Titrations, Indicators and Titration Curves
The technique of titration is used to find out accurately how much of a chemical substance is dissolved in a given volume of a solution, that is, the concentration of the solution.
The technique uses a set of apparatus with which volumes of solutions can be measured to an accuracy of greater than 0.1 cm3. Three important pieces of apparatus are:
||Measures accurately the volume of a solution added. Readings can be taken to an accuracy of half a division, that is ±0.05 cm3.
||Delivers an accurate volume of a solution. Often this is 25 cm3.
||Used to make up an accurate volume of a solution, for example, 250 cm3. This could be a standard solution (of exactly known concentration and known solute).
In a titration the pipette is used to transfer 25 cm3 (usually to ±0.05 cm3) of a solution into a conical flask. Another solution that reacts with the solution in the conical flask is carefully added from a burette until it has all exactly reacted. This is called the end point of the titration (or equivalence point of the reaction). There needs to be a way of knowing when the end point is reached. An indicator may be needed. Often a titration is repeated until successive titres are within 0.1 cm3.
An indicator is a substance that undergoes a change in colour when the end-point of a titration is reached. Acid-base indicators are used to signal the end of acid-base titrations.
Acid-base indicators are perhaps the most common types, but different types of indicators are used in precipitation reactions, such as in silver nitrate(V) titrations for chloride ion determination. In reactions where there is a colour change an indicator may not be needed, as in manganate(VII) titrations.