CO2SYS Marine Tank Table for PH/DKH/CO2

josephxsxn

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Some of you may have seen a planted tank table I have passed around that shows the relationship between CO2, DKH and PH. Personally this table has really helped me understand my system better hence my attempts to share it. By having my DKH supplement on a doser, skimmer set schedule with outdoor air (I assume its more stable with co2 concentration then indoor air) and a PH monitor I developed insights over repeat observations. When looking at the tank I have been able to correlate increased DKH consumption (and hence lower total tank DKH) and my PH being low together almost as a simple alert. My best guess is that because I have become in tune with my tanks PH range on a normal day and if the DKH is low you can see it in the PH. Anyhow I bring this up because I found it both fascinating and important to realize you can gain an understanding of your DKH based on PH.

@Randy Holmes-Farley let me know that because the table I was sharing was for a freshwater planted tank that it was incorrect. To try and put correct information out there I used the NOAA CO2SYS application to input the proper DKH (alkalinity) and PH along with other constants such as temperature and salinity. Using Python I put together a simple script (shared below) that uses CO2SYS and loops over a set of values for PH and DKH to calculate the CO2.

This application is well beyond my knowledge and hopfuly someone out there can help sanity check the values in the table! If you see something wrong please let me know.

Constants Used
  • Salinity 35 PPT
  • Temperature 25C
  • Silicate 50 umol/kg
  • Phosphate 4 umol/kg
  • PH Scale (2 Tables, see tabs)
    • NBS/NIST
      • What Hobby Probes use and what most of us refer to as PH
    • "Free" i.e. pH=−log10[H+]pH=−log10⁡[H+].
Instructions for PH CO2 Depression diagnostic
  1. Read Randy's Article on PH
    1. Perform "The Aeration Test" found here as independent verification of high CO2
  2. Gather inputs from your tank
    • DKH and PH
  3. Identify Intersection on table
    • PH across the top and DKH on far left side. The intersecting values are the tanks current CO2 ppm
  4. Internet Results (coming soon)


Table for Marine Tanks generated with CO2SYS

CO2SYS @ NOAA

CO2SYS Python Library

Script to generate table (work in progress, very MVP right now)

pH And The Reef Aquarium
 
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Aquarium Specialty - dry goods & marine livestock

taricha

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Woohoo!
I'm so glad someone is doing this. I've used the CO2SYS excel workbook, but I don't normally work in excel, so it's a pain for me to use an excel-running machine just for this.
Also I found out later I was using the wrong pH scale constants (yes there are multiple pH scales).

quickly checking against the graphs in Randy's Low pH Causes article
...it looks like your table generates CO2 values that are too low?

Figure2.jpg


"Elevated CO2" = 1000ppm
"Normal CO2" = 350ppm
3 meq/L = 8.4 dKH

maybe post your parameters used in the spreadsheet: the K's and pH scale etc.
 

Randy Holmes-Farley

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josephxsxn

josephxsxn

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The CO2SYS application allows for me to pick from 3 PH scales, is one of these better for us then another for our needs here? Total, Seawater, Free or NBS scale. Based on the presentation it doesn't seem like any of these will match the NIST standard.

Right now the tables were generated using "Total".
1608664742518.png



There are some other options way out of my knowledge base.

k1k2c = 4 # Choice of H2CO3 and HCO3- dissociation constants K1 and K2 ("4" means "Mehrbach refit")
kso4c = 1 # Choice of HSO4- dissociation constants KSO4 ("1" means "Dickson")
 

taricha

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These are what were recommended to me...
Just a suggestion: For better accuracy, try the NBS sale and the GEOSEC or Peng, et. al. constants. It will make a significant difference.
 
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josephxsxn

josephxsxn

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These are what were recommended to me...

I recalculated the table in the first post to use Free. It does seem much closer. When using Total {PH: 8.3, DKH 8.4, CO2 257ppm}, with Free {PH: 8.3, DKH 8.4, CO2 356ppm}, with NBS {PH: 8.3, DKH 8.4, CO2 388ppm}

Free seems closest to Randys values.
{PH: 8.3, DKH 8.4, CO2 356ppm}
{PH: 7.9, DKH 8.4, CO2 1079ppm}
 
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josephxsxn

josephxsxn

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I think NBS and NIST pH scales are the same. The old NBS (national bureau of standards) got renamed NIST in 1988.

That is what hobby reefers use.


Good old name changes :) Thank you for pointing that out. Included a tab so we have both Free and NBS tables in the first post. It seems to me that for our hobby level needs either of these is probably fine, not even sure exactly what I would do with the co2 information in the tables other then educate people more on the relationship. :D
 

Randy Holmes-Farley

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Good old name changes :) Thank you for pointing that out. Included a tab so we have both Free and NBS tables in the first post. It seems to me that for our hobby level needs either of these is probably fine, not even sure exactly what I would do with the co2 information in the tables other then educate people more on the relationship. :D

One thing it is good for, which I have done more crudely, is to tell people that a drop of about 0.3 pH units is a doubling of CO2 in the water and hence potentially in the air if it is being driven by high CO2 in the air, and that folks with pH down in the mid 7's have a serious CO2 elevation in their home air that might even impact their health.
 

taricha

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not even sure exactly what I would do with the co2 information in the tables other then educate people more on the relationship. :D

Sometimes people post concerning pH numbers and the Alk that goes with it implies a really unlikely CO2 level.

Which tells you the pH probe / calibration is most likely wrong.

...or dangerously high CO2 as Randy says.
 
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Randy Holmes-Farley

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A bunch of folks use CO2 meters to gauge their indoor air. I'm not sure how accurate any given model is, but a comparison of indoor air to outside air could be interesting for many folks. :)
 

taricha

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I have and occasionally use a CO2 meter. It will let you calibrate it with outdoor air. I have no idea how accurate it is when it measures high indoor CO2 in a room with people in it, but the elevated CO2 and the pH drop of the tank water are both significant and easily measured.


I don't think you'd be able to use the meter to detect errors in the table (like wrong parameter choices) but it would absolutely tell you that you aren't crazy and an observed pH drop would happen due to the indoor CO2 increase.
 

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