Determination of copper in food

Cu is an essential trace element of the human body and plays an important role in the metabolism of the human body
.
Cu is an important component of a variety of metal enzymes in the body.

1.
Principle

After the sample is digested, under alkaline conditions, copper ions react with sodium diethyldithiocarbamate (copper reagent, DDC-Na for short) to form a yellow complex, which is soluble in CCl ₄ and interacts with the standard at a wavelength of 440nm The series is quantitative and colorimetric
.

2.
Reagents

(1) Carbon tetrachloride
.

(2) Ammonium citrate-disodium edetate solution: Weigh 20g of ammonium citrate and 5g of disodium edetate, dissolve in water, and dilute to 100mL with water
.

(3) Sulfuric acid (1+17): Measure 20 mL of sulfuric acid , pour it into 300 mL of water, and dilute to 360 mL with water after cooling
.

(4) Ammonia (1+1)
.

(5) Phenol red indicator solution (1g/L): Weigh 0.
1 g of phenol red and dissolve it to 100 mL with ethanol
.

(6) Copper reagent solution: sodium diethyldithiocarbamate [(C ₂ H ₅ ) ₂ NOS ₂ Na·3H ₂ O] solution (1g/L), if necessary, it can be filtered and stored in the refrigerator
.

(7) Nitric acid (3+8): Measure 60 mL of nitric acid and dilute to 160 mL with water
.

(8) Copper standard solution: accurately weigh 1.
0000g of metallic copper (99.
99%), add nitric acid (4+6) in portions to dissolve, the total amount does not exceed 37mL, transfer to a 1000mL volumetric flask, and dilute to the mark with water
.
This solution is equivalent to 1.

(9) Copper standard liquid: draw 10.
0mL steel standard solution, place it in a 100mL volumetric flask, dilute to the mark with 0.
5% nitric acid solution, shake well, dilute it so many times to each 1mL equivalent to 1.
0ug copper
.

3.
Apparatus

(1) Spectrophotometer
.

(2) other instruments: a separatory funnel (125 mL of), pipettes, pipette, flask, beaker and the like
.

4.
Operation steps

(1) Sample digestion

①Cereals (without the outer shell), tea, coffee, etc.
: ground, pass through a 20-mesh sieve, and mix well
.
Take the edible part of the samples of vegetables, fruits, etc.

Take the same amount of nitric acid as the digested sample, and do a reagent blank test in the same way
.

②Aquatic products: mash the edible part into a homogenate
.
Weigh 1.

③Milk, condensed milk, milk powder: Weigh 2.
00g mixed sample, and operate according to step ② from "place in quartz or porcelain crucible"
.

④Eats and fats: Weigh 2.
00g mixed sample, first heat the solid oil to melt into liquid, place it in a 100mL separatory funnel, add 10mL petroleum ether , extract 2 times with nitric acid (10%), 5mL each time, shake for 1min , Combine the nitric acid solution in a 50ml volumetric flask, add water to dilute to the mark, mix well, and set aside
.
And do a reagent blank test at the same time

⑤Beverage, wine, vinegar, soy sauce and other liquid samples: direct sampling and determination, when there are more solids or when the sensitivity of the instrument is insufficient, the above samples can be concentrated and operated according to step ①
.

(2) Determination.
Pipette the 10.
0mL solution and the same amount of reagent blank solution after a constant volume, put them in a 125mL separatory funnel, and dilute to 20mL with water
.
Pipette 0, 0.

Add 5 mL of ammonium citrate , disodium edetate solution and 3 drops of phenol red indicator solution to the sample digestion solution, reagent blank solution and copper standard solution, mix well, and adjust to red with ammonia (1+1)
.

Add 2 mL of copper reagent solution and 10.
0 mL of carbon tetrachloride to each , shake vigorously for 2 min.
After standing for stratification, the carbon tetrachloride layer is filtered into a 2cm cuvette through absorbent cotton, and the zero point is adjusted with carbon tetrachloride.
Measure the absorbance at 440nm.
After subtracting the zero absorbance from the absorbance at each point of the standard, draw a standard curve or calculate a linear regression equation.
Compare the absorbance value of the sample with the curve, or obtain the content by the generation equation
.

5.
Result calculation

Where x-the content of copper in the sample, mg/kg or mg/L

m ₁ ——The quality of copper in the digestion solution of the sample used for determination, ug

m ₂ ——The quality of copper in the reagent blank solution, ug

m——sample mass (volume), g (mL)

V ₁ ——Total volume of sample digestion solution, mL

V ₂ ——The volume of the sample digestion solution for determination, mL

Expression of results: report two significant digits of the arithmetic mean of parallel determinations
.
When the sample content exceeds 10mg/kg, three significant figures are reported

6.
Tips

(1) After adding 2mL copper reagent solution and 10.
0mL CCl ₄ , be sure to shake vigorously for 2 minutes
.
If the shaking is not enough (not violent or insufficient time), it may lead to incomplete extraction, emulsification, turbidity and no stratification, or even if stratification, the CCl ₄ layer can be seen turbid and opaque, indicating that there is a small amount in the CCl ₄ layer Water droplets, at this time, you should pick up the funnel and shake it vigorously a few times.

(2) The CCl ₄ layer should be filtered into a cuvette through absorbent cotton to remove impurity particles
.
The absorbent cotton is torn into small pieces and stuffed in the outlet of the funnel tube.
It should not be too tight or too loose.
The filtrate is filtered too slowly, or even no liquid can be released; too loose, it will not filter
.

(3) After color development, the measurement must be carried out within 1h, otherwise the yellow complex decomposes and errors occur
.

(4) When adding CCl _4, move the CCl ₄ reagent bottle to the funnel and add it quickly to avoid CCl ₄ dripping
.