Standardization of Hydrochloric Acid and Sulphuric Acid

1.1 Principle

Sodium Carbonate is frequently used to standardize acid solutions. Analytical grade sodium carbonate of 99% purity contains a little moisture and must be dehydrated at 250 oC. The titration of sodium carbonate involves two end-points. The first, corresponding to the conversion of carbonate to bicarbonate at a pH of about 8.3. The second, involving the formation of carbonic acid is observed at a pH of 3.8. It is this second end-point which is used for standardization.






1.2 Reagents

1.2.1 Sodium carbonate (Na2CO3), AR dried at 250 oC for 4 hours.

1.2.2 Sodium carbonate, 0.05 N: Weigh 2.5 ± 0.2 g (to nearest 0.1 mg) Na2CO3 and record this weight. Transfer to 1 L volumetric flask and make up to the mark with reagent grade water. Store in polyethylene bottle in fridge. Remake weekly. Calculate normality of Na2CO3 (to 3 significant figures) as per 1.5 formula 2.

1.2.3 Methyl orange, AR

1.2.4 Indigo Carmine, AR

1.2.5 Methyl orange – indigo carmine indicator: dissolve 0.25 g methyl orange and 0.625 g indigo carmine in 250 mL of DI water. Store in glass bottle in refrigerator. Indicator will turn brown with shelf life has expired.

1.2.6 Hydrochloric acid (HCl), 32% AR or Sulphuric Acid (H2SO4), 98%, AR.

1.2.7 Standard hydrochloric acid or Sulphuric acid, ~0.1 N: Dilute 9.3 mL of conc. HCl or 2.8 mL conc. H2SO4 in 500 mL reagent grade water, transfer to a 1 L volumetric flask and make up to the mark with water. Standardise as per 1.3.1.

1.2.8 Standard hydrochloric acid or Sulphuric acid, ~0.02 N: Dilute 200 mL ~ 0.1 N standard acid to 1 L with reagent grade of water. Standardise as per 1.3.2.


1.3 Procedure:

1.3.1 Standardisation of ~0.1 N Hydrochloric Acid or Sulphuric Acid.

Accurately weigh about 0.1 g sodium carbonate (to the nearest 0.1 mg) into a conical flask and dissolve in 50 mL reagent grade water. Record this weight on the work book. Or alternatively pipette 25.0 mL of ~0.05 N Na2CO3 into a conical flask and dilute with 50 mL reagent grade water. Add 2 drops methyl orange – indigo carmine indicator and titrate with ~0.1 N HCl or H2SO4 until the colour changes from green to magenta with a blue-gray colour at approximately pH 4. Carry out standardization in triplicate. Calculate normality of HCl or H2SO4 as per 1.5 formula (1) or (3).

1.3.2 Standardisation of ~0.02 N Hydrochloric Acid or Sulphuric Acid.

Pipette 5.0 mL of 0.05 N Na2CO3 into a conical flask and dilute with 50 mL reagent grade water. Add 2 drops methyl orange – indigo carmine indicator and titrate with ~0.02 N HCl or H2SO4 until the colour changes from green to magenta with a blue-gray colour at approximately pH 4. Carry out standardization in triplicate. Calculate normality of HCl or H2SO4 as per 1.5 formula (3).

1.4 Frequency of Standardisation

Standardise hydrochloric acid and Sulphuric solutions on preparation. Solutions of hydrochloric acid and Sulphuric acid are stable and may be stored and used over a lengthy period of time without restandardising, provided that the contents of the storage vessel are thoroughly mixed before each use.


1.5 Calculations




1.6 Reference:

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