The assumption in the chart is that the CO2 in the air is in equilibrium with the water. The point was to make it easy for hobbyists to see what the CO2 concentration in their house does to their pH.So CO2 in the air does not change the pH directly. CO2 in water does. CO2 in air will only tell us which way the CO2 is moving, in or out, compared to the pCO2 in the water. Secondly, at dKH of ~7.5, natural seawater, the pH is 7.9 at 400 uatm pCO2 using CO2sys. I think your calculations are off
Adding skimmer to raise PH?
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mook1178
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That assumption is incorrect in my experience though. Rarely are our CO2 levels equilibrated. That is why the mantra is to pull water from the tank an measure the pH after an hour. High bioloads will elevated CO2 in the water. Coral and macro algae heavy tanks will have lowered CO2. High CO2 in the house compared to outside is going to lower pH and vice versa. However, if you are wanting to correct pH by looking at CO2, the pCO2 of the water is more important than the ambient air concentration.
This also does not even cover that the calculations are off, by a fair amount. According to your chart, the pH of typical Seawater at 400 ppm is just under 8.2. That is a large difference from 7.9.
The only reason I bring all this up, is that someone will see that chart and take it for gold. Your assumptions were not pointed out in your original post and the calculations are off. The typical reefer could look at your graph and expect a pH. That pH would be wrong.
Uncle99
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After years of playing the PH game and dumping tons of dollars, I quit this game and have not paid any attention to the PH in two decades.
And NOTHING changed. Same growth same colour or at least, what my eye sees.
Glad I got off that crazy train.
And NOTHING changed. Same growth same colour or at least, what my eye sees.
Glad I got off that crazy train.
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So CO2 in the air does not change the pH directly. CO2 in water does. CO2 in air will only tell us which way the CO2 is moving, in or out, compared to the pCO2 in the water. Secondly, at dKH of ~7.5, natural seawater, the pH is 7.9 at 400 uatm pCO2 using CO2sys. I think your calculations are off
Natural seawater does not have an alkalinity of 7.5 dKH except in highly saline areas like the Red Sea.
So I could include a full writeup of the assumptions and calculation basis when I posted it, but the vast majority of people won’t have the interest or aptitude to dig into it.
The assumption that the water is at equilibrium with the CO2 and air is not an incorrect assumption, it’s simply an assumption that serves the basis for the chart. Everyone’s tank pH is dynamic with photosynthesis and the fact that CO2 is not constant. The assumption was made for simplification.
I actually don’t agree that the calculation in the chart is off, but I don’t mind being proven wrong. I am familiar CO2sys though I don’t have it downloaded anymore. Great tool, but like most models the inputs are key. Our oceans are not currently pH 7.9, ocean alkalinity is not 7.5 dKH. I don’t think I understand all the assumptions you plugged into CO2sys.
If you consider published ocean alkalinity, pH, and atmospheric CO2 concentrations, I think you’re you’ll find the chart matches published data reasonably well.
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A simple airline connected to a skimmer raised my daily pH from 7.6 - 7.9 to 8.0 - 8.3 according to my Milwaukee monitor but it isn't a long run of hose as the tank is next to an outside wall so i drilled through the wall.
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A few comments here, but notably, such graphs are very useful for what CAN be attained by aeration, not for saying what tank pH will be (unless fully aerated).
As noted above, your seawater alk is not accurate.
Also, there are different pH scales used in seawater. One has to be careful because reefers use an NBS scale that ordinary labs use, while chemical oceanographers sometimes use a different scale (sometimes called the SWS scale).
In order to make the chart, pCO2 is determined. But hobbyists don’t know what to do with that so the data is plotted using CO2 in air so people can understand it.
Ac = (K1KHPCO2 / [H+]) + 2(K1K2KHPCO2 / [H+]2)
mook1178
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Oceanographers use total pH scale, but that scale is not going to change by 0.3-4 pH units in conversion to NBS
my alkalinity input was 2300umol/kg which is typical SW alkalinity at 35 ppt. Dickson CRMs at typically 33 ppt are at 2200 umol/kg. These CRMs are taken from the SIO dock in La Jolla, Ca.
