pKa = 3.5
c=10**(-linspace(0,6))
H=ConcentrationOxonium(c, Ka=10**(-pKa))
# Note: p(c) = -log10(c)
plot(p(c),p(H),lw=3,color='red',zorder=1)
plot(p(c),.5*(pKa+p(c)))
plot(p(c),p(c))
xlabel("pc")
ylabel("pH")