Dissolve 42 g of Sodium hydroxide in sufficient carbon dioxide-free water to produce 1000 ml.
Weigh accurately about 0.5g of potassium hydrogen phthalate previously powdered and dried at 120°C for 2 hours. Dissolve it in 75 ml carbon dioxide free water. Add 0.1 ml phenolphthalein solution and titrate with prepared sodium hydroxide solution until a permanent pink colour is produced.
Each ml of 1 M sodium hydroxide ≡ 0.2042 gm of potassium hydrogen phthalate.
Perform a duplicate and calculate the molarity factor (MF).
Note down the average value
MF =W/ 0.2042 x V
V = Volume of Sodium hydroxide consumed in ml.
W = Weight of potassium Hydrogen phthalate in gram.
Restandardise before use.
Store in bottle with well fitted suitable stoppers, which prevents access to atmospheric carbon dioxide.
Discard the solution after 30 days.
For storage and labeling follow the general procedure.