Method of Test determination of water-soluble and acid-soluble chlorides in mortar and concrete, Part 2: Hardened mortar and concrete
2001 Edition

The procedure involves collecting a core sample using a drill with a diameter at least 2.5 times the maximum aggregate size, breaking the sample into pieces smaller than 25 mm, crushing these fragments further using grinding equipment, and sieving through an 850-micron IS sieve. The pulverized material is then thoroughly mixed by transferring between glazed papers multiple times to ensure homogeneity. For chloride extraction, 1000 ±5 g of the pulverized sample is weighed. For acid-soluble chlorides, 100 ml of 6N nitric acid mixed with 900 ml distilled water is added, stirred, and gently warmed for 30 minutes before settling and filtration. For water-soluble chlorides, 1000 ml distilled water is added, stirred, warmed for 15 minutes, allowed to settle for 24 hours, then decanted and filtered through a Whatman No. 1 filter paper.

Water-soluble chlorides are those extracted by soaking the hardened concrete or mortar sample in distilled water, representing free chlorides immediately available to cause corrosion. Acid-soluble chlorides include both water-soluble chlorides and those bound within the concrete matrix, extracted by acid digestion (without adding nitric acid as per the latest updates). Both types are quantified via volumetric titration using standardized 0.02 N silver nitrate solution, with ammonium thiocyanate serving as the titration indicator. The chloride percentage is calculated using the formula: Chloride % = (2 × 0.709 × (X - Y)) / m, where X is the volume of silver nitrate added, Y is the volume of ammonium thiocyanate consumed, and m is the mass of the sample.

Essential reagents include analytical grade 0.02 N or 0.2 N silver nitrate (AgNO3), 0.02 N or 0.2 N ammonium thiocyanate (NH4SCN), 6 N nitric acid (HNO3), ferric alum as an indicator, nitrobenzene to coagulate precipitates, and distilled water complying with IS 1070 standards. The ferric alum reacts with thiocyanate ions producing a faint reddish-brown color at the titration endpoint. Potassium chromate is used as an indicator during standardization of silver nitrate against sodium chloride.

Silver nitrate solution is prepared by dissolving 1.7 g of AgNO3 in distilled water and diluting to 500 ml to achieve approximately 0.02 N concentration. It is standardized against 0.02 N sodium chloride using potassium chromate as an indicator to determine exact normality. During testing, 50 ml of the sample filtrate is pipetted into a conical flask, followed by 5 ml of 6 N nitric acid. A known volume (usually 25 ml) of the standardized silver nitrate solution is added, then 1 ml ferric alum and 5 ml nitrobenzene are introduced and the mixture shaken to coagulate precipitate. The excess silver nitrate is then back titrated with ammonium thiocyanate until a faint reddish-brown endpoint is observed.

To avoid interference, 5 ml of 6 N nitric acid must be added to maintain an acidic environment preventing unwanted precipitation. The addition of 1 ml ferric alum and 5 ml nitrobenzene helps coagulate silver chloride precipitates ensuring clear separation. Vigorous shaking facilitates coagulation. Using standardized silver nitrate solution with known normality is essential. The titration endpoint is indicated by the appearance of a faint reddish-brown color upon titrating with ammonium thiocyanate, assuring precise endpoint detection without ambiguity.