Chemical Forums

Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: lbjohnson on September 10, 2018, 04:15:14 AM

Title: Calculationg percentage ionisation of a diprotic acid or base?
Post by: lbjohnson on September 10, 2018, 04:15:14 AM

Trying to calculate percentage ionisation of a compound at pH 7.4 with two pKa values of 1.5 and 10.5.

I know that for calculating percent ionised for a compound with one pKa value it's:

[tеx] Percent unionised = 100/1+10^pKa-pH [/tеx]

How do I calculate the % ionised for both pKa values simultaneously at pH 7.4?

Title: Re: Calculationg percentage ionisation of a diprotic acid or base?
Post by: Borek on September 10, 2018, 05:30:22 AM

First of all: how do you define percentage of ionization for the multiprotic acid?

Title: Re: Calculationg percentage ionisation of a diprotic acid or base?
Post by: lbjohnson on September 26, 2018, 04:20:34 AM

Quote from: Borek on September 10, 2018, 05:30:22 AM

First of all: how do you define percentage of ionization for the multiprotic acid?

Do you mean this:

H₂A ::equil::HA^-+H⁺

Title: Re: Calculationg percentage ionisation of a diprotic acid or base?
Post by: Borek on September 26, 2018, 07:29:41 AM

That's dissociation reaction, I asked how do you _define_ the value you want to calculate.

For a monoprotic acid HA of a total concentration C_A percentage ionisation is rater trivial:

[tex]\% = \frac {[A^-]}{C_A} \times 100\%[/tex]

But how are you going to define it for H₂A?