Coulometric titrations advantage

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. 

Some of the reasons for considering coulometric titrations in nonaqueous solvents are that many organic compounds are not soluble in water, metals can exist in oxidation states that are not found in water, and advantage can be taken of the acidity or basicity of the solvent to improve the basic or acidic strength of a base or acid, respectively. 

An advantage of the technique is the use of an electrical standard to replace chemical standards and the problems associated with their preparation and stability. The coulometric titration also permits the generation of reagents such as copper(I) or bromine, which are difficult to employ as standard solution, or others such as silver(II) or chlorine, which are virtually impossible to use in any other way. A disadvantage of the coulometric titration is its lack of specificity. 

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... 

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. 

The measurements of metaUurgically prepared alloys, which were used as alloy electrodes in cells as in Figure 3.7, are limited in their number of data points. A big advantage that was introduced by Wagner is the coulometric titration. The cell is either a two-electrode cell or a three-electrode cell with a solid electrolyte or a molten salt electrolyte. The alloy is not preformed but is formed instead by a slow electrochemical deposition of component A into the component B (current 1). The mole number of A deposited is determined by the equation... 

The advantages of the coulometric method are its simplicity of operation, its increased sensitivity, and the fact that it does not require a standardized reagent, such as water saturated 1-octanol, to calculate the water content but only to assess the accuracy of the instrument. The advantage of the volumetric method is that it permits the use of a wider range of very polar solvents as well as nonpolar solvents for dissolving the sample in the titration vessel. Furthermore, the volumetric titration vessel can be heated to enhance the dissolution of slightly soluble samples. Either the volumetric or the coulometric titration instrument can be joined with an oven (evaporation) or a distillation apparatus (azeotropic distillation). In this configuration, the moisture that is volatilized from the sample can be transported to the titrator with a dry gas and the evaporated water measured either coulometrically or volumetrically. The recent... 

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. 

Where small quantities of reagent are required, a coulometric titration offers a considerable advantage. By proper choice of current, microquantilies of a substance can be introduced with ease and accuracy. The equivalent volumetric process requires dispensing small volumes of very dilute solutions, which is always difficult. 

The silver bromide does not interfere with the neutralization reaction as would the hydrogen ions that are formed at most anodes. Both potentiometric and indicator end points can be used for these titrations. The problems associated with the estimation of the equivalence point are identical with those encountered in a conventional volumetric analysis. A real advantage to the coulometric method, however, is that interference by carbonate ion is far less troublesome. It is only necessary to eliminate carbon dioxide from the solution containing the analyte by aeration with a carbon dioxide free gas before beginning the analysis. The coulometric titration of strong and weak bases can be performed with hydrogen ions generated at a platinum anode. 

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. 

Coulometric titrations are very useful since one can titrate unknowns with unstable reagents, such as radicals or other intermediates. A further advantage is the absence of the need to prepare and standardize solutions. The disadvantage of coulometric titrations is that they are not generally applicable, every system has to be developed and evaluated independently. 

The chemical follow-up reactions or homogeneous reactions that can be detrimental to simple electrochemical ionization can be analytically useful in their own right. In such a case, the initial products of the electrolysis react with the analyte of interest and the product of this reaction provides an analytical advantage in the experiments. In some ways this mirrors the coulometric titrations used in classic electrochemistry. 

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]. 

The use of a pair of identical metallic electrodes to establish the equivalence point in amperometric titrations offers the advantages of simplicity of equipment and elimination of the need to prepare and maintain a reference electrode. This type of system has been incorporated into equipment designed for the routine automatic determination of a single species, usually with a coulometric generated reagent. An example of this type of system is an instrument for the automatic determination of chloride in samples of serum, sweat, tissue extracts, pesticides, and food products. Here, the reagent is silver ion coulometrically generated from a silver anode. The indicator system consists of a pair of twin silver electrodes that are maintained at a potential of perhaps 0.1 V. Before the equivalence point in the titration of chloride ion, there is essentially no current because no easily reduced species is present in the solution. Consequently, electron transfer at the cathode is precluded and that... 

---