H4A
H
4
This Demonstration shows the chemical equilibria in a polyprotic acid dissociation reaction at a given pH:
H
4
k
1
⇌
H
3
-
A
+
H
H
3
-
A
k
2
⇌
H
2
2-
A
+
H
H
2
2-
A
k
3
⇌
3-
HA
+
H
3-
HA
k
4
⇌
4-
A
+
H
The expressions for the fraction of an acid in each form (A,, , , ) as a function of pH are obtained by taking account of the equilibrium constant and the mass balance. The fraction of molecules in a certain species is called , where is the number of hydrogens on the undissociated acid molecule and is the number of dissociated hydrogen ions. These are calculated as follows:
H
4
H
3
-
A
H
2
2-
A
3-
HA
4-
A
α
H
n-x
x-
A
n
x
α
H
4
4
[]
+
H
Δ
α
H
3
-
A
k
1
3
[]
+
H
Δ
α
4-
A
k
1
k
2
k
3
k
4
Δ
where
Δ=+[]+⋯+
4
[]
+
H
3
k
1
+
H
k
1
k
2
k
3
k
4
and
[]=
+
H
-pH
10
Moreover, the concentration (M) of the generic species is given by:
H
n-x
x-
A
[]=
H
n-x
x-
A
C
T
α
H
n-x
x-
A
where is the total concentration of the undissociated acid set to 0.1.
C
T
When "fractional composition diagram" is selected, you can change the , , and controls to observe the variation of the values as a function of pH. By changing the pH control, you will obtain the individual values, as highlighted by the dots on the diagram.
k
1
k
2
k
3
k
4
α
α
When "fractional composition bar chart" is selected, you can change the , , , and pH controls to see the resultant species concentration variations at equilibrium, where the sum of the species concentrations equals . Further, we show the of the different addends in , which accounts for the approximations often used in calculations [1].
k
1
k
2
k
3
k
4
C
T
wt%
Δ