Nonaqueous Solvents

The activity of the hydrogen ion is affected by the properties of the solvent in which it is measured. Scales of pH only apply to the medium (single solvent or mixed solvents, eg, water-alcohol) for which they are developed. The comparison of the pH values of a buffer in aqueous solution to one in a nonaqueous solvent has neither direct quantitative nor thermodynamic significance. Consequently, operational pH scales must be developed for the individual solvent systems. In certain cases, correlation to the aqueous pH scale can be made but, in others, pH values are used only as relative indicators of the hydrogen-ion activity.

Other difficulties of measuring pH in nonaqueous solvents are the complications that result from dehydration of the glass pH membrane, increased sample resistance and large liquid-junction potentials. These effects are complex and are highly dependent on the type of solvent or mixture used (1,5).

INDICATOR pH MEASUREMENTS

The indicator method is especially convenient when the pH of a well-buffered colorless solution must be measured at room temperature with an accuracy no greater than 0.5 pH unit. Under optimum conditions an accuracy of 0.2 pH unit may be obtainable. A list of representative acid-base indicators is given in Table 2 with their corresponding transformation ranges. A more complete listing, including the theory of the indicator color change and of the salt effect, is given in Reference 1.

Because they are weak acids or bases, the indicators may affect the pH of the sample, especially in the case of a poorly buffered solution. Variations in the ionic strength or solvent composition, or both, also can produce large uncertainties in pH measurements, presumably caused by changes in the equilibria of the indicator species. Specific chemical reactions also may occur between solutes in the sample and the indicator species to produce appreciable pH measurement errors. Examples of such interferences include binding of the indicator species by proteins and colloidal substances and direct reaction with sample components, for example, oxidizing agents and heavy metal ions.

Table 2. Acid-Base Indicators

Indicator

pH range

Acid color

Base color

Acid cresol red

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