ph high on RODI water?

[acr0p0ra]

Hi all,
I was making some freshwater to dip a fish in and i checked the PH on my RODI water. I have never done this before until now and I am getting a reading of 8.6. My tap water is 7.5. I also have 160 gallons of saltwater mixed at 1.024 and it is reading 8.2 which i expected to be higher.

I have checked my RO water to make sure there is no salt in it. It reads 1.000.

what could be causing this?

 

[acr0p0ra]

i should mention, this has been tested with 2 probes.

 

[Randy Holmes-Farley]

It may be high, or not, but it doesn't matter. Mine read high too, but I do not know if that was accurate. The pH or RO/DI water is unimportant and not accurately measured.

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

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).

 

[acr0p0ra]

Thank you for the reply. I am more concerned about it because when I am freshwater dipping fish that are in 7.8-8.2 I want to make sure the PH matches.

 

[Randy Holmes-Farley]

Thank you for the reply. I am more concerned about it because when I am freshwater dipping fish that are in 7.8-8.2 I want to make sure the PH matches.

IMO, pH matching is meaningless/unimportant in 0 ppm TDS RO/DI water.

 

[acr0p0ra]

I use a small amount of tap water to get the temp correct prior to dipping the fish. then i use a dechlor.

 

[Randy Holmes-Farley]

I use a small amount of tap water to get the temp correct prior to dipping the fish. then i use a dechlor.

The tap water is often high in pH as well.

 

[Drewbacca]

The mixed SW in my bin has a ph of 8.5 my RO water 10.0
I've been using kalk and my tanks ph is 8.7 now. I'm looking into setting up dosing pumps & 2 part so i can ween of the kalk. (Been hooked on it for years)
Does the high ph in my ro & SW mix mean I will have an easier time keeping good ph in my tank?
I noticed it go up when spring came and windows all opened but i also upped the kalk too.

 

[Randy Holmes-Farley]

The mixed SW in my bin has a ph of 8.5 my RO water 10.0
I've been using kalk and my tanks ph is 8.7 now. I'm looking into setting up dosing pumps & 2 part so i can ween of the kalk. (Been hooked on it for years)
Does the high ph in my ro & SW mix mean I will have an easier time keeping good ph in my tank?
I noticed it go up when spring came and windows all opened but i also upped the kalk too.

Not directly. The pH of the RO/DI water is not meaningful, and even if it read pH 7, adding it to reef tank water at pH 8.2 will likely raise (not lower) the pH of that water.

High pH of new salt water aerated in your home air suggests you may not have high CO2 levels, which can make the tank also run higher.