Method of Test determination of water-soluble and acid-soluble chlorides in mortar and concrete, Part 1: Fresh mortar and concrete
2001 Edition

For chloride analysis, samples must be collected within two hours after water addition to the mixture. At least three increments, aggregating to 0.02 cubic meters, should be taken at approximately one-quarter, one-half, and three-quarters of the discharge from the mixer. These increments are combined in a clean, non-absorbent container, preferably metallic, to prevent segregation and are thoroughly mixed to form a representative bulk sample.

Water-soluble chlorides are those that dissolve readily in distilled water from the fresh concrete or mortar, representing the free chlorides available in the pore solution. Acid-soluble chlorides include both the free chlorides and those chemically bound within the concrete matrix, extracted through acid treatment. The test separately measures these fractions to provide a comprehensive quantification of total chloride content.

The volumetric titration requires a 0.02 N silver nitrate solution prepared by dissolving 1.7 grams of AgNO3 in distilled water and diluted to 500 ml, standardized against 0.02 N sodium chloride using potassium chromate indicator. Additionally, 6 N nitric acid (prepared by diluting concentrated acid), ferric alum, nitrobenzene, and 0.02 N ammonium thiocyanate solution are used. The filtrate sample volume is typically 50 ml, to which these reagents are added sequentially during titration.

Chloride content is calculated using the difference in volumes of silver nitrate added (X) and ammonium thiocyanate consumed (Y), the normality (N) of silver nitrate, the sample mass (m), and the atomic mass of chlorine (35.45). The formula is: Chloride (%) = ((X - Y) × N × 35.45 × 100) / (m × 1000). This calculation accurately quantifies chloride concentration from volumetric titration data.

To minimize interference, the protocol advises adding 5 ml of 6 N nitric acid to maintain an acidic environment, which prevents the formation of unwanted precipitates. Ferric alum (1 ml) and nitrobenzene (5 ml) are introduced to coagulate silver halide precipitates, enhancing clarity. The mixture is shaken vigorously before titrating the excess silver nitrate with 0.02 N ammonium thiocyanate until a stable faint reddish-brown color indicates the endpoint. Accurate standardization of reagents is also essential.