Cerium coulometric titration

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... 

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 basic approach in coulometric titrations is to generate electrochemically (at constant current) a titrant in solution that subsequently reacts by a secondary chemical reaction with the species to be determined. For example, a large excess of cerium(III) is placed in the solution together with an iron(II) sample. When a constant current is applied, the cerium(III) is oxidized at the anode to produce cerium(IV), which subsequently reacts with the iion(II) ... 

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. 

In coulometric titrations, a constant current generates the litrant eleclrolytically. In some analyses, Ihe active electrode process involves only generation of the reagent." An example is the titration of halides by silver ions produced at a silver anode. In other titrations, the analyte may also be directly involved at the generator electrode. An example of this type of titration is the coulometric oxidation of iron(ll)-in part by elec-trolytic.illy generated cerium(l V) and in part by direct electrode reaction (Section 24B-2). Under any circumstance. the net process must approach 100% current efficiency with respect to a single chemical change in the analyte. 

The coulometry methods based on the physical law which sets the link between the weights of turned electricity and quantity of spent electricity. In many cases the electro generated coulometrical titrate enters in the oxidation process with organic substratum on the mechanism of reaction with the electron carriers. The most effective carriers ate the variable valence metals ions and its components oxidants—chrome (VI), manganese (III), cobalt (III), cerium (IV), vanadium (V), copper (II) deoxidants—cobalt (II), chrome (II), vanadium (HI), titanium (III), iron (II), copper (I), tin (II). The wide area of practical use of the halide ions (chloride-, bromide-, iodide-ions) highlights them apart Ifom a number of reagents—electron carriers. Halide—ions are... 

In coulometric titrations, cerium(IV) is generated in situ by electrolysis. Pastor et al. (1982) studied the anodic generation of cerium(IV). The formal redox potential of the Ce" +/Ce + system decreases as concentration of potassium acetate in the solution increases. 

---