HG/T 2784-2012 Ammonium Sulfite for Industrial Use

4.2.3.1 Principle

In the sample, ammonium bisulfite is oxidized to ammonium bisulfate by hydrogen peroxide. Then, using methyl red-indigo mixed solution as an indicator, titration is performed with sodium hydroxide standard titration solution until the solution turns yellow-green, which marks the endpoint. The mass fraction of ammonium bisulfite is calculated based on the consumption of sodium hydroxide standard titration solution.

The reaction is:

NH4HSO3+H2O2→NH4HSO4+H2O

2NH4HSO4+2NaOH→(NH4)2SO4+Na2SO4+2H2O

4.2.3.2 Reagents

4.2.3.2.1 Hydrogen peroxide solution: 3%. Using methyl red-indigo blue mixed indicator (4.2.3.2.3) as the indicator, neutralize the solution with sodium hydroxide standard titration solution (4.2.3.2.2) until the solution changes from red to yellow-green.

4.2.3.2.2 Standard sodium hydroxide titration solution: c(NaOH) = 0.1 mol/L.

4.2.3.2.3 Methyl Red-Indigo Mixture Indicator; prepared by mixing one part methyl red indicator (1 g/L) with two parts indigo solution (1 g/L). The preparation method for the indigo ethanol solution (1 g/L) is as follows: weigh 0.1g of indigo solution and dissolve it in 100 mL of 95% ethanol solution.

4.2.3.3 Analysis Steps

Weigh approximately 0.5g of the sample quickly using the difference subtraction method, with precision to 0.0001g, and place it in a 250ml Erlenmeyer flask containing 15ml hydrogen peroxide solution (4.2.3.2.1) and about 30ml water. Shake well. Add 5 drops of methyl red-indigo mixed indicator (4.2.3.2.3), at which point the solution turns red. Titrate with sodium hydroxide standard solution (4.2.3.2.2) until the solution turns yellow-green, marking the endpoint.