HG/T 2784-2012 Ammonium Sulfite for Industrial Use

4.2.5.1 Principle

In the test sample, ammonium sulfate reacts with formaldehyde to form hexamethylenetetramine and sulfuric acid in stoichiometric amounts with ammonium sulfate. Using a phenolphthalein-thymol blue mixed solution as the indicator, titration is performed with sodium hydroxide standard titration solution until the solution turns purple, which marks the endpoint. The mass fraction of ammonium sulfate is calculated based on the consumption of sodium hydroxide standard titration solution.

The reaction is:

2（NH4）2SO4+6CH2O→（CH2）6N4+2H2SO2+6H2O

        H2SO4+2NaOH→Na2SO4+2H2O

4.2.5.2 Reagents

4.2.5.2.1 Neutral formaldehyde solution: 1+1.

Using the phenolphthalein-thymol blue mixed indicator (4.2.5.2.4) as the indicator, neutralize with sodium hydroxide solution (4.2.5.2.2) until the solution turns purple. If the solution volume exceeds 100 mL, measure the pH value with a pH meter, and the pH value of the test solution is 8.5.

Warning-This reagent emits highly irritating toxic gases. Operations must be conducted in a fume hood with appropriate protective measures.

4.2.5.2.2 Sodium hydroxide solution: 80g/L.

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

4.2.5.2.4 Phenolphthalein-Thymol Blue Mixed Indicator: Prepared by mixing one part phenolphthalein (50g/L) with one part thymol blue (50g/L).

4.2.5.2.5 Methyl Red Indicator Solution: 1 g/L.