Sodium Formaldehyde Sulfoxylate
(soe' dee um for mal' de hyde sul fox' i late).
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CH3NaO3S 118.09

Methanesulfinic acid, hydroxy-, monosodium salt.
Monosodium hydroxymethanesulfinate [149-44-0].

Dihydrate 154.11 [6035-47-8].
» Sodium Formaldehyde Sulfoxylate contains an amount of CH3NaO3S equivalent to not less than 45.5 percent and not more than 54.5 percent of SO2, calculated on the dried basis. It may contain a suitable stabilizer, such as sodium carbonate.
Packaging and storage— Preserve in well-closed, light-resistant containers, and store at controlled room temperature.
Clarity and color of solution— Dissolve 1 g in 20 mL of water, and transfer 10 mL to a 20- × 150-mm test tube. Compare with water in a similar test tube: the liquids are equally clear and, when viewed transversely by transmitted light, exhibit no apparent difference in color.
A: Dissolve about 4 g in 10 mL of water in a test tube, and add 1 mL of silver-ammonia-nitrate TS: metallic silver is produced, either as a finely divided, gray precipitate or as a bright metallic mirror on the inner surface of the tube.
B: Dissolve about 40 mg of salicylic acid in 5 mL of sulfuric acid, add about 50 mg of Sodium Formaldehyde Sulfoxylate, and warm very gently: a permanent, deep red color appears.
Alkalinity— Dissolve 1.0 g in 50 mL of water, add phenolphthalein TS, and titrate with 0.10 N sulfuric acid: not more than 3.5 mL is required for neutralization.
pH 791: between 9.5 and 10.5, in a solution (1 in 50).
Loss on drying 731 Dry it at 105 for 3 hours: it loses not more than 27.0% of its weight.
Sulfide— Dissolve 6 g in 14 mL of water in a test tube, and wet a strip of lead acetate test paper with the clear solution: no discoloration is evident within 5 minutes.
Iron— Transfer 1.0 g to a suitable crucible, and carefully ignite, initially at a low temperature until thoroughly charred, and finally, preferably in a muffle furnace, at 500 to 600 until the carbon is all burned off. Cool, dissolve the residue in 2 mL of hydrochloric acid, and dilute with water to 50 mL. Add about 50 mg of ammonium persulfate and 5 mL of ammonium thiocyanate TS, mix, and transfer to a color-comparison tube. Treat in the same manner 5.0 mL of a solution of ferric ammonium sulfate, prepared by dissolving 43.2 mg of ferric ammonium sulfate in 10 mL of 2 N sulfuric acid and adding water to make 1000 mL, each mL representing 5 µg of Fe. The color of the test solution is not deeper than that of the solution containing the standard iron solution (0.0025%).
Sodium sulfite— Transfer 4.0 mL of the solution prepared for the Assay to a conical flask containing 100 mL of water. Add 2 mL of formaldehyde TS, and titrate with the same 0.1 N iodine VS that is used for the Assay, adding 3 mL of starch TS as the endpoint is approached. Calculate the percentage of Na2SO3 in the Sodium Formaldehyde Sulfoxylate taken by the formula:
(1.25)(63.02)(V2 V1)(N / W)
in which 63.02 is the equivalent weight of sodium sulfite; V1 and V2 are the volumes, in mL, of 0.1 N iodine VS consumed in this titration and in the titration performed in the Assay, respectively; N is the exact normality of the iodine solution; and W is the weight, in g, of Sodium Formaldehyde Sulfoxylate taken for the Assay: not more than 5.0% of Na2SO3, calculated on the dried basis, is found.
Assay— Transfer about 1 g of Sodium Formaldehyde Sulfoxylate, accurately weighed, to a 50-mL volumetric flask, dissolve in about 25 mL of water, dilute with water to volume, and mix. Reserve a portion of this solution for the test for Sodium sulfite. Transfer 4.0 mL of this solution to a conical flask containing 100 mL of water, and titrate with 0.1 N iodine VS, adding 3 mL of starch TS as the endpoint is approached. Each mL of 0.1 N iodine is equivalent to 1.602 mg of SO2.
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Topic/Question Contact Expert Committee
Monograph Robert H. Lafaver, M.S.
Scientific Liaison
(EXC2010) Monographs - Excipients
USP35–NF30 Page 1954