[educateme]

Hello everyone,

I have done a titration experiment. I did not calibrate the pH meter. Would this affect the endpoint or concentration that I have determined in this experiment. Please explain each if possible.
Thank you very much for your time.

[Borek]

Please read forum rules.

You have to show your attempts to receive help. This is a forum policy.

This is a very simple question. Take a look at the plot - what will happen to the curve if the pH is off by +1 pH unit? By -1 pH unit? Will it shift end point?

[marquis]

The real question is- why can't you standardize the pH meter?  If the standard slope is outside of the acceptable range, you have a problem (probably with the buffers).  If the standards aren't accepted by the instrument, there is a major problem (its either with your electrode or your standards). 

Can you say more about the instrument issues?

[Borek]

I think it was a theoretical question being part of a lab report, not any real problem with pH meter.

Besides, you have not addressed the question - is pH meter calibration important for determination of titration end point?

[marquis]

There are some circumstances where calibration would not be needed.  However, they are relatively few.  They also require the pH meter to be working correctly.  If the meter can't be standardized, there is a moderate chance it isn't working.  That functioning meter problem needs to be addressed first.

If you are titrating to a specific pH as an endpoint, then the calibration of the pH meter is critical.  Likewise, if you are titrating a multiprotic acid (say phosphoric acid), you need to know which form of the acid you are titrating.
 
One exception would be a potentiometric titration where you are measuring mv vs ml of titrant. You need to know what form of acid you are titrating.  From this information, you could plot out the curve and determine the endpoint without knowing the exact value of pH. Say, using a first derivative plot on semilog paper. 

[marquis]

Lets explain a little more.  The exact pH wouldn't be required.  So calibration of the pH meter would not be needed.

It's Newton and his funny math at work. You're calculating the endpoint based on the derivative.  So the absolute value of the pH doesn't count.

[Borek]

If you are titrating to a specific pH as an endpoint, then the calibration of the pH meter is critical.

Correct.

Likewise, if you are titrating a multiprotic acid (say phosphoric acid), you need to know which form of the acid you are titrating.

First at first, second as a second and so on. If you are titrating sample of a known pure acid shape of the titration curve is still enough, its exact position is not important for the end point determination (unless - as stated above - you are titrating to a specific pH).