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Chapter: Modern Analytical Chemistry: Gravimetric Methods of Analysis

Theory and Practice of Precipitation Gravimetry: Rinsing the Precipitate

Filtering removes most of the supernatant solution.

Rinsing the Precipitate 

Filtering removes most of the supernatant solution. Resid- ual traces of the supernatant, however, must be removed to avoid a source of deter- minate error. Rinsing the precipitate to remove this residual material must be done carefully to avoid significant losses of the precipitate. Of greatest concern is the po- tential for solubility losses. Usually the rinsing medium is selected to ensure that solubility losses are negligible. In many cases this simply involves the use of cold solvents or rinse solutions containing organic solvents such as ethanol. Precipitates containing acidic or basic ions may experience solubility losses if the rinse solution’s pH is not appropriately adjusted. 

When coagulation plays an important role in determining particle size, a volatile inert electrolyte is often added to the rinse water to prevent the precipitate from reverting into smaller particles that may not be re- tained by the filtering device. This process of reverting to smaller particles is called peptization. The volatile electrolyte is removed when drying the precipitate.

When rinsing a precipitate there is a trade-off between introducing positive de- terminate errors due to ionic impurities from the precipitating solution and intro- ducing negative determinate errors from solubility losses. In general, solubility losses are minimized by using several small portions of the rinse solution instead of a single large volume. Testing the used rinse solution for the presence of impurities is another way to ensure that the precipitate is not overrinsed. This can be done by testing for the presence of a targeted solution ion and rinsing until the ion is no longer detected in a freshly collected sample of the rinse solution. For example, when Cl is known to be a residual impurity, its presence can be tested for by adding a small amount of AgNO3 to the collected rinse solution. A white precipitate of AgCl indicates that Cl is present and additional rinsing is necessary. Additional rinsing is not needed, however, if adding AgNO3 does not produce a precipitate.

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