Can't believe this thread is still going on. Like Michigan, the pH of Illinois rain water through the 1990's was around 4.4 +/-. A brief Google search suggests that it is better today, maybe around 5.5, but can get lower depending on how much local air pollutants it absorbs while coming down.
Using acid rain water to mix coolant seems like it would use up some of the buffering capacity, maybe reducing the effective lifespan in your engine. Just a hunch that needs to be confirmed. At least you saved a couple of dollars vs. buying 88 cents/gallon store water.