Can I Use Rain Water In Plating Tanks

A. That's an interesting question. Due to the phenomenon of acid rain, we know that rain water is not necessarily pure, but it seems likely that it would be most of the time.

Picking out a page that looks clever from a list of google search results:
http://www.whollyh2o.org/rainwater-stormwater/item/122-rainwater-quality-and-filtration.html
tells us things like:

"The water in a raindrop is one of the cleanest sources of water available. Rainwater can absorb gases such as carbon dioxide, oxygen, nitrogen dioxide, and sulfur dioxide from the atmosphere. It can also capture soot and other microscopic particulates as it falls through the sky. Nevertheless, rainwater is almost 100% pure water before it reaches the ground. ... Although rainwater can be contaminated by absorbing airborne chemicals, most of the chemicals present in harvested rainwater are introduced during collection, treatment, and distribution. By properly designing and operating your rainwater harvesting system, you can minimize your exposure to a variety of chemical contaminants"

"The degree to which rainwater needs to be filtered depends upon its end use, and how long the water will be stored. For simple rain barrel systems that use gravity to supply a hose bib, an inlet screen that filters large leaf debris is all that is needed, provided the water is used within several weeks. For more complex systems which supply drip irrigation, indoor plumbing fixtures, or water for potable uses, the rainwater will need to be filtered to a higher degree."

Somehow I suspect that collected rainwater might not be up to the standards of DI water, but may well be easier to convert to DI water than city water generally is.

A. You should check the conductivity of your rain water. If it is less than 5 microsiemens, then you can probably use it in place of purified water. If it is higher conductivity than this, then as Ray suggested, it will not cost much to deionize it.

If it is stored in an unlined concrete tank, it will also pick up some ionic contaminants from this. Biological contamination is also of concern, so you will probably need to use UV or ozone for disinfection.