determining alkalinity via pH and co2*

[orcafood]

Maybe using some dye to tranduce the signal to the visible spectrum is the way to go. Green laser pointers are cheap, IR optics are much too expensive

[jrhupp]

Originally Posted by orcafood

I'm taking the optical grade mirrors from the cameras, they should work well as the entry and exit mirrors they are probably close to a wavelength flat.

The concave collimating mirrors would be pricey your right. Perhaps less so out of old equipment though. Next question why even use the collminating mirrors? Why not just a tube with a silicone collimating lens (passes 1-10 um) at the end, no mirrors, just one lens which runs around $200. Of course the bandpass filter is still needed. Gas standards would be tricky to make. Could they not be made using a working CO2 meter? For example remove the CO2 from a gas space with soda lime and see how low it can go. That is the first point on the calibration. Add CO2 with a tank until it goes up 50 ppm on the working meter then set the point. 5-10 points would probably do?

Good information on the IR-lasers I was trying to find more about them, I was worried they would be cost prohibitive. (Off topic but holy cow I wish I had one of those Xe/Ar lasers, so much fun to use in the labs doing Raman. I never set it up, but I watched them do it for a little bit, takes foooreeevverrr)

A=-log(I/I0)=Elc brings me back, extinction coefficients are so useful. Absorbance and concentration seem linear in that equation or were you referring to how the linear range breaks down at the maximum linear response?

CO2 only has 4 vibration modes. This is its symmetric stretch, asymmetric stretch, and then the two scissors. Only 3 of these are IR active, the asymmetric stretch at 4.3 um and the degenerate scissors at 15 um which have a much weaker IR intensity. So it seems like the best choice is to focus on the 4.3 um area.

All that said I agree that a sensor would be much easier to setup and repeatable for all of us reefers. Please let me know if you can find a sensor with ppb resolution for a decent price, that would be a game changer! The $100 NDIRs seem to have a resolution around 20 ppm which is not good enough.

I couldn't find one so far that fit the bill, thus looking for alternatives.


Another thing I was reading about was that many of the HVAC NDIRs have a built in baseline correcting algorithm that assumes that the lowest value it can get to is around atmospheric CO2 levels. The detector drifts to lower CO2 concentrations so the baseline constantly gets changed under the assumption that indoor co2 will only be above atmospheric CO2 and never below. Not good when a headspace in contact with seawater may have less than atmospheric co2.

To the mirrors there is not necessarily a need for them. There is no inherent reason the components can't be laid out linearly removing the need to reflect the light. This is good because standard mirrors used in NDIR instruments are gold plated.

You don't inherently need to collimate the beam. It helps greatly with optical through put and eliminates/minimizes sensitivities to surface characteristics of the optical bench though.

For standards, sure you could mix gas as a transfer standard from a working meter. But that means you have a meter already that has a greater accuracy than what you desire in your DIY version. You can also use precision syringes and a precision mixing volume to generate standards. This might actually be doable pretty cheaply as I think about.

As to absorption as a function of gas density. It may appear linear over a narrow range, but it is inherently not. Probably best to think about it on a narrow wavelength basis and build up from there. This is important for thinking about where a gas absorbs as well. Because while you are correct on major regions of absorption, CO2 has individual lines and small groups of lines peppered through out the IR region. Some more useful than others depending on your IR source, filters and detection limit.

In any case, absorption on a per wavelength bases (Av) can be expressed as:

Av = 1-e^(-kv*l*d)

Where l is the optical path length and d is absorber density. kv is the absorption coefficient at v, which is a function of line strength and halfwidth. Line strength saturates at some point.

I have lofty dreams of trying out some of the cheap NDIR sensors but haven't gotten to yet; too many more important projects to get through first. But when I do I will report what I find.

I had seen most had limited resolution and had hoped that there may be some way of working around that. Hopefully we shall see at some point.

I wasn't aware of the drift correction. The direction of the correction seems curious to me though. Do you know if they are using a reference gas cell? Diffusion into the reference cell over time would make sense for the need to correct upward. Or maybe its a source aging issue.

[jrhupp]

Originally Posted by orcafood

Maybe using some dye to tranduce the signal to the visible spectrum is the way to go. Green laser pointers are cheap, IR optics are much too expensive

If you are going that route, why not just measure alkalinity with an automated colorimetric test?

There should be some visible dye suitable for this. I know we typically use a titration method (and that seems the approach used in the automated monitors coming out). But I am pretty Hatch or somebody along those lines make a single reagent that segregates into two forms with different peak absorption wavelengths in the visible spectrum based on alkalinity.

[orcafood]

sound like we want a colorimetric drop checker

[karimwassef]

Another incomplete project from years ago

http://www.reefcentral.com/forums/sh...tic+alk&page=9

Need to clone myself or make enough $ to retire so I can tinker more

[orcafood]

I've been trying to find a way around titration. Use a chuvette as the drop checker (headspace) with an led, proper dye and a photo sensor. A reference would be nice too. Raising the response rate would require an interesting setup.

