# Question #8298c

##### 3 Answers

**!! LONG ANSWER !!**

So, you have your diprotic acid,

This means that your solution will contain three species: *constant*, regardless of what actual proportions you have.

Each of these three species will have a fraction of the total concentration that depends on their respective concentrations. For example,

Now, I can't show you the actual calculations because that would make for a very, very long answer. If you use the definitions of the **acid dissociation constants**,

These fractions will look like this

This form will help you with the actual speciation diagram because you can calculate these fractions for a given pH without *too much* trouble.

Start with **pH = 0**, determine

and the values of

For the entire pH range your diagram will look like this

That speciation diagram belongs to

- The
#color(green)("green")# curve will be#[H_2A]# ; - The
#color(red)("red")# curve will be#[HA^(-)]# ; - The
#color(blue)("blue")# curve will be#[A^(2-)]# ;

Now, your

For the intermediate species, in your case

Here's a sample calculation of you you can derive the fractions formulas that only use

According to the definitions of

**(1)**:

**(2)**:

I'll use this notation for simplicity

The fraction of

**(MAIN)**

Use **(1)** to write a value for

**(A)**

Use **(2)** to get a value for

**(B)**

Plug **(B)** into **(A)**

Now plug everything into **(MAIN)** and you'll get

Finally,

The speciation diagram is

Your equilibria are

H₂A ⇌ H⁺ + HA⁻;

HA⁻ ⇌ H⁺ + A²⁻;

There are three species: H₂A, HA⁻, and A²⁻.

The material balance is given by

The fraction of each species is given by

I will omit the detailed calculations because, Stefan Zdre has done them for you.

Let

Now we can use Excel to plot the fraction of each species as a function of pH.

The data are

I actually plotted the data at 0.2 pH intervals to get more points. The plots are shown at the beginning of this answer.

Note how the plots cross over at exactly

The

But it does peak at pH 7.6, which is the average of