Sciencemadness Discussion Board

Citric acid for A/B titrations

mycotheologist - 24-11-2012 at 13:27

I need to determine the concentration of an ammonia solution but I have no acids of known concentration. I have pure solid citric acid though, what I'm wondering is the stoichiometry. Citric acid has two carboxyl groups so will it react with ammonia in a 2:1 ratio? I know with H2SO4, the acidity drops significantly after the first proton is removed so it often forms the bisulfate salt rather than the sulfate salt. I'm wondering if the same applies for citric acid which doesn't have geminal protons.

Vargouille - 24-11-2012 at 13:58

Citric acid has three carboxyl groups. I wouldn't recommend using a weak acid to titrate a weak base, as it's quite a bit more complex. A triprotic weak acid is even worse. Like all other polyprotic acids, the acidity does drop with each successive deprotonation. This is due to coulombic forces: as the molecule become more negatively charged, it is more difficult to deprotonate it.

In any case, the best solution is to get a strong acid of known concentration and preform the titration with that. Unless it's an assignment to use the weak acid, in which case I wish you luck.

arsphenamine - 24-11-2012 at 14:21

Yes, what he said.

Since you can dry and weigh sodium carbonate reproducibly, make a ~1M solution from as your standard.

Then titrate your acid standard (say ~1M HCl) from the Na2CO3 standard. Titration is done around pH 7 when CO2 effervescence stops.

Knowing your acid concentration, you can titrate the ammonia solution.

A good overview is here.

Alternatively, measure the ammonia solution with a pH meter, and do the complex equilibrium calculation accordingly.

[Edited on 24-11-2012 by arsphenamine]