Titration of HCl with Borax

1. Acid – Base Titration

A- Standardization of Hydrochloric acid with anhydrous sodium carbonate
Sodium carbonate is a salt of a weak acid. When titrated with hydrochloric acid carbonate decomposes, yielding carbon dioxide and water:
Na2CO3 + 2HCl ? 2NaCl + CO2 + H2O
Evolving carbon dioxide acidifies the solution, and the end point in its presence is detected too early. To avoid titration errors we boil titrated solution to remove carbon dioxide.
Procedure to follow:
• Place 10 ml of 0.1 N of anhydrous sodium carbonate into 250 mL Erlenmeyer flask.
• Add 1-2 drops of methyl orange solution.
• Titrate with about 0.1M HCl solution till the first color change from red to yellow

Calculations

NHCl x VHCl = NNa2CO3 x VNa2CO3

B - Standardization of Hydrochloric acid with Borax

Borax has relatively high equivalent mass, which means potential errors in the standardization are smaller than in the case of other substances. The advantages of sodium tetraborate decahydrate (borax) are: (i) it has a large relative molecular mass,381.44 (that of anhydrous sodium carbonate is 106.00); (ii) it is easily and economically purified by recrystallisation; (iii) heating to constant weight is not required; (iv) it is practically non-hygroscopic; and (v) a sharp end point can be obtained with methyl red at room temperatures, since this indicator is not affected by the very weak boric acid.

Reaction taking place during neutralization is
Na2B4O7 + 2HCl + 5H2O ? 4H3BO3 + 2NaCl
Boric acid is so weak, that its presence doesn t interfere with end point detection (compare with standardization against sodium carbonate).
Procedure
• Place 10 ml of 0.1 N of dried Borax into 250 mL Erlenmeyer flask.
• Add 2-3 drops of Methyl red indicator
• Titrate against HCl solution till the first color changed from red to yellow.

Calculations
NHCl x VHCl = NBorax x VBorax