Types Of Gravimetric Methods 383o6y

Types of gravimetric methods 1. Precipitation methods – A gravimetric methods in which the signal is the mass of the precipitate. 2. Volatilization gravimetric – A gravimetric method in which the loss of a volatile species gives rise to the signal.

Precipitation methods The eight steps of precipitation methods 1. Preparation of solutions 2. Precipitation 3. Digestion 4. Filtering 5. Washing 6. Drying 7. Weighing 8. Calculation 1. Theory and methods Solubility considerations -

Require the solubility to be minimal. Usually the total accuracy should be +/- 0.1% Solubility losses can be minimized by controlling the composition of solution Reacts with analyte to form insoluble material. Precipitate has known composition or can be converted to known composition

Avoiding Impurities -

Inclusion – potential interfering ions whose ion and charge are similar to a lattice ion may substitute into the lattice structure by chemical adsorption. It may cause the precipitate masses to be larger than the expected but does not change the percentage of the analyte. Occlusion – when physical adsorbed interfering ions become trapped within the growing precipitate. ( mass larger) 2 cases of occlusions – when rapid precipitation traps a pocket of solution.(mass lesser) Inclusions can be only minimized by the re-precipitation and occlusions can be reduced by the digestions.

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Digestion is the process of maintaining the ppts in equilibrium with its supernatant solution for an extended time. Solution containing ppts are heated before filtering Heating the ppts within the mother liquor (or solution from which it precipitated) for a certain period of time. Done for crystalline ppts (e.g: BaSO4) diameter >10-4cm. During digestion, small particles tend to dissolve and reprecipitate on larger ones. The additional ppts can be minimized by controlling solution conditions.

Controlling Particle Size -

A solute relative supersaturation,RSS can be expressed as Relative supersaturation = Q : degree supersaturation ( [ ] Of mixed reagents before ppt) S : solubility of ppts at equilibrium

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Relative supersaturation can also be known as a measure of the extent to which a solution, or a localized region of solution, contains more dissolved solute than that expected at equilibrium.

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A solution with high RSS can have a solutions which highly supersaturated. Such solutions are unstable and show high rates of nucleation. It produces numerous small particles. When RSS is small, precipitation is likely to occur by particle growth than by nucleation. How to minimize the RSS? By decreasing the solute’s concentration or increasing the precipitate’s solubility. Practically, increase the temperatures, adjusting the pH . Precipitate that are usually insoluble and a big RSS is unavoidable. If RSS is small, it may need a very long time to form a precipitate.

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Filtering the precipitate -

Filtration is done by filter paper/ filtering crucible. The most common filtering crucible is cellulose based filter paper. Filter paper is also rated as fast, medium and fast. If the filtration is too fast, there might have a ppts loss. If the filtration is too slow, it might have clogged. Filter paper is hygroscopic and not easily dried to constant weight. Igniting the filter paper is a must. Ash is being left. If was quantitative analytical procedure, use the low ash filter paper.

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When the filter paper is made into a cone, the water is dampened to seal the cone and the funnel. Any supernatant solution is being left into the funnel. Supernatant solution is the solution that remains after the precipitate forms. Filtering crucible is also another way to filter the precipitate.

Rinsing the ppts -

The residual traces of supernatant must be removed in order avoid the sources of determinate error.