Titrations (titrimetric methods) are quantitative analytical techniques which help us determine the amount of certain substances based on the stoichiometry of different reactions.
Generally, we want to determine the unknown concentration of a solution, which we call the analyte, using another solution of a known concentration, called the titrant.
In most titrations, the analyte is placed in an Erlenmeyer flask and the titrant in a burette (see the apparatus guide)
In direct titrations, the titrant reacts with the analyte and, when the reactions stops, we can stoichiometrically calculate how much analyte we have.
Example:
In a substitution titration, the analyte reacts with another substance, releasing a compound which will then be titrated.
Example:
Back titrations are similar to substitution titrations, except we don't titrate a product of the first reaction, but the excess left from a reactant.
Example:
After reading everything, you probably have one big question left: how do we know when the reaction ends? The answer is indicators. Indicators vary, based on the substances involved. For example, in titrations involving iodine, one might use starch as an indicator. In the presence of iodine, starch turns blue and so we can find out when iodine forms/is completely consumed.
In the following pages about specific titrations, we will get into more detail about the choice of indicator.
Sometime, although it might seem counterintuitive, we can put the analyte in the burette and the titrant in the Erlenmeyer flask. When we choose to do that, there is a logical reason behind the decision and the choice is not arbitrary.
One example, is a titration with \(HCl\), to find the concentration of \(NaOH\) in a solution. In this case, the indicator is phenolphtalein, a substance which turns pink in the presence of base, but is colorless in acidic and neutral solutions. Naturally, when only a small amount of base is present, the pink is very pale.
If we were to do a regular titration, after putting phenolphtalein over the \(NaOH\) in the flask, it would turn into a very strong pink. When the titration nears completion, the pink fades away and, when the pink is too pale, we could confuse it with a colorless solution, stopping the titration too early.
However, if we have \(HCl\) in the flask, the solution (with phenolphtalein added) will be colorless, until completion, at which point it will turn slightly pink. This color change is easily noticeable if we use a white background (such as a sheet of paper).
In titrations, it is good practice to swirl the Erlenmeyer flask, such that the solutions are quickly mixed and the reaction is basically instantaneous. While doing this, with one hand, the other hand should constantly be on the burette faucet, ready to close it. Which hand does which thing is up to you, but, in the case of some burettes, keeping the left hand on the burette might be inconvenient, as they could be built for right-handed people.
You can let the solution from the burette continously pour out, in the beginning, but as the titration nears the end, you should only let it go dropwise. You can see that you're nearing the end of the titration, when the indicator starts changing color for short periods of time, before reverting to the original color. You have reached the end point only once the color change persists for a longer time.
Video courtesy of Auckland University of Technology (YouTube). Used for educational purposes under YouTube’s embedding policy.