Edition: BP 2025 (Ph. Eur. 11.6 update)
DEFINITION
Magnesium Trisilicate Mixture is an oral suspension containing 5% w/v each of Magnesium Trisilicate, Light Magnesium Carbonate and Sodium Bicarbonate in a suitable vehicle with a peppermint flavour.
The mixture complies with the requirements stated under Oral Liquids and with the following requirements.
Content of magnesium, Mg
1.70 to 2.20% w/v.
Content of sodium bicarbonate, NaHCO3
4.60 to 5.40% w/v.
IDENTIFICATION
A. Filter 50 mL of the mixture using a Buchner funnel, reserving the filtrate, wash the filter with 50 mL of water and dry the residue at 100° to 105° for 1 hour. The filtrate yields reaction B characteristic of sodium salts and reaction A characteristic of bicarbonates, Appendix VI.
B. Shake 0.5 g of the residue obtained in test A with 5 mL of 1M hydrochloric acid for 5 minutes, add 5 mL of 1M sodium hydroxide and centrifuge. The supernatant liquid yields reaction B characteristic of magnesium salts, Appendix VI.
C. The residue obtained in test A yields reaction A characteristic of carbonates, Appendix VI.
D. 30 mg of the residue obtained in test A yields the reaction characteristic of silicates, Appendix VI.
Neutralising capacity
To 5 g add 100 mL of water and heat to 37°. Add 100 mL of 0.1M hydrochloric acid VS previously heated to 37° and stir continuously using a suitable paddle stirrer rotating at a rate of about 200 revolutions per minute, maintaining the temperature at 37°. The pH of the suspension at 37° after 10 and 20 minutes is not less than 2.0 and not less than 2.4 respectively and at no time is it more than 4.0. Add 10 mL of 0.5M hydrochloric acid VS previously heated to 37°, stir continuously for 1 hour, maintaining the temperature at 37°, and titrate with 0.1M sodium hydroxide VS to pH 3.5. Subtract the volume of 0.1M sodium hydroxide VS from 150 to obtain the number of mL of 0.1M hydrochloric acid VS required for the neutralisation. Determine the weight per mL of the mixture, Appendix V G, and hence the volume of 0.1M hydrochloric acid VS required to neutralise 5 mL of the mixture. Not less than 100 mL of 0.1M hydrochloric acid VS is required.
ASSAY
For magnesium
To 5 g add 7 mL of hydrochloric acid and 7 mL of water, stir and heat on a water bath for 15 minutes. Add 30 mL of hot water, mix, filter while still hot and wash the filter with hot water. Cool the combined filtrate and washings and dilute to 100 mL with water. To 25 mL of the resulting solution add 150 mL of water and 5 mL of ammonia buffer pH 10.9 and titrate with 0.05M disodium edetate VS using mordant black 11 solution as indicator. Each mL of 0.05M disodium edetate VS is equivalent to 1.215 mg of Mg. Using the weight per mL of the mixture calculate the percentage of Mg, weight in volume.
For sodium bicarbonate
Boil 20 g with 100 mL of water for 5 minutes, filter, boil the residue with 100 mL of water for 5 minutes and filter. Combine the filtrates, cool and titrate with 0.5M hydrochloric acid VS using methyl orange–xylene cyanol FF solution as indicator. Add 10 mL of ammonia buffer pH 10.9 and titrate with 0.05M disodium edetate VS using mordant black 11 solution as indicator. After subtracting one-fifth of the volume of 0.05M disodium edetate VS, each mL of 0.5M hydrochloric acid VS is equivalent to 42.00 mg of NaHCO3. Using the weight per mL of the mixture calculate the percentage of NaHCO3, weight in volume.
