Pyridine is half protonated in aqueous phosphate buffer at pH 5.2. If you mix 45 mL of phosphate buffer with 55 mL of methanol, the buffer must have a pH of 3.2 to half protonate pyridine. Suggest a reason why.

The reaction between pyridine and acid is as follows.

The polarity of a compound is directly proportional to its dielectric constant. So, it can be noted that the dipole moment of water is higher than that of methanol. As a result, the polarity of methanol is less than that of water.

It is given that pyridine is half protonated in aqueous phosphate buffer at pH=5.2. Now if 55mL of methanol is mixed with phosphate buffer, neutral molecules of pyridine are favourable over the protonated ones. To protonate the pyridine ions higher concentration of acidic solution is required for the mixture. The pK_a value of pyridine ions decreases after adding methanol. Therefore, the phosphate buffer must have a pH of 3.2 to half protonate pyridine if methanol is added