Isohydric indicator solutions

Isohydric indicator solutions. Reliable information concerning unbuffered solutions is obtained only when the pH of the liquid is unaffected by the added indicator. Actually the indicator solution should have a composition such that its pH equals that of the solution being investigated. Indicator solutions of this kind have been called isohydric by E. H. Fawcett and S. F. Agree. ... 

Preparation of suitable indicator solutions. The prepar on of indicator solutions for use in the colorimetric pH determination has already been described in detail in Chapter Five ( 3). These solutions, however, were suited only for measurements of buffered solutions. Isohydric indicator solutions have to e prepared in another manner. H. T. Stern titrates the indicator with sodium hydroxide and measures the pH during neutralization with the quinhydrone electrode. A similar procedure has been described by Pierre and Fxjdge. Fawcett and Acree keep in stock a large series of neutralized indicator solutions and determine their pH approximately by colorimetric means. 

Errors in Measurements with Indicators.—Three chief sources of error in connection with pll determinations by means of indicators may be mentioned. In the first place, if the test solution is not buffered, eg., solutions of very weak acids or Imsos or of neutral salts of strong acids and bases, the addition of the indicator may produce an appreciable change of pH this source of error may be minimized by employing small amounts of indicator which have been previously adjusted, as a result of preliminary experiments, to have approximately the same pH as the test solution. Such indicator solutions are said to be isohydric vith the test solution. 

The acid-base error of indicators. Isohydric indicators. The measurement of pH in unbuffered or slightly buffered solutions. 

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