Unusual method of measuring alkalinity

[Sral]

Interesting, if I understand the manual correctly it says that it actually tops up the reference reservoir with tankwater if it gets low. "For daily operation, it will adjust water level to target level automatically." in the section about doing waterchanges to the reservoir. There's no inlets or outlets except the tankwater inlet and the entirely separate doser.

And for the reservoir waterchange it then presumably pumps the reservoir content into your tank? That's not nice for nano or pico tanks, but then again neither is returning a pH 4 sample from a titrator.


Bit curious how that fits with the claim that evaporation effects on calibration can be neglected.

But who knows, the whole thing seems like one big feedback loop, maybe it all finds an equilibrium longterm

If I can get the pH parts I have on hand to give at least 0.002 precision with some averaging and stuff then I think I'll build one, only thing I'm missing is tiny airstones and an extra air pump I can build into it for recirculation

Nice info !
This suggests to me that one could improve long-term stability massively by introducing a RODI water cleaning back to the tank and ATO process into the reference.
That way one could probably also use room air for the aeration, as the increased evaporation losses can be offset with the ATO. Additionally one would not need to clean the salt from the air pump.
I wonder why they didn’t go that route, when the device is already at 800$. Suggests to me that I’m overlooking something.
Does alkalinity evaporate with the water when one aerates a sample for a (very) long time, as it seems to do with salt (the reason for cleaning the pump) ?

 

[Dennis Cartier]

Nice info !
This suggests to me that one could improve long-term stability massively by introducing a RODI water cleaning back to the tank and ATO process into the reference.
That way one could probably also use room air for the aeration, as the increased evaporation losses can be offset with the ATO. Additionally one would not need to clean the salt from the air pump.
I wonder why they didn’t go that route, when the device is already at 800$. Suggests to me that I’m overlooking something.
Does alkalinity evaporate with the water when one aerates a sample for a (very) long time, as it seems to do with salt (the reason for cleaning the pump) ?

No, salt and alkalinity will not evaporate. However, it's not really evaporation, but atomization that we are dealing with. Small water droplets being physically blasted into the air and sucked into the pump over time.

 

[SDchris]

Does alkalinity evaporate with the water when one aerates a sample for a (very) long time

No, but alkalinity will increase with evaporation

 

[Sral]

No, salt and alkalinity will not evaporate. However, it's not really evaporation, but atomization that we are dealing with. Small water droplets being physically blasted into the air and sucked into the pump over time.

Thanks again, that makes a lot more sense. I was wondering about that for a long time ^^

 

[SDchris]

Similar theme, different method.

Reagentless Acid–Base Titration for Alkalinity Detection in Seawater​

 

[Dennis Cartier]

Similar theme, different method.

Reagentless Acid–Base Titration for Alkalinity Detection in Seawater​

That's a really cool method. Though the need to replace the films would probably limit it's application for reefing.

They state the film can be used up to 74 times over a 2 week period. I am currently testing 56 times a week.

I would also expect that the sample would need to be finely filtered. Another challenge in a reef tank.

Nonetheless, very cool, thanks for posting.

 

[Randy Holmes-Farley]

IMO, it sounds like an academic invention of a film with particular properties that went looking for a use, and found one.
Whether anyone actually uses it for that purpose is a different question.

 

[elysics]

Haven't messed with transferring liquids around with pumps yet, but got mixed results. Having 2 samples in a waterbath in a bucket covered with clingfilm, after airating a while, i get ±0.001 stability with some filtering for at least a few minutes up to half an hour. My guess is temperature drift, but that's one of the things that would cancel out in theory.

As soon as i remove the clingfilm to move the probe from one container to the other and later again to move it back, i lose equilibrium for a while, quick drift around -0.003 that goes back up after a while, probably from co2 intrusion and/or disturbing the bubbles that have collected at the probe. Whole setup is a bit janky, need smaller airstones and possibly a probe without plastic shields around the glass bulb to have less bubbles collect there, but not sure how big of a problem that really is, especially when the samples are pumped instead.

Next is hooking up peristaltic pumps to keep the container sealed, have also ordered cheap 12v air pumps to make it recirculating instead of using my existing outside air supply.

