Potassium Chromate PWD

Potassium Chromate is used as an indicator in the determination of Chloride by titration with standard Silver Nitrate solution. This method is called the Mohr method of determining Chloride and is based on the formation of a red precipitate of Silver Chromate at the endpoint after all of the Chloride has been precipitated as white Silver Chloride. A 5% solution of Potassium Chromate that has been Chloride neutralized (Group No. 6000) is commonly used. Approximately 1 mL of this indicator should be used per 100 mL of sample volume.

The accuracy of the Mohr method decreases as the Silver Nitrate concentration decreases due to the amount of excess Silver Nitrate that must be added to produce enough Silver Chromate to be seen visually. The Mohr method must also be performed in the pH range of 6.5 to 9. If the pH is too high, brownish Silver Hydroxide forms and masks the endpoint. If the pH is too low, the Chromate is converted to Dichromate and the endpoint comes too late or can not be perceived at all. In the presence of Ammonium ions, the pH must be kept below 7.2 to prevent partial conversion of the precipitate to soluble Ammine complexes.

For comparison purposes, all Mohr titrations should be carried out at about the same temperature, since the solubilities of Silver Chloride and Silver Chromate depend on temperature. Good stirring during the addition of the Silver Nitrate is also required; otherwise, Silver Chromate that forms locally before the endpoint can become occluded in the Silver Chloride precipitate instead of redissolving. Poor agitation could lead to an endpoint that is neither sharp nor reproducible. The reactions for the Mohr titration are:

Ag+ + Cl- → AgCl (white precipitate)

2 Ag+ (excess) + CrO42- → Ag2CrO4 (red precipitate at endpoint)