Edta is a hexaprotic system with the pka values: pka1=0.00 , pka2=1.50 , pka3=2.00 , pka4=2.69 , pka5=6.13 , and pka6=10.37 . the distribution of the various protonated forms of edta will therefore vary with ph. for equilibrium calculations involving metal complexes with edta , it is convenient to calculate the fraction of edta that is in the completely unprotonated form, y4− . this fraction is designated − . calculate − at two ph values.