What is the chemical difference between an endpoint and equivalence point?

Question

In our acid-base titration lab, we used phenolphthalein to monitor the neutralization reactions. What are the chemical differences between an equivalence point and an endpoint in titration and use this to explain whether phenolphthalein could be used as an indicator in any acid/base neutralization.
There's got to be some difference between them...otherwise my chem lab nazi TA wouldn't have asked.

Answer 1

Let us start by sorting out a few definitions:

Equivalence point: this is when exactly equal stoichiometric quantities of acid and base have been mixed and reacted together.

Endpoint: this refers to the situation where the indicator changes colour.



Now consider what happens with four different combinations of acid/ base titrations:

Phenolphthalein is going to be used as the indicator - this indicator is accepted to change colour at pH 8.3 - If added to an acid solution, it is colourless, and when a base is added, when the pH reaches 8.3, it starts to go pink.

1) strong acid / strong base: titrating 25.00mL of 0.100M HCl with 0.100M NaOH

Equation: HCl + NaOH → NaCl + H2O.

Here the product NaCl is neutral, and the equivalence point will be at pH = 7.00 when exactly 25.00mL of the NaOH solution has been added.

But because the base is strong, during the titration, the pH of the solution will be found to change from the acid pH approx 4.0 to basic pH 9.0 by the addition of 1 drop of NaOH solution. This range spans the endpoint of the phenolphthalein indicator, and as a result it is a suitable indicator for this titration.



2) Strong acid/weak base. Titrating 25.00mL of 0.100M HCl with 0.100M NH3

Equation: HCl + NH3 → NH4Cl

Here the product NH4Cl is an acidic salt and the equivalence point will be at an acid pH - probably around pH = 4.5. This will be attained when exactly 25.00mL of the 0.100M NH3 has been added to the 25.00mL of 0.100M HCl. In order to bring the pH up to the phenolphthalein endpoint, pH = 8.3, considerable extra NH3 solution must be added . Remember NH3 is a weak base, and it takes quite a high concentration of base to increase the pH from about 4.5 to 8.3. So in this case phenolphthalein is an unsuitable indicator for this type of titration - its endpoint does not match the equivalence point of the titration.



3) Strong base/ weak acid. Using the same argument as above - titrating a weak acid- CH3COOH with a strong base NaOH - the salt is CH3COONa - which is basic, so the equivalence point is at pH well above 7.00 Here a drop of the NaOH solution will change the pH of the titration solution from acidic to pH approx. 9.0 . Again this rapid and instantaneous pH change spans the endpoint pH of the phenolphthalein indicator - it is therefore a suitable indicator for this titration.



4) weak base / weak acid. Here the salt is neutral - consider CH3COOH + NH3 → CH3COONH4. The equivalence point occurs at pH = 7.00 - well below the endpoint pH of the phenolphthalein indicator. But because both acid and base are weak, the change in pH throughout the titration occur very slowly. In general this type of titration is considered by chemists to be avoided if possible - and no indicator is really suitable - definitely not phenolphthalein.



So there is your answer: phenolphthalein

1) suitable for strong acid / strong base: weak acid / strong base

2) unsuitable for weak base / strong acid or weak base / weak acid.

Answer 2

The equivalence point in the titration represents the complete neutralization of the acid by the base. The endpoint of a titration is the signal produced by an acid/base indicator. The endpoint is usually signaled by a color change as the pH of the solution changes. Ideally, the chemist would choose an indicator which will change color near the equivalence point.



Can phenolphthalein be used in any acid/base titration? Of course it can, but the endpoint may not be anywhere close to the equivalence point. Phenolphthalein is often used for strong-acid, strong-base titrations where the equivalence point should be at a pH of 7, yet the indicator changes color between a pH of 8 and 9. For instructional purposes, this is close enough. But if the equivalence point of a titration occurs at a pH of 5, then it is not close enough. So "no", phenolphthalein is not a "universal" indicator.



============= Follow up ==============



T.U. made the following statement, "Stoichoimetric point/ equivalence point is hypothetical and theoretical. It is not possible to detect this point in a practical experiment (by practical means) so the best estimate (closest estimate) would be the end-point."



Not true. You may not have a colorimetric indicator that changes color at the exact pH of the equivalence point, but it is possible to experimentally determine the equivalence point. You just don't do it with an acid/base indicator. A potentiometric titration will determine the equivalence point and shows up nicely with a plot of the second derivative. A conductometric titration will also give a good indication of the equivalence point.



Find out more...

potentiometric titration:

http://rgmlab.chem.uconn.edu/teaching/chem-232/Laboratory_Manual/GA7_potentio_Titr_rev7_99.pdf

http://en.wikipedia.org/wiki/Potentiometric_titration



Conductometric titration

http://www.vrml.k12.la.us/rpautz/documents/Chemistry/ConductometricTitration.pdf

Answer 3

Hello, For titration purposes, there is not a significant difference between the endpoint and equivalence point - both are often used interchangeably (though it is not recommended) to mean when the indicator changes color. However, in terms of being an indicator, there are various chemicals used as indicators for different reactions. For example, phenolphthalein is used often as a strong base is added to a weak acid because it changes color at approximately 8-9, the equivalence point for the reaction between the strong base and weak acid - the result is a slightly basic solution. Since this change happens around 8-9, phenolphthalein is ideal for this titration. However, there are other indicators used for different pH ranges - you can Google it and it will show you how various compounds change over the pH spectrum. It all depends on how the indicators' components react with the amount of acid/base present.

Answer 4

Conductometric Titration Definition