distilled water v R O water

here I normally only see de-ionized water in 1 L bottles, in the past distilled water was more common but due to car batteries being sealed for life mostly it is harder to get and due to lower sales prices go up, I can get larger amounts if I go somewhere more specialised but today I was talking to a place that does tropical fish and they sell very cheaply water treated by reverse osmosis, they call it R O water and they say it is pure and can be used instead of distilled in many things.

is this correct?

if it is pure enough is there an easy way to test?

they sell 25 liters for less than the local garage sells 0.75 liters of de-ionised or the cost of about 2 liters at the local large store, thought I would ask here

We use RO water on tap for rinsing glass before a DI rinse.

The RO still has ~ 100 ppm impurities on our conductivity meter before it goes through a series of resin columns to give us deionized water (which makes no AgCl upon adding to AgNO3).

RO is probably just fine for most uses, including silver digestion. It is not fine for making up samples or doing assay work. You need type 1 water/conductivity water for that.

Lou

thank you Lou

There are many different types of RO. How pure it is depends on the pore size of the membrane they push it through. The smaller the micro-membrane, the more expensive and time consuming the task.

For a fish tank they probably have the least pure of any RO. All they need is to remove the bulk of minerals, but mostly they probably pass it through an activated charcoal filter to remove VOC's and chlorine.

When I ran a glass lab we ran city water through a UV filter to kill anything living, then charcoal, then RO membranes, then a bank of ion exchange tanks, then polished it off with a continuously re-circulating bank of ion exchange tanks to yield 18 mega-ohm DI water. We used over 500 gallons of it a day just to clean glass plates with.

The accountants told me the final cost per gallon was about $5. Our vendor was in every week changing the ion tanks and monthly changing the UV lights. When the tanks got full you could really see the quality of our product deteriorate.