Magnesium Sulfate
(mag nee' zee um sul' fate).

Sulfuric acid magnesium salt (1:1), hydrate;    
Magnesium sulfate (1:1) monohydrate     138.36 [14168-73-1].
Magnesium sulfate (1:1) heptahydrate     246.47 [10034-99-8].
Anhydrous     120.37 [7487-88-9].
Magnesium Sulfate, rendered anhydrous by ignition, contains NLT 99.0% and NMT 100.5% of MgSO4.
•  A. Identification Tests—General, Magnesium 191 and Sulfate 191
Sample solution:  50 mg/mL
Acceptance criteria:  Meets the requirements
•  Procedure
Sample:  250 mg of the ignited Magnesium Sulfate obtained in the test for Loss on Ignition
Titrimetric system 
Mode:  Direct titration
Titrant:  0.05M edetate sodium VS
Endpoint detection:  Colorimetric
Analysis:  Dissolve the Sample in 100 mL of water and the minimum amount of 3 N hydrochloric acid required for a clear solution. Adjust the reaction of the solution (using pH indicator paper; see Reagents, Indicators, and Solutions—Reagents—Indicator and Test Papers) with 1 N sodium hydroxide to a pH of 7, add 5 mL of ammonia–ammonium chloride buffer TS and 0.15 mL of eriochrome black TS, and titrate with the Titrant to a blue endpoint. Caclulate the perecentage of MgSO4 in the portion of the ignited Magnesium Sulfate taken:
Result = [V × N × F × 100]/W
V== Sample titrant volume (mL)
N== titrant molarity (mmol/mL)
F== equivalency factor, 120.36 mg/mmol
W== weight of Sample (mg)
Acceptance criteria:  99.0%–100.5% on the anhydrous by ignition basis
•  Limit of Chloride 221
Sample:  1.0 g
Acceptance criteria:  The Sample shows no more chloride than corresponds to 0.20 mL of 0.020 N hydrochloric acid (0.014%).
•  Limit of Iron 241
Magnesium Sulfate intended for use in preparing nonparenteral dosage forms 
Sample solution:  Dissolve 0.50 g in 40 mL of water.
Analysis:  Proceed as directed in the test for Iron 241.
Acceptance criteria:  NMT 20 µg/g
Magnesium Sulfate intended for use in preparing parenteral dosage forms 
[Note—Rinse all glassware used in this test with Dilute hydrochloric acid. ]
Dilute hydrochloric acid:  1 mL of hydrochloric acid diluted with water to 1000 mL
Solution A:  500 mg/mL of ammonium acetate in water
Solution B:  13.4 mg/mL of ascorbic acid in water. [Note—Use this solution on the day prepared. ]
Color reagent:  3.8 mg/mL of 3-(2-pyridyl)-5,6-di-(2-furyl)-1,2,4-triazine-5¢,5¢¢-disulfonic acid, disodium salt in Solution A. Shake by mechanical means if necessary. Use this solution on the day prepared.
Standard stock solution:  1.0 µg/mL of iron, from Standard Iron Solution in Dilute hydrochloric acid
Standard solutions:  To three separate 50-mL volumetric flasks transfer 2.0, 5.0, and 10.0 mL of Standard stock solution, and dilute each with Dilute hydrochloric acid to 35 mL. These solutions contain 2.0, 5.0, and 10.0 µg of iron, respectively.
Sample solution:  Transfer 10.0 g of Magnesium Sulfate to a 50-mL volumetric flask, add Dilute hydrochloric acid to 35 mL, and sonicate, if necessary, to dissolve.
Blank:  Transfer 35 mL of Dilute hydrochloric acid to a 50-mL volumetric flask.
Instrumental conditions 
Mode:  UV-Vis
Analytical wavelength:  594 nm
Samples:  Standard solutions, Blank, and Sample solution
To each of the flasks containing the Standard solutions, the Sample solution, and the Blank add 5 mL of Solution B and 5 mL of Color reagent. Dilute each solution with Dilute hydrochloric acid to volume, mix, and allow to stand for 10 min.
Plot the absorbance values of the Standard solutions versus their iron contents in µg and draw the straight line best fitting the three plotted points. From the graph, determine the iron content, C, in µg, of the Sample solution.
Calculate the content, in µg/g, of iron in the portion of Magnesium Sulfate taken:
Result = C/W
C== content of iron in the Sample solution in µg, determined from the graph
W== weight of Magnesium Sulfate in the Sample solution (g)
Acceptance criteria:  NMT 0.5 µg/g
•  Selenium 291
Test solution:  200 mg in 50 mL of 0.25 N nitric acid
Acceptance criteria:  NMT 30 µg/g
•  Heavy Metals 231
Sample solution:  2 g in 25 mL of water
Acceptance criteria:  NMT 10 ppm
•  pH 791
Sample solution:  50 mg/mL
Acceptance criteria:  5.0–9.2
•  Loss on Drying 731: Dry a sample at 105 for 2 h: the anhydrous form loses NMT 2% of its weight.
•  Loss on Ignition 733
Sample:  1 g
Analysis:  Weigh the Sample in a crucible, heat at 105 for 2 h, then ignite in a muffle furnace at 450 ± 25 to constant weight.
Acceptance criteria 
Monohydrate:  Loses 13.0%–16.0% of its weight
Dried form:  Loses 22.0%–28.0% of its weight
Heptahydrate:  Loses 40.0%–52.0% of its weight
•  Packaging and Storage: Preserve in well-closed containers.
•  Labeling: The label states whether it is the monohydrate, the dried form, or the heptahydrate. Magnesium Sulfate intended for use in preparing parenteral dosage forms is so labeled. Magnesium Sulfate not intended for use in preparing parenteral dosage forms is so labeled. In addition, it may be labeled also as intended for use in preparing nonparenteral dosage forms.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Hariram Ramanathan, M.S.
Associate Scientific Liaison
(SM42010) Monographs - Small Molecules 4
USP35–NF30 Page 3755
Pharmacopeial Forum: Volume No. 29(6) Page 1921