SOP: Determination of Copper
GIFSQ — Global Institute for Food Safety & Quality
Principle
Copper is determined colorimetrically as copper diethyldithiocarbamate at pH 8.5 in the presence of ethylenediaminetetraacetic acid (EDTA) as chelating agent. Copper reacts with sodium diethyldithiocarbamate in alkaline solution producing a yellow to brown colour depending on the amount of metal present; the colour is soluble in organic solvents, is extracted from the aqueous solution with carbon tetrachloride and is measured colorimetrically.
Apparatus required
- Short-stem separating funnels, 250 ml
- Glass-stoppered tubes or flasks
- Colorimeter (400 nm or 42 green filter)
Reagents
a. Sodium diethyldithiocarbamate [(C₂H₅)₂NCS₂Na]: dissolve 1 g of the salt in water, dilute to 100 ml and filter; store in a refrigerator and prepare weekly
b. Citrate-EDTA solution: dissolve 20 g of dibasic ammonium citrate [(NH₄)₂HC₆H₅O₇] and 5 g of EDTA disodium salt in water and dilute to 100 ml; remove traces of copper by adding 0.1 ml of carbamate solution and extracting with 10 ml of carbon tetrachloride; repeat the extraction until the carbon tetrachloride extract is colourless
c. 6 N ammonium hydroxide: dilute 349.7 ml of ammonia (not less than 27.9% NH₃ by weight, sp. gr. = 0.9) to 1 litre with water
d. 0.1% thymol blue: dissolve 0.1 g in water, add enough 0.1 N NaOH to change the colour of the dye to blue and dilute to 100 ml
e. 2 N sulphuric acid: dilute 56.8 ml of analytical-grade H₂SO₄ to 1 litre
f. Redistilled carbon tetrachloride (CCl₄)
g. Standard copper solution: dissolve 0.25 g of pure copper wire or foil in 15 ml of HNO₃ in a conical flask; cover with a watch glass and warm to completion; boil to expel fumes, cool and dilute to 250 ml (solution A); dilute 25 ml of solution A to 250 ml (1 ml = 0.1 mg) (solution B); to prepare the working standard, dilute 5 ml of solution B to 250 ml with 2 N H₂SO₄ (1 ml = 2.0 μg). As an alternative, dissolve 0.3928 g of copper sulphate (CuSO₄·5H₂O) in water and make up to 1 litre in a volumetric flask; dilute 2 ml to 100 ml before use (1 ml = 2 μg of Cu)
Sample preparation
Destroy the organic matter by wet digestion or dry ashing as described in SOP: Destruction of Organic Matter; the ash solution from any of the ashing procedures may be used.
Procedure
Pipette 25 ml (or an aliquot of the sample diluted to 25 ml with 2 N H₂SO₄) from the wet-digested solution into a 250-ml short-stem separating funnel
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Add 10 ml of citrate-EDTA reagent; add 2 drops of thymol blue indicator and 6 N NH₄OH dropwise until the colour turns green or blue-green
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Cool and add 1 ml of carbamate solution and 15 ml of CCl₄; shake vigorously for 2 min; allow the layers to separate and drain off the CCl₄ layer through a cotton wad into a glass-stoppered tube or flask
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Prepare a sample blank by the same procedure using a similar aliquot from the blank digest
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Measure the colour at 400 nm (or with a 42 green filter), setting the blank at 100% transmission
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Prepare the standard curve: pipette 0, 1, 2, 5, 10, 20 and 25 ml of the standard copper solution (2 μg/ml) into separating funnels; add 2 N H₂SO₄ to make the volume to 25 ml; extract and develop the colour as for the sample; plot absorbance against concentration; since there is usually some deviation from linearity, read sample values from a smoothed curve.
Calculation
Copper, mg/100 g = (μg of Cu in the aliquot × Total volume of digest × 100) ÷ (ml of digest taken for extraction × Wt of sample taken for digestion)
Copper, ppm = mg of copper per 100 g × 10
Note: the printed expression is given as above; the standard reading is in μg, so the expression as printed does not include the ÷1000 needed to convert the result to mg/100 g — flagged as a source inconsistency.
Where,
- μg of Cu in the aliquot = copper read from the standard curve (μg)
- Total volume of digest = volume to which the digest was made up (ml)
- ml of digest taken for extraction = aliquot extracted (ml)
- Wt of sample taken for digestion = weight of sample digested (g)
References
- Ramsey, L.L., J. Assoc. Offic. Agric. Chem., 43, 605 (1960).
- Callan, T. & J.A.R. Henderson, Analyst, 54, 650 (1929).