Volumetric Analysis and Titration go hand in hand, as titration is the main method used to carry out volumetric analysis. There are three types that are commonly happened upon in the study of inorganic chemistry.
Acid-Base Titrations
As the name suggests, acid-base titrations are where the two reactants are an acid and a base. In the majority of acid-base titrations an appropriate indicator has to be added, as the reactants do not act as their own indicators.
Indicators for Use in Acid-Base Titrations
Normally, indicators are weak acids, where the acid's colour is noticeably different than the colour of the conjugate base(of the acid). A good indicator is one where the two colours are distinctively different from one another.
For example, if we had a weak acid indicator that was red in colour, and the conjugate base of this acid (the ionised indicator) was green in colour, we could compare the dissociation in water of the two in a reaction.
If an acid was added, the position of the equilibrium of the dissociation would shift towards the acid side. It would turn red in colour.
If an alkali was added, the position of the equilibrium of the dissociation would shift towards the alkaline side, and so would go green in colour.
Indicators, The Equilibrium Constant and pH
From the equilibrium constant equation, it can be determined that the indicator's colour at any time during a titration is dependent upon the concentrations of the indicator molecules that have not yet been ionised, and its conjugate base(the ionised indicator).
This is equal to 1, when the association constant of the indicator (pK) is equal to the pH - when the pK(of the indicator) =pH.
From this, we can conclude that when the indicator changes colour, the pK of the indicator is equal to the pH.
It is only when the concentration of the un-ionised indicator ions and the concentration of the conjugate base have a difference between them of a factor of 10 that the colour change is noticeable, so the pH range in which the colour change of the indicator( and so the end-point of the reaction) occurs is around two pH units -- around pH=pK +/- 1.
pH changes in a titration happen most rapidly around the end-point of the reaction, and so the pH range of any indicator must match with the point of most rapid change in pH (e.g., the end-point).
For the different combinations of acid and base titrations, there is a different range of pH within which the indicator must lie.
- For strong acid-strong base titration, the pH range must be between around pH3 to pH11 ( most indicators fall in this range, so most indicators have the ability to be used in these titrations).
- For strong acid-weak base titrations the pH range of the indicators must be between 3 and 7 approximately.
- For weak acid-strong base titrations, the pH should be between 7 and 11.
For weak acid-weak base titrations, the pH range is almost non-existent, so no indicator is suitable for these titrations - this also explains why weak acids and weak bases do not generally react.
Redox Titrations
Redox is short for reduction and oxidation titrations. During a redox reaction (and titration) a reducing agent reacts with an oxidising agent.
A reducing agent is oxidised during the reaction, as it is an electron donor, and the oxidising agent is reduced during the reaction, due to it's nature of being an electron acceptor.
Potassium Permanganate and Redox Titrations
In redox titrations a very useful oxidising agent is acidified permanganate as it is both a very good oxidising agent, and is self indicating - so acts as the indicator in the reaction. An example of a redox titration including acidified permanganate, is that the purple potassium permanganate solution is in the burette, whilst the reducing agent, and some sulphuric acid (this contributes the acidic H+ ions) is placed in the conical flask.
The permanganate ions are reduced to colourless ions as they are dispensed into the conical flask. The reaction is over when all of the reducing agent has been used up, so the purple permanganate ions are no longer reduced. The end-point occurs, and the permanent colour change observed is from colourless to a light pink/purple colour.
Complexometric Titrations
Complexometric titrations are titrations that result in the formation of complexes. EDTA, a chemical reagent and a hexadentate ligand, is very useful and important in complexometric titrations that is used to find out the concentration of metal ions in solution.
Sources
- Mark Bishop, Preparatory Chemistry.
- Archie Gibb, David Hawley, BrightRed Advanced Higher Chemistry, pg 30, 38.
Hayleigh Hunter - Hayleigh describes herself as the eternal student - she loves to learn. Her articles aim to provide the best quality information ...
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