Controlled potential methods coulometric

Coulometry. Two methods of coulometry are used coulometry at controlled potential and coulometric titrations. The main advantage of the coulometric method is the elimination of the necessity of standardization as the Faraday constant is a standard. In analysis of complicated samples encountered in environmental analysis the coulometric titrations are more advantageous where 100% current efficiency can be more readily attained by suitable choice of the reagent-solvent system. Coulometric titrations are suitable for determining the amount of substance in the range 0.01 to 100 mg (and sometimes below 1 iJg). Under optimum conditions these titrations can be carried out with a precision and accuracy of 0.01%. Automatic coulometric analyzers for the determination of gaseous pollutants (SO2, O3, NO, etc.) have proven to be useful in environmental chemistry. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

A second approach to coulometry is to use a constant current in place of a constant potential (Figure 11.23). Controlled-current coulometry, also known as amperostatic coulometry or coulometric titrimetry, has two advantages over controlled-potential coulometry. First, using a constant current makes for a more rapid analysis since the current does not decrease over time. Thus, a typical analysis time for controlled-current coulometry is less than 10 min, as opposed to approximately 30-60 min for controlled-potential coulometry. Second, with a constant current the total charge is simply the product of current and time (equation 11.24). A method for integrating the current-time curve, therefore, is not necessary. 

Coulometry may be used for the quantitative analysis of both inorganic and organic compounds. Examples of controlled-potential and controlled-current coulometric methods are discussed in the following sections. 

Control led-Potential Coulometry The majority of controlled-potential coulometric analyses involve the determination of inorganic cations and anions, including trace metals and halides. Table 11.8 provides a summary of several of these methods. 

Controllcd-Currcnt Coulomctry The use of a mediator makes controlled-current coulometry a more versatile analytical method than controlled-potential coulome-try. For example, the direct oxidation or reduction of a protein at the working electrode in controlled-potential coulometry is difficult if the protein s active redox site lies deep within its structure. The controlled-current coulometric analysis of the protein is made possible, however, by coupling its oxidation or reduction to a mediator that is reduced or oxidized at the working electrode. Controlled-current coulometric methods have been developed for many of the same analytes that may be determined by conventional redox titrimetry. These methods, several of which are summarized in Table 11.9, also are called coulometric redox titrations. 

Representative Method Every controlled-potential or controlled-current coulo-metric method has its own unique considerations. Nevertheless, the following procedure for the determination of dichromate by a coulometric redox titration provides an instructive example. 

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. 

The way in which these alternatives with their particular measuring characteristics are carried out can be best described by (1) controlled-potential coulometry and (2) coulometric titration (controlled-current coulometry). Both methods require an accurate measurement of the number of coulombs consumed, for which the following instrumental possibilities are available (a) chemical coulometers, (b) electrochemical coulometers and (c) electronic coulometers. 

Now returning to the coulometric analysis proper we can. say that any determination that can be carried out by voltammetry is also possible by coulometry whether it should be done by means of the controlled-potential or the titration (constant-current) method much depends on the electrochemical properties of the analyte itself and on additional circumstances both methods, because they are based on bulk electrolysis, require continuous stirring. 

The process may be illustrated with the following example. Table 21 gives some data from two methods, B and C, for estimating the silver content in photographic media. Method B is based upon an X-ray fluorescence procedure and Method C is a controlled potential coulometric method. 

Coulometry employs either a constant current or a controlled potential. Constant-current methods, like the preceding Br2/cyclohexene example, are called coulometric titrations. If we know the current and the time of reaction, we know how many coulombs have been delivered from Equation 17-2 q = / t. 

Attempts to reduce anthracene with an alkali metal in acetonitrile causes solvent decomposition, whereas controlled-potential electrolysis produces stable anion radicals. Thus the working electrode of a coulometric cell can be considered as a continuously adjustable reagent, capable of producing a wide variety of radical species in diverse solvent systems. The versatility of electrochemical EPR methods is best illustrated by citing a few specific examples from the extensive literature. More complete compilations appear in the reviews listed in Appendix I, but the studies mentioned next provide some appreciation for the techniques. 

