I'm probably gonna regret this but here goes; proper PH is 8.5. Anything above 11 and the coolant attacks aluminum and copper parts. Anything below 7 and the ferrous metals try to return to ore. (rust) I have never heard of plastics being affected by PH. That's one reason they're used to hold products that have extreme PH on either end of the scale, i.e. acid and soda. Also keep in mind that if you put in 8.5 PH anti-freeze and mix in water that isn't neutral, you undid everything you were trying to do.