Why is sodium sulfate a neutral salt [duplicate]








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[Interesting ionization of sulfuric acid](/questions/16812/interesting-ionization-of-sulfuric-acid)

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When I have a solution of $\ce{Na2SO4}$, the sodium sulfate should ionize:
$$\ce{Na2SO4(aq) -> 2Na+(aq) + SO4^{2-}(aq)}$$


In my mind, the sodium ions should just float around, but because $\ce{HSO4-}$ is a weak acid:


$$\ce{HSO4- (aq) + H2O <-> H3O+ (aq) + SO4^2- (aq)}$$


The sulfate ion should be a weak conjugate base, and hence the sodium sulfate salt should be a weak base. This was my line of reasoning, until today my friends told me it is actually a neutral salt, which was confirmed by searching online. So why is it a neutral salt? (i.e. what's wrong with my reasoning?)




For a solution of $\ce{Na2SO4}$, the sodium sulfate will ionize as you indicated:  




> 
> 
> > 
> > $\ce{Na2SO4(s) ->[aq] 2Na+(aq) + SO4^{2-}(aq)}$
> > 
> > 
> > 
> 
> 
> 


This reaction is essentially irreversible in that the ions in an unsaturated solution won't suddenly form solid $\ce{Na2SO4}$ again. 


$\ce{HSO4-}$ is a not a weak acid, but has a $\text{pK}\_\text{a2} = 1.99$ which makes a moderately strong acid. So for pH > 3 essentially all the $\ce{HSO4-}$ will ionize:



> 
> 
> > 
> > $\ce{HSO4-(aq) + H2O ->[pH > 3] H3O+ (aq) + SO4^2- (aq)}$
> > 
> > 
> > 
> 
> 
> 


Thus $\ce{SO4^{2-}}$ is such a weak base that reverse reaction doesn't happen when you dissolve $\ce{Na2SO4}$, and a solution of $\ce{Na2SO4}$ in distilled water stays neutral rather than becoming basic. 



> 
> 
> > 
> > $\ce{\require{cancel}H2O + SO4^2- \cancel{->} HSO4- + OH-}$
> > 
> > 
> > 
> 
> 
> 


To think about a different example acetic acid is a weaker acid, $\text{pK}\_\text{a} = 4.76$, so the acetate anion is a stronger base than the sulfate anion. A solution of sodium acetate would become weakly basic. 


One last point - the statement should be "sodium sulfate a neutral salt *in aqueous solution.*" In other solvents sodium sulfate could act as a base. 


 2

Here is the somewhat weak explanation given by [Wikipedia](https://en.wikipedia.org/wiki/Sodium_sulfate):



> 
> Sodium sulfate is a neutral salt: its aqueous solutions exhibit a pH
> of 7. The neutrality of such solutions reflects the fact that sulfate
> is derived, formally, from the strong acid sulfuric acid. Furthermore,
> the Na+ ion, with only a single positive charge, only weakly polarizes
> its water ligands provided there are metal ions in solution. Sodium sulfate reacts with sulfuric acid to give the acid salt sodium bisulfate.
> 
> 
> 


It definitely leaves some unanswered questions like what they mean by "metal ions in solutions": what metal ions (sodium ion?), what concentrations?


But the gist of it makes sense and makes a reasonable answer to me.


 1