The TOTAL H+ concentration (numerical balance) is negative in alkaline systems, because the (true) concentration of OH- dominates over the (true) concentration of H+. This is because H+ and H2O are used as primary species, and OH- is used as a derived species (i.e., function of H+ and H2O, with OH- + H+ = H2O) thus with a negative H+ stoichiometric coefficient. Thus the total numerical mole balance (per kg water) for H+ is: totH+ = mH+ - 1 mOH- (m is molality). totH+ is negative when mOH- > mH+. For pure water at neutral pH, mOH- = mH+ and thus totH+ is zero. Note that the actual (true) concentration of any individual species can never be negative (e.g., here, mH+ and mOH- are always positive). See Reed (1982, Geochimica, 46, 513-528) for more details.