Acid-Base Titrations

The titration of acids and bases is an experimental technique used to determine the amount of an acid or base in a solution, i.e. the concentration of the solution. By definition, a titration is an analytical procedure for the quantitative analysis of a substance by means of detecting an essentially complete reaction in solution with a reagent of known concentration.

To perform a titration, a buret (a volumetric cylinder calibrated in divisions of 0.1 ml) is used to deliver an accurate volume of solution. The buret is filled with base of a known concentration, and is placed above a beaker or flask containing an acidic solution for which the concentration will be determined. This type of titration is referred to as 'titrating an acid with a base'; the opposite is also common. The acidic solution contains an indicator, a dye that changes color as a function of pH when the reaction reaches completion.

The base is slowly added to the acidic solution. When the number of moles of OH- added is exactly equivalent to the number of moles of H+ supplied from the acid, the reaction has reached its equivalence point. Laboratory scientists, however, do not actually observe the equivalence point; instead, they determine the concentration of the solution at the reaction's end point, which is accompanied by the dye's color change. Stating the titration has reached its 'equivalence point' when we note a solution's color change is incorrect. Although there is error associated with the discrepancy, indicators are chosen so the error is typically minimal.

This module simulates the useful laboratory technique of titration using an indicator. In this acid-base titration, NaOH is added from a buret to a container having 25 ml acid of unknown concentration.. Three indicators are available: Methyl Orange, Phenolphthalein, and Thymolphthalein, each of which can potentially mark the equivalence point by changing color.

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INSTRUCTIONS

  1. Select an indicator from the three available.
  2. Select the concentration of the NaOH base solution, from the three available.
  3. To titrate, click on one of the three Add Base buttons. Each button adds base aliquots of different volumes; use any combination of buttons to add the desired amount of base.
  4. Use the Redo Titration button to repeat titration.
  5. Use New Unknown button to choose a different acid concentration.
  6. You can see the answer anytime by clicking at the bottom right hand corner label, saying "Click here for answer".
Last modified: April 13, 2000