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Polyprotic base

  • Thread starter temaire
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  • #1
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Homework Statement


Which of the following, Na3PO4 or C2H4(OH)2 could produce a pH curve with two buffer zones?

The Attempt at a Solution


I think the answer is Na3PO4 because it can react twice, but the third reaction may not complete. C2H4(OH)2 is ethylene glycol. Is it possoble for it to react with water to produce a pH curve? Any help would be appreciated.
 

Answers and Replies

  • #2
symbolipoint
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Your first compound is an alklaline salt, and your second compound is an alcohol with two hydroxyl functional groups. Those groups are not hydrixide ions, so no significant contribution to alkalinity. The sodium orthophosphate may go through maybe three steps of equilibria process. You can titrate and create a curve (pH titration), but you can not well resolve the third equivalence point from the second equivalence point (too close together).
 
  • #3
epenguin
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The sodium orthophosphate may go through maybe three steps of equilibria process. You can titrate and create a curve (pH titration), but you can not well resolve the third equivalence point from the second equivalence point (too close together).
Surely they are rather well separated? pKs of phosphate 2.15, 6.86 and 12.32

Maybe it is rather a trick question? - you could "produce a pH curve with two buffer zones" by e.g. titrating Na3PO4 with HCl, amd stopping when you get to about pH 4-5. ?
 
  • #4
symbolipoint
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Surely they are rather well separated? pKs of phosphate 2.15, 6.86 and 12.32

Maybe it is rather a trick question? - you could "produce a pH curve with two buffer zones" by e.g. titrating Na3PO4 with HCl, amd stopping when you get to about pH 4-5. ?
In practice the last two endpoints are difficult to resolve. There is nothing fundamentally wrong in actually TRYING such a titration. A typical lower division (and sometimes upper division) lab exercise is to titrate orthophosphoric acid and make a titration curve of volume of titrant versus pH. ( orthophosphoric acid, H3PO4 )
 
  • #5
Borek
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Ah, there is nothing like being an author of a program that calculates such curves :wink:

phosphoric_acid_titrated_with_sodium_hydroxide.png


This is phosphoric acid tritrated with sodium hydroxide (both 0.01M).

phosphate_titrated_with_hydrochloric_acid.png


This is phosphate tritrated with hydrochloric acid (both 0.01M).
 

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