Question: from implicit to function

Hi,

I obtain  curve pH = f(Vb) with following code

restart;with(plots);

 Vtot := V0+Vb+Vind;h := 10^(-ph);C0 := 10^(-2);Cind := .83;Vind := 0.1e-1;V0 := 10;Ka1 := 10^(-4.8);Ka2 := 10^(-9);Cb := 0.1e-1;Kaind := 10^(-8);

eq := h+Cb*Vb/Vtot = 10^(-14)/h+C0*V0/(Vtot*(1+h/Ka1))+Cind*Vind/(Vtot*(1+h/Kaind));

 implicitplot(eq, Vb = 0 .. 40, gridrefine = 3, ph = 0 .. 14, numpoints = 400, gridlines, tickmarks = [spacing(2), default], thickness = 2);

I would like now :

- obtain a function ph = f(Vb), trace it by plot,

- obtain the curve diff(ph(Vb) = f'(Vb);

- obtain the tangent on any point

But i can't transform implicit plot in function Vb -> ph

I've tried with PH:=solve(eq,ph) but don't succeed.

any suggestion welcomed

Phil