Coulometric titrations procedures

Coulometric titration procedures have been developed for a great number of oxidation-reduction, acid-base, precipitation, and complexation reactions. The sample systems as well as the electrochemical intemediates used for them are summarized in Table 4.1, and indicate the diversity and range of application for the method. An additional specialized form of coulometric titration involves the use of a spent Karl Fischer solution as the electrochemical intermediate for the determination of water at extremely low levels. For such a system the anode reaction regenerates iodine, which is the crucial component of the Karl Fischer titrant. This then reacts with the water in the sample system according to the... 

The accepted reference method for determining chloride in blood serum, plasma, urine, sweat, and other body fluids is the coulometric titration procedure. In this technique, silver ions are generated coulometrically. The silver ions then react with chloride ions to form insoluble silver chloride. The end point is usually detected by amperometry (see Section 23B-4) when a sudden increase in current occurs on the generation of a slight excess of Ag. In principle, the absolute amount of Ag" needed to react quantitatively with Cl can be obtained from application of Faraday s law. In practice, calibration is used. First, the time required to titrate a chloride standard solution with a known number of moles of chloride (nci )s using a constant current I is measured. The same constant current is next used in the titration of the unknown solution, and the time r is measured. The number of moles of chloride in the unknown (ncr)u is then obtained as follows ... 

Reagent dispensers are chiefly volumetric or coulometric In nature. Formerly, the automation of coulometric titration procedures developed at a higher speed than that of volumetric titration —this was probably a result of the ease with which the current intensity used for reagent generation could be regulated by means of relatively simple electronics. 

V, the relationship between Eq, and [Py ] needs to be defined only over this narrow potential region. While the Nernst equation should, in principle, define this relationship, these films are non-Nernstian (see Figure 6). Therefore, the relationship between Eqc and [Py ] was determined empirically using a coulometric titration procedure (58-60). This procedure is described in detail in reference (30). 

By virtue of its inherent accuracy, coulometric titration is very suitable for the determination of substances present in small amount, and quantities of the order of 10 7-1(U5 mole are typical. Larger amounts of material require very long electrolysis times unless an amperostat capable of delivering relatively large currents (up to 2 A) is available. In such cases, a common procedure is to start the electrolysis with a large current, and then to switch to a much lower output as the end point is approached. 

A selection of coulometric titrations of different types is collected in Table 14.2. It may be noted that the Karl Fischer method for determining water was first developed as an amperometric titration procedure (Section 16.35), but modern instrumentation treats it as a coulometric procedure with electrolytic generation of I2. The reagents referred to in the table are generated at a platinum cathode unless otherwise indicated in the Notes. 

The result of the entire procedure, being a 100% conversion of Fe(II) to Fe(III), thus represents a so-called coulometric titration with internal generation of course, it seems possible to titrate Fe(II) with Ce(IV) generated externally from Ce(III), but in this way one would unnecessarily remove the solution of the 100% conversion problem hence the above titration with internal generation in the presence of a redox buffer as an intermediary oxidant represents an extremely reliable method, unless occasional circumstances are prohibitive for the remainder internal generation offers the advantage of no dilution of the analyte solution. 

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. 

Recently, Mazzocchin et al. have studied several titration methods for determining relatively large amounts of technetium. The most precise results have been obtained by coulometric titration of TcO ions With electrogenerated tin (II) according to the procedure suggested by Bard and Lingaiie . The supporting electrolyte consists of 2.5 M sodium bromide, 0.15 M stannic chloride and 0.2 M hydrochloric acid. The titration reaction is very fast and currents up to 40 mA can readily be employed in the detection of the equivalence point. 

Clement and Paris [17] have devised a pair of methods for the determination of cobalt in steels, especially materials encountered in the nuclear industry. In the first technique, suitable for the analysis of solutions containing 8 to 160 mM cobalt(II), iron(III) is used to oxidize cobalt(II) in a picolinic acid medium, after which the resulting iron(II) is titrated po-tentiometrically with a standard solution of cerium(IV). An alternative procedure, for concentrations of cobalt(II) below 8 mM, involves a constant-current coulometric titration with electrogenerated cerium (IV) to measure the iron(II) that arises from the original reaction between cobalt(II) and iron(III). 

The top of the metrological traceability chain is given in figure 7.6. The procedure and system referred to in the figure may be either for the production of the calibrator, such as the international prototype of the kilogram, or for a primary measurement procedure governing a measuring system, as in the use of a coulometric titration. 

Some method of signaling is required to indicate when the amount of titrant generated is equivalent to the amount of unknown present, and all of the endpoint detection methods used in volumetric titrimetry are, in principle, applicable to coulometric titrations. A list that covers most of the published coulo-metric titration procedures is given in Table 25.2. It is beyond our scope here to describe any of these in detail because each of these methods is a subject for discussion in its own right. Discussions of the equations for a number of types of titration curves are found in texts by Lingane [15], Butler [16], and Laitinen and Harris [17]. 

The uses of constant-current coulometry for the determination of drugs in biological fluids are few, basically due to sensitivity restriction. Monforte and Purdy [46] have reported an assay for two allylic barbituric acid derivatives, sodium seconal and sodium sandoptal, with electrogenerated bromine as the titrant and biamperometry for endpoint detection. Quantitative bromination required an excess of bromine hence back titration with standard arsenite was performed. The assay required the formation of a protein-free filtrate of serum with tungstic acid, extraction into chloroform, and sample cleanup by back extraction, followed by coulometric titration with electrogenerated bromine. The protein precipitation step resulted in losses of compound due to coprecipitation. The recoveries of sodium seconal and sodium sandoptal carried through the serum assay were approximately 81 and 88%, respectively. Samples in the concentration range 7.5-50 pg/mL serum were analyzed by this procedure. 

