SOP: Determination of Salt Content in Butter by Mohr's Method

GIFSQ — Global Institute for Food Safety & Quality

1. Principle

In this method, the butter sample is melted in hot water, and the chlorides present in the mixture are titrated with a solution of silver nitrate using potassium chromate as indicator.

Reaction
AgNO₃ + NaCl → AgCl + NaNO₃
2AgNO₃ + K₂CrO₄ → Ag₂CrO₄ + 2KNO₃ (brick-red ppt)

2. Reagents

a. Standard silver nitrate solution (0.1 N): standardized against standard sodium chloride. Dissolve slightly more than the theoretical quantity of silver nitrate (equivalent weight 169.89) in halogen-free water and dilute to volume. Dissolve between 17 g and 19 g of silver nitrate in 1 L of water which is practically free from carbon dioxide. Standardize the silver nitrate solution against standard sodium chloride solution. Store the solution away from direct sunlight.

b. Potassium chromate indicator (5%, w/v): dissolve 50 g of potassium chromate (K₂CrO₄) in 1 L of water.

c. Calcium carbonate: analytical grade, free from chloride.

3. Apparatus

a. Conical flask: 250 mL capacity

b. Burette: 50 mL capacity, graduated to 0.1 mL

c. Pipette: capable of delivering 2.0 mL

d. Measuring cylinder: 100 mL capacity, graduated

4. Procedure

Weigh accurately 5 g of butter sample into the 250 mL conical flask

↓

Carefully add 100 mL of boiling distilled water and mix the contents of the conical flask

↓

Allow to stand with occasional swirling for 5 to 10 minutes

↓

After cooling to 50 to 55°C (titration temperature), add 2 mL of potassium chromate solution and mix by swirling

↓

Add about 0.25 g of calcium carbonate and mix by swirling

↓

Titrate at 50 to 55°C with standard silver nitrate solution while swirling continuously, until the brownish colour persists for half a minute

↓

Carry out a blank test with all the reagents in the same quantity, except the butter sample

Precision: the maximum deviation between duplicate determinations should not exceed 0.02% of sodium chloride.

5. Calculation

NaCl, % by mass = [5.844 × N × (V₁ − V₂)] ÷ M

Where,
– N = normality of silver nitrate solution (0.1 N)
– V₁ = volume in mL of silver nitrate used in the sample titration
– V₂ = volume in mL of silver nitrate used in the blank titration
– M = mass in g of the butter sample

References

Manual of Methods of Analysis of Foods, Milk and Milk Products, FSSAI, 2016, Method 12.4.2.