Unusual method of measuring alkalinity

[arking_mark]

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.

Calculated Alk is 8.68 instead of 8.61...

 

[Dennis Cartier]

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.

They show examples of the proper amount of bubbling in the manual and state that insufficient bubbles, will impact the accuracy. Each air line has a valve to adjust the aeration for that side of the test chambers.

I don't know if they show the current pH, I doubt it, but they do mention that the test time can fall into a range of times. So I wonder if they use the stabilization of the pH as an indicator for when the samples are equalized.

 

[Dennis Cartier]

I should add, that the pH on the device would almost certainly be incorrect as you never calibrate the probe. They do show a pH in their online site graphs, which I assume is calculated from the alkalinity. Though I have no idea how that would be accurate without knowing how much CO2 is within the local air.

I originally assumed that a CO2 sensor would be included somewhere in the system, but I no longer think that is the case.

 

[SPS2020]

Aquawiz...sounds like something kids do in pools.

 

[arking_mark]

So it appears that this Aquawiz may work as follows...

Basically the reference solution is ASW with a trusted Alk measurement and a pH probe.

In a small chamber, aerated reference ASW pH is measured and CO2 is calculated based on measured pH and known Alk (CO2Sys).

Then aerated tank water is mixed in and pH is measured for the mix and mixed Alk is calculated from CO2 and pH (CO2Sys)

Using weighted average for concentration, we can calculate tank Alk.

ALKmix = ((Vref * ALKref) + (Vtank * ALKtank))/(Vref +Vtank)

If Vref = Vtank then ALKmix = (ALKref + ALKtank)/2

Solving for ALKtank = 2 * ALKmix - ALKref

I might build me one of these..

 

[pap2]

sounds like its complicated to me.

 

[Peter Houde]

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

I long for the day someone invents a continuous realtime alkalinity meter that doesn't require reservoirs and continuous maintenance. I'm still longing. Sorry to hear this isn't it.

 

[Sral]

I long for the day someone invents a continuous realtime alkalinity meter that doesn't require reservoirs and continuous maintenance. I'm still longing. Sorry to hear this isn't it.

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.

 

[Randy Holmes-Farley]

I long for the day someone invents a continuous realtime alkalinity meter that doesn't require reservoirs and continuous maintenance. I'm still longing. Sorry to hear this isn't it.

Mindstream was close. Too bad they never made it to market.

 

[Sral]

Mindstream was close. Too bad they never made it to market.

This one here ? Looks like a realy interesting way of doing it, although the chemistry and physics of the test-foils seems complicated aka expensive. Thanks for bringing it up !

 

[Randy Holmes-Farley]

This one here ? Looks like a realy interesting way of doing it, although the chemistry and physics of the test-foils seems complicated aka expensive. Thanks for bringing it up !

Yes. They had some technical issues, though I'm not certain if it was those that killed it.

 

[hotdrop]

I think you guys have proved that it can be done but I wonder if it’s cost effective to do. Seems like a really complicated setup.

I really think that you could probably get really close just automating a Hanna tester for way less cost.

Moreover we don’t need absolute measurements for our tanks, just relative stability. There is not much value in knowing a range from 6-12dkh when all we’re really looking for is a drift of +- .2/.3 from the set point.

 

[Dennis Cartier]

I think you guys have proved that it can be done but I wonder if it’s cost effective to do. Seems like a really complicated setup.

I really think that you could probably get really close just automating a Hanna tester for way less cost.

Moreover we don’t need absolute measurements for our tanks, just relative stability. There is not much value in knowing a range from 6-12dkh when all we’re really looking for is a drift of +- .2/.3 from the set point.

The photos that they show of the device internals show that it uses much less technology than the titration units. The unit is a carrier board with the pH electronics on it, the pump drivers, and a socket for what looks like an Arduino Nano clone and an off the shelf wifi board. All the pumps are DC so again very basic, no precision stepper pumps here. All the reservoirs are cut and glued together acrylic.