It's more of a fun project, feels like it's going to be a bit overengineered and complex compared to a titrator in the end.

 

[elysics]

OK the weird inconsistencies are gone with proper small airstones instead of the big eheim felt ones I had lying around, even when manually moving the probe instead of transferring water. Maybe it was a bubbles inside the probe shields issue then, now the bubbling of the stones doesn't touch the bottom of the probe.

Half an hour of airation seems appropriate too, took about 35 minutes to get tankwater that was left stagnant overnight to as high as it was going to get

 

[Peter Houde]

Well … if you have an „accurately“ calibrated pH meter (to within 0.01 pH ?) one could try measuring the CO2 content in the water by using the same method as I have seen for fresh-water: a small air dome that is completely submerged in the water and whose air will therefore slowly follow the temperature and CO2 content of the water. Measure the CO2 in that air (to within 50ppm ?) and calculate alkalinity.

Problems that will likely arise:
You need very accurately calibrated probes, otherwise the alkalinity result will be off by 0.3 or maybe even more, as we have seen calculated above.
The method of measuring CO2 is often subjected to heavy drift, additionally to being subjected to „salty“ air that’s not nice to electronics.
A different method might be a pH fluid inside that dome that will tell you it’s pH resulting from the CO2. That’s likely much less prone to corrosion and drift, but also not easily read out electronically.
You could move the alkalinity reference into this dome, but then you will again need a second calibrated pH meter that can tell you the precise difference to the pH meter in the main tank.
Funny idea would be to use RODI or low alkalinity reference water as reference since that should in my mind have a much larger swing in pH from small changes in CO2, which might decrease the pH accuracy requirement inside the dome considerably.

In the end though, the CO2 in the dome air will only follow the tank slowly, so you aren’t necessarily faster than using an external tank for the measurement.

Sounds too onerous for a lazy bum like me

 

[Sisterlimonpot]

What's the end goal here? Just testing the theory? or collectively going to diy a working prototype?

 

[Sral]

Sounds too onerous for a lazy bum like me

Yeah, sometimes my thoughts go a little wide

What's the end goal here? Just testing the theory? or collectively going to diy a working prototype?

I‘d absolutely be up for the latter. Does anybody know if they have a patent (aka monopoly) on that process ?

 

[Sral]

OK the weird inconsistencies are gone with proper small airstones instead of the big eheim felt ones I had lying around, even when manually moving the probe instead of transferring water. Maybe it was a bubbles inside the probe shields issue then, now the bubbling of the stones doesn't touch the bottom of the probe.

Half an hour of airation seems appropriate too, took about 35 minutes to get tankwater that was left stagnant overnight to as high as it was going to get

Interesting, wouldn’t that mean that your tank Water had more dissolved CO2 than the equilibrium with the air if the pH rose due to aeration ?
I guess that’s due to all of the organisms in the water creating CO2, right ?

 

[elysics]

Interesting, wouldn’t that mean that your tank Water had more dissolved CO2 than the equilibrium with the air if the pH rose due to aeration ?
I guess that’s due to all of the organisms in the water creating CO2, right ?

Yeah, those experiments were with outside air, haven't gotten to do recirculating experiments yet, that's next.

What's the end goal here? Just testing the theory? or collectively going to diy a working prototype?

Idk about the others, mine is investigating something interesting until I'm bored of it. If I'm still not bored when I hold a working prototype then that, if it gets too annoying or doesn't want to work I'll stop and rebuild into a titrator and be done with it.

 

[Sisterlimonpot]

My immediate thought when reading through this thread that it can be a great opportunity for the community to calibrate and design a prototype that if pans out can be a working project open to the community. We have scientists and engineers here that can bring this to a point to test the theories.

And if it's just a few peristaltics, pH probe, a processor and perhaps a custom pcb, I think people would be willing to build one just to eliminate reagent costs.

 

[taricha]

If I were making up an automated alkalinity system, i'd be tempted to do this...(but I guess I just don't trust pH probes much)
Acid + bromcresol green + two colorimeter LEDs.

Impact of impurities in bromocresol green indicator dye on spectrophotometric total alkalinity measurements

 

[akaimal]

Got tired of Tirdent servicing and filling up the reagent so decided to get in contact with Aquawiz to give it a try. i installed it on Dec 20, 2023 and so far it had been giving me reliable readings.

