Reaction coulometer

For quantitative determination of the rate and the amount of heat release, we have developed a method based on complete combustion of the volatiles on a platinum catalyst and measurement of amount of oxygen required (J8). The required oxygen is generated by a reaction coulometer (Figure 27) and is related to the heat of combustion as shown in Figure 28. As discussed in Chapter 14, the amount of combustible, volatile pyrolysis products formed and the rate of heat release may be reduced drastically by the addition of flame retardants. 

Figure 27. Block diagram of the thermal evolution analysis system coupled to a reaction coulometer detector.

A reaction coulometer has been used to determine the rate of heat release from these combustible volatiles (65). Table VIII shows these results on the effect of inorganic additives that were obtained by using reaction coulometry. The treated cellulose samples decomposed at lower temperatures and produced less heat than the untreated. Addition of 5% NaOH reduced the heat of combustion of cellulose volatiles at 500 °C to less than one-half of untreated (65). 

It is apparent (Fig. 1.21) that at potentials removed from the equilibrium potential see equation 1.30) the rate of charge transfer of (a) silver cations from the metal to the solution (anodic reaction), (b) silver aquo cations from the solution to the metal (cathodic reaction) and (c) electrons through the metallic circuit from anode to cathode, are equal, so that any one may be used to evaluate the rates of the others. The rate is most conveniently determined from the rate of transfer of electrons in the metallic circuit (the current 1) by means of an ammeter, and if / is maintained constant it can eilso be used to eveduate the extent. A more precise method of determining the quantity of charge transferred is the coulometer, in which the extent of a single well-defined reaction is determined accurately, e.g. by the quantity of metal electrodeposited, by the volume of gas evolved, etc. The reaction Ag (aq.) -t- e = Ag is utilised in the silver coulometer, and provides one of the most accurate methods of determining the extent of charge transfer. 

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. 

C19-0099. In a silver coulometer (see Problem ), both electrodes are silver metal. Draw a molecular picture that illustrates the reactions that occur during operation of this coulometer. 

A meter placed in the circuit (see Figure 5.1) tells us the charge (or current) that flows through the electrodes. Following from equation (5.3), though, we see that in fact an ammeter (or coulometer) also tells us how much charge (or current) has flowed through the solution of the cell. In other words, the meter tells us how many electrons have been consumed by electrode reactions in solution. We see... 

The electrolysis current (not to be confused with the detector current) for the Br2-generating electrodes can be controlled by a hand-operated switch. As the detector current approaches 20.0 p,A. you close the switch for shorter and shorter intervals. This practice is analogous to adding titrant dropwise from a buret near the end of a titration. The switch in the coulometer circuit serves as a stopcock for addition of Br2 to the reaction. 

For halogenated compounds, combustion gives C02, H20, N2, and HX (X = halogen). The HX is trapped in aqueous solution and titrated with Ag+ ions in a coulometer (Section 17-3). This instrument counts the electrons produced (one electron for each Ag+) during complete reaction with HX. 

Coulometers, like the balance, are basic instruments for absolute analysis and they are still used as the most reliable and precise instruments for the analysis of absolute standards. Coulometers are frequently used in elucidating electrochemical reactions because they allow determining the number of transferred electrons when the molar amount of electrolyzed compound is known (-> Faraday s law). When the charge is measured as a function of time, the technique is called chrono-coulometry. See also coulometric titration. 

A direct determination of n values n = F/mol in a reaction) can be made by microcou-lometry. However, n values derived from electrolyses with macroelectrodes should be considered with some care because of the possibility of further reactions during the time of the experiment. A reliable procedure for compounds that can be dissolved fully is to follow the concentration of the electroactive substance analytically, plot corresponding values of the response and coulometer readings, and calculate the n value from the slope of the possibly straight line obtained during the first part of the electrolysis. If a straight line is not obtained, this indicates that secondary reactions take place. 

PotentiostatS and Coulometers For controlled-potential coulometry, we use a potentiostat similar in de.sign to that shown in Figure 22-8. Generally, however, the potentiostat is automated and equipped with a computer or an electronic current integrator that gives the charge in coulombs necessary to complete the reaction, as shown in Figure 22-12. 

The sample was dissolved in 100 mL of methanol after electrolysis for 30 min, the reaction was judged complete. An electronic coulometer in series with the cell indicated that the reduction required 31.23 C. Calculate the percentage of C6H5NO2 in the sample. 

Coulometer A device that permits measurement of the quantity of charge. Electronic coulometers evaluate the integral of the cur-rent/time curve chemical coulometers are based on the extent of reaction in an auxiliary cell. 

Tests of Faraday s Law Tinder Varying Conditions. We have already seen that, if disturbing effects are taken into account, Faraday s law applies to all electrochemical reactions which have been carefully studied. The tests so far mentioned, however, have all been made at ordinary temperature, under atmospheric pressure, and in aqueous solutions. A number of researches have been carried out to find out whether variations in the nature of the solvent, or variations in the physical conditions, such as temperature and pressure, have any influence on the constant in Faraday s law. No real variation in the constant has yet been observed. There are, to be sure, many apparent deviations from the law, such as that observed with the copper coulometcr, which gives a deposit at the cathode which is lighter than the computed value. In this case, as has been seen, the cause of the discrepancy has been found to be the occurrence of a disturbing reaction, In every similar case a simple explanation of the apparent deviation has been readily found. The comparison of the iodine, and of the copper coulometer, with the silver coulometer, as has been described in previous sections, affords precise evidence, for these reactions at least, that Faraday s law is indc-... 

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