SOP: Determination of Total Chlorides

GIFSQ — Global Institute for Food Safety & Quality

Principle

This is a precipitation titration: the chloride in the dissolved sample is titrated with standard silver nitrate, using potassium chromate as the indicator; the end point is the first persistent reddish-brown tinge after brisk shaking.

Apparatus required

• Conical flask
• Burette (for the silver nitrate titration)

Reagents

a. 0.1 N silver nitrate, AgNO₃, standard solution.

b. 5% potassium chromate indicator.

Sample preparation

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

Procedure

Dissolve 0.25 g of the sample in 100 ml of water in a conical flask

↓

Add 1 ml of potassium chromate indicator

↓

Titrate against 0.1 N AgNO₃ solution until the reddish-brown tinge persists after brisk shaking

↓

Carry out a blank determination using water only

Calculation

                      (Sample titre − Blank titre) × N of AgNO₃ × 35.46 × 100

Total chlorides % = —————————————————————-
Weight of sample free of moisture (mg)

Where,

• Sample titre = ml of AgNO₃ used in the sample titration
• Blank titre = ml of AgNO₃ used in the blank determination
• N of AgNO₃ = normality of the silver nitrate solution
• 35.46 = equivalent weight of chloride
• Weight of sample free of moisture (mg) = weight of the moisture-free sample taken, in mg

Note: the fraction layout of the book’s printed calculation is garbled in the extraction; the formula above follows the printed variables and the standard form of the Mohr calculation.

Precautions

• Shake briskly at the end point and take the first persistent reddish-brown tinge.
• Always run the blank determination with water only.

References

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