Magnesium content—
Transfer about 800 mg, accurately weighed, to a 200-mL volumetric flask. Dissolve in and dilute with water to volume, and mix. Stir the resulting solution continuously for about 15 minutes and filter, discarding the first 10 mL of the filtrate, into a flask. Transfer 50.0 mL of the filtrate to a 250-mL conical flask, add 50 mL of water, 5 mL of ammonia–ammonium chloride buffer TS, and 0.15 mL of
eriochrome black TS, and titrate with 0.05
M edetate disodium VS to a blue endpoint. Each mL of 0.05
M edetate disodium is equivalent to 1.215 mg of magnesium: between 6.3% and 6.7% of magnesium is found.
Assay—
Transfer 3.0 mL of the filtrate prepared in the test for
Magnesium content to a 500-mL volumetric flask, dilute with water to volume, and mix. Concomitantly determine the absorbances of this solution and a solution of
USP Salicylic Acid RS in water having a known concentration of about 18 µg per mL, in 1-cm cells at the wavelength of maximum absorbance at about 296 nm, with a suitable spectrophotometer, using water as the blank. Calculate the quantity, in mg, of C
14H
10MgO
6·4H
2O in the Magnesium Salicylate taken in the test for
Magnesium content by the formula:
(370.59 / 276.24)(33.3C)(AU / AS),
in which 370.59 is the molecular weight of magnesium salicylate tetrahydrate; 276.24 is twice the molecular weight of salicylic acid;
C is the concentration, in µg per mL, of
USP Salicylic Acid RS in the Standard solution; and
AU and
AS are the absorbances of the test solution and the Standard solution, respectively.
;)
197U
—
for 4 hours.