Why can't pH of diprotic acids be calculated like that of amphiprotic?

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In summary, the website discusses calculating the pH of amphiprotic salts but the same approach cannot be applied to diprotic acids like H2S because of the uncertainty of the dissociation of the first step. The website also explains the stoichiometry of the reactions and the reasoning behind considering H2AsO42- as amphiprotic despite its Ka value being much higher in the acidic reaction. The approach for calculating pH can be used as a rough estimate, but it is not always accurate and should be checked with known equilibria and mass balances.
  • #1
krackers
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I was reading this website (http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt) on calculating pH of amphiprotic salts.

My question is why can't you do something similar for diprotic acids? Take for example the diprotic acid H2S.

Because you have the following reactions:

H+ + HS- ⇔ H2S

HS- ⇔ H+ + S2-

can't you treat HS- like an amphiprotic substance and calculate the pH the same way you calculated pH for other amphiprotic substances?

—————————————————————

I also have another question relating to these topics. In this image,
Frfxa.png


I don't understand how they came to the conclusion that [H3AsO4-] ≈ [HAsO42-]. Further, why is H2AsO42- considered amphiprotic when the value of K-1, that is the equilibrium constant for H2AsO42- + H+ ⇔ H3AsO4, is much higher than that of the reaction where it acts an acid?
 
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  • #2
krackers said:
I was reading this website (http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt) on calculating pH of amphiprotic salts.

My question is why can't you do something similar for diprotic acids? Take for example the diprotic acid H2S.

Because you have the following reactions:

H+ + HS- ⇔ H2S

HS- ⇔ H+ + S2-

can't you treat HS- like an amphiprotic substance and calculate the pH the same way you calculated pH for other amphiprotic substances?

It can be done, but only if you can be sure HA- dissociation won't change first dissociation step. That can work for a sulfuric acid, where we can safely assume first step is always 100% dissociatied.

I don't understand how they came to the conclusion that [H3AsO4-] ≈ [HAsO42-].

Simple stoichiometry.

Further, why is H2AsO42- considered amphiprotic when the value of K-1, that is the equilibrium constant for H2AsO42- + H+ ⇔ H3AsO4, is much higher than that of the reaction where it acts an acid?

Amphiprotic means it can react both ways, it doesn't tell much about relative strengths of both reactions. And the difference of 4 orders of magnitude in such a case is not much.
 
  • #3
Borek said:
It can be done, but only if you can be sure HA- dissociation won't change first dissociation step. That can work for a sulfuric acid, where we can safely assume first step is always 100% dissociatied.
How would you know whether this holds true for HS-?

The Ka values are as follows:

H+ + HS- ⇔ H2S (Ka = 107)

HS- ⇔ H+ + S2- (Ka = 10-19)

Borek said:
Simple stoichiometry.

The amphiprotic reactions are as follows:

H2AsO4 ⇔ HAsO42- + H+ (Ka = 1.1 *10-7)

H2AsO4- + H+ ⇔ H3AsO4 (Ka = 1.6 * 102)

Doesn't the acidic reaction have a much higher Ka and thus won't the concentration of H3AsO4 greater than that of HAsO42-?
 
  • #4
krackers said:
How would you know whether this holds true for HS-?

Calculate pH using this approach, plug it into equilibrium equations and see if they still make sense.

H+ + HS- ⇔ H2S (Ka = 107)

More like 10-7.

Have you seen http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-simplified ?

Doesn't the acidic reaction have a much higher Ka and thus won't the concentration of H3AsO4 greater than that of HAsO42-?

They ASSUME system is dominated by the reaction they wrote and they use it to calculate pH. Assumption doesn't have to be correct - but once again, after calculating pH you can do the simple sanity check, seeing if the numbers fit all known equilibria and mass balances. Turns out they do.

It is discussed at the page you linked to - there are tables showing how accurate the results calculated using equivalent assumption are. Quotient [H2A]/[A2-] being close to 1 is equivalent to the idea that [HAsO42-]≈[H3AsO4] (these may look like completely unrelated, but try to understand the acid/base reaction happening and you will see they are the same).
 
  • #5


The reason why you cannot calculate the pH of diprotic acids in the same way as amphiprotic substances is because diprotic acids have two ionization constants, while amphiprotic substances only have one. In the case of H2S, it can lose two protons, creating two different ions: HS- and S2-. This means that there are two different equilibrium reactions happening simultaneously, making it more complicated to calculate the pH.

Treating HS- as an amphiprotic substance would not give an accurate calculation of the pH because it does not take into account the second ionization reaction of H2S. Each reaction has its own equilibrium constant, and both reactions must be considered in order to accurately calculate the pH of a diprotic acid.

As for the second question, the approximation of [H3AsO4-] ≈ [HAsO42-] is based on the assumption that the second ionization reaction of H2AsO4- is negligible compared to the first. This means that the concentration of HAsO42- is approximately the same as H3AsO4-. However, this is only an approximation and may not hold true in all cases.

H2AsO42- is considered amphiprotic because it can act as both an acid and a base. While the equilibrium constant for its reaction as an acid may be higher, it does not mean that it cannot also act as a base. The equilibrium constant for the reaction where it acts as a base may be lower, but it still has the ability to donate a proton and therefore is considered amphiprotic.
 

What is a diprotic acid?

A diprotic acid is a type of acid that has two acidic hydrogen atoms in its molecular structure. This means that it can donate two protons in a chemical reaction.

What is an amphiprotic substance?

An amphiprotic substance is a substance that can act as both an acid and a base. This means that it can donate a proton in an acidic environment or accept a proton in a basic environment.

Why can't the pH of diprotic acids be calculated the same way as amphiprotic substances?

The pH of a substance is a measure of its acidity or basicity, which is determined by the concentration of hydrogen ions (H+) in the solution. Diprotic acids and amphiprotic substances have different chemical properties and behave differently in solution, therefore their pH cannot be calculated in the same way.

How is the pH of a diprotic acid calculated?

The pH of a diprotic acid is calculated using the dissociation constants (Ka) for each of its acidic hydrogen atoms. These constants are specific to the acid and are used to determine the concentration of H+ ions in solution, which is then used to calculate the pH.

Can a diprotic acid also be amphiprotic?

Yes, a diprotic acid can also be amphiprotic, but not all diprotic acids are amphiprotic. For example, sulfuric acid (H2SO4) is a diprotic acid that can act as an amphiprotic substance, while hydrochloric acid (HCl) is a diprotic acid that can only act as an acid and not a base.

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