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is rainwater comparable to demineralised water?
As the title suggests, is captured rain water comparable to demin water?
My father used to catch rain water on the back lawn in plastic containers and always told me that it was just as good.
Was he right?
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Precipitation naturally condenses around impurities in the air, dust, ash, and other stuff. Downstream of power plants in the eastern USA we famously had/have acid rain. So the answer is that it depends on what it's condensing around and pulling out of the air.
Mark Barendt, Ignacio, CO
"The mind that opens to a new idea never returns to its original size." Albert Einstein
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Demineralised water is more pure than rainwater is more pure than tapwater.
If you live in an urban environment, it's closer to tapwater from absorbing carbon dioxide (turns it to weak carbonic acid) and etc from pollution in the air.
If you live in a rural place, it's much closer to demineralised water.
Either way, wait out the first 10 minutes or so of rain to 'wash out' the air, and then collect water after that. It's the cleanest you're going to get.
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 Originally Posted by quejai
Demineralised water is more pure than rainwater is more pure than tapwater.
If you live in an urban environment, it's closer to tapwater from absorbing carbon dioxide (turns it to weak carbonic acid) and etc from pollution in the air.
If you live in a rural place, it's much closer to demineralised water.
Either way, wait out the first 10 minutes or so of rain to 'wash out' the air, and then collect water after that. It's the cleanest you're going to get.
OK, so my father was kind of right, because we lived on a farm in a rural area, 150KM away from the closest city.
But for me....OK, I suppose I would also then need to consider filtration, which in the end might just end up being just as easy to buy the 4 litre bottles from the super market.
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Even if you live in an area with very hard water you can soften it by vigorously boiling tapwater for 5 minutes and allowing it to stand overnight. The temporary hardness (calcium ions) will precipitate out. The permanent hardness (magnessium ios) have no effect on photographic uses. The next day decant or filter off of the clear portion for use in developers.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.
~Antoine de Saint-Exupery
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I collect and use rain water, (we live a good distance from any cities), and have had good results. We have a lot of limestone in the water here, so I use the rain for a final rinse and with the wetting solution. I'm pretty sure the film is drying a lot cleaner. But, as a couple of people have noted, I wouldn't even think of it in a big city.
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I only drink distilled water, or rainwater, and only pure grain alcohol.
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Originally Posted by adelorenzo
I only drink ...... pure grain alcohol.
Fixed it for you! (OK, maybe I am thinking about me and maybe this is wishful thinking!)
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Another source for what is basically distilled water is the ice build-up you have to chip away from a freezer.
(De-frosting a non, frost-free freezer, of course). If you're in the city, I'd just buy the distilled bottled water.
I suppose the photography folks living in Bermuda have it easy, (for distilled/demineralized water),
as the Island's entire water supply is captured rain water.
Marc
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Acid rain falls everywhere. Rain water is hardly what I would consider pure chemically.
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