FAJANS METHOD FOR THE DETERMINATION OF CHLORIDE


Preparation of AgNO3 Solution Use the top-loading balance to weigh about 9.0 g of AgNO3 into a clean, dry beaker. Dissolve the solid and dilute to 500 mL with DI water. Mix well, and store the solution in an amber glass bottle. [I would HIGHLY recommend wearing latex gloves for the duration of this experiment!]

Standardization of AgNO3 Dry about 2 g of primary standard sodium chloride for 1 h at 110oC. When cool weigh a sample into a 500 mL conical flasks, and dissolve it in about 200 mL of DI water. To this first (preliminary) sample add about 0.1 g of dextrin and 3-4 drops of dichlorofluoroscein indicator. Titrate the sample until a pink color remains throughout the solution for at least 30 seconds. Use the data from the first titration to calculate the sample size and the approximate volume of titrant for the next four (4) samples, but do not use the data from the first titration for calculation of AgNO3 molarity. For the remaining 4 samples add the dextrin and indicator only after about 95% of the volume of AgNO3 has been added, as the AgCl precipitate is sensitive to photo-decomposition.

Calculate the molarity of your silver nitrate solution from the volume titrated and the mass and purity of the primary standard sodium chloride. The formula weight of NaCl is 58.4425.

Determination of Chloride Dry your soluble chloride unknown for 1 hour at 110oC and cool in your desiccator. Be sure to choose sample sizes that will require between 30 and 50 mL of titrant. Treat your unknown samples exactly like the standard samples were treated (remember to reject the first titration in your calculations.) Calculate the % chloride in your sample.


Miscellaneous Information Do not discard your titrated samples down the sink. There is a bottle in the hood that you can pour your samples into when you are finished with them. There is also a bottle available for you to dispose of any excess AgNO3 solution. Be certain you understand the distinction between waste silver chloride (solution in your conical flask) and waste silver nitrate (excess standard solution) when disposing of your solutions in the containers provided in the hood. The silver chloride (solid; pink) should go in the wide mouth bottle; the silver nitrate (clear; colorless) should go into the narrow mouth bottle.

Silver nitrate will turn your skin black because of the reduction of the silver ion by ascorbic acid (Vitamin C). Although this discoloration is harmless, it is a good idea to minimize the amount of contact AgNO3 makes with your skin. Gloves are available for you to use to help minimize contact with your skin.

To calculate the amount of titrant needed for your titrations from your preliminary data realize that the volume/mass ratio is fixed:

(VOL1 / MASS1) = (VOLx / MASSx)

You should use about 95% of this calculated value for your titration before addition of the indicator and the dextrin.


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