Coulometric generation of titrant

In this automatic system, the authors preferably used coulometric generation of titrant (cf., microcoulometric determination of deviations in the above end-point titration ), e.g., H, OH, Ag, Hg2+, Br2,12, Fe(CN) (cf., Table 1 in ref. 63). The detection method may be potentiometric (logarithmic signal), amperometric (linear signal), biamperometric, conductometric, oscillometric, etc. Moreover, the authors evaluated triangle programmed titration curves by... 

The Karl Fischer titration,30 which measures traces of water in transformer oil, solvents, foods, polymers, and other substances, is performed half a million times each day.31 The titration is usually performed by delivering titrant from an automated buret or by coulometric generation of titrant. The volumetric procedure tends to be appropriate for larger amounts of water (but can go as low as 1 mg H20) and the coulometric procedure tends to be appropriate for smaller amounts of water. 

The coulometric generation of titrants is widely applicable to redox, pre cipitation, acid-base and complexing reactions. Of particular value is the determination of many organic compounds with >romine and of mercaptans and halides with the silver ion. Amperometric equivalence point detection is the most common. An attractive feature of the technique is that the need to store standard and possibly unstable reagent solutions is obviated. In fact many applications involve the use of electrdgenerated reagents such as halogens and chromium(II) which are difficult or impossible to store. The technique is especially useful for the determination of veiy small amounts. 

Figure 14.2(a) is a schematic diagram of a suitable circuit for coulometric titration with internal generation of titrant and using the dead-stop or... 

Another example of the application of the principles of coulometric titrations to a continuous on-stream analyzer is the moisture analyzer developed by Kei-del.18 It illustrates one of the outstanding advantages of the coulometric generation of a titrant namely, an intermediate is produced as a titrant that would not be available in standard solutions. The principle of the moisture analyzer is to place a phosphoric acid solution between two closely spaced platinum electrodes (helically wound in a glass tube). When current is passed between the two electrodes, the water in the phosphoric acid is electrolyzed... 

Figure 5.36 Flow cell for external coulometric generation of a titrant.

As an alternative for the external application of a stepwise change in pH, as described above, titrant can be added by coulometry. Coulometric generation of protons or hydroxyl ions is possible by sending a current between a noble metal actuator electrode, situated closely around the ISFET gate, and a distant counter electrode 115]. 

Figure 4.5. Apparatus for the external generation of titrant for coulometric titrations.

Fig. 2. Spectra of cytochrome c (17.5 M) - cytochrome c oxidase (12.5 heme a) mixture during a reduction cycle by coulometrically generated MV. Titrant generated in increments of 0.5 me (1 me = 1.04 x 10 equivalents). Spectrum recorded after each addition of MV. Spectra correspond to titration from totally oxidized to totally reduced forms. The last two spectra at 605 nm were taken after excess was present. 550 nm band of cytochrome c 605 nm cytochrome c oxidase. (Reproduced from [28], courtesy of the publisher).

The principle of coulometric titration. This involves the generation of a titrant by electrolysis and may be illustrated by reference to the titration of iron(II) with electro-generated cerium(IV), A large excess of Ce(III) is added to the solution containing the Fe(II) ion in the presence of, say IM sulphuric acid. Consider what happens at a platinum anode when a solution containing Fe(II) ions alone is electrolysed at constant current. Initially the reaction... 

The limitations of coulometric titration with internal generation of the titrant include the following. 

The sample concentration can be calculated from (5.62). The handling of the results is more complicated than in normal titrations because two end-points must be determined and thus a desk-top calculator was recommended for the purpose [100]. From the point of view of the accuracy and precision, it is advantageous to generate the titrants coulometrically [98, 99]. 

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]. 

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. 

Equation (10) shows that the response of the 1SFET with respect to a coulometrically generated titrant perturbation is inversely proportional to the buffer capacity of the analyte, if the titrant perturbation is small. It indicates that the buffer capacity can be determined. It can be seen that this dynamic way of... 

A coulometric titration uses an electrolytically generated titrant for reaction with the analyte. In some analyses, the active electrode process involves only generation of the reagent. In other titrations, the analyte may also be directly involved at the generator electrode. The current in a coulometric titration is carefully maintained at a constant and accurately known level. The product of this current and the time required to reach the equivalence point for the reaction yield the number of coulombs and thus the number of equivalents involved in the electrolysis. The coulomb (C) is the quantity of electricity that is transported in 1 by a constant current of 1 ampere. The Faraday constant (F) is the quantity of electricity that produces one equivalent of chemical change at an electrode. ... 

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