A water is in equilibrium with the atmosphere (partial pressure of CO2 = 5*10^-4 atm) at 25 degrees celsius, and has a pH of 8.1. The total phosphate concentration is 0.001 M. No other weak acids or bases are present. How many millimeters of 0.5 M H2SO4 are needed to decrease the pH of 1 liter of this water to 6.0?