Na2S2O5 190.11

Disulfurous acid,disodium salt.
Disodium pyrosulfite [7681-57-4].

Packaging and storage— Preserve in well-filled,tight containers,and avoid exposure to excessive heat.

Identification— Asolution (1in 20)responds to the tests for Sodium á191ñand for Sulfite á191ñ.

Limit of chloride— Dissolve 1.0g in 10mLof water,filter,if necessary,through a small chloride-free filter,and add 6mLof 30percent hydrogen peroxide.Add 1Nsodium hydroxide until the solution is slightly alkaline to phenolphthalein,dilute with water to 100mL,and mix.Dilute 2.0mLof this solution with water to 20mL,and add 1mLof nitric acid and 1mLof silver nitrate TS.Mix,allow to stand for 5minutes protected from direct sunlight,and compare the turbidity,if any,with that produced from a 2-mLaliquot of a control (prepared by diluting 0.71mLof 0.020Nhydrochloric acid to 100mL)in an equal volume of total solution containing the quantities of reagents used in the test (see Visual Comparisonin the section Procedure,under Spectrophotometry and Light-scattering á851ñ):any turbidity produced by the test specimen does not exceed that of the control preparation (0.05%).

Iron á241ñ— Dissolve 500mg in 14mLof dilute hydrochloric acid (2in 7),and evaporate on a steam bath to dryness.Dissolve the residue in 7mLof dilute hydrochloric acid (2in 7),and again evaporate to dryness.Dissolve the resulting residue in a mixture of 2mLof hydrochloric acid and 20mLof water,add 3drops of bromine TS,and boil to expel the bromine.Cool,and dilute with water to 47mL:the limit is 0.002%.

Limit of thiosulfate— Mix 2.2g with 10mLof 1Nhydrochloric acid in a 50-mLbeaker.Boil gently for 5minutes,cool,and transfer to a small test tube.Any turbidity is not greater than that produced by 0.10mLof 0.10Nsodium thiosulfate,similarly treated (0.05%).

Heavy metals,Method Iá231ñ— Dissolve 1g in 10mLof water,add 5mLof hydrochloric acid,evaporate on a steam bath to dryness,and dissolve the residue in 25mLof water:the limit is 0.002%.

Assay— Transfer about 200mg of Sodium Metabisulfite,accurately weighed,to a glass-stoppered conical flask containing 50.0mLof 0.1Niodine VS,and swirl to dissolve.Allow to stand for 5minutes,protected from light,add 1mLof hydrochloric acid,and titrate the excess iodine with 0.1Nsodium thiosulfate VS,adding 3mLof starch TSas the endpoint is approached.Each mLof 0.1Niodine is equivalent to 3.203mg of SO2.

Expert Committee:(EMC)Excipients:Monograph Content

USP28–NF23Page 3079

Phone Number:1-301-816-8262