Not to discount their ingenuity, but they did not put a lot of expense in the design. Though the v3, the current design, is a lot more sleek than the previous versions, based on photos. Those looked really hacky.

 

[arking_mark]

The photos that they show of the device internals show that it uses much less technology than the titration units. The unit is a carrier board with the pH electronics on it, the pump drivers, and a socket for what looks like an Arduino Nano clone and an off the shelf wifi board. All the pumps are DC so again very basic, no precision stepper pumps here. All the reservoirs are cut and glued together acrylic.

Not to discount their ingenuity, but they did not put a lot of expense in the design. Though the v3, the current design, is a lot more sleek than the previous versions, based on photos. Those looked really hacky.

Hence, why I said I might build me one of these...

 

[C4ctus99]

Interesting…

what about using distilled water for the reference? How much does aeration affect the pH of it? I loved chemistry in high school, but this was not the part we spent a lot of time on

 

[elysics]

I think you guys have proved that it can be done but I wonder if it’s cost effective to do. Seems like a really complicated setup.

I really think that you could probably get really close just automating a Hanna tester for way less cost.

Moreover we don’t need absolute measurements for our tanks, just relative stability. There is not much value in knowing a range from 6-12dkh when all we’re really looking for is a drift of +- .2/.3 from the set point.

It's way cheaper than a titrator, doesn't need high end accurate pumps, actual volumes don't matter.

Maybe $25 for the 4 low end dosing pumps, $10 for an air pump, $40 for the acrylic vessel (just a guess, but diy doesn't have to be like that anyway, can just be a bucket), $20 for the housing, $15 for tubing and airstones and valves and stuff, $10 for ph probe, $10 for wifi MCU, let's be generous and say $50 for various other electronics to get accuracy.


That's a pretty generous estimate of $180 for materials. Theres not much of webdesign or complicated manufacture going into it either from the looks of it. It is selling for $800.

 

[Sral]

Hence, why I said I might build me one of these...

Please report on the progress, would be very interesting indeed !

Slight idea for design improvement: wash the measurement chamber with a bit of RODI water after both measurements are done and flush that back through the tank sample chamber into the main tank. That way you can also make an ATO for the reference to offset evaporation.

Another way might be to coat the chambers with something water repellant (like PTFE), as long as that's not harmfull to the tank. That way there will be much less water drops left in the chamber for contamination.

 

[Randy Holmes-Farley]

Interesting…

what about using distilled water for the reference? How much does aeration affect the pH of it? I loved chemistry in high school, but this was not the part we spent a lot of time on

The pH of distilled water changes more than buffered water, and also is not accurately measured by pH meter unelss there are soem salts in it.

Totally pure fresh water has a pH around 7, while aeration of that water with normal outside air gives a pH down in the mid 5's.

 

[Sral]

The pH of distilled water changes more than buffered water, and also is not accurately measured by pH meter unelss there are soem salts in it.

Totally pure fresh water has a pH around 7, while aeration of that water with normal outside air gives a pH down in the mid 5's.

Well, the first property (large change) should make it easier to measure the effect of CO2 more precisely. Do you know why the CO2 to pH relation is not accurate without additives ? Is it because of the unknown salt background creating a large-ish deviation in pH for a given CO2 content ?

 

[Randy Holmes-Farley]

Well, the first property (large change) should make it easier to measure the effect of CO2 more precisely. Do you know why the CO2 to pH relation is not accurate without additives ? Is it because of the unknown salt background creating a large-ish deviation in pH for a given CO2 content ?

The relationship is accurate, the issue is that pH meters themselves become inaccurate at measuring pH at very low ionic strength.

The article below discusses the issue, and describes the response in low ionic strength as "– slow, noisy, drifting, non-reproducible, and inaccurate." In essence, not what you want in an analytical method.

https://assets.thermofisher.com/TFS-Assets/LSG/Application-Notes/AN-PHLOWION-E%200914-RevA-WEB.pdf