RODI pH - hmmmm

[alan mcilvried]

Might be a few quesitons and observations here if you want to pick through my mumbo jumbo.

 

I was checking pH on my new RODI water, comparing readings from a Aquarium Pharmaceuticals, Low pH test kit and my digital pH meter. Generally the digital meter is within 0.1 of my API tests for saltwater, so I expected them to be the same for the RODI water, but they were not. Here's what I got. (I don't normally measure freshwater pH.)

 

1) RODI with the AP kit measured a pH of 6.4 with the initial color. No wait time is indicated on the instructions.

2) The digital meter readings are ALL OVER THE PLACE. With saltwater, it zeros in very quickly and remains stable. In the RODI it started at about 7.45 and VERY slowly (like 5 minutes) dropped to 6.9. I stirred the water with the meter to make sure it was stablized and lo and behold, it continued to drop, all the way down to 6.25. When I stopped stirring, it started to rise again, but only back to 6.7.

3) I went and got some more water and tried again in a separate clean glass. With the meter, I started at 7.3 and dropped to 7.1 where it stopped. Stirred and got a big drop again, ultimately rising back to 6.9 when I stopped. API tested at 6.4 again for initial pH.

4) After screwing around for a little bit, I went to empty the vials and noticed that color of the first vial of AF had darkened to a color around pH 7.2 to 7.4 - up almost a full point from the initial color was. The 2nd vial was getting darker also.

 

Questions I'm pondering:

 

1) API card does not mention a need to wait on the directions for the Low pH card color match. Should I be waiting 5 minutes like Saltwater? Color definitely changed over time.

2) Why would pH drop by stirring the meter around in the RODI water? Could it be CO2 creating carbonic acid from the air? Seems like a fairly extreme response. And why would it go back up again when I stop stirring? I don't see the same response in saltwater or even tap water when I stir them.

3) I was expecting RODI water to be almost exactly pH 7.0. No matter what I measure by, it's not that, but I guess I don't know what it is either. (My tap water is around 7.6 by both tests)

 

I got the meter because I got tired of not being able to do the color match. You know. "It's pink....well, maybe the purple one........no I'd definitly say it's the red one.......you think? I'd say it's more orange ....."

 

Al

[dshnarw]

Might be a few quesitons and observations here if you want to pick through my mumbo jumbo.

 

I was checking pH on my new RODI water, comparing readings from a Aquarium Pharmaceuticals, Low pH test kit and my digital pH meter. Generally the digital meter is within 0.1 of my API tests for saltwater, so I expected them to be the same for the RODI water, but they were not. Here's what I got. (I don't normally measure freshwater pH.)

 

1) RODI with the AP kit measured a pH of 6.4 with the initial color. No wait time is indicated on the instructions.

2) The digital meter readings are ALL OVER THE PLACE. With saltwater, it zeros in very quickly and remains stable. In the RODI it started at about 7.45 and VERY slowly (like 5 minutes) dropped to 6.9. I stirred the water with the meter to make sure it was stablized and lo and behold, it continued to drop, all the way down to 6.25. When I stopped stirring, it started to rise again, but only back to 6.7.

3) I went and got some more water and tried again in a separate clean glass. With the meter, I started at 7.3 and dropped to 7.1 where it stopped. Stirred and got a big drop again, ultimately rising back to 6.9 when I stopped. API tested at 6.4 again for initial pH.

4) After screwing around for a little bit, I went to empty the vials and noticed that color of the first vial of AF had darkened to a color around pH 7.2 to 7.4 - up almost a full point from the initial color was. The 2nd vial was getting darker also.

 

Questions I'm pondering:

 

1) API card does not mention a need to wait on the directions for the Low pH card color match. Should I be waiting 5 minutes like Saltwater? Color definitely changed over time.

2) Why would pH drop by stirring the meter around in the RODI water? Could it be CO2 creating carbonic acid from the air? Seems like a fairly extreme response. And why would it go back up again when I stop stirring? I don't see the same response in saltwater or even tap water when I stir them.

3) I was expecting RODI water to be almost exactly pH 7.0. No matter what I measure by, it's not that, but I guess I don't know what it is either. (My tap water is around 7.6 by both tests)

 

I got the meter because I got tired of not being able to do the color match. You know. "It's pink....well, maybe the purple one........no I'd definitly say it's the red one.......you think? I'd say it's more orange ....."

 

Al

 

 

 

It is exactly as you suspected - introducing even minimal amounts of CO2 into the water will drastically drop the pH of RODI since there is nothing to buffer the effect. The pH of RODI at room temperature eventually bottoms out and stabilizes below pH 6. The more you introduce air, the faster that process will happen.

 

Given your tap water pH, it wouldnt surprise me that your RODI comes out above pH 7 and begins to fall.

 

I wouldn't rely on any test of your RODI water for pH until it's had a chance to sit for a long while. And then, you're really looking for something around pH 6.

[Highland Reefer]

More information is available in Randy Farley-Holmes article, Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria

 

http://reefkeeping.com/issues/2005-05/rhf/index.php

 

 

From this article:

 

Final Effluent pH

 

Aside from the issues discussed above concerning the effluent

Edited February 4, 2009 by Highland Reefer