Determination of bleaching agent in food (2)

(4) Operation steps

①Sample processing:

a.
For water-soluble solid samples such as white sugar, weigh about 10.
00g uniform sample (the amount of sample depends on the content), dissolve it with a small amount of water, place it in a 100mL volumetric flask, and add 4mL sodium hydroxide solution ( 20g/L), 5min later add 4mL sulfuric acid (1+71), then add 20mL sodium tetrachloromercury absorption solution, dilute to the mark with water
.

b.
For other solid samples such as biscuits, weigh 5.
0～10.
0g of uniformly ground sample, moisten it with a small amount of water and transfer it into a 100mL volumetric flask, add 20mL sodium tetrachloromercury absorption solution, soak for more than 4h, if the upper layer solution is not clear , You can add 2.
5mL each of potassium ferrocyanide solution and zinc acetate solution, and finally dilute to 100mL with water, and filter for use
.

c.
Liquid samples such as wine, etc.
can be directly absorbed 5.
0～10.
0mL sample, placed in a 100mL volumetric flask, diluted with a small amount of water, add 20mL sodium tetrachloromercury absorption solution, shake well, finally add water to the mark, mix well, if necessary Filter it for later use
.

②Determination: Pipette 0.
50～5.
0mL of the above sample treatment solution into a 25mL colorimetric tube with stopper
.

(5) Result calculation

Where x—— the content of sulfur dioxide in the sample , g/kg

m ₁ ——Determine the content of sulfur dioxide in the sample solution , ug

m ₂ ——the mass of the sample, g

V——The volume of the sample solution used for determination, mL

The calculation result is expressed to three significant figures
.

The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 10%
.

(6) Tips

① sulfite and foods aldehydes (acetaldehyde), ketones ( ketoglutaric acid , pyruvic acid , etc.

② After adding sodium tetrachloromercury to the sample for absorption, the sulfur dioxide content of the solution is stable within 24 hours, and the measurement needs to be carried out within 24 hours
.

③The amount of hydrochloric acid in pararosaniline hydrochloride affects the color development.
When the amount of hydrochloric acid is large, the color is light, and the color is dark when the amount is small, which has a significant impact on the measurement result.
Therefore, the dosage should be strictly controlled
.

④Dark samples need to be decolorized with activated carbon
.

⑤ dioxide standard solution with a concentration of placing prolonged gradually decreased, a new calibration must be diluted with sulfur dioxide standard solution immediately before use
.

⑥ Nitrous acid interferes with this method, so add sulfanilic acid to decompose nitrous acid
.

HNO ₂ +NH ₂ SO ₂ ONH ₄ =NH ₄ HSO ₄ +N ₂ ↓+H ₂ O

⑦Strictly control the color development time and temperature
.

⑧Pararosaniline hydrochloride is added to hydrochloric acid to adjust it to yellow.

Related Links: Determination of Bleaching Agents in Food (1)