View Full Version : Possible for ro water to have an alk of <1?

09/21/2017, 05:56 PM
So I'm setting up my tank and just got my water system running and since I have well water I wanted to test the alkalinity and pH of my ro water so I could find out how much CO2 is in it but when I tested the alkalinity it tested really low .2KH in dKH. Is this possible? Before when I tested the ro water with this test kit I was getting a completely different result.

09/21/2017, 06:18 PM
That's probably below detectable results for the test. Why are you testing alk of RO water? What is the TDS of the RO water? That is what matters, and it should be very low (below 10, depending on some variables)

09/21/2017, 06:22 PM
I'm testing the alk of the ro water so I can use it to determine the co2 amount in my water (I use a well). The ph tested at 7.4. The tds after the ro membrane is 5. The tds going into the system is about 950, yes 950.

09/21/2017, 08:24 PM
KH is a measure of alkalinity of water caused by carbonates and bicarbonates. Stripping the water of dissolved solids, as an effective RO unit should, would of course strip it of those carbonates/bicarbonates also, resulting in 0 dKH. So in that sense, it's not only possible, it's rather part of the whole job of RO filtration.

RO water will absorb CO2 gas from the atmosphere until equilibrium (with Henry's law), bringing the dissolved CO2 up and the PH of the RO water down to approximately 5.6 (if I recall correctly). I don't quite remember *how* to work out exactly how much CO2 dissolves into water (without carbonates/bicarbonates) at regular atmospheric pressure/a certain temperature.

That said, I really don't understand the bigger picture of what you are trying to do, maybe an explanation would help us get you sorted.

09/21/2017, 08:42 PM
Alright so my water is well water. Well water tends to have high levels of co2.this co2 chews through resin very quickly. To determine the level of co2 ( in ppm) you can do a simple calculation using your alkalinity and the ph of your water. With that said I was trying to test the alk and ph of my ro water so I could then determine the co2 levels in my water. I am doing this to determine if I will need a degassing container to let the co2 levels equalize before I run the water through the di stage.
Hope that clears things up.

09/22/2017, 01:16 AM
I don't really have any experience with well water, but I have to figure somewhere in that formula you are either multiplying or dividing by the hardness (leaving you with 0 or an angry calculator respectively), which after RO filtration should be as close to zero as possible, as you are removing the carbonates/bicarbonates that determine water hardness. So, as you have probably experienced, that throws any use of that formula out as far as I can figure.

Hanna makes a dissolved CO2 test kit http://hannainst.com/hi3818-carbon-dioxide-test-kit.html, which, while at $23 is not nearly as cheap as a formula (but that isn't too expensive either for this hobby), and should yield an accurate result for the dissolved CO2 remaining in your well water after it has passed the RO stage(s).