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My text asks  to calculate  pMg(transition) for transition of Eriochrome black T in a Mg- tritation ,using edta in a buffered solution,pH=10

I know the constant formation for my Indicator-complex is: logK (Mg-Ind)= 7.1
Eriochrome black T :  :  pk1=1.6 ; pk2=6 3 ; pk3 l= 11.6

Now I know that :
K(Mg-Ind)= [Mg]/[M']*[Ind']

SO when [ M']= [Ind']
log[Mg']=logK'(Mg-Ind)= logK(Mg-Ind)* α_Ind

Now for Eriochrome black T,  HI^2- is the complexing species

So  I need to calculate α_HI^2-= (Ka1 ka2 ka3)/ ([H⁺]³+ ka1*[H⁺]²+Ka1 ka2 [H⁺] + ka1 ka2 ka3)

Where [H+]= 10^-10

But using thos formula I don't get the correct answer (pMg= 5.4)

Some helps?
Thanks