Investigation to Determine the Percentage of Citric Acid in Lime Juice

by magisterartium (student)

A – The Standardisation of Sodium Hydroxide

Discussion and Method

A primary standard is a that is extremely pure, stable, has no waters of hydration, and has a high . This enables a concentration to be calculated from a direct weighing of its pure form.

NaOH is cannot be classed as a primary standard as a concentration cannot be calculated directly from a weighing. This is due to the fact that over time, in its pure form, NaOH will absorb atmospheric water and react with atmospheric CO2. With NaOH solutions, again, over time, they will absorb atmospheric CO2 and will also slowly react with glass bottles in which it is stored. Therefore, before using NaOH as a standard in a titration, the solution must be standardised to determine the concentration which is being used.

Using good laboratory techniques, potassium hydrogen phthalate was weighed by difference into a conical flask. 25cm3 distilled water was added and was then mixed until the potassium hydrogen phthalate had dissolved. Phenolphtalein indicator was added and the solution was carefully titrated with an unknown concentration of NaOH solution until a pale-pink end point was reached. The titration was repeated for accuracy.

Results

Potassium hydrogen phthalate: