Why are a calico strainer and chloroform used in a British Pharmacopoeia assay of milk of magnesia?

The following is an assay for milk of magnesia in the British Pharmacopoeia (emphasis added):

Dissolve the sodium hydroxide in $\pu{150 cm^3}$ of purified water, add the light magnesium oxide, mix to form a smooth cream and then add sufficient purified water to produce $\pu{2500 cm^3}$. Pour this suspension in a thin stream into a solution of the magnesium sulfate in $\pu{2500 cm^3}$ of purified water, stirring continuously during the mixing. Allow the precipitate to subside, remove the clear liquid, transfer the residue to a calico strainer, allow to drain and wash the precipitate with purified water until the washings give only a slight reaction for sulfate. Mix the washed precipitate with purified water, dissolve the chloroform in the mixture.

What is a calico strainer, and why is it used here? Is it better than using, say, Büchner filtration?

As well, why is chloroform used (see emphasized text, above)?

• Are you certain that chloroform is used in the assay, in my BP , milk of magnesia is assayed with hydrochloric acid, and a complometrix titration with sodiun edetate – xavier_fakerat Apr 26 '17 at 15:16
• Pour this suspension in a thin stream into a solution of the magnesium sulfate in 2500 cm3 of purified water, stirring continuously during the mixing. Allow the precipitate to subside, remove the clear liquid, transfer the residue to a calico strainer, allow to drain and wash the precipitate with purified water until the washings give only a slight reaction for sulfate. Mix the washed precipitate with purified water, dissolve the chloroform in the mixture <-- this last line. – Simran Apr 26 '17 at 19:05

Ok I understand where you got chloroform from.

This is not the assay of milk of magnesia, but the extemporaneous preparation.

In extemporaneous preparations chloroform is used as:

• a general anaesthetic
• a carminative
• flavouraing agent
• preservative

So in the case of mik of magnesia it has two main roles that is a carminative and a preservative.

Hope this helps

• Thank you so much! But, why is the calico strainer used? In the experiment I had done, we had to use the buchner funnel instead even though the monograph had called for the calico.. would this affect the results in any way? – Simran Apr 26 '17 at 20:36
• They essentially perform more or less similar functions, though a bunchner funnel is more efficient as it additionally dries up quickly the precipitate (if vacuum setup is used) – xavier_fakerat Apr 28 '17 at 10:49