Quantitative Analysis
2010/2011
Babylon University
Practical chemistry
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 nonhygroscopic; 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
1
Quantitative Analysis
2010/2011
Babylon University
Practical chemistry
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
Burette containing
hydrochloric acid
Conical Flask containing
Borax and Methyl Red
Experiment 2
2
Quantitative Analysis
2010/2011
Babylon University
Practical chemistry
Standardization and titration of Sodium hydroxide against HCl solution
Theory
Sodium hydroxide reacts with hydrochloric acid solution as follows:
NaOH + HCl
1mol. of NaOH
NaCl + H2O
1 mol. of HCl
Depending on the above equation
eq.wt of HCl = Mo.wt.
eq.wt of NaOH = Mo.wt.
is not primary standard substance because I's
hygroscopic where it absorbs the water vapor from air , and it
forms a layer of carbonate around it
Sodium hydroxide
3