Magnesium Aluminosilicate

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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1 DEFINITION

Magnesium Aluminosilicate is a synthesized material that contains NLT 20.5% and NMT 27.7% of magnesium oxide (MgO), NLT 27.0% and NMT 34.3% of aluminum oxide (Al₂O₃), and NLT 14.4% and NMT 21.7% of silicon dioxide (SiO₂), calculated on the dried basis.

2 IDENTIFICATION

A. IDENTIFICATION TESTS-GENERAL, Aluminum (191)

Sample: 0.5 g

Analysis: Transfer the Sample to a suitable container, add 5 mL of a sulfuric acid solution (1 in 3), and heat until white fumes are observed. Cool, add 20 mL, of water, and filter. Neutralize the filtrate with ammonia TS, and retain for use in Identification test B. Collect the precipitate, and dissolve in 3 N hydrochloric acid.

Acceptance criteria: Meets the requirements

B. IDENTIFICATION TESTS GENERAL, Magnesium (191).

Sample solution: The filtrate retained from Identification test A

Acceptance criteria: Meets the requirements

C.

Analysis: Prepare a bead by fusing a few crystals of sodium ammonium phosphate on a platinum loop in the flame of a Bunsen burner. Place the hot, transparent bead in contact with Magnesium Aluminosilicate, and again fuse.

Acceptance criteria: The silica floats about in the bead, producing, upon cooling, an opaque bead with a weblike structure.

3 ASSAY

ALUMINUM OXIDE

Edetate disodium titrant: Prepare a solution with a concentration of 18.6 g/L of edetate disodium in water, and standardize as follows. Weigh 2 g of aluminum wire, transfer to a 1000-mL volumetric flask, and add 50 mL of a mixture of hydrochloric acid and water (1:1). Swirl the flask to ensure contact of the aluminum and the acid, and allow the reaction to proceed until all the aluminum has dissolved. Dilute with water to volume. Pipet 10 mL of this solution into a 250-mL beaker, and add, in the order named and with continuous stirring, 25.0 mL of Edetate disodium titrant and 20 mL of acetic acid-ammonium acetate buffer TS. Boil gently for 5 min. Cool, and add 50 mL of alcohol and 2 mL of dithizone TS. Titrate with 0.05 M zinc sulfate VS to a bright rose-pink color. Perform a blank determination, substituting 10 mL of water for the aluminum solution, and make any necessary correction. Calculate the molarity of the solution taken:

Result = W/(A_r x V)

W = weight of aluminum in the portion of solution taken (g)

A_r = atomic weight of aluminum, 26.98 g/mol 

V = volume of Edetate disodium titrant consumed (mL)

Sample solution: Transfer 1.25 g of Magnesium Aluminosilicate to a conical flask, add 10 ml of 3 N hydrochloric acid and 50 mL of water, and heat on a water bath for 15 min. To this solution add 8 mL of hydrochloric acid, and heat on a water bath for 10 min. After cooling, transfer the solution to a 250-mL volumetric flask, rinse the conical flask with water, and add the washings to the volumetric flask. Dilute with water to volume. Centrifuge, and use the supernatant as the Sample solution.

[NOTE-Retain a portion of the Sample solution for use in the Assay for Magnesium Oxide.]

Blank: 10 mL of 3 N hydrochloric acid and 50 mL of water

Titrimetric system

(See Titrimetry (541).)

Mode: Residual titration

Titrant: Edetate disodium titrant

Back-titrant: 0.05 M zinc sulfate VS

Endpoint detection: Visual

Analysis: Transfer 20.0 ml. of the Sample solution to a beaker, and add 20.0 mL of Titrant. To this solution add 15 mL of acetic acid-ammonium acetate buffer TS and 20 mL of water, and boil for 5 min. After cooling, add 50 mL of alcohol and 2 mL of dithizone TS, and titrate with the Back-titrant until the color of the solution changes from green-violet to rose-pink. Perform a blank determination, and make the necessary correction. Each mL of 0.05 M Edetate disodium titrant is equivalent to 2.5490 mg of aluminum oxide (Al₂O₃).

Acceptance criteria: 27.0%-34.3% on the dried basis

MAGNESIUM OXIDE

Sample solution: Use the portion retained from the Sample solution prepared in the Assay for Aluminum Oxide.

Titrimetric system

(See Titrimetry (541).)

Mode: Direct titration

Titrant: 0.05 M edetate disodium VS

Endpoint detection: Visual

Analysis: Transfer 50.0 ml of the Sample solution to a suitable container, add 50 mL of water and 25 mL of a trolamine solution (500 mg/mL), and shake well. Add 25 ml of ammonia-ammonium chloride buffer TS and 0.04 g of eriochrome black TS trituration as the indicator. Titrate with Titrant until the red-purple color changes to blue and persists for 30 s. Each ml. of 0.05 M edetate disodium VS is equivalent to 2.0152 mg of magnesium oxide (MgO).

