The buret is a device designed to measure a delivered volume of a solution to a high level of accuracy.  This requires assurance that

  • its contents (which we call the titrant) are carefully and consistently characterized
  • the delivered volume of the solution is able to be measured accurately  and precisely
The use of the buret involves several separate operations.

1. Selecting the proper buret
In the introductory laboratory, each buret is numbered uniquely.  Students are assigned the use of a specific pair of burets throughout the semester.  This practice minimizes the impact of any possible manufacturing differences from one buret to the next.  Here is an example of the impact of such manufacturing differences.

2. Preparation of the buret to receive and deliver its contents.
Burets are generally stored filled with distilled water and capped with a rubber stopper when not in use.  This prevents foreign materials from entering the buret between uses.  Before it is used, the buret must be conditioned to receive its contents without changing their composition.  This would seem to require emptying the buret completely (including the tip) to insure that the material with which the buret is subsequently filled is not diluted by any residual distilled water.  This is not easy to achieve.  An alternative to complete emptying is to insure that any remaining drops of distilled water are replaced by the titrant.  Rinsing the buret with the titrant is a way to accomplish the same condition. insing)

  • empty the contents of the buret (be sure to remove the rubber stopper from the top of the buret to allow the liquid to escape through the tip).
  • if the buret does not discharge a substantial stream, ask the instructor to inspect the tip and clean it if it is clogged
  • put a small amount ( 5 - 7 mL) of the titrant in the buret and rinse the barrel of the buret
  • discharge the rinse solution through the tip -- to insure that the tip does not contain any remaining distilled water
  • rinse with a second small amount of the titrant as in the previous step
  • empty this through the tip of the buret
The buret should now contain no residual distilled water.  It is now ready for the next step.

3. Fill the buret with the titrant and adjust the initial volume
The solution with which the buret is to be filled has usually been carefully prepared.  It may be a solution of known concentration that you have prepared yourself or a stock solution that you have obtained from a labeled container.  In the latter case, be sure the container that you have used to obtain the solution is clean and dry so that you do not change its nominal concentration.

  • fill the buret to a level above the 0 marking
  • mount the buret in a buret clamp on a buret stand (be sure the buret is level -- i.e., perpendicular to the desk)
  • discharge the titrant into a container until the level is between 0 and 5 mL (do not try to set the level to 0.00 mL)
  • be sure that there are no bubbles in the tip of the buret.  Bubbles will add to the measured volume even though no corresponding solution is delivered.
  • record the initial reading of the buret (to the appropriate precision -- i.e., to two decimal places)
    • When reading a buret, make sure the line of sight from your eye to the buret is perpendicular to the buret.
4. Perform the titration
Many titrations require initial slow addition of the titrant to the titrated solution.  In any even, it is essential that the titrant be mixed thoroughly with the solution being titrated throughout the titration.  Be sure to stir or swirl the solution into which the titrant is being added to insure complete mixing of the constituents.
  • Erlenmeyer flasks are ideally designed to minimize any loss of solution during swirling.  When using a beaker, stirring is the preferred method
5. The end point
The end point of any titration will be a signal (often a color change).  The titrant should be added slowly when approaching the end point.  When approaching the endpoint, evidence of the final color is often detectable before the solutions are completely mixed.  Near the end point, the titrant should be added as slowly as possible.  Remember that the buret is capable of delivering and measuring a volume smaller than a whole drop (~0.05 mL).  If a fraction of a drop is necessary, it can be washed off the buret tip into the titration vessel with a wash bottle.

6. Measure the final volume

  • record the final reading of the buret (to the appropriate precision -- i.e., to two decimal places)
  • the volume of the titrant consumed in the titration is the ( final volume - initial volume ).  It is known to two decimal places.
7. Preparing the buret for storage
When the last of a series of titrations is completed, the buret must be prepared for storage.  Since burets are shared from one section of the laboratory course to the next, it is essential that the buret be stored so as to minimize problems for the next user.  The steps are as follows.
  • empty the contents of the buret -- through the tip
  • rinse the buret with water -- tap water can be used for rinsing -- and empty the rinse solution through the tip
  • rinse the buret a second time and empty it as completely as possible
  • fill the buret with distilled water (including the tip) and replace the rubber stopper
  • place the buret in the appropriate slot of the buret rack

    The impact of manufacturing differences -- an extreme example.
    Suppose in the construction of a particular buret, the markings are consistently 5% more than the true volume (I.e., the true volume is 0.95 times the reported volume).  If a solution of given concentration is used in this buret, the actual amount of the titrant will always be 5% less than the amount computed from the difference.

    E.g.   Suppose the solution is truly 0.1000 M NaOH and the initial and final buret readings in a titration are 3.25 mL and 29.86 mL respectively, i.e. 26.61 mL net.

    • we would compute the number of mmols of NaOH delivered to be 0.1000 * ( 29.86 - 3.25 ) = 2.661 mmol
    • the actual number of mmol would be 0.1000 *( 0.95 * 29.86 - 0.95 * 3.25) = 2.528 mmol, which is 5% less than the amount we compute

    In this instance, we would suffer a 5% error in accuracy due to the device.  If a second titration were conducted with a different buret having no error in its markings, the results would differ by 5%.

    Suppose however, that we had used the same defective buret in the determination of the concentration of the NaOH solution (which we assume is actually 0.1000 M) in a standardization.  For this purpose, we prepared a solution containing 2.500 mmol of an acidic standard (prepared using an analytical balance).  This amount of standard should require exactly 25.00 mL of our NaOH.  The initial and final volumes of the NaOH we used would each have been 5% larger than the true volume that we used so we would have measured this volume as 26.32 mL.  This would have led us to the conclusion that the concentration of the NaOH was actually
    2.500 / 26.32 = 0.0950 M, or 95% of its true concentration.

    Using 26.61 mL of this solution in our defective buret (as we did above) would have delivered 26.61 * 0.0950 = 2.528 mmol of NaOH which is the actual number of mmols of NaOH used in the titration.

    By using the same (defective) buret in both the standardization and in a titration with the same solution, we have caused the error to cancel in this case.


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Last Update: 2005-06-13