Edition: BP 2025 (Ph. Eur. 11.6 update)
DEFINITION
| Dried Magnesium Sulfate | A sufficient quantity |
| Phenol | 0.5 g |
| Glycerol | 55 g |
Extemporaneous preparation
The following directions apply.
Dry about 70 g of Dried Magnesium Sulfate at 150° for 1.5 hours or at 130° for 4 hours and allow to cool in a desiccator. Mix 45 g of the dried material in a warm mortar with the Phenol dissolved in the Glycerol.
In preparing larger quantities of the paste the period of heating the Dried Magnesium Sulfate should be increased, if necessary, to ensure that the dried material contains not less than 85% of magnesium sulfate, calculated as MgSO4.
The paste complies with the requirements stated under Topical Semi-solid Preparations and with the following requirements.
Content of magnesium sulfate, MgSO4
36.0 to 41.0% w/w.
Content of phenol, C6H6O
0.45 to 0.55% w/w.
ASSAY
For magnesium sulfate
Dissolve 5 g in sufficient water to produce 100 mL. To 10 mL of the resulting solution add 150 mL of water and 10 mL of ammonia buffer pH 10.9 and titrate with 0.05M disodium edetate VS, using mordant black 11 solution as indicator, to a full blue colour. Each mL of 0.05M disodium edetate VS is equivalent to 6.018 mg of MgSO4.
For phenol
Dissolve 5 g in 25 mL of water. To this solution add 25 mL of 0.05M bromine VS and 5 mL of hydrochloric acid, stopper the flask, allow to stand for 30 minutes with occasional swirling and then allow to stand for a further 15 minutes. Add 5 mL of a 20% w/v solution of potassium iodide, shake thoroughly, titrate with 0.1M sodium thiosulfate VS until only a faint yellow colour remains, add 0.25 mL of starch mucilage and 10 mL of chloroform and complete the titration with vigorous shaking. Repeat the operation without the substance being examined. The difference between the titrations represents the amount of bromine required. Each mL of 0.05M bromine VS is equivalent to 1.569 mg of C6H6O.
