106.3.2.32 TM-32, Determination of Purity of Sodium Chloride

This method determines the percent of sodium chloride in commercial grades of sodium chloride.

106.3.2.32.1 Reagents and Apparatus

a) Millivolt meter equipped with a combination chloride electrode.

b) Nitric Acid (HNO₃), chloride free, 1.42 specific gravity.

c) Sodium Chloride (NaCl), Reagent Grade, dried at 105-110°C for 1 hour prior to use.

d) Silver Nitrate (AgNO₃), Reagent Grade.

e) A source of chloride-free distilled or deionized water.

106.3.2.32.2 Preparation of Standard Solution

Standard Sodium Chloride Solution (0.0100N). Weigh 0.5844 g dried NaCl, dissolve in distilled water and dilute to 1L.

Standard Silver Nitrate Solution (0.01N). Weigh 1.699 g AgNO₃, dissolve in distilled water and dilute to 1L. Standardize to the nearest 0.0001 N against 0.0100 N NaCl.

106.3.2.32.3 Procedure

Using a suitable sample splitting technique, divide the salt, as received, to obtain a representative sample of about 75 g. Dry at least 3 hours at 105-110C and cool to room temperature in a dessicator. Weigh the sample to the nearest 0.1 mg and transfer to a 1000-mL beaker. Add approximately 500 mL of distilled water and 20 mL of HNO₃. Heat near boiling 1 hour, occasionally stirring and crushing any insoluble matter against the bottom of the beaker with the flattened end of a stirring rod. Cool to room temperature, quantitatively transfer to a 1000-mL volumetric flask and dilute to volume with distilled water. Transfer a 10-mL aliquot to a 1000-mL volumetric flask and dilute to volume with distilled water. Transfer a 10-mL aliquot to a 250-mL beaker, adding 90 mL of distilled water and 1 mL of HNO₃. Determine the chloride concentration by potentiometric titration with the standard silver nitrate solution.

106.3.2.32.4 Calculation and Report

Report percent sodium chloride to the nearest 0.1% as follows:

% Sodium Chloride (NaCl) = $100\times {\frac {A\times C\times 584.5}{D}}$

Where:

A = Milliliters of AgNO₃ solution to titrate sample

C = Normality of AgNO₃ solution

D = Grams of sample