why does the apparent partition coefficient (Papp or D) decrease as the pH increases for benzoic acid?

Question

please help, i understand that as pH increses the [H+] decreases, can anyone giv eme detailed answer to mek me undertand?????

but why does it partition into the water phase just because the [H+] goes down?? im still confused!!

Answer 1

Assume HA is benzoic acid.

In aqueous phase, HA(w) dissociates into H3O+ and A- partially since it is a weak acid.

Equation for Kapp:

Kapp = K [H30+] / (Ka + H3O+)

K is the true partition coefficient (constant).
Ka represents the degree of ionization.

Hence when pH increases, [OH-] increases. HA(w) will dissociate more into H30+ and A-. From the equation, if [OH-] increases, this will lead to higher [H3O+] and Ka, while K remains as a constant. So you can see that upper term in the equation is now divided by a larger lower term, resulting in the decrease in Kapp.

In simpler terms, as pH increases, benzoic acid dissociates more into ions. Ions prefer the aqueous phase more since its hydrophilic. The partition coefficient measures the concentration of the benzoic acid in the oil phase over its concentration in the aqueous phase. Since now there is a higher concentration of benzoic acid in the aqueous phase, K will decrease.

Answer 2

Apparent Partition Coefficient

Answer 3

The acidity of vinegar would decrease if water was added, but on the pH scale the number would increase. The more Acidic something is, the lower the number on the pH scale for example battery acid would be around 0, vinegar would be around 3 , milk would be around 6.5 and fresh distilled water would be around 7. So the acidity would decrease but the number of its acidity on a pH chart would increase. The less acidic a solution becomes means that the more alkaline it becomes. water is more alkaline than battery acid..

Answer 4

As pH goes up, [H+] goes down and [OH-] goes up. Benzoic acid, C6H5-COOH is increasingly converted to C6H5-COO(-), which partitions into the water phase rather than the organic phase. C6H5-COOH has greater affinity for the organic phase.