HG/T 2784-2012 Ammonium Sulfite for Industrial Use

4.3.1 Appearance

Determined by visual inspection.

4.3.2 Determination of Ammonium Sulfite Mass Fraction

4.3.2.1 Principle

In neutral or weakly acidic solutions, iodine oxidizes ammonium sulfite and ammonium bisulfite in the test solution to ammonium sulfate and ammonium bisulfate, respectively. A starch solution is then used as an indicator. The excess iodine is titrated with a sodium thiosulfate standard titration solution until the blue color just fades, marking the endpoint. The mass fraction of sodium sulfite is calculated based on the consumption of the sodium thiosulfate standard titration solution.

The reaction is:

（NH4）2SO3+I2+H2O→（NH4）2SO4+2HI

NH4HSO3+I2+H2O→NH4HSO4+2HI

            2Na2S2+I2→Na2S4O6+2NaI

HG/T 2784—2012

4.3.2.2 Reagents

4.3.2.2.1 Iodine solution: 254 g/L.

4.3.2.2.2 Sodium thiosulfate standard titration solution: c(Na2S2O3) = 0.1 mol/L.

4.3.3.3.3 Starch indicator solution: 10g/L.

4.3.2.3 Analysis Steps

Weigh approximately 0.6g of the sample with precision to 0.0001g, place it in a 250ml volumetric flask containing about 150ml of water, and dilute with water to the mark, then shake well. Measure 5.00ml of the above solution and transfer it to a 250ml iodine titration flask containing 25.00ml of iodine solution (4.3.2.2.1), shaking thoroughly. Titrate with sodium thiosulfate standard titration solution (4.3.2.2.2) until the solution turns pale yellow. Add 2ml of starch indicator solution (4.3.2.2.3) and continue titration until the solution just begins to turn blue, marking the endpoint.

A blank test was performed simultaneously with the aforementioned measurements.