Polarography accuracy

Standard addition. A known amount of the constituent being determined is added to the sample, which is then analysed for the total amount of constituent present. The difference between the analytical results for samples with and without the added constituent gives the recovery of the amount of added constituent. If the recovery is satisfactory our confidence in the accuracy of the procedure is enhanced. The method is usually applied to physico-chemical procedures such as polarography and spectrophotometry. 

For the rapid determination of Tc in a mixture of uranium fission products. Love and Greendale have used the method of amalgam polarography. It consists in a selective reduction of technetium at a dropping mercury electrode at a potential of —1.55 V vs. SCE in a medium of 1 M sodium citrate and 0.1 M NaOH. Under these conditions, technetium is reduced to an oxidation state which is soluble in mercury. The amalgam is removed from the solution of fission fragments and the amount of Tc determined in nitric acid solution of the amalgam by a y count. For Tc the measurement accuracy is within 1 %, and the decontamination factor from other fission products 10 . 

Generally, sensitivity in the analytical sense is greater if the technique employed is faster, i.e. the electrolysis time is shorter, or the frequency of a periodic electrolysis is higher. Resolution of half-wave potentials, and thus accuracy of standard potentials and stability constants, is better if a derivative technique such as differential pulse polarography, a.c. polar-ography, and, preferably, the second derivative technique second-harmonic a.c. polarography, is employed. 

The sensitivity of this technique is excellent and its accuracy is greater than that of conventional polarography. This method has found wide application in analytical chemistry. For addnl info on this subject see Refs 6,7,8,10,12,14,16,17,18,19,20,21,22,24,25,26 28,29,30,31 32... 

In the more advanced kinetic measurements, which were carried out by using chronopotentiometry [118], chronocoulometry [124, 139], linear [146] and convolution [18, 147] potential sweep voltammetry, or phase-sensitive ac polarography [142, 143], the ohmic drop was either numerically subtracted [118], or compensated [124, 139, 142, 143, 146, 147] with the help of the positive feedback. The feedback adjustment was based either on the assumption that the separation of the current peaks measured by the slow potential sweep voltammetry should reach the value of (59/z)mV [124, 139, 146, 147], or on the value of the solution resistance obtained by an ac bridge technique [142, 143]. However, the former adjustment is not very sensitive, whereas the estimated accuracy of 10 Q [142] in the latter case may not be... 

The methods of metal analysis described in this chapter can be employed with the highest sensitivity and accuracy of classical polarography (about 50 jag 2%, at higher concentrations 1%). 

The accuracy of these indirect methods is somewhat lower than in normal polarography, i.e. 3-5%. ... 

After 15 min of oxidation, dimethylformamide is added and insoluble particles are filtered off at decreased pressure. An aliquot of the solution is mixed with a Britton-Robinson buffer pH 1.8. The final solution gives a two-electron cathodic wave of the sulphoxide. The precision amounts to 1.42% as compared with 1.48% in ultraviolet spectroscopy. Surprisingly enough in the analysis of tablets by direct polarography, an accuracy of about 2-3% has been obtained on average [213,226,227]. 

Feroci, Fini, Badiello, and Breccia [97FER/FIN] studied the complex formation between and SeO by classical polarography in 0.15 M NaN03. They found no evidence for an interaction, see Appendix A. The review estimates from an assumed accuracy of the polarographic measurement of 2 mV, the equilibrium constant of the reaction ... 

In addition, DC polarography has a more specific limitation. Measurement of a more positive wave in the presence of an excess of material reduced at more negative potentials (wave A in Fig. 3.14) can be carried out with maximum accuracy but when the trace material to be determined is reduced at more negative potentials (wave B in Fig. 3.14)—that is, when a small wave follows a large one—measurement of the small, more negative, wave can be carried out only when the concentration ratio between the excess component and the analyzed species is less than about 10 1. At larger excess the accuracy of the determination of the trace component decreases considerably. 

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