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2.1: Precipitation
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In a precipitation reaction, the solid product separates out from the clear solution making the solution opaque or turbid called a suspension. The solid product i.e., precipitate, may be filtered out, but usually, it is forced to settle at the bottom of the test tube as sediment or a solid pellet, by centrifugation process leaving a clear solution, i.e., supernatant, at the top.
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2.2: Water bath
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Heating a reaction mixture in a water bath is more uniform with less fire hazard than heating on a bunsen burner or directly on a hot plate. A 200 mL beaker filled to ~150 mL mark and gently boiling on a hot plate with surface temperature ~350-degree Celcius is recommended for use as a water bath for qualitative analysis of caitons.
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2.3: Centrifugation
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A precipitation reaction usually results in the formation of a suspension. The precipitate can be separated by filtration, but a more effective, faster, and easier approach is to force the precipitate to form sediment or pellet at the bottom of the test tube under the action of a centrifugal force in a centrifuge machine.
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2.4: Separation of the precipitate
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Separation of the precipitate from the clear supernatant after centrifugation is achieved by decantation, i.e., pouring out, by aspiration, i.e, drawing out using a pasture pipette, or by gravity-filtration are described.
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2.5: pH measurement
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In a qualitative analysis of cations, litmus paper is used to determine the solution is acidic or basic, and a pH paper is used to measure the approximate pH value of the solution. The solution is applied to the end of the pH paper strip and the color change in the pH paper is observed.
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2.6: Flame test
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Metal salt solutions, particularly metal chloride solutions impart a characteristic color to the flame when they are evaporated in a non-Luminus flame. The flame color is dependent on the metal and it is used to identify the metal in a test called flame test. Heavy alkaline earth metals and alkali metals are often identified by a flame test.
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2.7: Common qualitative analysis reagents, their effects, and hazards
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Reagents commonly used in the qualitative analyses of cations in water are listed along with their effects and hazards.