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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)?

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  • $\begingroup$ 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 $\endgroup$ Apr 26, 2017 at 15:16
  • $\begingroup$ 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. $\endgroup$
    – Simran
    Apr 26, 2017 at 19:05

2 Answers 2

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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

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  • $\begingroup$ 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? $\endgroup$
    – Simran
    Apr 26, 2017 at 20:36
  • $\begingroup$ 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) $\endgroup$ Apr 28, 2017 at 10:49
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Chloroform is used as an emulsion stabilizer. The calico strainer is used as it is less prone to clogging than a buchner funnel. The mix will clog frits and normal filter paper.

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    $\begingroup$ Milk of magnesia has a pH ~10.5, perhaps not high enough to react with chloroform - or would it? Or what other process results in dispersion stabilization? $\endgroup$ Dec 25, 2021 at 20:13

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