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  #1  
Old 01/09/2008, 10:25 AM
dmilne85 dmilne85 is offline
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treating Ro/di water, who does this?

i just read some advice by Anthony Calfo and he said that ro/di water should always be treated and re-mineralized and airated before adding salt mix. i have never heard of this for saltwater, and im wondering if anyone out there does this? He stated that by adding the minerals back it will help stabalize the waters ph/alk...
  #2  
Old 01/09/2008, 11:03 AM
Randy Holmes-Farley Randy Holmes-Farley is offline
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Nonsense. That is not useful or desirable, IMO, unless the salt mix itself is missing something that you choose to supplement in the raw mix.
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  #3  
Old 01/09/2008, 11:30 AM
David Grigor David Grigor is offline
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I believe that is outdated info. Most common brands salts are designed now to be used with RO/DI and even say so in the literature.
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  #4  
Old 01/09/2008, 04:42 PM
manderx manderx is offline
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i've read his comments on this several times and he seems to think that the low pH of DI water brings down the pH of mixed saltwater, or the tank when added as topoff. from what i can tell he feels that when you add 2 liquids together, you simply get the weighted (by volume) average of the pHs.


one example from: http://forum.marinedepot.com/Topic21343-13-1.aspx
Quote:
the gist of the demineralized water issue is rather short and sweet. RO/DI water is neutral to acidic and very unstable (crashed to low pH easily). For you to get it to saltwater useful condition... it must be buffered op to an average mineral hardness. If you do not, then adding it raw to your system is a huge burden on your ALK, CA, etc as they are reduced (ameliorated really) by the demineralized what that is coming in. A bad dilution.

Yet so much of it can be avoided. Aerating raw RO or DI water offgasses nitric acid that is lowering the RO water pH. Then adding back good minerals to buffer the demineralized water will be less of a burden on display water when they are averaged.

Using raw RO or DI water is a common cause for flow ALk or Ca in aquaria.


whenever someone says he's wrong and points him to a chemistry thread here, he says this:
Quote:
I do not see how Randy and I disagree here much if at all.
  #5  
Old 01/09/2008, 04:49 PM
Randy Holmes-Farley Randy Holmes-Farley is offline
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RO/DI water is never a cause of low pH in a reef aquarium, IMO.

I address the pH issue here:

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

from it:

http://www.reefkeeping.com/issues/20...hf/index.php#8

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).
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  #6  
Old 01/09/2008, 10:19 PM
Icefire Icefire is offline
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As soon as you add salt to the water it buffer itself out, it's total non sense what he say.
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