Question

The Henderson-Hasselbalch Equation is: pH = pKa + log ( [A-]/[HA] ). Here is phosphoric acid at pH 0. It is polyprotic with pKas of 2.12, 7.21, and 12.67 At what pHs would the average charge on the phosphate species be -0.5, -1.0, and -1.5?

Answer 1

Henderson-Hasselbalch equation is used for estimation of PH of a buffer solution and isoelectric points of proteins.

Pkas value - 2.12,7.21,12.67

Low pka value of an acid means that the acid ready dissociates and molecule will exist as its conjugate base at physiological PH.

Now if PH is increased (greater than Pka value of 2.21 and less than pka value 7.21), the deprotonation will takes place, dihydrogen phosphate anion will be formed and the charge on the molecule will be - 1.

But if the PH is approximately equal to Pka then equimolar mixture of phosphoric acid and dihydrogen phosphoric acid is formed.

So at PH ~2.12 charge will be - 0.5.

Similarly at PH ~7.21 charge will be - 1.5.

If PH is greater than 7.21 then hydrogen phosphate anion will be formed.