How to Detect Sodium Sulfite Residues in Food?

There are various methods to detect sodium sulfite residue in food, the following are some common testing methods:
Para-rose aniline hydrochloride method
Principle: Sodium sulfite reacts with sodium tetrachloromercuric acid to form a stable complex, and then interacts with formaldehyde and pararosaniline hydrochloride to form a purplish-red substance, and then determines the absorbance at a certain wavelength, and compares it with the standard curve for quantification.
Operation steps:
Sample treatment: weigh a certain amount of sample, add appropriate amount of water immersion or homogenization, and then add sodium tetrachloromercurate absorbent, filtration to get the sample treatment solution.
Color reaction: take a certain amount of sample treatment solution in the colorimetric tube, add ammonium sulfamate solution, formaldehyde solution and pararosaniline hydrochloride solution in turn, shake well, and leave it for a period of time to make the color stable.
Determination: Determine the absorbance at 550nm with spectrophotometer and calculate the residual amount of sodium sulfite in the sample according to the standard curve.
Scope of application: Suitable for the determination of sodium sulfite residue in various types of food, such as candied fruit, dried fruit, fruit juice, etc..
Distillation method
Principle: Under acidic conditions, sodium sulfite is converted into sulfur dioxide, which is released by distillation and absorbed by excess hydrogen peroxide solution to produce sulfuric acid, and then titrated with sodium hydroxide standard solution, and the content of sodium sulfite is calculated according to the volume of consumed sodium hydroxide standard solution.
Operation steps:
Distillation device installation: put the sample into the distillation flask, add an appropriate amount of dilute sulfuric acid, connect the distillation device, heating and distillation.
Absorption and titration: absorb the distilled sulfur dioxide with a certain amount of hydrogen peroxide solution, after absorption, add phenolphthalein indicator, titrate with sodium hydroxide standard solution until the solution is slightly red, and the color does not fade within 30 seconds as the end point.
Calculation: Calculate the residual amount of sodium sulfite in the sample according to the concentration and consumption volume of sodium hydroxide standard solution.
Scope of application: Suitable for all kinds of food, especially for some dark-colored or complex food, distillation method can effectively eliminate the interference and accurately determine the residual amount of sodium sulfite.
Iodometric method
Principle: Sodium sulfite is reductive and can react with iodine in a redox reaction. Under acidic conditions, sodium sulfite reacts with excess iodine, and the remaining iodine is titrated with standard solution of sodium thiosulfate, and the amount of sodium sulfite is calculated according to the amount of iodine and sodium thiosulfate consumed.
Operation steps:
Sample treatment: weigh the appropriate amount of sample, add the appropriate amount of water to dissolve, then add an excess of iodine standard solution, then add hydrochloric acid to make the solution acidic.
Titration: titrate the remaining iodine with sodium thiosulfate standard solution, until the solution is light yellow, add starch indicator, continue titration until the blue color disappears as the end point.
Calculation: Calculate the residual amount of sodium sulfite in the sample according to the concentration and dosage of iodine standard solution and sodium thiosulfate standard solution.
Scope of application: It is suitable for the determination of food samples with high residual amount of sodium sulfite, but for some food containing other reducing substances, it may cause interference and need to be pre-treated appropriately.