SOP: Limit Test for Heavy Metals in Salt

GIFSQ — Global Institute for Food Safety & Quality

Principle

This is a limit test for the common metallic impurities that are coloured by hydrogen sulphide (Ag, As, Bi, Cd, Cu, Hg, Pb, Sb and Sn). The colour developed by the test solution on adding hydrogen sulphide is compared with that of a standard lead solution; a solution of 4 g of sample in 25 ml of water meets the requirements if its colour is equivalent to, or lower in intensity than, that developed with 20 µg of lead in the control. The test shows that the substance is not grossly contaminated and does not exceed the prescribed limits in terms of ppm of lead. Twenty micrograms of lead ion in 50 ml of solution is the optimum concentration for matching.

Apparatus required

• 50-ml Nessler tubes (a matched pair)
• Crucible (for Procedure B)
• Muffle furnace (500–600°C, for Procedure B)
• Narrow-range pH paper

Reagents

a. Dilute ammonia: dilute 400 ml to 1000 ml with water.

b. Lead nitrate stock solution: dissolve 159.8 mg of lead nitrate, Pb(NO₃)₂, in 100 ml of water containing 1 ml of HNO₃; make up to 100 ml and mix. Prepare and store in glass containers free from lead salts.

c. Standard lead solution: immediately before use, dilute 10 ml of the lead nitrate stock solution to 100 ml with water (1 ml = 10 µg of Pb).

d. Hydrogen sulphide solution: prepare a saturated solution by passing H₂S into cold water. Store in a cool, dark place, in a small dark amber-coloured bottle filled to the brim. The solution is not useful unless it has a strong smell of H₂S and gives a copious precipitate of sulphur with an equal volume of 9% ferric chloride (FeCl₃·6H₂O) solution in water.

e. 6% acetic acid.

f. Dilute ammonia (1 + 1.5).

g. Sulphuric acid, H₂SO₄.

h. Nitric acid, HNO₃.

i. Hydrochloric acid, HCl (1 + 2).

Sample preparation

Dissolve 4 g of the sample in 25 ml of water (test solution).

Procedure — A (direct comparison)

Into a 50-ml Nessler tube, pipette an aliquot of the standard lead solution containing the lead ion corresponding to the heavy metal limits specified; add water to make 25 ml

↓

Adjust the pH to between 3.6 and 4.0 using narrow-range pH paper, 6% acetic acid or dilute ammonia (1 + 1.5); dilute to 40 ml with water and mix — this is solution 1

↓

Into a matching 50-ml Nessler tube, pipette 25 ml of the test solution (4 g of sample in 25 ml of water); adjust the pH to between 3.0 and 4.0 as for solution 1; dilute to 40 ml with water and mix — this is solution 2

↓

To each tube, add 10 ml of freshly prepared H₂S solution; mix and allow to stand for 5 min

↓

Observe the solutions downward over a white surface — the test sample solution (2) should not be darker than solution (1)

Procedure — B (ashing, when specified)

Weigh the specified quantity of sample (4 g) in a crucible; add sufficient H₂SO₄ to wet the sample; ignite carefully over a flame at low temperature until thoroughly charred, keeping the crucible loosely covered with a suitable lid during ignition

↓

After ignition, add 2 ml of HNO₃ and 5 drops of H₂SO₄; heat gently until white fumes evolve

↓

Place in a muffle furnace between 500 and 600°C until the carbon has burnt off; cool

↓

Add 4 ml of HCl (1 + 2); cover and heat on a water bath for 10 to 15 min

↓

Remove the cover and evaporate to dryness on the water bath; moisten the residue with 1 drop of HCl; add 10 ml of hot water and digest for 2 min

↓

Add dilute ammonia and adjust the pH to between 3.0 and 4.0 using 6% acetic acid; filter if necessary

↓

Wash the crucible and the filter paper with 10 ml of water; dilute the combined filtrate and washings with water to 40 ml and mix — use this for developing the colour as solution 2 of Procedure A, with solution 1 prepared as in Procedure A

Precautions

• Use freshly prepared hydrogen sulphide solution — it must have a strong smell of H₂S.
• Keep the pH within the stated ranges (3.6–4.0 for the standard, 3.0–4.0 for the test).
• Observe the tubes downward over a white surface under the same conditions.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Osborne, D.R. & P. Voogt, The Analysis of Nutrients in Foods, Academic Press, New York, 1978 — cited in the book.