Acceptance criteria: 20.5%-27.7% on the dried basis

SILICON DIOXIDE

Sample: 1 g

Analysis: To the Sample add 30 mL of 3 N hydrochloric acid, and evaporate on a water bath to dryness. Moisten the residue with hydrochloric acid, and again evaporate on a water bath to dryness. To the residue add 8 mL of hydrochloric acid and 25 mL of hot water, and stir. Allow to stand, then decant the supernatant through an ashless filter paper. To the residue in the container add 10 mL of hot water, stir, and decant the supernatant through the filter paper. Wash the residue in the container with three additional 10-ml portions of hot water, stir, and decant as described above. Treat the residue in the container with 50 ml of water, and heat on a water bath for 15 min. Filter, and rinse the residue on the filter paper with hot water until no precipitate is obtained when 1 mL of silver nitrate TS is added to 5 ml. of the washing Transfer the filter paper and its contents to a tared platinum crucible, heat to dryness, incinerate, and continue to heat at 800 ± 25° for 1 h. Cool, and weigh. Moisten the residue with 6 ml, of hydrofluoric acid, evaporate to dryness, and ignite for 5 min, Cool, and weigh. The loss in weight represents the weight of SiO₂

Acceptance criteria: 14.4%-21.7% on the dried basis

4 IMPURITIES

CHLORIDE AND SULFATE, Chloride (221)

Analysis: A 20-mL portion of the diluted filtrate retained from the test for Soluble Salts shows no more chloride than corresponds to 0.75 mL

of 0.020 N hydrochloric acid.

Acceptance criteria: NMT 0.053%

CHLORIDE AND SULFATE, Sulfate (221)

Analysis: A 2-ml portion of the diluted filtrate retained from the test for Soluble Salts shows no more sulfate than corresponds to 0.5 ml. of

0.020 N sulfuric acid.

Acceptance criteria: NMT 0.480%

Change to read:

ARSINIC (211), Procedures. Procedure 1 (CN 1-Jun-2023): NMT 3 µg/g

Change to read:

IRON (241), Procedures. Procedure 1 (CN 1-June-2023)

Sample: 0.11 g

Analysis: To the Sample add 8 ml, of 2 N nitric acid, boil for 1 min, and cool. Dilute with water to 100 ml, and centrifuge. Dilute 30 ml of the supernatant with water to 45 mL.

Acceptance criteria: NMT 0.03%

5 SPECIFIC TESTS

ACID-CONSUMING CAPACITY

Sample solution: Transfer 0.2 g of Magnesium Aluminosilicate to a glass-stoppered flask, and add 100.0 mL of 0.1 N hydrochloric acid VS.

Stopper the flask tightly, shake at 37 ± 2 for 1 h, and filter. Use the filtrate.

Analysis: Transfer 50.0 mL of the filtrate from the Sample solution to a beaker, and while stirring, titrate the excess hydrochloric acid with 0.1N sodium hydroxide VS to a pH of 3.5. Perform a blank determination, and make any necessary corrections.

Acceptance criteria: NLT 250 mL of 0.1 N hydrochloric acid is consumed per g of Magnesium Aluminosilicate, calculated on the dried basis.

PH (791)

Sample: 2 g

Analysis: Add 50 ml of water to the Sample. While stirring, immerse the pH electrodes in the suspension, and after 2 min, record the pH.

Acceptance criteria: 8.5-10.5

LOSS ON DRYING (731),

Analysis: Dry at 110° for 7 h.

Acceptance criteria: NMT 20.0%

SOLUBLE SALTS

Sample: 10.0 g

Analysis: Transfer the Sample to a suitable container, add 150 mL of water, and boil gently for 15 min, with shaking. After cooling, dilute with water to 150 mL, and centrifuge. Dilute 75 mL of the clear filtrate with water to 100 mL, and retain the diluted filtrate for use in the tests for Chloride, Sulfate, and Alkalinity. Evaporate 25 mL of the diluted filtrate on a water bath, and heat at 700° for 2 h.

Acceptance criteria: NMT 1.6%; the residue weighs NMT 0.020 g.

ALKALINITY

Sample: 20 mL of diluted filtrate retained from the test for Soluble Salts

Analysis: Add 2 drops of phenolphthalein TS to the Sample, containing 1 g of Magnesium Aluminosilicate.

Acceptance criteria: If a pink color is produced, NMT 0.50 mL of 0.1 N hydrochloric acid is required to discharge it.

6 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in tight containers, and prevent exposure to excessive heat.