Electrolysis at controlled potential can also serve as an elegant method of removing interfering metals from samples to be analyzed by other methods such as spectrophotometry or polarography. The electrogravimetric and coulometric procedures mentioned above represent such separations. The electrolysis can, however, be carried out primarily as a selective separation, with the actual determination being... 

Table 22-2 lists some other separations performed by controlled-potential electrolysis. Because of limited sensitivity and the time required for washing, drying, and weighing the electrodes, many electrogravimetric methods have been replaced by the coulometric methods discussed in the next section. 

Two methods have been developed that are based on measuring the quantity of charge controlled-potential (potentiostatic) coulometry and controlled-cur-rent coulometry, often called coulometric titrimetry. Potentiostatic methods are performed in much the same way as controlled-potential gravimetric methods, with... 

Controlled-potential coulometric methods have been used to determine more than 55 elements in inorganic compounds. Mercury is a popular cathode methods have been described for the deposition of more than two dozen metals at this electrode. The method has found use in the nuclear-energy field for the relatively interference-free determination of uranium and plutonium. 

Spreadsheet Summary In the first experiment in Chapter 11 of Applications of Microsoft Excel in Analytical Chemistry, numerical integration methods are investigated. These methods are used to determine the charge required to electrolyze a reagent in a controlled-potential coulometric determination. A trapezoidal method and a Simpson s rule method are studied. From the charge, Faraday s law is used to determine the amount of analyte. 

In controlled-potential coulometry the total number of coulombs consumed in an electrolysis is used to determine the amount of substance electrolyzed. To enable a coulometric method, the electrode reaction must satisfy the following requirements (a) it must be of known stoichiometry (b) it must be a single reaction or at least have no side reactions of different stoichiometry (c) it must occur with close to 100% current efficiency. 

Controlled-potential coulometric methods have been applied to more than fifty elements in inorganic com-pounds. Historically, mercury was widely used as the working electrode material, and methods for the deposition of two dozen or more metals at this electrode have been described. In recent years, many other electrode materials have been used, including platinum and various forms of carbon. Coulometry is used widely in the nuclear energy field for the relatively interference-free determination of uranium and plutonium. For example, the uranium-to-oxygen ratio in spent nuclear fuel has been determined by controlled-potential coulometry with a precision of 0.06% relative standard deviation (RSD). ... 

These methods are divided into two parts the first is controlled potential coulometry and the second is coulometric titration. The principles of the two methods are quite different, the only common features are that the quantity measured is the amount of electricity used and the concentration is calculated using Faraday s law. 

No direct controlled-potential coulometric methods for cyanide have been reported although Anson, Pool, and Wright (100) have generated cyanide ion by controlled-current reduction of Ag (CN)J at platinum cathodes in slightly alkaline media. Baker and Morrison (101) were able to determine 0-14 /ig of cyanide in 0.1 N sodium hydroxide by measurement of the current produced by spontaneous electrolysis between a platinum cathode and silver anode. Hypochlorite and sulphide interfered, but moderate quantities of nitrate, nitrite, chloride, sulphite, sulphate, phosphate, and ammonia did not. 

A direct controlled-potential coulometric method for fluoride has... 

It must be realized that the constant current (-1) chosen virtually determines a constant titration velocity during the entire operation hence a high current shortens the titration time, which is acceptable at the start, but may endanger the establishment of equilibrium of the electrode potentials near the titration end-point in an automated potentiometric titration the latter is usually avoided by making the titration velocity inversely proportional to the first derivative, dE/dt. Now, as automation of coulometric titrations is an obvious step, preferably with computerization (see Part C), such a procedure can be achieved either by such an inversely proportional adjustment of the current value or by a corresponding proportional adjustment of an interruption frequency of the constant current once chosen. In this mode the method can be characterized as a potentiometric controlled-current coulometric titration. 

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