determining alkalinity via pH and co2*

[orcafood]

Given that the pH of a reef tank is being read at a very accurate level, is it not feasible to determine the co2* gas dissolved in a reef tank via a small IR device, then plug and chug the salinity and temperature into equilibrium equations to get the K0, K1, K2, effectively giving the amount of HCO3- and CO3--?

https://www.co2meter.com/products/k-...iABEgIQQ_D_BwE

How about measuring the ppm of co2 gas dissolved in a head space only in contact with tank water. A relationship must exist between the headspace co2 and the amount dissolved into the tank, though it might differ a little bit from the expected Henry's laws.

[dartier]

Interesting idea. Too bad that the sensor is not able to directly measure the dissolved CO2 directly in water as opposed to the content in a gas. Perhaps Jonathon can say if the CO2 level (in a contained headspace) would be an equilibrium function like PH.

Dennis

[karimwassef]

yes. done it

http://www.reefcentral.com/forums/sh...alinity&page=7

Originally Posted by karimwassef

Here's the spread over the 5 variables: Temp, Salinity, Pressure, pH, and CO2



For each chart where one variable is being changed vs. CO2 to determine Alk, the fixed variables are based on the base case:


If this is right, it would be possible to make an alkalinity monitor that measures air pressure, water temperature, CO2 in the air, salinity and pH and constructs the dKH...

This would be a continuous monitor over time... All variables except for air pressure and CO2 are already being measured by my Apex.

There's an excel file on page 2

[orcafood]

I read the exact same paper haha

[bertoni]

If my memory is correct, there are meters to measure carbon dioxide in water samples, although I'm not sure that they are useful in saltwater. In theory, this can be done, but in practice, it's going to be costly or inaccurate, depending on how much you want to spend. I haven't looked into the technology for a while, though. There might be some better alternatives available.

[karimwassef]

I'm using atmospheric CO2 in the air going into my heavy air injector.
My formula uses atmospheric CO2 anyways.

[orcafood]

Excellent job on your alkalinity detection system Karim! I bet a kalman filter would really help to get a more stable value. Hmm or a machine learning algorithm would probably help too.

[karimwassef]

I think it works for me because of my air injector. I push gallons of air into my water constantly through a very fine bubble foamer. It's a dual penductor fed by outside fresh air through a high pressure pump. This means that the time variable for the diffusion of gas into water is reduced significantly. Basically, I force the assumption that I'm at steady state to be more true.

Air CO2 sensors are only ~ $100. You could, in theory, drive it from an Apex and have all the data you need.

Unfortunately, Apex won't let you enter a math formula that returns a function of known values but this app works for me: iFxCalc iFxCalc:

https://itunes.apple.com/us/app/ifxc...786018193?mt=8

It's clunky to figure out but easy once you do

The formula is
Alk=(2*D3*0.0334211*C3/29.41*0.000001*10^(-(-8.712-0.00946*F3+0.0000856*F3^2+1355.1/(((E3-32)*5/9)+273)+1.7976*ln((((E3-32)*5/9)+273))))/10^(-G3)*10^(-(17.0001-0.01259*F3-0.000079334*F3^2+936.291/(((E3-32)*5/9)+273)-1.87354*ln((((E3-32)*5/9)+273))-2.61471*F3/(((E3-32)*5/9)+273)+0.07479*F3^2/(((E3-32)*5/9)+273)))/10^(-G3)*44.01+D3*0.0334211*C3/29.41*0.000001*10^(-(-8.712-0.00946*F3+0.0000856*F3^2+1355.1/(((E3-32)*5/9)+273)+1.7976*ln((((E3-32)*5/9)+273))))/10^(-G3)*61.02)/2*1000/17.9

Where Alk(D3, C3, F3, E3, G3) has
D3 = barometric pressure (mmHg) = 30
C3 = CO2 (ppm) = 450
F3 = salinity (ppt) = 34.5
E3 = temperature (F) = 77.5
G3 = pH = 8.3

Returns 9.8 dKH

Can machine learning reduce the impact of measurement errors- probably but you'll need an accurate and frequent Alk measurement to feed into it.

[orcafood]

So you assume the CO2 is always the same? Or that the CO2 concentration is equal to that outside and you measure it? The CO2 concentration in the air must change with temperature and humidity?

What I am suggesting would help get more accurate results would be some form of anomaly detection or outlier rejection system but you would need to use a PC with an I/O board and some programming language.

[karimwassef]

I measure CO2 in the air. Since I use outside air, and heavily aerate with it, I don't see much change over time.

Using fresh air has been key to my stability. I still measure it though.

[orcafood]

Thanks for your time Karim!

[dartier]

Karim, do you use a calcium reactor? Just curious if your alkalinity calculation technique works in the presence of injected CO2 from things like CalRx?

Dennis

[karimwassef]

I don't. I dose Kalk.

However, the air injection should neutralize the effect of any CO2 that leaks through. I'm injecting gallons of fresh air. I wouldn't trust this method if I didn't.

For reference, I first used my injector when I was curing concrete in the tank. It took pH down from 11 to 8 ina couple of hours and kept it there as the concrete cured. That's when I realized the power of fresh air injection for stability.

[dartier]

Originally Posted by karimwassef

For reference, I first used my injector when I was curing concrete in the tank. It took pH down from 11 to 8 ina couple of hours and kept it there as the concrete cured. That's when I realized the power of fresh air injection for stability.

So as to not take this thread off topic, do you have any threads about your air injection system?

Dennis

[orcafood]

You only dose kalk? How does your pH vary over the day?

[karimwassef]

Yes. It's basically my skimmer injectors.. without the cup or column

I built a double penductors injected skimmer that's 12' tall. It was a beast. Now it's powered by the jebao pressure pump DCP-18000

http://www.reefcentral.com/forums/sh....php?t=2634328

http://www.reefcentral.com/forums/sh....php?t=2637568

http://www.reefcentral.com/forums/sh....php?t=2571877

[karimwassef]

My pH doesn't vary at all because my pH aligns to my Alk and I use a pH feedback loop to dose Kalk

Results speak for themselves

[orcafood]

This is exactly what I was planning on doing haha, all I really want to do is get my pH up and very constant, it was then when I started thinking about calculating alkalinity from the CO2. Amazing that your just doing kalk, I will have to give it a shot. You are using two pH probes though right? I was thinking about getting one of the really nice ones that goes out to hundredths or thousandths meant for continuous monitoring.

How much resolution does your pH setup have? your pH probably still varies a little but you can't detect it.

Very nice color and growth tips on those corals!

[karimwassef]

Yes. Must have two and look for volatility on either... stopping due to a false positive is better

[Horace]

I must admit I am a bit shocked you are able to dose enough kalk to maintain your alkalinity. I assume you are doing it via topoff? Unless you are able to dose the powder form, accurately, and in a way that does not cause issues, I found in the past I didnt evap enough water to allow for enough kalk to keep alk up on its own.

[karimwassef]

No. Dosing on a feedback loop based on pH that is pegged to my Alk.

[karimwassef]

I consume a lot of kalk. No vinegar. I run a hot tank so evaporation is very high anyway.

[dartier]

Karim, your CO2 monitor is monitoring the outside CO2 level or the level around your tank? If outside, I guess the amount of air you are pushing prevents an elevated indoor CO2 level from depressing the PH too much.

That you are able to do that in a controllable fashion is fairly impressive.

Dennis

[karimwassef]

I'm measuring the intake air since I'm forcing that in.

The air around the tank aligns.

I don't use the CO2 measure to run anything. It's just to confirm where I am.

[dartier]

Ah, so you are part of the feedback loop.

Dennis