Perfoming the Titration
The accuracy of the results of your titration will be a reflection of the
care you took while performing it. When done carefully, titrations give
very accurate, precise results. Before performing a titration, you should
familiarize yourself with the proper cleaning procedures and use of the equipment you will be using.
Titrations of unknown solutions are done in two steps: a scout titration
used to determine the approximate amount of titrant needed followed by the
actual titration that you will use to make your calculations.
When performing the scout titration, you will most likely overshoot the
endpoint. The purpose of this titration is to give you a rough idea how
much titrant is needed before you begin the actual titration.
- Use a pipet to deliver a known amount of the analyte to the appropriate container (usually
an Erlenmeyer flask) which has been cleaned and rinsed with distilled water.
Since you are concerned with the number of moles in the volume of analyte
dispensed from the pipet, the flask does not need to be completely dry before
adding the analyte.
- If you are going to be using a magnetic stir plate, put a clean, dry stir
bar into the flask and place the flask on the stir plate. Adjust the water
flow so that the stir bar is spinning but not splashing solution onto the
walls of the container.
- Record the initial buret reading in your notebook. Add titrant from the buret to the flask while stirring
magnetically or swirling the flask with your hand. Close the stopcock when
the endpoint is seen. Record the final buret reading in your notebook. Calculate the amount of titrant needed to reach the endpoint.
- Set up just as you did for the scout titration.
- Record the initial buret reading and calculate the reading you expect at the endpoint.
- Add titrant rapidly just as you did in the scout titration, but stop about
1 mL before your expected endpoint reading.
- Rinse the walls of the flask with a little bit of distilled water from your
wash bottle. Again, you are concerned with the volume of titrant added to
the flask, not the total volume of the flask, so the added water will not
affect your calculations.
- Resume delivery of the titrant from the buret one drop at a time until you
think you have reached the endpoint.
- Record the final buret reading. Add one more drop of titrant to be sure
you have reached the endpoint. If this looks more like the endpoint, record
the new reading. Continue adding drops and reading the buret until you are
convinced that you have reached the endpoint.
- Calculate the amount of titrant needed using the appropriate final buret
Usually at least one more titration is performed for comparison. If the
volume of titrant needed in each case is not sufficiently close, more titrations
can be performed until the desired precision is reached.