Unusual method of measuring alkalinity

[Dennis Cartier]

I saw someone mention a different type of automatic alkalinity tester that does not do a titration and so requires no re-agent, so I just had to look it up and read about it. Its a product called the AquaWiz and it uses aeration and pH differentials to calculate the alkalinity of the sample water as compared to a known reference sample.

From reading the manual, it appears to work like this. A reservoir holds the reference water within the device. There are 2 test chambers, a left and a right. The right side contains an air stone and a couple of tubes for moving water into and out of. The left side contains an air stone, tubes and a pH probe. Tank water gets added to right chamber, and the left side gets it's reference sample refreshed from the reservoir. Then the 2 sides are equally aerated for 15 minutes. Once the test chambers are fully aerated, the pH of the left side with the reference sample is measured, and then the left side gets drained back to the reservoir. Now the right side, with the aerated tank water, gets pumped to the left chamber, and the pH is measured. Then they calculate the dKH based on the difference in pH from the aerated reference sample and the tank water.

The odd bits are that they don't calibrate the pH probe as the specific pH does not matter, only the differential between the 2 aerated samples.

They claim a 0.05 accuracy, but they also mention that re-calibration is required based on how far apart the dKH of the reference sample is compared to the tank dKH, with a 1.0 dKH difference requiring a weekly re-calibration, and a 0.5 dKH difference able to go up to a month between re-calibrations. Re-calibrating involves measuring the dKH of the reference sample in the reservoir and entering into their online app.

It certainly is a unique and different way of trying to determine the tank alkalinity. Thoughts on the method?

(You can read the installation manual below to better understand the concept)

AquaWiz

www.aquawiz.net

 

[Pistondog]

Need someone to vet the concept.

 

[elysics]

The concept seems almost too good to be true, what I am wondering is how stable is the dkh of seawater in a box (the reference)?

I guess what it comes down to how much calibration does this take in practice vs titration

Also, come to think of it, the thing is essentially 2 little skimmers in a box, what about cleaning

 

[taricha]

This "works" but it shifts the demands and errors around a bit.
Given that pH is essentially only a function of
1) alk and
2) CO2...

and further given that it requires you to have a known stable alk solution in the reference reservior, then this can calculate the tank alkalinity by aerating both solutions equally thus removing the CO2 difference and leaving only the alk difference of the two determining the pH difference.

But the alk / pH function is mathematically complicated so they need you to provide water that already matches the target alk for your tank pretty closely.


"The source of reference seawater can be natural seawater, your existing tank of seawater that you previously maintained, or newly synthesized seawater with stable alkalinity. It has to be stable over time. An unstable reference seawater produces inaccurate measurement. Please note that the KH of that in the reference seawater should be kept near the target KH value."

That is pretty restrictive and, IMO harder to achieve than just titration. I don't think I trust myself to make a totally stable alk solution that would hold alkalinity over days to week+.
But just aerating and measuring pH is easier to automate, so that's an advantage.

( I think I'm getting the concept right.)

 

[JimWelsh]

It's an interesting concept, in theory, but I've run some numbers through the CO2SYS calculator, and in order to get +/- 0.05 dKH accuracy, you would need to be able to read the pH probe within +/- 0.002 pH units. I'm dubious about that degree of precision out of a pH probe.

Also, it assumes that the reference water and the tank water will be the same temperature when the measurements are taken. What if the tank is at 78F and the reference water is more like room temp at 68F? I'm not sure whether the tank water will cool down to 68F with 15 minutes of aeration, and every 1 degree F temperature difference equals appx. 0.02 dKH calculated difference.

 

[elysics]

I think the temperature swing might not be so bad, it's a tiny amount in vial kept a big water bath.

Regarding probe precision, isn't the whole point that you don't need actual pH values so it really doesn't matter how much the value drifts over a span of days, only how much random noise there is in the voltage in the span of minutes. Seems plausible to me with good electronics and throwing pH accuracy out entirely.

The money for the thing has to go somewhere, certainly didn't go to sleek design, integrating the components, or webdesign

 

[Randy Holmes-Farley]

I saw someone mention a different type of automatic alkalinity tester that does not do a titration and so requires no re-agent, so I just had to look it up and read about it. Its a product called the AquaWiz and it uses aeration and pH differentials to calculate the alkalinity of the sample water as compared to a known reference sample.

