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Chappy
09/10/2006, 04:10 PM
I just checked my RODI water and the pH is 8.9. I mixed my salt for a water change and it is still the same. What can I do to bring it down before I do the water change. The pH in the tank is 8.2.

Any suggestions?

Billybeau1
09/10/2006, 04:28 PM
Yes. Dont worry about the pH of freshly mixed water. Too many factors. As long as your tank is 8.2 you should be pleased. Many of us wish we had 8.2 this time of year.

And FWIW, I highly doubt your ro/di, before salt is added is 8.9

How are you measuring ?

Chappy
09/10/2006, 04:29 PM
I'm using an electronic ph meter. I havn't calibrated it in a while, but I think its close.

Billybeau1
09/10/2006, 04:34 PM
Hmmm, pH that high is almost always testing error. I'm surprised your tank reads 8.2 and your ro/di reads 8.9

Something is not right here :confused:

Is it possible you are picking up some electrical interferance where you are testing the ro/di water ?

Chappy
09/10/2006, 04:42 PM
Well, I just calibrated and checked again. Tap water is 8.6. RODI is 7.6. Tank is 8.0. Saltwater mixed about 8 hours ago is 8.6. I guess that's about right. I should have calibrated it sooner.

Billybeau1
09/10/2006, 04:44 PM
:thumbsup:

Randy Holmes-Farley
09/10/2006, 06:18 PM
FWIW, my RO/DI water has an "apparent" pH above 9, but I do not know if that is real or not. In any case, it will not push up the pH of the salt water too much.

I address the pH of RO/DI here:

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria
http://reefkeeping.com/issues/2005-05/rhf/index.htm
from it:


Final Effluent pH

Aside from the issues discussed above concerning the effluent’s pH when the DI resin becomes depleted, the final pH coming out of an RO/DI system should not significantly concern reef aquarists. Many aquarists with low pH problems have asked, for example, if their aquarium’s low pH may be caused by their replacing evaporated water with RO/DI water that they measure to have a pH below 7. In short, the answer is no, this is not a cause of low pH nor is it something to be generally concerned about, for the following reasons:

1. The pH of totally pure water is around 7 (with the exact value depending on temperature). As carbon dioxide from the atmosphere enters the water, the pH drops into the 6’s and even into the 5’s, depending on the amount of CO2. At saturation with the level of CO2 in normal (outside) air, the pH would be about 5.66. Indoor air often has even more CO2, and the pH can drop a bit lower, into the 5’s. Consequently, the pH of highly purified water coming from an RO/DI unit is expected to be in the pH 5-7 range.

2. The pH of highly purified water is not accurately measured by test kits, or by pH meters. There are several different reasons for this, including the fact that highly purified water has very little buffering capacity, so its pH is easily changed. Even the acidity or basicity of a pH test kit’s indicator dye is enough to alter pure water’s measured pH. As for pH meters, the probes themselves do not function well in the very low ionic strength of pure freshwater, and trace impurities on them can swing the pH around quite a bit.

3. The pH of the combination of two solutions does not necessarily reflect the average (not even a weighted average) of their two pH values. The final pH of a mixture may actually not even be between the pH’s of the two solutions when combined. Consequently, adding pH 7 pure water to pH 8.2 seawater may not even result in a pH below 8.2, but rather might be higher than 8.2 (for complex reasons relating to the acidity of bicarbonate in seawater vs. freshwater).

Chappy
09/11/2006, 05:33 PM
Very nice. I love reading those articles. It brings me back to my 6 semesters of chemistry in college. Then I remembered the best thing about chemistry class was the the minute after you turned in your final exam!

Thanks for the explaination. I do appreciate it.

Randy Holmes-Farley
09/12/2006, 06:31 AM
You're welcome.

Happy reefing. :)