Standardisation of Sodium Hydroxide (NaOH)

1.1 Principle:

Potassium hydrogen phthalate (KHP) is an excellent primary standard for the standardization of sodium hydroxide due to its high purity and non-hygroscopic nature. It is a weak organic acid, which requires the use of an indicator, in this case phenolphthalein, with a basic transition range.





1.2 Reagents

1.2.1 CO2 free water: Boil reagent grade water for 15 minutes and cool to room temperature.

1.2.2 Potassium hydrogen phthalate (KHC8H4O4) (KHP), AR: Dry at 120oC for 2 hours and allow to cool in a covered vessel in a desiccator.

1.2.3 Potassium hydrogen phthalate (KHP) primary standard solution, 0.049 N: Accurately weigh 10.0 g of KHP and dissolve in ~800 mL of reagent grade water. Make up to 1000 mL in a volumetric flask.

1.2.4 Phenolphthalein, AR

1.2.5 Ethanol, AR

1.2.6 Phenolphthalein indicator solution: Dissolve 5 g of the reagent in 500 mL of ethanol and add 500 mL of reagent grade water with constant stirring. Filter if a precipitate forms.

1.2.7 Sodium Hydroxide (NaOH), AR

1.2.8 Standard Sodium Hydroxide, ~0.1 N: Dissolve 4.0 g NaOH in 1 L or reagent grade water in a glass beaker and store in a plastic bottle. Standardise against KHC8H4O4 as directed in Section 1.3.

1.2.9 Standard Sodium Hydroxide, ~0.02 N: Dilute 200 mL ~0.1 N NaOH to 1 L and store in a plastic bottle. Standardise against KHC8H4O4 as directed in Section 1.3.


1.3 Procedure

1.3.1 Standardisation of 0.1 N NaOH

Weigh approximately 0.2 g (to the nearest 0.1 mg) KHP into a conical flask and add a 20 mL aliquot of CO2 free water, cover flask and dissolve KHP. Or alternatively pipette 20.0 mL of 0.049 N KHP into a conical flask. Titrate each solution with NaOH using phenolphthalein as the indicator to the first permanent pink tinge. Record the information on the work book. Carry out standardization in triplicate. Calculate normality of NaOH as per 1.5.

1.3.2 Stantardisation of 0.02 N NaOH

Weigh approximately 0.1 g (to the nearest 0.1 mg) KHP into a conical flask and add a 20 mL aliquot of CO2 free water, cover flask and dissolve KHP. Titrate each solution with NaOH using phenolphthalein as the indicator to the first permanent pink tinge. Record the information on the work book. Carry out standardization in triplicate. Calculate normality of NaOH as per 1.5.

1.4 Frequency of Standardisation

Sodium hydroxide solutions must be standardized prior to use. Due to the nature of these solutions, the absorb CO2 from the atmosphere.

1.5 Calculations




1.6 Reference:

Vogel’s, ‘Textbook of Quantitative Inorganic Analysis’, 4th Edition; pp. 242, 301-305.