From reading the manual, it appears to work like this. A reservoir holds the reference water within the device. There are 2 test chambers, a left and a right. The right side contains an air stone and a couple of tubes for moving water into and out of. The left side contains an air stone, tubes and a pH probe. Tank water gets added to right chamber, and the left side gets it's reference sample refreshed from the reservoir. Then the 2 sides are equally aerated for 15 minutes. Once the test chambers are fully aerated, the pH of the left side with the reference sample is measured, and then the left side gets drained back to the reservoir. Now the right side, with the aerated tank water, gets pumped to the left chamber, and the pH is measured. Then they calculate the dKH based on the difference in pH from the aerated reference sample and the tank water.

The odd bits are that they don't calibrate the pH probe as the specific pH does not matter, only the differential between the 2 aerated samples.

They claim a 0.05 accuracy, but they also mention that re-calibration is required based on how far apart the dKH of the reference sample is compared to the tank dKH, with a 1.0 dKH difference requiring a weekly re-calibration, and a 0.5 dKH difference able to go up to a month between re-calibrations. Re-calibrating involves measuring the dKH of the reference sample in the reservoir and entering into their online app.

It certainly is a unique and different way of trying to determine the tank alkalinity. Thoughts on the method?

(You can read the installation manual below to better understand the concept)

AquaWiz

www.aquawiz.net

Norton tells me that the Aquawiz link is a known dangerous web site and they blocked it.

 

[gbroadbridge]

I saw someone mention a different type of automatic alkalinity tester that does not do a titration and so requires no re-agent, so I just had to look it up and read about it. Its a product called the AquaWiz and it uses aeration and pH differentials to calculate the alkalinity of the sample water as compared to a known reference sample.

From reading the manual, it appears to work like this. A reservoir holds the reference water within the device. There are 2 test chambers, a left and a right. The right side contains an air stone and a couple of tubes for moving water into and out of. The left side contains an air stone, tubes and a pH probe. Tank water gets added to right chamber, and the left side gets it's reference sample refreshed from the reservoir. Then the 2 sides are equally aerated for 15 minutes. Once the test chambers are fully aerated, the pH of the left side with the reference sample is measured, and then the left side gets drained back to the reservoir. Now the right side, with the aerated tank water, gets pumped to the left chamber, and the pH is measured. Then they calculate the dKH based on the difference in pH from the aerated reference sample and the tank water.

The odd bits are that they don't calibrate the pH probe as the specific pH does not matter, only the differential between the 2 aerated samples.

They claim a 0.05 accuracy, but they also mention that re-calibration is required based on how far apart the dKH of the reference sample is compared to the tank dKH, with a 1.0 dKH difference requiring a weekly re-calibration, and a 0.5 dKH difference able to go up to a month between re-calibrations. Re-calibrating involves measuring the dKH of the reference sample in the reservoir and entering into their online app.

It certainly is a unique and different way of trying to determine the tank alkalinity. Thoughts on the method?

(You can read the installation manual below to better understand the concept)

AquaWiz

www.aquawiz.net

If it sounds too good to be true ...

 

[Randy Holmes-Farley]

Re-calibrating involves measuring the dKH of the reference sample in the reservoir and entering into their online app.

Do they tell you how to do that to high accuracy?

 

[Dennis Cartier]

Do they tell you how to do that to high accuracy?

Not really. They point out that most alkalinity test are only accurate to 0.3 - 0.6 dKH. They leave it up to the user to select a method of testing the reference water that they are comfortable with. We need something like the TM High Precision Hydrometer, but for alkalinity, lol.

Also, the time between recalibration appears to be caused by residual tank water slowly contaminating the reference water as they both use the compartment with the pH probe. That is why the difference in alkalinity between the tank water and reference water affects the time between recalibration.

 

[Randy Holmes-Farley]

Not really. They point out that most alkalinity test are only accurate to 0.3 - 0.6 dKH. They leave it up to the user to select a method of testing the reference water that they are comfortable with. We need something like the TM High Precision Hydrometer, but for alkalinity, lol.

Also, the time between recalibration appears to be caused by residual tank water slowly contaminating the reference water as they both use the compartment with the pH probe. That is why the difference in alkalinity between the tank water and reference water affects the time between recalibration.

