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Use of Various Relations

Let us consider three situations: (i) a solution of CH3COOH, (ii) a solution of CH3COONa, and (iii) a solution of CH3COOH and CH3COONa (buffer). In all the three solutions, there will be some CH3COO and CH3COOH. But the relations used for calculating the pH differ. For (i), the relation used is: pH = ½(pKa – log c); for (ii), the relation used is: ½ (pKw + pKa + log c); and for (iii), the relation used is Description: 22177.png To understand as to when a particular relation is to be used, the following chart should be kept in mind.
Description: 22185.png
Although it is difficult to give an exact limit up to which a buffer can be used, it is generally accepted that a buffer solution can be used for practical purposes when its [Salt]/[Acid] lies within the range of 0.1 to 10. Therefore, it can be concluded that a particular weak acid (or base) can be employed for making useful buffer solution of pH (or pOH) lying within the range of (pKa ± 1) or (pKb ± 1). For example, acetic acid (pKa = 4.75 at 25°C) and sodium acetate mixture can be used for preparing buffer solutions whose pH values are roughly in the range of 3.75–5.75. Outside this range, the buffer capacity of acetic acid–sodium acetate solution is too small to be used for any practical purpose.

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