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Home > Technical Articles > Titrations
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Home > Technical Articles > Titrations
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Titrations are generally used in analytical chemistry for very accurate
and precise determinations of % level components as opposed to trace
amounts (<1 ppm). The most common example of a titration is determining
the concentration of acid or base in solution by measuring how much of the
other (base or acid) it takes to neutralize the sample. The precision and
accuracy of titrations make it an excellent choice for performing assays
with tight specifications.
Lets take a closer look at a titration of a basic sample, say a potassium hydroxide solution. We want to determine the concentration of hydroxide in the sample, so we will measure out a small volume (50 mL) |
and add, very slowly with constant swirling, small amounts of hydrochloric acid. Now the acid has been previously standardized so we know it’s exact concentration (0.100 molar). And we measure the amount of acid added to the sample using a long, volumetric device called a buret, able to measure volumes of 0.1 mL or less. Before we add anything to the sample, the potassium hydroxide solution has a very high pH (>12). As we add acid, the pH does not change very rapidly until we get close to the end point, the point at which the moles of hydrochloric acid almost equals the moles of hydroxide in the sample. Then the pH changes very rapidly. If we continue to add acid, the pH will become low (<3) with only a slight excess in the number of moles of acid. |
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Now we don’t need to take sub-samples out of the acid-base mixture to
measure the pH with a meter. Instead, we add a very small amount of an
indicator that changes color near the pH range of the end point. In this
case the end point has a pH of 7.00. As soon as we get close to the end
point, we will note that the new color lasts longer with each drop of
titrant added.
So how do you determine how much hydroxide was in your original sample? |
Molarity (i.e. the number of moles per liter
of hydroxide) =
If it takes 10.0 mL of 0.100 molar HCl to titrate 50 mL of hydroxide solution to its end point (pH = 7), then the original sample contains 0.020 molar hydroxide (molecular weight 17), or 0.85 g/L of hydroxide. |
| Now this is an example of a very simple acid-base titration. Other examples include oxidation-reduction or redox (e.g. iodine-thiosulfate), precipitation (e.g. chloride-silver nitrate and | calcium-EDTA), potentiometric, and a large number of variations and combinations involving all kinds of exciting and interesting chemistry. |
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