Water at 7 is neutral....but as I understand it...when Antifreeze hits 7...it's corrosive. I found this on another website

"Why is the Antifreeze pH Level Important?
Antifreeze is made with a number of corrosive chemicals, because these chemicals have a low enough freezing point to keep from freezing in the cold winter temperatures. However, these chemicals can themselves eat away at the metallic portions of the car. Over time, the antifreeze becomes more corrosive, and its pH level decreases, as the ingredients in it meant to inhibit the corrosive properties of the chemicals break down. As your antifreeze becomes more corrosive, it can begin to eat away at the metallic portions of your engine, causing damage that you will later need to repair. The pH level of your antifreeze should ideally be between 9.5 and 10.5 and definitely be above 8.3."

http://autorepair.answers.com/cooling-and-heating/learning-about-antifreeze-ph-levels