Precipitation Titration Mohr Method

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Precipitation Titration Mohr Method

Experimental

The chloride ion (Cl-) is an important anion found in solids and solutions. In this experiment, the amount of chloride ion in an unknown sample J (NaCl + KCl mixture) of water using the Mohr method is determined, which relies on the solubility differences of two anions and the titration endpoint of a precipitate. The net ionic reaction during the titration is as follows:

Ag+ + Cl- → AgCl(s)

The Ksp for AgCl is 1.8 x 10-10 and that for Ag2CrO4 is 1.2 x 10-12. Thus, as silver ions are slowly added to solution, AgCl precipitates first, removing the chloride ions from solution. As can be inferred, Ag+ ions react with almost all chloride ions before precipitation of Ag2CrO4 begins.

Thus, the secondary reaction is

2Ag+(aq) + CrO42-(aq) → Ag2CrO4(s)

and results in a color change from yellow to reddish-brown at the end point. Knowing the molarity of the titrant allows for calculation of the chloride concentration.

Materials: volumetric pipettes, tubes, 250 mL Erlenmeyer flasks, 50 mL buret, 16.7 g/1L of AgNO3, CrO4 solution.

Weigh out the sample (NaCl + KCl mixture) into a dry tube in a range of 0.150 g to 0.250 g. Dissolve the salt with water to 10 mL/15 mL/20 mL. Pipette a 5 mL aliquot of diluted salt into a 250 mL flask and add distilled to 100 mL mark. Then, add 1 mL of CrO4 indicator to the solution. Titrate the sample with silver nitrate solution. The endpoint of the titration is identified as the first appearance re-brown color of silver chromate.

Important details: Silver nitrate is obtainable in primary standard purity. It has a high equivalent weight and dissolves readily in water. The solid as well as its solutions must be scrupulously protected from organic matter and from sunlight. The reagent is expensive; every effort should be made to avoid waste. Unused solutions should be collected rather than discarded; similarly, appreciable amounts of silver...
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