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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: teddypunkkari on March 16, 2006, 12:40:54 PM

Title: third equivalent point
Post by: teddypunkkari on March 16, 2006, 12:40:54 PM

So, I titrated Na2PO4 with HCl and now I need to find the third equivalent point. First two can be found directly from the titration curve, but I really have no idea how to find the last one....
Title: Re:third equivalent point
Post by: Alberto_Kravina on March 16, 2006, 12:46:27 PM
Quote from: teddypunkkari on March 16, 2006, 12:40:54 PM

So, I titrated Na2PO4 with HCl and now I need to find the third equivalent point. First two can be found directly from the titration curve, but I really have no idea how to find the last one....
Na2PO4? I think you mean Na3PO4

I think that the last one can be found from the titration curve as well!
Title: Re:third equivalent point
Post by: teddypunkkari on March 16, 2006, 01:02:36 PM

Yes of course. Na3PO4! It can´t be found from the titration curve.I understood that this is becouse of the acidity constant is now so small that it can´t be experimentally found. Or something like that...
Title: Re:third equivalent point
Post by: Borek on March 16, 2006, 01:15:55 PM
Quote from: Alberto_Kravina on March 16, 2006, 12:46:27 PM
I think that the last one can be found from the titration curve as well!

Not so easy, see titration curve (0.1M vs 0.1M). Guess how I made it ;)

teddypunkkari: no idea what you mean exactly by 'find third equivalence point' - but if you know positions of the first two you know how much acid must be added to reach third point, don't you?
Title: Re:third equivalent point
Post by: teddypunkkari on March 16, 2006, 02:58:16 PM

Is this really the "official way"? I mean, is this the professional way to do it.
 From my titration curve: first equivalent consumption of HCl is 14 ml and second is 14,5. So which do I use to calculate the third or do I calculate average value? Hope you get my point, since english isn´t my native language...
Title: Re:third equivalent point
Post by: Borek on March 16, 2006, 03:51:10 PM
Quote from: teddypunkkari on March 16, 2006, 02:58:16 PM
Is this really the "official way"? I mean, is this the professional way to do it.

If there is not enough data you have no choice.

Quote
From my titration curve: first equivalent consumption of HCl is 14 ml and second is 14,5. So which do I use to calculate the third or do I calculate average value?

You mean 14mL to first and 28.5mL total to the second endpoint?

You should use average.
Title: Re:third equivalent point
Post by: teddypunkkari on March 16, 2006, 04:26:02 PM
Quote from: Borek on March 16, 2006, 03:51:10 PM
If there is not enough data you have no choice.

Well my data is the titration curve. And of course consentration of HCl and mass (and consentration) of Na3PO4.

You mean 14mL to first and 28.5mL total to the second endpoint?

Yes 28.5 ml.

Title: Re:third equivalent point
Post by: HCaulfield on April 08, 2006, 01:13:07 PM
Are you sure it is pure Na3PO4?

If it is a mixture with hydrogen phosphate, there can be a difference, and no averaging should take place.