Citric (tri)acid ([C3H5O(COO)3]H3) deprotonation reactions' thermodynamic constant at 25 °C, and corresponding Sillen / Bjerrum diagram
Deprotonation reactions and their K's at 25 °C
$$\ce{(Citrate)H_3^0 <=> (Citrate)H_2^- + H^+} \qquad pK(25 °C)=3.13$$
$$\ce{(Citrate)H_2^- <=> (Citrate)H^{2-} + H^+} \qquad pK(25 °C)=4.76$$
$$\ce{(Citrate)H^{2-} <=> (Citrate)^{3-} + H^+} \qquad pK(25 °C)=6.40$$
'Citrate' being the C3H5O(COO)33- (tri)anion ; and the 25 °C pK values taken from the citric acid Wikipedia page.
Database snippet
SOLUTION_MASTER_SPECIES
Citrate Citrate-3 3 189 189
SOLUTION_SPECIES
Citrate-3 = Citrate-3
Citrate-3 + H+ = (Citrate)H-2
log_k 6.40
(Citrate)H-2 + H+ = (Citrate)H2-
log_k 4.76
(Citrate)H2- + H+ = (Citrate)H3
log_k 3.13
Citrate Bjerrum diagram at 25°C ([total citrate]=1e-3 mol/kg of water)
The following diagram was created with PHREEQC using an input file inspired by the 'Titration Example' of the Notepad++ 3.8 version Help.
