Question

If 3 drops of HCl is added to two vials of liquid, one containing distilled water, the other containing phosphate buffer, what will happen to the pH in each and how will they compare?

Answer 1

If we add only 3 drops of HCl in vial containing-

Liquid Water-

Surely due to the ionization of the HCl, more $\large H^{+}$ ions will be released according to the following equation and decrease in pH would be observed-

HCl + H2O= H3O+ + C1-

Phosphate Buffer

The addition of only 3 drops of HCl into phosphate buffer will not result in any significant change in pH because the buffer system, as we know, resits the change in pH if you add only some small amounts of acid/base. As we are not provided with any data of the molarities of the HCl or the pHor pKa values of the phosphate buffer, we cannot calculate the amount of change in pH. But obviously, the change will range from 0.2 to 0.5 and not more than that. The buffer system equation would be-

.

HC1+ Na2HPO4 = NaH,PO4 + NaCl

The above equation illustrates how the phosphate buffer system neutralizes the effect of the addition of a small amount of HCl without a significant change in pH.

So, from the discussion above, it is clear that lowering of pH will be to a greater extent in the vial containing distilled water only because in the case of phosphate buffer, the buffer system will resist the change in pH if we add a little amount of HCl.