The equivalence point, or stoichiometric point, of a chemical reaction is the point at which chemically equivalent quantities of reactants have been mixed. For an acid-base reaction, the equivalence point is where the moles of acid and the moles of base would neutralize each other according to the chemical reaction. This does not necessarily imply a 1:1 molar ratio of acid:base, merely that the ratio is the same as the ratio in the chemical reaction. It can be found by means of an indicator, such as phenolphthalein or methyl orange.

The endpoint refers to the point at which the indicator changes color in a colorimetric titration. The endpoint is related to the equivalence point, but they will not necessarily be the same.

Methods to determine the equivalence point

Different methods to determine the equivalence point include:

pH indicator

: A pH indicator is a substance that changes color in response to a chemical change. An acid-base indicator, such as phenolphthalein, changes color depending on the pH. Redox indicators are also frequently used. A drop of indicator solution is added to the titration at the start; when the color changes, the endpoint has been reached. This is an approximation of the equivalence point.

Amperometry

: Amperometry can be used as a detection technique (amperometric titration). The current due to the oxidation or reduction of either the reactants or products at a working electrode will depend on the concentration of that species in solution. The equivalence point can then be detected as a change in the current. This method is most useful when the excess titrant can be reduced, as in the titration of halides with Ag<sup>+</sup>. (This is also useful because it ignores precipitates.)

See also

  • Outline of chemistry
  • Inflection point
  • Isoelectric point

References

  • Equivalence points of virtual and real acid-base titrations - Software Program
  • Example of robust industrial endpoint detector
  • Graphical method to solve acid-base problems, including titrations
  • Graphic and numerical solver for acid-base problems -Software Program for phone and tablets