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Testing CO2 by sampling PH?

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7 Sep 2007
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Hi,
I'm struggling to remember how to measure CO2 concentration using a PH meter... I recall something about measuring a sample of aquarium water after 1 hour. If the PH has dropped 1 point then the concentration is 30ppm but I'm not sure if I've got that quite right... Anyone help fill in the gaps?
Thanks
 
Take glass of aquarium water and wait 24hours. Measure pH in the glass - this is baseline pH. Measure pH in aquarium every hour from CO2 ON until CO2 OFF. Ensure you have stable 1 pH drop throughout the photo period by adjusting CO2 injection rate and re-doing pH profile after every change. Make note of stabilised pH time. Adjust injection time to match stable 1 pH drop with time when lights turn ON.
 
Take glass of aquarium water and wait 24hours. Measure pH in the glass - this is baseline pH. Measure pH in aquarium every hour from CO2 ON until CO2 OFF. Ensure you have stable 1 pH drop throughout the photo period by adjusting CO2 injection rate and re-doing pH profile after every change. Make note of stabilised pH time. Adjust injection time to match stable 1 pH drop with time when lights turn ON.
and take pH at lights on time, I normally take pH every 30mins from lights on for couple of hours at least.

Once stable best to see how long it takes to hit target pH the next day after a full overnight degas, and light should not come on till target pH is reached, also not after a WC as that water will have a higher [CO2] to start with ;)
 
Thank you, both. My CO2 runs 24/7 (although planning to get a solenoid soon!). These are the readings I will take today:

EVENTPH
Removed tank water +24hrs7.2
At LIGHTS ON (1600)6.3
+30mins (1630)6.3
+30mins (1700)6.3
+30mins (1730)6.3
+30mins (1800)6.3
At LIGHTS OFF? (2300)6.3
 
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So... Given the above profile... I'm just under 1pH drop so can assume CO2 saturation is slightly under 30ppm? An, in theory, once a solenoid is fitted, it should read 7.2pH some time during the CO2 off period? All things considered... A good result?
 
in theory, once a solenoid is fitted, it should read 7.2pH some time during the CO2 off period?
Hi,
It might not reach PH7.2 if there is residual CO2 i.e. your aquarium hasn't fully degassed during lights/CO2 off (depending on whether you have skimmer, airstone etc running during this period which will affect this parameter). That is why we measure our starting PH in water that has been removed from the aquarium and degassed for 24 hours prior to taking a reading.
My PH only goes up from 5.9 to 6.6, but I only run an airstone for 1 hour during the light ramp down period due to bursting bubbles wetting the wall.
Cheers!
 
Event (note: Lily Pipe surface skimmer running 24/7)pH
Removed tank water degassed (after 5 hr) (tested every hour until pH stopped moving)7.2
just before CO2 on (1430)6.8
Lights on (1600)6.4
+60min (1700)6.4
+60min (1800)6.4

I just did my pH profile. coincidentally, my degassed water is the same pH at 7.2 but it seems that I have plenty of residual CO2 (or added CO2 from plant respiration?)
I'm reasonably happy that CO2 is stable because at the same time, the mist level is manageable. I'll try to increase CO2 a little bit more to see if I can get pH to 6.3 on lights on.
 
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@erwin123 good to see you did the glass test till it stopped moving :thumbup:

I think to hit the 1.0 drop

I have two pH probes, it I calibrate both with same solutions then take a reading unless its 4.0pH or 6.89pH ( the reference pH of the test buffer solitons) they will show different pHs slightly so the 1.0pH drop is a guide not an absolute IMO, I go off the DC (Drop Checker) Colour mainly and watch the livestock carefully once I enter the yellow colours of the DC, Have had the DC nearly clear at times and livestock I had was fine
 
Event (note: Lily Pipe surface skimmer running 24/7)pH
Removed tank water degassed (after 5 hr) (tested every hour until pH stopped moving)7.2
just before CO2 on (1430)6.8
Lights on (1600)6.4
+60min (1700)6.4
+60min (1800)6.4
2100 (at 1900 increased CO2 a tiny bit)6.3
2200 CO2 off6.3
2235 lights off6.3

Noted on the +- accuracy for pH probes and always good to doublecheck the livestock.

After getting a steady 6.4 I increased CO2 a tiny bit and achieved a steady 6.3.

30 minutes after CO2 was turned off, pH was still 6.3 suggesting that plants weren't utilising that much CO2 by then. The Rotala Wallichis had already closed even though this was 'just' a 6.5 hr photo period. I could probably 'save' some CO2 by switching it off at 2150 instead of 2200 (as long as its still pH 6.3 at 2230 I guess).
 
After getting a steady 6.4 I increased CO2 a tiny bit and achieved a steady 6.3.
Which may affect the CO2 on time also - may need a little longer - looking good all the same.

I do tend to cheat a little when doing a pH profile with my PLC, I can set it up very easy so once light on if the pH creeps up a little I can have say the CO2 on a repeat cycle of on for 12mins then off for 2 mins or whatever time scale suits, find it easier than adjusting the injection rate (BPS)
 
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