Use of a Buret

A buret is a very valuable piece of equipment to have in the laboratory.  The primary use for burets is in titrations.

Titration is the process of adding one solution of known concentration (titrant) to another solution of unknown concentration in order to determine the unknown concentration.  Dispensing the titrant starts a chemical reaction which proceeds, as the more titrant is added to a recognizable end-point (change in color, meter reading, etc.).  At this point the titration is complete.

A buret is calibrated to be read from the top down.  For example, a 25 mL buret will have the zero mark at the top and 25 mL mark at the bottom, above the stopcock.  To determine the volume dispensed from the buret, use the graduation that is closest to the lowest point of the meniscus.  As an example, in the diagram, the meniscus is between the 11.4 and 11.5 mL marks.  Since the lowest point is closest to the 11.5 graduation, the column of liquid dispensed would be recorded as 11.5 mL, assuming that the titration was begun at zero.
                                                      

Using a buret is not difficult, however, good technique is needed to titrate accurately.  Fill the buret so that the meniscus is above the zero mark, then drain the excess out until the zero mark is reached.  Because of the accuracy required from the buret, there should be no air bubbles either trapped in the tip or on the sides of the buret.  If air bubbles are present in the tip, open the stopcock completely to force them out, then refill.  If there are air bubbles on the sides of the buret, gently tap the buret to get them to rise to the surface, then check to make sure that the meniscus still reads zero.  If necessary, add additonal liquid to refill the buret.

The stopcock is designed to allow you to control the rate of flow from the buret.  When the wings of the stopcock on a standard buret are in a horizontal position, the opening is completely closed.  When the wings are in a vertical position, the opening is completely open and flow is maximum.  By adjusting the angle of teh wings, the rate of flow can be controlled.  Standard burets are designed so that the stopcock can be turned in any direction.

There are several precautions which should be observed when using a buret:

  1. Never use a solution that has been left in the buret overnight.  After the day's testing has been completed, any liquid remaining in the buret should be discarded and the buret cleaned, rinsed  and allowed to dry.


  2. It is recommended that the buret be acid cleaned on a regular basis.  It is especially important to acid clean the buret before using a different chemical.


  3. Be extremely careful not to break or chip the buret tip.  The calibration and accuracy of a buret is dependent upon the tip remaining undamaged.  Breaking or chipping the buret tip can make the buret worthless by lowering the accuracy of the buret.