Titer of NaOH 0.1 mol/L

Solid NaOH cannot be accurately weighed as it is highly hygroscopic (it absorbs water from the air). It also absorbs carbon dioxide from the air, forming sodium carbonate and thereby reducing the amount of sodium hydroxide present.

NaOH cannot be used as a primary standard. NaOH solution of known molarity cannot be directly prepared without resorting to an air-and-water-free environment. Therefore, NaOH solution is standardized by titrating weighed samples of a primary standard acidic substance, potassium hydrogen phthalate (KHC₈H₄O₄, "KHP", molar mass 204.2 g). KHP has one acidic hydrogen atom, and reacts with NaOH on a 1:1 stoichiometric basis.

Reaction:

     KHC₈H₄O₄ + NaOH --------> NaKC₈H₄O₄  + H₂O         

This application is used to standardize NaOH titrant with potassium hydrogen phthalate (KHP). The titration is monitored by potentiometric measurement with a pH sensor and the titration curve is evaluated at the inflection point to determine the equivalence point (EQP).