Chapter 12 - Chapter 12 Gravimetric Methods of Analysis...

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Chapter 12 Gravimetric Methods of Analysis Gravimetric methods of analysis are based on the measurement of mass. There are two major types of gravimetric methods: Precipitation methods: in this method the analyte is converted to a sparingly soluble precipitate. This precipitate is then filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment, and the product is weighed. Volatilization methods: in this the analyte or its decomposition products are volatilized at a suitable temperature. The volatile product is then collected and weighed, or, alternatively, the mass of the product is determined indirectly from the loss in mass of the sample.
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PROPERTIES OF PRECIPITATES AND PRECIPITATING REAGENTS A gravimetric precipitating agent should react specifically, and selectively with the analyte. The ideal precipitating reagent would react with the analyte to give a product that is 1. Readily filtered and washed free of contaminants 2. Of sufficiently low solubility so that no significant loss of the solid occurs during filtration and washing 3. Unreactive with constituents of the atmosphere 4. Of known composition after it is dried or, if necessary, ignited
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Particle Size and Filterability of Precipitates Precipitates made up of large particles are generally desirable in gravimetric work because large particles are easy to filter and wash free of impurities. In addition, such precipitates are usually purer than are precipitates made up of fine particles.
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What Factors Determine Particle Size? The particle size of solids formed by precipitation varies enormously. At one extreme are colloidal suspension , whose tiny particles are invisible to the naked eye (10 -7 to 10 -4 cm in diameter). Colloidal particles show no tendency to settle from solution, nor are they easily filtered. At the other extreme are particles with dimensions on the order of tenths of millimeter or greater. The temporary dispersion of such particles in the liquid phase is called a crystalline suspension . The particles of a crystalline suspension tend to settle spontaneously and are readily filtered.
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experimental variables as precipitate solubility, temperature, reactant concentrations, and the rate at which reactants are mixed. The particle size is related to a single property of the system called its relative supersaturation , where relative supersaturation = (Q – S) / S In this equation, Q is the concentration of the solute at any instant and S is its equilibrium solubility. When (Q – S)/ S is large, the precipitate tends to
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Chapter 12 - Chapter 12 Gravimetric Methods of Analysis...

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