It would seem that the accuracy in measuring the reference solution alkalinity has to be at least as good as the claimed accuracy of the method overall.

Evaporation of the reference solution (or condensation into it) during it's aeration will cause it to drift.

 

[Dennis Cartier]

It would seem that the accuracy in measuring the reference solution alkalinity has to be at least as good as the claimed accuracy of the method overall.

Evaporation of the reference solution (or condensation into it) during it's aeration will cause it to drift.

They sort of deal with the evaporation by drawing air from the top of the reference solution chamber and the bubbled air returns to that area. It does not look like the compartment has any seal though, just resting on top of the chamber, so I expect that their efforts are moderated by that.

 

[arking_mark]

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!!

 

[arking_mark]

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!!

I made a user input error and corrected this post...

 

[taricha]

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.

Since it's mostly just a dKH comparison meter, then for calibration just put reference solution in both sides, after aeration set the pH meters to equal...

Under that scenario, based on your numbers, I wouldn't worry about the pH meters drifting from each other by <0.05 over a modest time frame.

Mostly to me the issue is it's just measuring alk relative to a standard of whatever accuracy the user can provide.

 

[elysics]

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!!

What effect does make pH "low" by 0.1 though? Slow calibration drift of the probe? That would be almost consistent for both samples and cancel out. The reference would also measure closer to 8.1 instead of 8.2, the delta would remain close to 0.05.
The error would come from random electrical noise and from the extent to where the drift affects the reading at 8.2 differently than at 8.25

 

[Randy Holmes-Farley]

They sort of deal with the evaporation by drawing air from the top of the reference solution chamber and the bubbled air returns to that area. It does not look like the compartment has any seal though, just resting on top of the chamber, so I expect that their efforts are moderated by that.

OK, that reduces evaporation, but I don't see how it aerates the reference and tank samples to equilibrate them with the same air.

Perhaps I'm not understanding the setup. Aerating the reference with air that is on top of the reference already won't do much, and then aerating the sample with air that is (where?) top of the reference or top of the sample, or room air?

In either case, I don't understand how they are aerating both samples with air with the same CO2 level.

 

[elysics]

OK, that reduces evaporation, but I don't see how it aerates the reference and tank samples to equilibrate them with the same air.

Perhaps I'm not understanding the setup. Aerating the reference with air that is on top of the reference already won't do much, and then aerating the sample with air that is (where?) top of the reference or top of the sample, or room air?

In either case, I don't understand how they are aerating both samples with air with the same CO2 level.

Both the reference sample and tank sample are sitting in vials/containers and those containers sit inside the reference reservoir like a waterbath. The airlines feeding the airation stones in both containers are connected and come from above the reservoir.

They airate both at the same time, test the pH of the reference sample, dump the reference sample back into the reservoir and then pump the tank sample from it's vial over into the reference vial and test the pH

 

[JimWelsh]

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.

Now, what if the measured pH of the aerated tank water is 8.253 instead of 8.250? How would that affect the calculated alkalinity? Please recalculate with pH 8.253 instead of 8.250 as the only change. My point being that even that tiny change in the pH measurement gives a greater error than the stated precision. I understand that the absolute pH value isn't important, and that it's the relative change in pH between the two aerated samples that matters. That said, just because a pH meter may be able to "read" to three decimal places doesn't mean that one can reproducibly measure pH that precisely.

 

[Randy Holmes-Farley]

Both the reference sample and tank sample are sitting in vials/containers and those containers sit inside the reference reservoir like a waterbath. The airlines feeding the airation stones in both containers are connected and come from above the reservoir.

They airate both at the same time, test the pH of the reference sample, dump the reference sample back into the reservoir and then pump the tank sample from it's vial over into the reference vial and test the pH

OK, so the reference will drift from contamination with the sample residue, as others have noted.

I wonder how many experiments (with different CO2 levels) they did to see if 15 minutes of aeration of the two samples always reaches an equilibrium. It's a lot more complicated system than aerating a sample with room air, because CO2 can be coming in or out of the tank sample, in or out of the reference sample, and in or out of the reference reservoir water and air.

Can a user see the pH in real time to know for themselves if the pH is stabilized?

Sorry for asking questions I could read for myself, but the Norton warning has me wary.