Titration

Titration is a standard laboratory method of chemical analysis[?] which can be used to determine the concentration of a known reactant.

A reagent, called the titrant, of known concentration and volume is used to react with a measured volume of reactant. Using a calibrated burette to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached.

Many methods can be used to indicate the endpoint of a reaction; titrations often use visual indicators (the reactant mixture changes colour). In simple acid-base titrations a pH indicator may be used, such as phenolphthalein, which turns (and stays) pink when a certain pH is reached or exceeded. Different types of titrations include: acid-base, redox, precipitation, and compleximetric. The example below is an acid-base reaction. Although the vast majority of titrations are carried out in aqueous solution, other solvents such as glacial acetic acid[?], are used for special purposes.

Procedure

1. Accurately measure a volume of the reactant into to a beaker or Erlenmeyer flask.
2. Add a suitable indicator to the flask.
3. Pour the titrant into the buret, read the start-point of the liquid on the burette.
4. Turn the tap of the burette to allow the titrant to slowly fall into the reactant. Swirl the flask with the other hand or with a magnetic "flea".
5. The indicator should change colour as the titrant is added, but then quickly return to its original colour.
6. As the end-point is approached, the indicator takes longer to turn back to its starting colour. Add the titrant more slowly at this point.(one drop at a time)
7. When the indicator remains at its end colour, the reaction has reached the end point. Measure the amount of titrant liquid used, as shown on the scale of the buret.
8. Repeat twice more, then average the volumes.

Analysis

For this analysis it is assumed that the concentrations are molar. If the concentration of the titrant is not known as mole/litre, it is better to convert it first. (See converting concentrations[?]) By subtracting the start-point from the end-point, the total volume of titrant added (V_t) can be calculated. Also known are the concentration of the titrant (C_t) and the volume of the reactant (V_r). Multiplying the concentration by the volume of the titrant gives The number of moles:

V_t * C_t = M_t

The reaction formula must be known in order to determine how many moles of the reactant combined with the titrant.For example Hydrochloric acid reacts with sodium hydroxide in the ratio 1:1

H⁺ + Cl^- + Na⁺ +OH^- --> Na⁺ + Cl^- H₂ O

In this case the number of moles of reactant equals the number of moles of titrant.

Another example would be sulphuric acid reacting with sodium hydroxide. This time the acid and base react in a 1:2 ratio

2H⁺ +SO₄^2- + 2{Na⁺ +OH^-} --> 2Na⁺ + SO₄^2- +2H₂O

In this case the number of moles of the base is twice the number of moles of the acid.

Once you know the number of moles of reactant that have been neutralised then it is easy to calculate the concentration in Moles per litre.

C_r = M_r /V_r

Variations

The titration process can also be monitored by different means than visually

• using the change of potential: potentiometry electrochemistry
• using the change of conductivity of the solution: conductometry[?]
• using the heat of reaction in an isothermal titration calorimeter
• using spectroscopy