Question

1. Look up the structure of citric acid and the pKa values associated with disassociation of each proton. a) Write the full disassociation (overall and each step) reaction starting with the fully protonated acid and ending with the fully deprotonated salt. b) Sketch a titration curve (y axis pH and x axis equivalents of base) identify each pka on the graph and note the acid and the salt at each step (similar to the graph for phosphate in the notes) – write the equilibria on the graph. c) Determine how you would prepare 500ml of a 200mM citric acid buffer at pH 6.5. Assume you have all sodium salts available in the lab. Show all calculations and indicate how you would make the solution. d) Using your graph show what would happen (where on the graph you would be) if you added 2 molar equivalents of base to your buffer. Mark the point with an *

Answer 1

Citric acid falls into the category of polyprotic acids, which are acids that have more than one acidic hydrogen that can react with water to produce the hydronium ion, H₃O⁺.

Its molecular formula is C₆H₈O₇, and it is known as a weak organic acid. It is actually a triprotic acid which means it has 3 acidic hydrogen atoms in its structure -

When placed in water, citric acid will ionize in a step-wise manner-

pKa values for all the three steps are-

Titration curve for citric acid (0.1M) titrated with 0.1M solution of strong base gives-