SOP: Thiocyanate Test for Porosity of Tin Coating
GIFSQ — Global Institute for Food Safety & Quality
Principle
The plate sits in a bath of acetic acid, hydrogen peroxide and ammonium thiocyanate; the iron it dissolves out is measured colorimetrically. Where the paper tests count spots, this one yields a number — milligrams of iron per unit area — a direct index of the exposed steel.
Reagents
a. 5% ammonium thiocyanate (NH₄SCN): 50 g in 1000 ml of water (the printed weight is illegible in the source; 50 g is the straight 5% equivalent — flagged).
b. 2.5% acetic acid solution.
c. 0.3% hydrogen peroxide (1 vol.) solution: dilute 10 ml of 30% H₂O₂ (100 vol.) solution to 1000 ml with water.
d. Dilute sulphuric acid: dilute 400 ml of concentrated acid to 1 litre with water.
Procedure
Use tinplate samples of 8 cm × 5.5 cm area (work with large sample areas — porosity differs across a sheet, and one stray scratch can fake a result on a small sample); clean, dry, degrease using carbon tetrachloride and dry again
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Coat the edges of each plate completely with paraffin wax — apply two coatings, the first at about 90°C and the second at 75°C
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Prepare the thiocyanate test solution by mixing 100 ml of ammonium thiocyanate solution, 100 ml of acetic acid and 50 ml of hydrogen peroxide in a 250-ml beaker
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Keep the test samples immersed vertically in the solution for 15 min, undisturbed
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Stir the solution quickly with the test plate and remove the plate; add 5 ml of dilute H₂SO₄ to the solution in the beaker and determine the iron content by colorimetric estimation of its red thiocyanate colour with suitable standards
Results
Express the thiocyanate value as milligrams of iron per square decimetre.
References
- Kerr, R., J. Soc. Chem. Ind., 61 (volume number uncertain in the source print), 181 (1942).
- Hoare, W. E. & S. C. Britton, Tinplate Testing, p. 30.