S(-2) speciation

[Szasza]

Hi,

I need some helps.

I made equilibrium model, and I calculate (from molality) the individual speciations how many percent from the total.
For example:
Ni [mol/L] = 6,95E-08            
Ni+2 [mol/L] =  4,46E-08  --> Ni2+ [%] = 64,2
NiSO4 [mol/L]  2,49E-08  --> NiSO4 [%] = 35,8
It gives 100%.

But the problem is with S(-2).
S(-2) [mol/L] = 1,11E-02
S5−2 [mol/L] = 2,22E-03 --> S5−2 [%] = 20,00
S4-2 [mol/L] = 1,04E-11 --> S42- [%] = 9,32E-08
This never gives 100%. Why? Where is the other 80%?
 S(6) speciations give 100%...

Could somebody help me?

Thank you!

Best regards,
Szasza

[dlparkhurst]

I suspect you are not accounting for the stoichiometry of the species. There are 5 moles of S in S5-2 (not that I am much of a believer in the polysulfide species anyway).