SOP: Iron Solution Test for Tinplate (Iron Solution Value)
GIFSQ — Global Institute for Food Safety & Quality
Principle
Discontinuities let the steel corrode in a bath where the tin’s slightly anodic behaviour partly protects it — 1.09 N sulphuric acid spiked with hydrogen peroxide and ammonium thiocyanate. A cleaned plate sits in that bath; afterwards the dissolved iron is measured colorimetrically. The micrograms of iron found is the Iron Solution Value (ISV).
Apparatus required
- Wide-mouthed bottles of 8 oz capacity with bakelite screw cap and an inner gasket of 2 in. diameter, preferably made of polyethylene or tygon
Reagents
a. 2.18 N sulphuric acid.
b. 3% (10 vols) hydrogen peroxide.
c. 4% ammonium thiocyanate.
d. Standard iron solution: dissolve 3.514 g of ferrous ammonium sulphate, FeSO₄·(NH₄)₂SO₄·6H₂O, in a minimum quantity of water; add 100 ml of dilute H₂SO₄ (5%) and mix; add 17 ml of concentrated HNO₃; allow to stand overnight, then make the volume to 1 litre. One ml of this solution contains 0.5 mg of iron. Dilute 2 ml of this solution to 100 ml (1 ml = 0.01 mg).
Procedure
Cut a tinplate specimen of area 25.8 sq. cm. (4 sq. in.); clean it cathodically in 0.5% sodium carbonate solution using 1.5 amps for 30 sec; rinse in water and dry (if cathodic cleaning cannot be done, clean using carbon tetrachloride as solvent)
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Fix the cleaned test specimen into the cap of the bottle with the help of the gasket
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Measure into the bottle 23 ml of H₂SO₄, 2 ml of hydrogen peroxide and 25 ml of ammonium thiocyanate solution
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Screw the cap containing the test piece onto the bottle; invert the bottle and allow to stand for 2 hr at 26.7°C without agitation
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After 2 hr place the bottle upright, remove the cap and specimen; add 1 ml of the hydrogen peroxide solution; rinse the lid, test plate and bottle with water and make up the volume to 100 ml
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Prepare a blank containing similar aliquots of the solution and make up to 100 ml
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Measure the colour at 485 nm adjusting the instrument to 100% transmission with the blank; from the standard curve, read the concentration of iron in the solution
Standard curve
Pipette 2, 4, 6, 8 and 10 ml of standard iron solution (1 ml = 0.01 mg) into separate 100-ml volumetric flasks; add 23 ml of H₂SO₄ (2.18 N), 3 ml of hydrogen peroxide (3.0%) and 25 ml of ammonium thiocyanate solution; dilute to mark with water and measure the colour at 485 nm. Plot absorbance against concentration to get the standard curve.
Result
Express the result as micrograms of iron dissolved in the course of the test carried out as described — the value so obtained is called the Iron Solution Value (ISV).
Source note
The book’s introduction gives the immersion as “two hr at 25°C” while the procedure gives “2 hr at 26.7°C” — both are carried as printed; flagged for printed-book verification.
References
- Willey, A. R., J. L. Krickl & R. R. Hartwell, Corrosion, 12, 433 (1956).
- Hoare, W. E. & S. C. Britton, Tinplate Testing, p. 45.