and another site states

"Extensive testing has shown that a coolant pH below 8.3 pH is not acceptable for use in engines due to its corrosive nature. The correct pH value should be maintained between 9.5 - 10.0 pH. Below 9.0 pH it is advisable to flush the cooling system and refill with a new coolant solution."

http://www.eutechinst.com/techtips/tech-tips10.htm