The effect of pH on solubility. The water solubility of an organic acid or base

Question

The effect of pH on solubility. The water solubility of an organic acid or base can be increased by converting it from a neutral molecule to a charged species. Shown below is the acid dissociation equilibrium of 2,4-dinitrophenol (2,4-DNP). The pKa of 2,4-DNP is 4.0.

b.)2,4-DNP acts as a metabolic poison of respiration, which takes place in mitochondria. When introduced into the inter-membrane space (IMS) of mitochondria, it diffuses into the mitochondrial matrix. In actively respiring mitochondria, the pH in the IMS is lower than the matrix pH (see below). What proportion (%) of 2,4-DNP is in its neutral form in the IMS? What proportion of 2,4-DNP is neutral in the mitochondrial matrix? Show your work please.

Explanation / Answer

Supose .....HA =un ionised DNP ..............A =ionised DNP

pH = pK - log (HA)/(A)

pH = 4 - log (HA)/(A) .................ratio of neutral to charged = (HA)/(A) =10-(pH-4)

In intermembrane space pH = 6.9 So ratio of neutral to charged = (HA)/(A) = 10-(pH-4) = 10-(6.9-4) = 10-2.9

So charged species will be 102.9 times more than uncharged so it will be soluble

proportion (%) of 2,4-DNP is in its neutral form in the IMS= 10-2.9 *100 = 0.00125*100 = 0.125%

proportion of 2,4-DNP is neutral in the mitochondrial matrix  10-(pH-4) = 10-(7.8-4) = 10-3.8 = 1.5*10-4

proportion in (%) of 2,4-DNP is neutral in the mitochondrial matrix = 1.5*10-4*100 = 1.5*10-2

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