Determination of Vitamin A in Food

Vitamin A is found in animal fats
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1.
Principle

Vitamin A interacts with antimony trichloride in chloroform to produce a blue soluble complex whose color is proportional to the amount of vitamin A in the solution
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Where x-the content of vitamin A in the sample, mg/100g (if in international units, every 1 international unit = 0.

p——Check the content of vitamin A in the sample solution from the standard curve , ug/mL

p ₀ ——The content of vitamin A in the sample blank solution obtained from the standard curve, ug/mL

m——sample mass, g

V——The quantitative volume of chloroform added after extraction, mL

2.
Tips

(1) As vitamin A is easily destroyed by light, strong light or brown glassware should be avoided as much as possible during the determination process
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(2) Diethyl ether or other ethers should be used in sample processing.
Before use, check whether the ether contains peroxides generated after long storage time and exposure to air or exposure to light to avoid explosion during distillation
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① Inspection method: Add an equal volume of 2% potassium iodide acetic acid solution.
If it contains ethyl peroxide, iodine will be freed and the starch solution will turn purple or blue
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② Treatment method: put a small amount of pure iron wire or iron filings into the bottle, redistill the ether, discard 10% of the first distillate and 10% of the residual distillate
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In addition, it should be noted that as an extractant, ether is prone to emulsification.
Before extraction, do not use too much washing operation.
If emulsification occurs, add a few drops of ethanol to eliminate emulsification
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(3) There should be no decomposition products in chloroform , otherwise vitamin A will be destroyed
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① Inspection method: Trichloromethane is unstable.
After being placed, it is easy to generate hydrogen chloride and phosgene under the action of oxygen in the air
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② Treatment method: If the chloroform contains decomposition products, it should be washed several times with water in the separatory funnel, dehydrated by adding anhydrous sodium sulfate or calcium chloride , and then distilled

(4) The reaction liquid must not contain water, otherwise the antimony trichloride will precipitate antimony oxychloride when it meets with water, which will interfere with the colorimetric determination

SbCl ₃ +H ₂ O→SbOCl↓+2HCl

Therefore, 1 drop of acetic anhydride is added to every 1 mL of chloroform to absorb trace water that may be mixed into the reaction solution

(5) Since the blue substance produced by antimony trichloride and vitamin A is very unstable, it usually starts to fade after 6 seconds, so the reaction is required to be carried out in a cuvette, and the absorbance value is read immediately after the blue color is produced

(6) If the sample contains β-carotene to interfere with the determination, the concentrated and evaporated sample can be dissolved in n-hexane , using alumina as the adsorbent and the acetone-hexane mixture as the eluent for column chromatography

(7) The antimony trichloride solution is hygroscopic, easily deteriorated, and corrosive.