Cysteine titration

Question

A solution of 0.2090.209 M cysteine is titrated with 0.05970.0597 M HNO3HNO3. The p𝐾ap�a values for cysteine are 1.70, 8.36, and 10.74, corresponding to the carboxylic acid group, thiol group, and amino group, respectively. Calculate the pH at the equivalence point of the titration.

Answer 1

To calculate the pH, we'll compare the pKa values with the pH at each relevant step:

1. Carboxylic acid group (pKa = 1.70): At this pH range, the carboxylic acid group will be fully protonated, and the pH will be less than the pKa value. We'll assume it to be pH = 1.70.
2. Thiol group (pKa = 8.36): At this pH range, the thiol group will be deprotonated, and the pH will be greater than the pKa value. We'll assume it to be pH = 8.36.
3. Amino group (pKa = 10.74): At this pH range, the amino group will be deprotonated, and the pH will be greater than the pKa value. We'll assume it to be pH = 10.74.

Since we're at the equivalence point, all three groups are in their conjugate acid forms, and the pH can be assumed to be the average of the pKa values:

pH = (1.70 + 8.36 + 10.74) / 3 = 6.60

Therefore, the pH at the equivalence point of the titration between cysteine and HNO3 is approximately 6.60.