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Endpoint and Equivalence Point in Titration
Name: Jacob
Status: student
Grade: 9-12
Location: AL
Country: USA
Date: September 2007
Question:
Can you please explain the difference between the endpoint
and the equivalence point in a titration?
Replies:
Jacob-
I think they are basically the same thing.
However, if one is trying to get precise,
the equivalence point is "the right answer"
and the end-point is real physical event that tells you to record the
volume that has been added.
They can be a little different.
Equivalence-point means the moment in your slow, dripping addition,
at which the amount of base added is chemically equivalent to the acid in
the flask,
(or vice-versa) so the two kinds of reagent are exactly cancelling out.
Only G-d and Maxwell's demons know when this really is,
and we are merely hoping the indicator tells you right at that time.
It might not.
On the other hand, the end-point is the perceptual event at which
the indicator actually does change, and you decide you are "done",
and you actually read the quantity of measuring reagent you have added.
Such as when the solution finally changes color and does not change back
right away.
The end-point may be slightly different from the true equivalence point,
due to the indicator you use, the slowness of pH changing with reagent
addition,
the rate of stirring and reaction, and maybe other things
like CO2 in the atmosphere slowly adding itself to the reaction.
Suppose it is an Acid-Base titration of NaOH and HCl,
and you use phenolphthalein indicator,
perhaps because that is all you had on hand.
When equivalent amount (same number of moles) of HCl and NaOH are together
in the flask, the pH is pretty near pH=7.
Nothing visible happens right then,
because the phenolphthalein indicator changes color closer to pH=10.
If adding base from the burette to acid in the flask,
you might need add an extra drop of NaOH solution to make the indicator
change color.
Then the volume you have added would be "wrong" by about 1 drop in 100ml.
This small volume difference, caused by a pretty big pH difference,
is often less than 1% of the milliliters used.
But some titrations, carefully done, can get precision better than 0.1%.
So other indicators with transition closer to pH=7 would be more ideal for
this.
And when titrating weak acids and bases, it matters a lot more.
Because they change pH slowly, the error can be bigger than 20%,
and it gets really important to talk about the difference,
and maybe estimate a correction.
Or, it is possible you are reading a paragraph where
somebody is talking about only one thing
but casually alternating between similar words,
just so they do not have to use the same word so many times.
You will have to judge that from the context.
Unfortunately I cannot be sure, because you did not mention why you are
asking, or what words you saw that implied such a distinction.
Jim Swenson
It is possible for a poly-protic acid or poly-hydroxyl base to have more
than one equivalence point. For example, H2SO4 has two, and (HO)3PO has
three, one for each titratable entity. However, the two terms are often
used interchangeably, particularly if the context makes clear the
distinction.
Vince Calder
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Update: June 2012
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