SOP: Measurement of Tin Coating Weight (Clarke's Test)

GIFSQ — Global Institute for Food Safety & Quality

Principle

Free tin and the tin–iron alloy layer are dissolved off the steel base selectively. The weight lost gives the coating weight, after correcting for the iron the alloy layer carries with it.

Reagents

a. Detinning stock solution: dissolve 20 g of antimony trioxide (Sb₂O₃) or 35.5 g of antimony trichloride (SbCl₃) in 1 litre of concentrated hydrochloric acid (sp. gr. 1.16).

Procedure

Note the area of the specimen — the book gives the specimen area as (8.0 × 5.5 cm) and notes that a specimen of 50 to 100 sq. cm. area gives a convenient weight difference (see source note)

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Clean the tinplate and degrease using carbon tetrachloride as solvent (cathodic treatment in 1% sodium carbonate solution for 1 to 2 min is preferable)

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Dry the specimen and weigh accurately

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Immerse the specimen in the detinning solution to remove the tin coating including the alloy layer from the steel — allow the specimen to remain for about 1 min after the gas evolution has ceased, then wash it immediately in running water

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Remove the loosely adherent deposit of antimony by wiping with a soft cotton-wool swab, dry the specimen and weigh again

Calculation

The alloy contains approximately 19% iron. Apply the correction for the iron in the tin alloy layer: subtract from the tin coating obtained 1 oz per base box for hot-dipped tinplate and 0.03 lb per base box for electrolytic tinplate.

Tin coating in oz per base box = (Initial weight of tinplate (g) − Weight of tinplate after test (g)) × 62.79 / (Area of tinplate used × 2 × 28.35) (see source note)

Where,

• Initial weight of tinplate (g) = weight of the cleaned, degreased specimen before detinning
• Weight of tinplate after test (g) = weight of the specimen after removal of the tin coating and alloy layer
• Area of tinplate used = area of the specimen, in sq. in.
• 2 = both faces of the plate are tinned
• 28.35 = grams per ounce

Source notes: the book prints the specimen area as “(8.0 x 5.5m)” (read as 8.0 × 5.5 cm) alongside the statement that a 50 to 100 sq. cm. specimen gives a convenient weight difference — carried as printed. The constant is printed “62.79”; the book separately states the total tinned area of a base box (both faces) as 62,720 sq. in. — carried as printed. The base box is defined by the book as 112 sheets of 14 × 20 in., i.e. 31,360 sq. in. of tinplate.

Reference

Hoare, W. E. & S. C. Britton, Tinplate Testing, p. 29.