Sodium Sulphite Heptahydrate
(Ph. Eur. monograph 0776)
Na2SO3,7H2O 252.1 10102-15-5
Action and use
Antioxidant.
DEFINITION
Content
48.0 per cent to 52.5 per cent of Na2SO3.
CHARACTERS
Appearance
Colourless crystals.
Solubility
Freely soluble in water, very slightly soluble in ethanol (96 per cent).
IDENTIFICATION
A. Solution S (see Tests) is slightly alkaline (2.2.4).
B. To 5 mL of solution S add 0.5 mL of 0.05 M iodine. The solution is colourless and gives reaction (a) of sulfates (2.3.1).
C. Solution S gives reaction (a) of sodium (2.3.1).
D. It complies with the limits of the assay.
TESTS
Solution S
Dissolve 5 g in water R and dilute to 50 mL with the same solvent.
Solution S1
To 10.0 g add 12.5 mL of water R. Shake until mostly dissolved, carefully and progressively add 7.5 mL of hydrochloric acid R and heat to boiling. Cool and dilute to 50.0 mL with water R.
Appearance of solution
Solution S is clear (2.2.1) and colourless (2.2.2, Method I).
Thiosulfates
Maximum 0.05 per cent.
To 4.00 g add 100 mL of water R. Shake to dissolve, then add 10 mL of formaldehyde solution R and 10 mL of acetic acid R. Allow to stand for 5 min, add 0.5 mL of starch solution R and titrate with 0.05 M iodine. Carry out a blank titration.
The difference between the volumes used in the titrations is not more than 0.15 mL.
Iron (2.4.9)
Maximum 5 ppm, determined on solution S1.
Zinc
Maximum 12 ppm.
Atomic absorption spectrometry (2.2.23, Method I).
Test solution: Dilute 2.0 mL of solution S1 to 10.0 mL with water R.
Reference solutions: Prepare the reference solutions using zinc standard solution (100 ppm Zn) R, diluting with water R.
Source: Zinc hollow-cathode lamp.
Wavelength: 213.9 nm.
Atomisation device: Air-acetylene flame.
ASSAY
Introduce 0.500 g into a 500 mL conical flask containing 50.0 mL of 0.05 M iodine. Shake until completely dissolved. Add 1 mL of starch solution R and titrate the excess of iodine with 0.1 M sodium thiosulfate. Carry out a blank titration.
1 mL of 0.05 M iodine is equivalent to 6.30 mg of Na2SO3.
