SOP: Determination of Sodium Ferrocyanide in Salt

GIFSQ — Global Institute for Food Safety & Quality

Principle

This is a colorimetric limit test for sodium ferrocyanide, added to brine to modify salt crystal growth. Ferrous sulphate and sulphuric acid develop a blue colour with ferrocyanide; the colour of the sample solution is compared in comparator tubes with a control containing a known amount of sodium ferrocyanide — the sample blue must not exceed the control.

Apparatus required

• Two 150-ml glass-stoppered cylinders
• Two comparator tubes

Reagents

a. Ferrous sulphate solution: dissolve 8 g of clear crystals of ferrous sulphate, FeSO₄·7H₂O, in about 100 ml of boiled and cooled distilled water. Prepare the solution afresh.

b. 10% (w/v) sulphuric acid: add 57 ml of concentrated H₂SO₄ gradually with stirring to about 100 ml of water, cool to room temperature, and dilute to 1000 ml with water.

c. Standard solution of sodium ferrocyanide: dissolve 99.5 mg of sodium ferrocyanide, Na₄Fe(CN)₆·10H₂O, in 500 ml of water (1 ml = 125 µg of Na₄Fe(CN)₆).

Sample preparation

Grind the salt sample to pass a No. 20 sieve and be retained on a No. 80 sieve; quarter and mix.

Procedure

In a 150-ml glass-stoppered cylinder, take 9.62 g of the sample; dissolve in water and make to 80 ml

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In another 150-ml glass-stoppered cylinder, take 1.0 ml of the standard sodium ferrocyanide solution (the control)

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To each cylinder, add 2 ml of ferrous sulphate solution and 1 ml of dilute H₂SO₄; dilute to 100 ml with water and mix

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Transfer 50 ml of the sample solution into one comparator tube and 50 ml of the control solution into another comparator tube

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Compare the blue colours — the blue colour in the sample solution should not be more than that of the control

Calculation

The control contains 125 µg of sodium ferrocyanide in 9.62 g of sample equivalent:

125 ÷ 9.62 = 13.0 µg/g = 13.0 ppm

The sample passes if its sodium ferrocyanide content does not exceed that of the control (13.0 ppm).

Where,

• 125 µg = sodium ferrocyanide in 1.0 ml of the standard solution (the control)
• 9.62 g = weight of the sample taken
• 13.0 ppm = the control level, matching the specification table’s maximum for sodium ferrocyanide

Precautions

• Prepare the ferrous sulphate solution afresh.
• Compare the two tubes under identical conditions.

References

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