Unusual method of measuring alkalinity

[Sral]

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

Thanks a lot ! That seems logical, although the noise part over large resistances should be AC and can likely be reduced by averaging, since it impacts precision, not accuracy. Another factor is of course the internal resistance of the pH meter vs the probe's, which does impact accuracy if I understand it correctly.

One should be able to remedy that part by applying pure NaCl, right ? That increases conductivity, helping the pH measurement, but doesn't impact the pH itself if I'm not mistaken.

 

[elysics]

Thanks a lot ! That seems logical, although the noise part over large resistances should be AC and can likely be reduced by averaging, since it impacts precision, not accuracy. Another factor is of course the internal resistance of the pH meter vs the probe's, which does impact accuracy if I understand it correctly.

One should be able to remedy that part by applying pure NaCl, right ? That increases conductivity, helping the pH measurement, but doesn't impact the pH itself if I'm not mistaken.

Do you want to prevent growth in the thing?

Maybe it's going a bit too far for a fun experiment, but one might look into the preservatives used in various dosing products

 

[Sral]

Do you want to prevent growth in the thing?

Maybe it's going a bit too far for a fun experiment, but one might look into the preservatives used in various dosing products

Not mainly, no. If it does that as well, I take it.

I'm mainly interested in thinking about RODI water as a reference solution and the implications thereof. NaCl was only a thought to make the pH of that reference measureable more easily.

 

[Randy Holmes-Farley]

Thanks a lot ! That seems logical, although the noise part over large resistances should be AC and can likely be reduced by averaging, since it impacts precision, not accuracy. Another factor is of course the internal resistance of the pH meter vs the probe's, which does impact accuracy if I understand it correctly.

One should be able to remedy that part by applying pure NaCl, right ? That increases conductivity, helping the pH measurement, but doesn't impact the pH itself if I'm not mistaken.

It solves part of the issue, but not all of the drift without a buffer helping to hold the pH more steady.

 

[Sral]

It solves part of the issue, but not all of the drift without a buffer helping to hold the pH more steady.

Does that still apply when one aerates it from a reservoir of steady CO2-content air and waits for a steady state pH as in the presented method ?

 

[hotdrop]

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.

The target is 50-100$ in parts for 250-300 in retail price. You could probably design an automated colorometer Based in a Hannah kh meter for 100-150$ depending on volume and sell it for 300$.

conceptually it’s a simple process. Mix 10:1 and read the color. But it would take a lot of fine tuning to get it right

 

[Randy Holmes-Farley]

Does that still apply when one aerates it from a reservoir of steady CO2-content air and waits for a steady state pH as in the presented method ?

I've had problems getting pH to stabilize even in buffered seawater. I think it would be a big problem in unbuffered water even if CO2 in the air is fixed (though it won't be if one is aerating it and also aerating a tank water sample with the same air).

I suggest just trying to see what you get for pH stability by putting some table salt in RO/DI water and watch the pH.

 

[elysics]

The target is 50-100$ in parts for 250-300 in retail price. You could probably design an automated colorometer Based in a Hannah kh meter for 100-150$ depending on volume and sell it for 300$.

conceptually it’s a simple process. Mix 10:1 and read the color. But it would take a lot of fine tuning to get it right

Yeah, no... $100 in parts maybe, if you are super thrifty, but then selling that for 300 retail? If you eat the losses and charge as much as hanna does for the reagent maybe... What's that, like 13€ per bottle that would last 1 day of hourly testing?

And if you want volume, youll need proper websites and apps and good design. Not to mention be rewarded for your R&D. That's not free.

 

[Jeff Kohlmann]

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 wonder if this is the technique major aquariums across the country use

 

[Randy Holmes-Farley]

I wonder if this is the technique major aquariums across the country use

Seems extraordinarily unlikely to me.

A simple acid titration is cheap and easy and highly accurate. It's been a standard procedure in chemistry labs for a hundred years.

 

[Dennis Cartier]

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.

They mention in the manual not to clean the built up slime coat off the test chambers as it is hydrophilic and helps their emptying.

 

[elysics]

They mention in the manual not to clean the built up slime coat off the test chambers as it is hydrophilic and helps their emptying.

You found an actual manual? where?

 

[Dennis Cartier]

You found an actual manual? where?

Lol, sorry, I have been referring to their installation instructions page as the manual. I posted the link earlier. The one that gave Randy a Norton warning.

 

[elysics]

Lol, sorry, I have been referring to their installation instructions page as the manual. I posted the link earlier. The one that gave Randy a Norton warning.

Ah, yeah found it, must have overlooked that there were two separate google doc frames, didn't exactly trust disabling any part of my script blockers etc

 

[thepotoo]

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

I beta tested the mindstream system and the alk numbers never matched the actual alk. It would read 11 when the actual alk was 8. They tried at least 3 different units in my tank to try to figure it out. I have to assume this was part of the reason everything collapsed for them.

 

[arking_mark]

So if you have a trusty pH probe, you can just aerate tank water outside for a bit and measure pH. Outdoor air is roughly 400ppm CO2. Then use CO2Sys to calculate Alk.

Unfortunately, our pH meters are too inaccurate to provide good numbers.

However, I use Alk and outdoor CO2 to check my pH meter accuracy...as I trust my Alk reading +/- 0.5.

The impact of being 0.5 off on Alk doesn't have a large effect on pH reading.

 

[arking_mark]

I beta tested the mindstream system and the alk numbers never matched the actual alk. It would read 11 when the actual alk was 8. They tried at least 3 different units in my tank to try to figure it out. I have to assume this was part of the reason everything collapsed for them.

I also worked with them on the same issue...it's the pH and CO2 readings readings they were using to derive Alk. Small variances or inaccuracies with the pH threw the numbers off.

 

[SDchris]

The article below discusses the issue, and describes the response in low ionic strength as "– slow, noisy, drifting, non-reproducible, and inaccurate."

After making a alkalinity titration meter and even going as far to design and build a pH sensor cuircut, that is how I would describe pH measurements in general.

 

[elysics]

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

 

[Dennis Cartier]

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

Ya, I was wondering what they were talking about when they mentioned adjusting the water level automatically and doing a water change. In one of the photos, you can see what must be a big float switch or water sensor extending down into the water reservoir.

I suspect that pulling air from the reservoir headspace helps with the aeration process for stabilizing the two samples, but causes other complications. They mention having to flush out the salt creep from the diaphragm of the air pump as part of the possible maintenance required. That may be why they did not attempt a better seal for the reservoir, and explain why the reservoir would need to be topped up.

They don't show the air pump in the manual, but I am assuming it is in the back panel, based on the set of wires going down that way along with the hoses. Some older docs showed an external air pump with some some added hoses rigged into the plastic case. The new design must use a 12v pump to be able to be powered from the single power supply.