I obtain curve pH = f(Vb) with following code but now, i would have tangent for two points near Veq. I don't know the procedure with implicitplot.
I think the best way is to attribute the points to a function but dn't know how to do
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 suppose this problem is quite simple