Sunday, March 22, 2009
♥ Endpoint & Equivalence point
Equivalence pointDefination: The equivalence point, or
stoichiometric point, of a chemical reaction occurs during a chemical titration when the amount of titrant added is equivalent, or equal, to the amount of analyte present in the sample.
cases were there are multiple equivalence points, are which multiples of the first equivalent point, such as in the titration of a
diprotic acid.
Ways to determine the equivalence pointpH indicator:
This is a substance that changes colour in response to a chemical change. An acid-base indicator (e.g., phenolphthalein) changes colour 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 colour changes the endpoint has been reached, this is an approximation of the equivalence point.Potentiometer:
This is an instrument which measures the electrode potential of the solution. These are used for titrations based on a redox reaction.
The potential of the working electrode will suddenly change as the equivalence point is reached.Conductance:
The conductivity of a solution depends on the ions that are present in it. During many titration, the conductivity changes significantly.
For instance, during an acid-base titration, the H+ and OH- ions react to form neutral H2O. This changes the conductivity of the solution. The total conductance of the solution depends also on the other ions present in the solution (such as counter ions). Not all ions contribute equally to the conductivity; this also depends on the mobility of each ion and on the total concentration of ions (ionic strength). Thus, predicting the change in conductivity is harder than measuring it.
Color change:
In some reactions, the solution changes colour without any added indicator.
This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colours.EndpointThe end point (similar, but not the same as the equivalence point) refers to the point at which the indicator changes color in a colorimetric titration.
Credits to http://en.wikipedia.org/wiki/Equivalence_point