 

[laercionit]

Vamos fazer o experimento mental sobre isso. Vamos supor que a solução de "referência" seja @7,5dKH e se comporte como um sistema de CO2 padrão. Arejar a água por 15 minutos fornece o CO2 em equilíbrio e uma leitura de pH. Vamos supor um pH de 8,2. Então:

Agora vamos supor que para a água do tanque aerada tenhamos um pH de pH 8,25. Podemos então usar o xCO2 de 460 para calcular o Alk:

...e Alk é 8,6.

No entanto, como mencionado, precisamos observar a fidelidade dos medidores de pH e verificar se a diferença relativa de pH é suficiente.

Não acho que a fidelidade do medidor seja um problema. Muitas sondagens lerão até a terceira casa decimal. No entanto, a precisão desses medidores geralmente não é melhor que +/- 0,1.

É aqui que o problema pode estar. Se usar o mesmo medidor, a diferença relativa de pH será precisa; caso contrário, dois medidores de pH calibrados não fornecerão uma diferença relativa boa o suficiente.

A medição de pH relativo funciona? Vamos supor que a medição do pH seja baixa em 0,1. Então o xCO2 da referência aerada seria 589, e o Alk calculado da água do tanque seria 8,3dKH.

Portanto, o DOES funciona como 8,6 vs 8,3, próximo o suficiente de outra precisão de medição Alk.

Erro do usuário corrigido no 1º cálculo!!

Primeiramente, parabéns pela sua análise. Tento estudar e pesquisar os cálculos, mas não consigo. Você poderia me explicar os cálculos e equações?

Translation:
Firstly, congratulations on your analysis. I try to study and research the calculations, but I can't. Can you explain the calculations and equations to me?

 

[laercionit]

Let's do the thought experiment on this. Let's assume the "reference" solution is @7.5dKH and behaves like a standard CO2 system. Aerating the water for 15 minutes provides them the CO2 at equilibrium and a pH reading. Let's assume a pH of 8.2. Then:

Now lets assume for the aerated tank water we have a pH of pH 8.25. We can then use the xCO2 of 460 to calculate the Alk:

...and Alk is 8.6.

However, as mentioned, we need to look at the fidelity of pH meters and see if relative difference in pH is sufficient.

I don't think fidelity of the meter is an issue. Many probes will read to the 3rd decimal. However, accuracy of these meters is usually not better than +/- 0.1.

Here is where the issue may be. If it uses the same meter, than the relative pH difference will be accurate, otherwise no two calibrated pH meters will give you a good enough relative difference.

Does relative pH measurement work? Let's assume the pH measurement is low by 0.1. Then the xCO2 from the aerated reference would be 589, and the calculated Alk of the tank water would be 8.3dKH.

So the DO

Let's do the thought experiment on this. Let's assume the "reference" solution is @7.5dKH and behaves like a standard CO2 system. Aerating the water for 15 minutes provides them the CO2 at equilibrium and a pH reading. Let's assume a pH of 8.2. Then:

Now lets assume for the aerated tank water we have a pH of pH 8.25. We can then use the xCO2 of 460 to calculate the Alk:

...and Alk is 8.6.

However, as mentioned, we need to look at the fidelity of pH meters and see if relative difference in pH is sufficient.

I don't think fidelity of the meter is an issue. Many probes will read to the 3rd decimal. However, accuracy of these meters is usually not better than +/- 0.1.

Here is where the issue may be. If it uses the same meter, than the relative pH difference will be accurate, otherwise no two calibrated pH meters will give you a good enough relative difference.

Does relative pH measurement work? Let's assume the pH measurement is low by 0.1. Then the xCO2 from the aerated reference would be 589, and the calculated Alk of the tank water would be 8.3dKH.

So the DOES work as an 8.6 vs 8.3 is close enough to other Alk measurement accuracy.

Corrected user error from 1st calculation!!

First, congratulations on your analysis. I try to study and research the calculations, but I couldn't. Could you explain the calculations and equations to me?

 

[arking_mark]

I'm just using https://pyco2sys.readthedocs.io/en/latest/

Mark