OCADS - Information on Batches of CO2 in Seawater Reference Material
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Oceanographers use total pH scale, but that scale is not going to change by 0.3-4 pH units in conversion to NBS
my alkalinity input was 2300umol/kg which is typical SW alkalinity at 35 ppt. Dickson CRMs at typically 33 ppt are at 2200 umol/kg. These CRMs are taken from the SIO dock in La Jolla, Ca.
OCADS - Information on Batches of CO2 in Seawater Reference Material
www.ncei.noaa.gov
The NBS scale is about 0.13 pH units higher than the seawater scale.
That 2.3 umol/kg alk is reasonable and is only about 6.4 dKH.
mook1178
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Well I can admit when I am wrong. I ran this at in CO2sys using K1,K2 from Millero 2010, pH on NBS. I used the Hamza reef to convert dKH to meq/L. That is a 1:1 conversion for mmol/L. I then converted to umol/KG. The pH at 400 uatm is similar to rishma's chart. But there is a large difference in the pH at higher pCO2 values.
Differences at high pCO2 might be due to uncertainties in the equilibrium constants that start to matter at high CO2. Maybe Randy knows.
Well I can admit when I am wrong. I ran this at in CO2sys using K1,K2 from Millero 2010, pH on NBS. I used the Hamza reef to convert dKH to meq/L. That is a 1:1 conversion for mmol/L. I then converted to umol/KG. The pH at 400 uatm is similar to rishma's chart. But there is a large difference in the pH at higher pCO2 values.
mook1178
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Those differences are for sure errors associated with K1 and K2. Which paper did you pull yours from? There are many to choose from in the literature and really depends on what the salinity range.
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I've not really looked into such details, but in general, the farther one gets from the data that supported assumptions, constant determinations, and even the actual equations, the more minor issues grow in magnitude and become substantial. For example, at high alk and high CO2, the chemical speciation of various very minor players may become more significant and play a role. For example, calcium is mostly Ca++ in seawater, with smaller amounts of the ion pairs (sort of temporary molecules and ions) of CaSO4, CaHCO3+ and CaCO3. But one might expect different amounts of the latter two species as CO2 and alk deviate from normal, and that may possibly impact these sorts of curves, depending on how these minor issues are accounted for (if at all).
While I know some folks will want them perfectly accurate, I think the curves are mostly useful for showing the relationships and shapes of curves. Whether the pCO2 curve at 1300 utam should really be where the 1100 uatm curve is would have no impact on 99.99% of reefers looking at such a graph.
I don’t recall but I do remember there being a lot of choices. I tried a number of published constants and the variations didn’t ultimately impact the point I was trying to make. Next time I’m home I could find the spreadsheet. I originally shared the graph without gridlines so people didn’t get too fixated on specific values. Miami Reef asked me to add the grid lines so I obliged. This thread reminds me why I originally left them off
fryman
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So many factors affect pH that IMO any individual's report may not be applicable to your tank. There's a ton of theory in this thread from ppl who understand the science better than I, so I'm not sure I can add to that aspect of the discussion.
I'm just a humble engineer but have done a lot of pH testing on my own display. Baseline was night-day swing of 7.8 to 8.2ish. Alk of approx 8.5. Outside air CO2 is 350-450 ppm, inside 400 to 1000 ppm. My skimmer is a Simplicity 240DC, total display volume ~120gal. I have two pH probes (Apex and Milwaukee) and calibrate them monthly.
- Running a skimmer pulling inside air had no significant impact on pH.
- Running a skimmer that pulls outside air had a detectable but modest increase in nightime pH, the nightime minimum increased to approx 7.9
- Running skimmer air intake through about 3ft of 2in PVC pipe filled with soda lime had a very significant impact on pH, and I was able to "peg" pH at ~8.3 all night long. It worked both using inside or outside air. Doing it in a "recirculating" setup allowed my soda lime to last 2 or 3 times longer. However even in recirculating mode, the soda lime was eventually consumed and then stopped working. I decided this way was not cost-effective for me.
- Using a powerful air pump placed outside (rated for 80lpm) to force more outside air into the skimmer increased nightime minimum pH to almost 8.0
- Using the same 80lpm air pump outside with large Pentaire airstones placed in a covered sump I am able to peg pH at ~8.3 all night. This is the most cost-effective method that I've found.
Hope this helps.
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