Preparation of standard buffer for calibration of a ph meter the glass electrode used in commercial ph meters gives an electrical response proportional to the concentration of hydrogen ion. to convert these responses to a ph reading, the electrode must be calibrated against standard solutions of known h+ concentration. determine the weight in grams of sodium dihydrogen phosphate (nah2po4 · h2o; fw 138) and disodium hydrogen phosphate (na2hpo4; fw 142) needed to prepare 1 l of a standard buffer at ph 7.00 with a total phosphate concentration of 0.100 m

Explanation:

Keeping in mind the equilibrium:

H₂PO₄⁻ ↔ HPO₄⁻² + H⁺

We use the Henderson-Hasselbalch equation (H-H):

pH = pka +

For this problem [A⁻] = [HPO₄⁻²] and [HA] = [H₂PO₄⁻]

From literature we know that pka = 7.21, from the problem we know that pH=7.00 and that

[HPO₄⁻²] + [H₂PO₄⁻] = 0.100 M

From this equation we can express [H₂PO₄⁻] in terms of [HPO₄⁻²]:

[H₂PO₄⁻] = 0.100 M - [HPO₄⁻²]

And then replace [H₂PO₄⁻] in the H-H equation, in order to calculate [HPO₄⁻²]:

With the value of  [H₂PO₄⁻], we calculate [HPO₄⁻²]:

[HPO₄⁻²] + 0.0381 M = 0.100 M

[HPO₄⁻²] = 0.0619 M

Finally, using the concentrations, the volume, and the molecular weights; we can calculate the weight of each substance:

it should be p1 v1 = p2 v2