Effect of pH

The pH of an aqueous sample will influence the success or failure of an extraction for acidic and basis analytes. Acids and bases involve an equilibrium between two forms, one neutral and one ionic. Each form has significantly different chemical and physical properties, as noted in Table 1.

Figure 2 Solubility of amino acids in water as a function of temperature.

Temperature (°C}

Figure 2 Solubility of amino acids in water as a function of temperature.

Figure 3 Vapour pressaure of common solvents as a function of temperature. (Calculated from data in Handbook of Chemistryand Physics (1971).)

Extraction of organic acids from water is only practical at pH values more than two units below the pKa of the acid. Only at this pH is most of the compound in the neutral form and amenable to extraction with an organic solvent. Similarly, to keep a base, such as an aromatic amine, in the neutral (extractable) form, the pH of the solution must be adjusted to at least two units above the pKb of the base. Readings with a pH meter are likely to be unstable and/or unreliable in the presence of organic solvents, and the equilibrium constant, Ka, is also

Table 1 Properties of individual forms in acid-base equilibria

Neutral form

Ionic form

HA + H2O 8

A- + H3O#

Acid

Conjugate base

B + H2O 8

BH# + OH-

Base

Conjugate acid

More soluble in organic solvents

Less soluble in organic solvents

Insoluble in water

Soluble in water

More volatile

Nonvolatile

Sour/bitter taste, bad odour

Little odour

Reproduced with permission of ACCTA, Inc.

likely to change, often in an unpredictable way. Any required pH adjustments and measurements must, therefore, be made before the organic component is added.

Solar Panel Basics

Solar Panel Basics

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