Should any iron(II) reach the anode, it also would be oxidized and thus not require the chemical reaction of Eq. (4.13) to bring about oxidation, but this would not in any way cause an error in the titration. This method is equivalent to the constant-rate addition of titrants from a burette. However, in place of a burette the titrant is electrochemically generated in the solution at a constant rate that is directly proportional to the constant current. For accurate results to be obtained the electrode reaction must occur with 100% current efficiency (i.e., without any side reactions that involve solvent or other materials that would not be effective in the secondary reaction). In the method of coulometric titrations the material that chemically reacts with the sample system is referred to as an electrochemical intermediate [the cerium(III)/cerium(IV) couple is the electrochemical intermediate for the titration of iron(II)]. Because one faraday of electrolysis current is equivalent to one gram-equivalent (g-equiv) of titrant, the coulometric titration method is extremely sensitive relative to conventional titration procedures. This becomes obvious when it is recognized that there are 96,485 coulombs (C) per faraday. Thus, 1 mA of current flowing for 1 second represents approximately 10-8 g-equiv of titrant. 

Both the automatic coulometric titration of petroleum streams and the continuous monitoring of pesticides and sulfur-halogen compounds indicate that the coulometric titrator method is amenable to the automatic maintenance of the concentration of a component in a solution system. A manual version of this approach has been used to study the kinetics of hydrogenation of olefins as well as to determine the rate of hydrolysis of esters.12 The latter system is a pH-stat that is based on the principles of coulometric titrations. Equations (4.9)-(4.11) indicate how this approach is applied to the evaluation of the rate constants for ester hydrolysis. A similar approach could be used to develop procedures for kinetic studies that involve most of the electrochemical intermediates summarized in Table 4.1. The coulometric titration method provides a convenient means to extend the range of systems that can be subjected to kinetic study in solution. 

Procedure Quickly inject the Test Preparation, or transfer the solid sample, into the anolyte, mix, and perform the coulometric titration to the electrometric endpoint. Read the water content of the Test Preparation directly from the instrument s display, and calculate the percent that is present in the substance. 

The anode compartment in a coulometric titration cell contains 250 ml of 1 M Na2S04, with the CO2 removed. Suppose that 1.2463 g of pure NajCOj is added and electrolyzed with platinum electrodes to completion of the neutralization. How long will this procedure take if the current is 194.36 mA ... 

Coulometric titration offers several significant advantages over a conventional volumetric procedure. Principal among these is the elimination of the problems asso-... 

A further advantage of the coulometric procedure is that a single constant-current source provides reagents for precipitation, complex formation, neutralization, or oxidation/reduction titrations. Finally, coulometric titrations are more readily automated, since it is easier to control electrical current than liquid flow. 

No practical apphcations are given in this area to cover food analysis, although the reader is referred to the References section, "additional recent references" portion, for some suggestions. The paper by Kalbus and Lieu represents, for example, a laboratory experiment in the coulometric titration determination of the iodine number of fats and oils. For a practical experiment the following determination of arsenic by coulometric titration is suggested. It is of interest since it presents to alternatives in the final estimation procedure. 

Three electroanalylical methods ai c based on electrolytic oxidation or reduction of an analyte for a sufficient period to assure its guantiUitive conversion to a new oxidation state. These methods are constanl-poieniial coulometry consiant-curre.ut coulometry, or coulometric titration and electrogravimetty. In eleclrogravi-metric methods, the product of the electrolysis is weighed as a deposit on one of the electrodes. In the two coulometric procedures, on the other hand, the ifuantiiyof electricity needed to congdete the electrolysis is a measure of the amount of analyte present. 

A coulometric titration, like a more conventional volumetric procedure, requires some means of detecting the point of chemical equivalence. Most of the end-point detection methods applicable to volumetric analysis are equally saiisfactory here. Visual observations of color changes of indicators, as well as poicn-tionieiric, amperometric, and photometric measurements have all been used successfully. 

To summarize, then, the eurrent-time measurements required for a coulometric titration are inherently as accurate as or more accurate than the comparable volume-molarity measurements of a classical volumetric analysis, particularly where small quantities of reagent are involved. Often, however, the accuracy of a titration is not limited by these measuremenis hut by the sensitivity of the end point in this respect, the two procedures are equivalent. 

Scott JR, Mcjunkin TR (2009) In 2nd Japan-IAEA workshop on advanced safeguards technology for the future nuclear fuel cycle, Tokai-mura Shinonaga T (2008) Analytical instructions for uranium and plutonium in environmental swipe samples (internal IAEA procedure SALCLI.5100, IAEA) Shults WD (1960) Controlled potential coulometric titration of plutonium. Application to PER samples, ORNL-2921... 

Different methods that are worked out to measure free chlorine concentration mostly take advantage of its strong oxidizing character. A broad scale of volumetric and coulometric titrations with different endpoint detection, as well as voltammetric as colorimetric methods, have been worked out. In practice, water analysis often involves classical titrimetric procedures, such as titration with arsenous acid or an appropriate iodometric approach. 

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. 

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