A mixture of 0.4M copper sulfate and 4M lactic acid has been prepared, and then 4M sodium hydroxide solution is added to adjust the pH, but the more sodium hydroxide is added, the more the concentration of the mixture is diluted, but it is difficult to adjust the pH to 10. Should we keep increasing the concentration of sodium hydroxide?
Add close to 1:1 mole ratio of NaOH to lactic acid and adjust to the desired pH with diluted NaOH (e.g., 0.1 M) to avoid overshooting the pH. Best practice is to do a calculation of the necessary amount of base needed and fine tune the pH. The pKa of lactic acid is 3.9. The pH of a weak acid is given by:
pH=1/2(pKa - log Cacid).
To calculate the concentration of base (in molar) needed for a certain pH, use the Henderson-Hasselbalch equation:
pH=pKa + log Cbase/Cacid
In addition to Andrei Blasko awnser I would also suggest to standardize the NaOH (by titration or pH meter measure) to ensure that it has the correct concentration (as it usually carbonates).
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