Coulometric titration accuracy

Accuracy The accuracy of a controlled-current coulometric method of analysis is determined by the current efficiency, the accuracy with which current and time can be measured, and the accuracy of the end point. With modern instrumentation the maximum measurement error for current is about +0.01%, and that for time is approximately +0.1%. The maximum end point error for a coulometric titration is at least as good as that for conventional titrations and is often better when using small quantities of reagents. Taken together, these measurement errors suggest that accuracies of 0.1-0.3% are feasible. The limiting factor in many analyses, therefore, is current efficiency. Fortunately current efficiencies of greater than 99.5% are obtained routinely and often exceed 99.9%. 

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. 

The current-time measurements required for a coulometric titration are inherently as accurate as or more accurate than the comparable volume/molarity measurements of a conventional volumetric method, particularly when small quantities of reagent are involved. When the accuracy of a titration is limited by the sensitivity of the end point, the two titration methods have comparable accuracies. 

Under proper conditions, coulometric titrations can be performed with typical accuracies of 0.1% or better, even with small quantities of compounds. (Work in the microgram range may, however, have errors on the order of 1%.) If special precautions are taken, accuracies can be obtained that are difficult to achieve by any other method. Taylor and coworkers, for example, have titrated milligram quantities of substances with precisions of 0.005% or better [5, 6]. For these titrations, series resistors in a constant-temperature oil bath are used to control and measure the current from a 48 V storage battery the oil bath dissipates heat and thus stabilizes the resistance. The current is determined by measuring the iR drop across a precision resistor with a very sensitive potentiometer and comparing it with a standard Weston cell maintained at 1.017875 V 0.8 /iV by careful thermostatic control. The titration time is measured with a quartz-crystal-controlled time-interval meter capable... 

In Figure 3.21 the second intermetallic compound LijSb also looks like a stoichiometric compound. The high accuracy of the coulometric titration method reveals that this compound has a very small region of variable stoichiometry of Lij+ Sb with = 10". This is shown in Figure 3.22. 

Figure 3.22 demonstrates the high precision of coulometric titration and the accuracy achievable for thermodynamic data by electrochemical potential measurements. 

The amount of titrant added is usually measured by volume (by dispensing the solution from a burette), and in this case, titrimetry is an example of volumetric analysis. Occasionally, the titrant is measured by weight (especially if greater accuracy is required) or by amount of electricity (as in coulometric titrations). 

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... 

Coulometric titration. This method was used by Ahn et al. (1988, 1990) for the determination of phase equilibria, and by Tetenbaum et al. (1989) for the determination of a possible miscibility gap. A drawback is that the absolute accuracy depends on the oxygen content of the starting material. This was determined by iodometry and therefore the accuracy was only Ax = 0.02. The near equilibrium conditions of the synthesis of their samples allowed the determination by EM of the various superstructures (sect. 5.1.1). 

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. 

Voltammetric and polarographic methods were, and still are, only exceptionally used for determining the main component of a sample (pure substance determination) since the accuracy of about 2 to 3% is not sufficient here. The other electroanalytical methods, e.g. coulometry and coulometric titrations are to be recommended here because of their higher accuracy. 

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 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]. 

In coulometry the stoichiometry of the electrode process should be known and should proceed with 100% current efficiency, and the product of reaction at any other electrode must not interfere with the reaction at the electrode of interest. If there are intermediate reactions, they too must proceed with the desired accuracy. In practice the electrolytic cell is designed to include isolation chambers. Losses of solute through diffusion, through ionic or electrical migration, and simply through bulk transfer must be minimal. Finally, the end point has to be determined by one of the many techniques used in titrations generally, whether coulometric or not. Both indeterminate and determinate end-point errors limit the overall accuracy achieved. Cooper and Quayle critically examined errors in coulometry, and Lewis reviewed coulometric techniques. 

Coulometric methods also excel when small amounts of analyte have to be titrated because tiny quantities of reagent are generated with ease and accuracy through the proper choice of current. With conventional titrations, it is inconvenient and often inaccurate to use very dilute solutions and small volumes. 

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