Acid and alkaline errors

Figure 15-J5 Acid and alkaline errors of some glass electrodes. A Coming 015, H2S04. B. Corning 015, HCI. C Corning 015,1 M Na+...

FIGURE 23-7 Acid and alkaline error of selected glass electrodes at 25°C. (From R. G. Bates, Determination of pH, 2nd ed,. p. 365. New York Wiley, 1973. yVilh permission.)... 

Figure 15-9 Acid and alkaline errors of some glass electrodes. A Corning 015, HjSO. B Corning 015, HCi. C Corning 015, IMNa. D Beckman-GP, 1 M Na. E L N Black Dot, 1 M Na. F Beckman Type E, 1 M Na. G Ross electrode. [From R. G. Bates, Determination of pH Theory and Practice, 2nd ed. (New York Wiley, 1973). Ross electrode data is from Orion, Ross pH Electrode Instruction Manual.]...

The acid and alkaline errors of the pH measurements by the glass electrode have baffled chemists ever since the glass electrode was created. After many studies, the conclusion has been that it is too complicated to be understood (30). The Nernst equation cannot explain the nonlinearity at the very low and very high hydrogen ion concentrations. If we accept the following concepts, the acid and alkaline errors are easily understood. 

Departures from the ideal behavior expressed by equation 7 usually are found in alkaline solutions containing alkaH metal ions in appreciable concentration, and often in solutions of strong acids. The supposition that the alkaline error is associated with the development of an imperfect response to alkaH metal ions is substantiated by the successhil design of cation-sensitive electrodes that are used to determine sodium, silver, and other monovalent cations (3). 

Assessment of indomethacin powder had been specified in the U.J5. XIX and the British Pharmacopeia, 1980 as a back titration with hydrochloric acid after alkaline hydrolysis(13,14). This method can attain a precision of + 0.8%(62). A direct titration for tablets and capsules is described using sodium hydroxide. If performed rapidly with phenolphthalein indicator, a precision of +0.3% is attainable. 8 The latter procedure serves to differentiate ester formation as well as hydrolysis products from intact material. The presence of parachlorobenzoic acid and 5-methoxy-2-methylindole-3-acetic acid are cause for positive error. 

The earlier work on this oxidation by Slater and Acree was in error because of their failure to distinguish between the iodine equivalence of thiosulfate in acid and in alkaline solution. In acid solution (reaction 15o) the equivalence is 1 1, with sodium tetrathionate as a product. In alkaline solution 1 mole of thiosulfate is equivalent to 8 atoms of iodine (reaction 156), with sodium sulfate as a product. 

The analyst has little choice in the selection of a redox analytical method. In acid solution ceric ion is the most effective oxidant however, in acid solution there is possibility of analytical error because of the volatility ofhydrazoic acid. In alkaline solution, reduction of the azide ion by homogeneous or heterogeneous reduction methods results in variable stoichiometry and cannot be used. The argentometric determination of azide ion in neutral solution is satisfactory but has one serious drawback the manipulation of silver azide, a high cxplosix c. Two basic approaches have evolved for the assay of azide ion ... 

What is the alkaline error and the acid error of a glass membrane pH electrode ... 

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