Experiment 28 Determination of the Effective Ingredient in Antacid Tablets.

A. Standardisation of Sodium Hydroxide `A'.
Standardisation of sodium hydroxide requires a pure solid acid. Amongst the most commonly used reagents for this purpose is potassium hydrogen phthalate, KHC8H4O4 , which is obtainable in 99.99% purity and is readily soluble in water. Phthalic acid is a dibasic acid but KHC8H4O4 acts as a monobasic acid.
Weigh accurately (on an analytical balance) using the method of weighing by difference, between 0.50 and 0.55 g of potassium hydrogen phthalate into a 250 cm3 conical flask and dissolve it in about 25 cm3 of water. Titrate with sodium hydroxide using phenolphthalein as indicator. Repeat the standardisation using a second sample of potassium hydrogen phthalate. For each titration, calculate the molarity of the sodium hydroxide solution and decide whether or not a third titration is necessary.

B. Determination of the hydroxide ion content of the antacid tablet.
The tablet is dissolved in an accurately known volume of standard acid. The unreacted (excess) acid is then determined by back titration using standard alkali.
Add by pipette, 25.00 cm3 of the hydrochloric acid `B' to one of the tablets in a conical flask. Swirl gently until the tablet has dissolved and then titrate the excess acid using screened methyl orange as indicator. Carry out the experiment in duplicate. Use the concentration of the standardised hydrochloric acid provided to determine the hydroxide ion content of the tablet. Check if your value is consistent with that on the label.

1. Give three reasons why potassium hydrogen phthalate is used as a primary standard.
2. Explain why phenolphthalein, rather than methyl orange, was used as indicator for the titration of the potassium hydrogen phthalate with sodium hydroxide.

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