LABORATORY EXPERIMENT 5 PRECIPITATION TITRATION WITH SILVER NITRATE. The AgNO 3 solution (~0.02 M) needs to be standardized using NaCl as a primary standard. You will perform standardization using Fajans method with adsorption indicator and using Mohr method with chromate indicator. Both titrations are to be done in triplicate. 1. The Determination of Chloride by Titration with an Adsorption Indicator Discussion In this titration, the anionic adsorption indicator dichlorofluorescein is used to locate the end point. With the first excess of titrant, the indicator is incorporated into the counter- ion layer surrounding the silver chloride and imparts color to the solid. To obtain a satisfactory color change, it is desirable to maintain the particles of silver chloride in the colloidal state. INDICATOR Dichlorofluorescein indicator (sufficient for several hundred titrations). Dissolve 0.2 g of dichlorofluorescein in a solution prepared by mixing 75 mL of ethanol and 25 mL of water. PROCEDURE Dry the NaCl sample at 110°C for about 1 hr; allow it to return to room temperature in a desiccator (this will we done for you). If 0.02 M AgNO3 is to be used, weigh a 0.5-g NaCl sample into a 500-mL volumetric flask, and take 25-mL aliquots for titration. To each, add 5 drops of indicator. Titrate with AgNO3 to the first permanent pink color of silver dichlorofluoresceinate. Note: Colloidal AgCl is sensitive to photodecomposition, particularly in the presence of the indicator; attempts to perform the titration in direct sunlight will fail. If photodecomposition appears to be a problem, establish the approximate end point with a rough preliminary titration, and use this information to estimate the volumes of AgNO3 needed for the other samples. For each subsequent sample, add the indicator only after most of the AgNO3 has been added, and then complete the titration without delay.