# Mixing RO and tap water



## lbj (Mar 11, 2011)

I got my API test kit a few days ago and have been doing my best impression of a mad scientist testing all kinds of things. (Sort of an addictive sub-hobby. *w3) In the process I found that my tap is somewhat alkaline at pH 8.0. In fact, in doing some online research, I found that my city is aiming to keep pH for their water between 8.2 and 8.6. I decided to see what mixing in some RO water would do for me in lowering pH.

I found some gallon jugs of RO water hiding in the corner of my pantry. Literally hiding, they've been there a long time judging by the layer of dust covering them, and the "use by" date is Dec 07. The pH of this water tested 6.0 or lower, the API test kit doesn't go lower than 6.

So, RO water is 6 (or lower), tap is 8. Using just the API test tubes, I tried a 1:1 mix of RO to tap. Surprisingly (to me), the pH barely budged from 8. With a 2:1 mix RO to tap, pH dropped to 7.5 or so. With a 3:1 mix, it reads about 7.2.

In a nutshell, it looks like a large proportion of RO is required to lower pH a significant amount. Does this jibe with other member experiences? I've forgotten my high school chemistry, otherwise I'd be able to run the numbers and figure it out...


----------



## NeonShark666 (Dec 13, 2010)

Ph is related to the powers of ten and is not linear. A ph of 7.0 has ten times more acid (H+ ion) than 8.0. Also, RO water has a very unstable ph and can change easily. High ph water is very stable (highly buffered) and doesn't move down fast. The reading you are getting are very typical of water with a high ph. In place of RO water you can also use distilled. Your readings would be about the same.


----------



## phys (Feb 4, 2011)

+1


----------



## jrman83 (Jul 9, 2010)

Haven't had that experience myself. My tap is 8.2 and once I got my tank to about a 50/50 mixture my ph read almost exactly half the difference. My RO water ends out at 6.6 once it has sat for 12-18hrs. Knocks my hardness, gh and kh, in half also. I'm surprised the water you had read that low of a ph. Most that I've read tested RO water tested at 7.0. Still a very effective way to lower your ph and arguably the safest.


----------



## lbj (Mar 11, 2011)

Thanks for the replies. I was expecting my results to be more along the lines of what jrman83 said. I think my tap water may be somewhat hard and has a lot of minerals buffering the pH, making it more stable. (Pure speculation on my part of course, since I have zero understanding of the underlying chemistry.)

Just looked at my city's water report, and they quote total hardness as about 190mg/L avg, where 180 and over is considered very hard. I wonder if this is causing the pH to be difficult to change? If I need more than a 1:1 mixture of RO water to have a significant effect, maybe I'll just live with (and have the fish live with) the tap water pH. Stable pH more important than a specific pH, right?


----------



## jrman83 (Jul 9, 2010)

My water is very hard - well water. Yes, more important to be stable.


----------



## slurik (Dec 19, 2010)

ph is logarithmic, that said pH 6, is 10 times more acidic than ph 7, and 100 times more acidic than ph 8.0. This is the amount of hydrogen available in the substance. As it grows by factors of 10, you need to use a substantial amount of your tap water, to budge the RO water upwards. Or you'd need a load of ro water to bring tap water down respectively. Its a little non-intuitive I'll admitt, but its how she rolls. I'm not sure how the impact of your hardness will really fare against pH, I havent explored this aspect before in much detail, though it does have a small impact, not to the degree we're talking. Stability in pH is critical, though it is also equally as important to maintain a tolerable level. Tap water is usually fine for tropical fish. I'm not sure if you have a marine system or not... If you do have a marine system your salt mix will adjust your water appropriately to reach a pH 8.3 or so (Depending on your SG and salt brand)


----------



## lbj (Mar 11, 2011)

jrman83:

Must be kH then, we have different kH levels? I don't have a way to test kH, maybe I'll pick up a test kit.


slurik:

Thanks! It's a freshwater tank.


----------



## jrman83 (Jul 9, 2010)

My kh is 9-10 from my tap.


----------

