Acid and Base Titration
Molarity quantitive analysis, knowledge of acid and bases.
The most common types of titration is acid-base titrations.
For example, if you are titrating hydrochloric acid with sodium hydroxide:
HCl + NaOH → NaCl + H2O
You can see from the equation there is a 1:1 molar ratio between HCl and NaOH. If you know that titrating 50.00 ml of an HCl solution requires 25.00 ml of 1.00 M NaOH, you can calculate the concentration of hydrochloric acid, [HCl]. Based on the molar ratio between HCl and NaOH you know that at the equivalence point:
moles HCl = moles NaOH
MHCl x volumeHCl = MNaOH x volumeNaOH
MHCl = MNaOH x volumeNaOH / volumeHCl
MHCl = 25.00 ml x 1.00 M / 50.00 ml
MHCl = 0.50 M HCl
Get Ready for Action!!!
Before starting the titration a suitable pH indicator must be chosen. The equivalence point of the reaction, the point at which equivalent amounts of the reactants have reacted, will have a pH dependent on the relative strengths of the acid and base used. The pH of the equivalence point can be estimated using the following rules:
- A strong acid will react with a strong base to form a neutral (pH = 7) solution.
- A strong acid will react with a weak base to form an acidic (pH < 7) solution.
- A weak acid will react with a strong base to form a basic (pH > 7) solution
Titration are often recorded on graphs called titration curves, which usually contain the volume of the titrant as the independent variable and the pH of the solution as the dependent variable because it changes depending on the composition of the two solutions.
The equivalence point on the graph is where all the starting solution has been neutralized by the base. This endpoint of titration is the point when the indicator changes color.
At this point moles H+ = OH-.
The pH of a weak solution being titrated with a strong base solution can be found at different points along the way. These points fall into one of four categories:
- initial pH
- pH before the equivalence point
- pH at the equivalence point
- pH after the equivalence point