Sliding Ph and RODI water

BaliReefBox · Nov 19, 2023

I live in the tropics and have a open area where the tank is so I dont think CO2 build up is an issue as the doors are open all day and the room isnt sealed ...Ph was always in the range of 8.2-8.4... I hadnt tested it for a long time and randomly decided to check it. it was 7.8.. I tested my ato top off rodi water from the new rodi machine and it was a low 6.8. I put and air bubbler hose into the storage tank and now have top ato off water at ph 7.4-7.6 after 48hrs of aeration ...did a water change today and after mixing with the salt i tested the ph and it was 8.2-8.4.
I have ordered some co2 scrubbing media and will put that on the skimmer air inlet to try and get a bit of ph back...

any thoughts on how to get the ato top off water up to a decent ph level.. keep it bubbling away or try and add a buffer?

redfishbluefish · Nov 19, 2023

Pure water has no buffering ability, so simply looking at it cross-eyed will give you a wonky value. That said, a typical test of CO2 in your source water is a pH test initially when the water is drawn, and then after a day or after bubbling. Another indication is how quickly you go through DI resin. CO2 in your source water will very quickly consume your DI.

Here's where I'm having an issue.....simply letting the RO/DI water sit, the CO2 will leave the water raising pH. So by the time you put it in your tank, it won't be there, so no pH effect. I'm thinking errors in reading/dirty probe/interference by other equipment close to the probe/etc.

Randy Holmes-Farley · Nov 19, 2023

Agreed. The pH of 0-1 ppm TDS Ro/DI water has zero impact on a reef tank pH. It also is not accurately measured by your meter.

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com

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).