You need to check the coolant with litmus paper
That checks the acidity or PH of the coolant,you need to be in the 7.0 range,thats neutral
Local parts stores may be able to help you there.
I know caterpilar has a solution that you pour into the coolant system and also has the litmus paper to test coolant systems.
That additive also helps with corrosion and keeps the cooling system happy
http://en.wikipedia.org/wiki/Litmus_paper