Calcium Hydroxide

                   Ca(OH)2                MW= 74.1

DEFINITION

Calcium hydroxide contains not less than 95.0 per cent and not more than the  equivalent of 100.5 per cent of Ca(OH)2. 

 

 CHARACTERS

A white or almost white, fine powder, practically insoluble in water. 

 

 IDENTIFICATION

A.    To 0.80 g in a mortar, add 10 ml of water R and 0.5 ml of phenolphthalein  solution R and mix. The suspension turns red. On addition of 17.5 ml of 1 M  hydrochloric acid , the suspension becomes colourless without effervescing. The  red colour occurs again when the mixture is triturated for 1 min. On addition of a  further 6 ml of 1 M hydrochloric acid  and triturating, the solution becomes  colourless.

B.    Dissolve about 0.1 g in dilute hydrochloric acid R and dilute to 10 ml with water  R. 5 ml of the solution give reaction (b) of calcium.

 

 TESTS

Matter insoluble in hydrochloric acid

Dissolve 2.0 g in 30 ml of hydrochloric acid R. Boil the solution and filter. Wash the  residue with hot water R. The residue weighs not more than 10 mg (0.5 per cent).

 

 Carbonates

Not more than 5.0 per cent of CaCO3.

Add 5.0 ml of 1 M hydrochloric acid  to the titrated solution obtained under Assay  and titrate with 1 M sodium hydroxide using 0.5 ml of methyl orange solution R as  indicator.

 

 1 ml of 1 M hydrochloric acid  is equivalent to 50.05 mg of CaCO3.

 

 Chlorides.

Dissolve 0.30 g in a mixture of 2 ml of nitric acid R and 10 ml of water R and dilute  to 30 ml with water R. 15 ml of the solution complies with the limit test for chlorides  (330 ppm).

 

 Sulphates.

Dissolve 0.15 g in a mixture of 5 ml of dilute hydrochloric acid R and 10 ml of  distilled water R and dilute to 60 ml with distilled water R. 15 ml of the solution  complies with the limit test for sulphates (0.4 per cent).

 

 Arsenic.

Dissolve 0.50 g in 5 ml of brominated hydrochloric acid R and dilute to 50 ml with  water R. 25 ml of the solution complies with the limit test  A for arsenic (4 ppm).

 

 Magnesium and alkali metals

Dissolve 1.0 g in a mixture of 10 ml of hydrochloric acid R and 40 ml of water R. Boil  and add 50 ml of a 63 g/l solution of oxalic acid R. Neutralise with ammonia R and  dilute to 200 ml with water R. Allow to stand for 1 h and filter through a suitable filter.  To 100 ml of the filtrate, add 0.5 ml of sulphuric acid R. Cautiously evaporate to  dryness and ignite. The residue weighs not more than 20 mg (4.0 per cent  calculated as sulphates).

 

 Heavy metals.

Dissolve 1.0 g in 10 ml of hydrochloric acid R1 and evaporate to dryness on a water- bath. Dissolve the residue in 20 ml of water R and filter. 12 ml of the filtrate complies  with limit test A for heavy metals (20 ppm). Prepare the standard using lead  standard solution (1 ppm Pb) R.

 

 ASSAY

To 1.500 g in a mortar, add 20 ml to 30 ml of water R and 0.5 ml of phenolphthalein  solution R. Titrate with 1 M hydrochloric acid  by triturating the substance until the  red colour disappears. The final solution is used in the tests for carbonates.

 

 1 ml of 1 M hydrochloric acid  is equivalent to 37.05 mg of Ca(OH)2.

Calculation

Result of later reaction: