The mean-field theory describing the conformation of an ionizable polymer chain(weak polyacid or polybase) grafted at one end onto an impermeable planar surface and immersed into a water-salt solution is presented. The dependences of the brush thickness and degree of chain ionization on grafting density and ionic strength of the solution are analyzed. It is shown that the annealing of the brush affects the brush structure mostly at low ionic strength of the solution and relatively sparse grafting. The nontrivial effect of an increase in the brush thickness with decreasing grafting density and/or increasing ionic strength is predicted.