Explain the Term Back Titration
Titration is the determination of the quantity of a substance with a known nature, but an unknown concentration, by the carefully measured addition of a solution of known concentration. An "indicator" (often through color change) reveals the endpoint. Calculations then reveal the concentration of the unknown.
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Example of Routine Titration
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A filled 50-ml. burette is positioned over a beaker sitting on a stir plate containing a solution of known composition but unknown concentration. The analyst adds an indicator, then carefully adds liquid of known concentration from the burette. When the color changes the endpoint is reached. Calculations reveal the concentration of the unknown.
Back Titration
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Sometimes this is not the easiest way to titrate. An alternative is "back" titration. The unknown substance is given an excess of a known reactant, and that excess is titrated instead of the unknown. To illustrate: if one has seven apples and oranges and they want to know the number of apples, they can count the apples, or they can count the oranges and subtract that from seven.
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Reasons to Back Titrate
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There are at least two reasons for using back titration. A sample may require stabilization before titration to preserve sample integrity, or the indicator may be more easily read if the process is reversed. Most technicians prefer a change from colorless to color, rather than the reverse.
An Example of When to Use
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If the unknown is a volatile acid, it may be given an excess of a known base. This avoids loss by fuming. The indicator is added. The excess base is titrated.
Another Use
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It may be the analyst accidentally bypasses the endpoint of a normal titration. If so, back titration can usually prevent a ruined analysis.
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