Question

Explain why you can't accurately determine molarity sodium hydroxide using number of grams dissolved in total amount of solution? What method does a chemist use to determine molarity of sodium hydroxide?

Answer 1

Solid sodium hydroxide can never be obtained in pure form. Alkali hydroxides like sodium hydroxide and potassium hydroxide are extremely hygroscopic. They always contain traces of water and alkali carbonate with it. so solution prepared by weighing a certain gram of NaOH will not contain the measured amount in the solution. Hence you can not accurately determine the molarity of sodium hydroxide using the no. of grams.

Hence it is important to remove the traces of Carbonate from the alkali. This is done by methods like Anion Exchange. The solution is passed through a strong base anion exchange column in chloride form. The resin is converted to its hydroxide form by sodium hydroxide. The carbonate will get attach to the resin since it has more affinity for this. The sodium hydroxide obtained from above method can be weighed and make a solution.

This is followed by Standardisation of the hydroxide using HCl or Potassium Hydrogen Phthalate (KHP) to get the exact molarity of the sodium hydroxide prepared. This is done as follows-

Analytical grade KHP is almost pure and not hygroscopic. 0.6-0.7 gm of KHP is taken in a conical flask and 75 ml water is added to it. then it is titrated against the NaOH solution., using Phenolphthalein as the indicator. Reaction is follows

NaOH(aq) + KHC₈H₄O₄(aq) -------> KNaC₈H₄O₄(aq) + H₂O(l)

Conc. of NaOH is subsequently using titrimetric calculations.