mercaptans and other sufur compounds by reaction with generated bromine. An early example was the Titrilog, manufactured by Consolidated Electrodynamics Corporation in the 1950s. Others, produced by European or Japanese firms, are intended for the determination of SO2 or CO2 from combusion analyzers (47). Stock (48) has summarized the development and present status of continuous "coulometric" titrators. 

Thc polarity of the working electrode is then reversed, and the excess bromine is determined by a coulometric titration involving the generation of Cu(I) ... 

A third design developed by Holland et al. [48,49] consisted of two wires at the end of the capillary, one inside the tube and a second across the end of the capillary (Figure 6C). The wire provided high coulometric transformation at the first electrode. The second wire was used to detect the product of the first reaction. Postcolumn amperometric reaction was evaluated for the indirect detection of thiols and thioethers. Bromide present in the run buffer was oxidized to bromine at the first electrode. The resulting bromine was detected at the second electrode downstream. Thiols and thioethers are oxidized by the bromine generated at the first electrode, leading to a reduced response at the second electrode... 

The first coulometric titration is credited to Szebel-ledy and Somogyi, who, in 1938, used electrolysis to generate bromine which reacted stoichiometrically with the substance to be determined. The quantity of substance which reacted was computed by Faraday s law from the number of coulombs of electricity rather than from a volume of standard reagent. By keeping the current constant and measuring the time, the number of coulombs was easily calculated. 

Within recent years several commercial constant-current sources have appeared on the market one of these is the Sargent Coulometric Current Source, capable of generating a constant current variable in steps from 4.825 to 193.0 ma. There was then a need for an automatic detection device which could be used with a commercially available source such as that manufactured by Sargent. The detection device shown in Fig. 1 will operate equally as well for slow reaction rates as for fast reaction rates, and has been found to be especially useful for the determination of unsaturated compounds by means of electrolytically generated bromine. 

For example, using a silver anode, the passage of a current produces silver ions, which react with any chloride in the solution. Bromine and acids may also be generated coulometrically. 

Amperometric-Coulometric Apparatus, automatic, suitable for bromine index titrations with variable generator current and timer. A typical circuit diagram of suitable equipment is shown in Fig. l. ... 

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