Sodium hydroxide cannot be used to prepare a primary standard titration solution

Question

Sodium hydroxide cannot be used to prepare a primary standard titration solution because the compound is not available in high purity. The most common impurity is water because sodium hydroxide is hygroscopic, meaning it absorbs water from the air. a. If sodium hydroxide that had absorbed water was used directly to prepare a standard solution how would the actual concentration differ from the presumed concentration? b. If this sodium hydroxide standard solution was used to titrate a hydrochloric acid solution of unknown concentration, how would the determined concentration compare to the actual concentration.

Explanation / Answer

a. If sodium hydroxide adsorbs water, then the actual weighed amount will include the weight of adsorbed water and so lower weight of sodium hydroxide than the measured amount needed to make the primary standard solution. So, the actual concentration of the solution will be lower than the presumed concentration.

b. If this solution is used to titrate a HCl solution of unknown strength, higher volume of NaOH will be required to neutralise the acid as its concentration is lower than the presumed concentration. This will imply an increased concentration of HCl than its actual concentration.

V1S1 = V2S2 measured V1'S2' =

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