# Absorbance vs titrant added in acid-base titration

I was working on this question:

Nitrophenol is a colorless weak monoprotic acid ($\mathrm{p}K_\mathrm{a} = 7.2$) whose conjugate base is bright yellow. To $2.00\ \mathrm{mL}$ of a solution of $\mathrm{0.0100\ M}$ nitrophenol is added $\mathrm{1.00\ M} ~\ce{NaOH}$ in $\mathrm{0.001\ mL}$ portions, and the absorbance of the solution at $\mathrm{485\ nm}$ is monitored. What does the graph of $A_\mathrm{{485}}$ as a function of added volume of $\ce{NaOH}$ look like?

The answers given were:

My reasoning for this question was that the absorbance is proportional to the concentration of conjugate base present in the solution. As the titration progressed towards the equivalence point ($\mathrm{0.02\ mL}~\ce{NaOH}$ added), the concentration of base increased. Then, as more base is added beyond the equivalence point, the concentration of base decreased, since no more base is being produced but the solution is being diluted. Therefore, I thought the correct graph was (A). However, the correct answer ended up being (B). So why would the concentration of base stay constant past the equivalence point?

• Once all the conjugate base is formed in the solution it doesn't get any more yellow. The twist here is that the small extra volume of titrant has a negligible effect on the overall volume of solution. – MaxW Dec 31 '15 at 17:19
• @MaxW Ohh the solution originally had a volume of 2 mL so adding the 0.02 mL of titrant doesn't change the concentration much. Thanks. – carbenoid Dec 31 '15 at 18:07