How does a back titration differ from a regular titration?
In a direct titration, you add a standard titrant to the analyte until you reach the end point.
In a back titration, you add an excess of standard titrant to the analyte, and then you titrate the excess titrant to determine how much is in excess.
It is often preferable to do a back titration if
- one of the reactants is volatile and some might escape during the titration (e.g., NH₃)
- the analyte is an insoluble salt (e.g., CaCO₃)
- a particular reaction is too slow
- direct titration would involve a weak acid-weak base titration (difficult to observe the end point)
Here's an example of a back-titration to determine the mass of calcium carbonate present in a sample of chalk.
She placed the sample in a 250 mL conical flask and added 50.00 mL 0.2000 mol/L HCl from a volumetric pipette.
She then titrated the excess HCl with 0.2500 mol/L NaOH and found an average titre of 40.00 mL.
Her calculations went something like this:
- She added 0.010 00 mol of HCl to the chalk.
- She used 0.008 000 mol of NaOH to neutralize the excess HCl.
- The chalk must have reacted with 0.002 00 mol of the HCl.
- Since 1 mol of CaCO₃ reacts with 2 mol of HCl, there must have been 0.001 00 mol of CaCO₃.
- The mass of CaCO₃ in the chalk was (0.001 00 × 100.09) g = 0.100 g.