1 Scope
This standard specifies the method for determination of sulfite in foods.
This standard is applicable to the determination of residue content of sulfur dioxide in foods.
The Detection concentration in this standard is 1 mg/kg.
Method I Prosaniline Hydrochloride Method
2 Principles
The sulfite reacts with mercuric sodium tetrachloride, generating stable complex compound and then reacts with formaldehyde and pararosaniline chloride, generating the complex compound in claret; fix quantity in comparison with standard series.
3 Reagents
3.1 Absorption solution of mercuric sodium tetrachloride: weigh 13.6 g mercuric chloride and 6.0 g sodium chloride and then dissolve in water for dilution till 1000mL; setting over night and then filtered for standby application.
3.2 Ammonium sulfamate solution (12 g/L).
3.3 Formaldehyde solution (2g/1); absorb 0.55mL formaldehyde without polymerized settles (36%) and diluted with water to 100mL and then blend average.
3.4 Starch indicating liquid: weigh 10.6 g soluble starch, work it with a little water into a paste and then pour it slowly into 100mL boiling water by stirring as it is added till it is boiled; and then cooled for standby application; this solution shall be prepared when it is needed.
3.5 Potassium ferrocyanide solution: weigh 10.6g potassium ferrocyanide [4Fe(CN)6•3H2O], dissolve with water, dilute to 100mL.
3.6 Zinc acetate solution: weigh 22g zinc acetate [Zn (CH3COO)•2H2O], dissolve it in a small amount of water and then added with 3mL glacial acetic acid, dilute with water to 100mL.
3.7 Pararosaniline chloride solution: weigh 0.1 g pararosaniline chloride (C19 H18 N2 Cl4•H2 O; P-rosaniline Hydrochloride) in mortar and then add a small amount of water to grind to make it dissolved and diluted to 100mL. Take out 20mL in the 100mL volumetric flask, add hydrochloric acid (1+1), sufficiently shake, the solution turns yellow from red; if it doesn't turn yellow, drop a small amount of hydrochloric acid till the yellow appears and then dilute with water to the scale division, blend average for standby application (if no pararosaniline hydrochloride is available, hydrochloric acid fuchsin may replace).
Refinement method of pararosaniline chloride: weigh 20 g pararosaniline chloride in 400mL water, acidize it with 50mL hydrochloric acid (1+5), stir slowly and add 4g~5g active carbon, heat and boil for 2 min. Pour the mixture into large funnel, filter ( filtering with hot funnel while it is hot). Set the filtered solution for a night till it shows crystal and then filter by drawing with buchner funnel; re-suspend the crystal in 1 000mL acetic ether-ethanol (10:1) mixed solution, shake for 3min~5 min, filter by drawing with buchner funnel, repeatedly wash with acetic ether till there is no color in alcohol ether layer dry it in sulphuric acid dryer, store it in brown flask after it is ground fine.
Foreword I
1 Scope
2 Principles
3 Reagents
4 Instruments
5 Analytical Procedures
6 Principles
7 Reagents
8 Instruments
9 Analytical Procedures
Appendix A (Informative) Explanation of Pararosaniline Chloride