Beautiful project by the way

[dartier]

Originally Posted by jrhupp

If you are going that route, why not just measure alkalinity with an automated colorimetric test?

There should be some visible dye suitable for this. I know we typically use a titration method (and that seems the approach used in the automated monitors coming out). But I am pretty Hatch or somebody along those lines make a single reagent that segregates into two forms with different peak absorption wavelengths in the visible spectrum based on alkalinity.

I have been researching the various methods of performing alkalinity tests, and the reason I was following this thread. Along the way I ran across a paper that described a system to perform alkalinity testing using Spectrophotometry and a reagent with a known absorption profile. Then a small amount of the sample was added and the resulting PH shift was used to calculate the alkalinity concentration to achieve the shift. It was quite interesting, but did require removing the carbolic acid from the sample by flowing the sample through a gas permeable hose in a NaOH bath. I am not sure if the added complexity is worth it though.

I am leaning towards doing a regular titration with a colorimeter to detect the end point. To get around the issues with volume accuracy, I plan to use a load cell to calculate the various weights of the sample and reagent rather than try to keep pumps from drifting.

Dennis

[orcafood]

Much smarter with weighing the sample, but you will still need to calibrate that pretty often?

The problem I see with titration is too many moving parts, each of which has some variability.

[karimwassef]

So... different direction:

Was talking to a reef buddy of mine about this and had an idea about measuring CO2 in the air by pH...

So visualize a 5 gallon jug with RODI low dKH water and an air stone (pump has intake filter). The water has a pH probe in it and there's a loop of hose that allows water to recondense back into the jug as the air leaves.

Kinda like a drop checker with a pH probe.

Now, the pH should be proportional to CO2...

[orcafood]

Exactly what I’m thinking, cycle the headspace with an inline air pump.instead of a pH meter, use a ph sensitive dye, laser pointer (already collimated and tight band) and a photosensor.

[karimwassef]

Why? pH meters are already integrated into an Apex and can easily be calibrated, sourced, etc...

[karimwassef]

Also, the reference jug could be any liquid where the pH and air CO2 can be correlated. I'm thinking of a thickened water formula to minimize any evaporation? As long as the inlet air for this test volume and the reef tank are the same, the formulas would apply directly to provide Alk. Almost all the needed parameters can be measured with conventional devices- pH, temp, salinity, pH(CO2). Barometric pressure is the only one to be assumed relatively constant.

The last gap is a fit that converts the test volume pH to air CO2 and an interface processor that collects all the data live and calculates Alk, then feeds it back as an analog or digital reading back to the Apex.

[orcafood]

Wish I had me an Apex, that is probably why I don't focus on integrating to one. I think the liquid needs to be water, what else would you use? Many other liquids are volatile and would end up in the tank water eventually. Agar has large pores but I don't think you want to slow the diffusion down through reference solution, I think quite the opposite we want air vigorously pumping through, like your air injectors, keeping the headspace equilibrium. pH meters are expensive for nice ones, like $100 for a good probe with longevity. Green laser diode $5, dye free essentially, photosensor $2-5, half mirror $10, I'm sure there will be hidden costs. Think hanna phosphate checker. Total cost of around $30 tops and likely better resolution than could be had with a pH probe.

If the reference solution is isotonic with the other liquid (tank water) in contact through the headspace there shouldn't be evaporation. The headspace will be saturated with humidity.

I'm sure a pH meter would work just fine

[karimwassef]

I think there's a confusion about the CO2 liquid pH sensor.

This is a completely separate and independent device that is sampling air not connected to the tank at all. It can be anywhere and can run oil if need be (not saying that's a good choice - just emphasizing that the liquid and pH sensor have nothing to do with the tank.)

The idea is that I'm not relying on headspace. I'm actively injecting air into the tank water and this sampling device simultaneously. I'm sampling the air going in and relying on my injectors to keep the gas in system at the measured rate.

[orcafood]

Makes sense to me, outside is your headspace. I still recommend the design should bubble air through the solution being tested for pH rather than rely on diffusion.

Should work with a serious air injection system on a tank, such as yours. Otherwise the headspace might need to be used.

Ideally we need to know the pH of the solution to at least the thousands place, which you are not going to get with a meter, not to mention the pH meter will drift.

[karimwassef]

I calibrate pH meters often

Yes. I was pushing the same air through the airstone. Maybe a surface treatment for the water would reduce evaporation.

[karimwassef]

Accuracy does matter but maybe there's a way to make the water very low in alkalinity so that it responds more aggressively to CO2

[orcafood]

A dkh around 0.1 might do. Once I get some time I will start to play with making a low range co2 drop checker, I have bromothymol blue sitting right here ready to go.

[orcafood]

Hey Karim how accurate is your pH meter? To the tenths or hundredths? Alternatively can you link me to the probe your using?

Do you know the standard error of your kH estimate? Or perhaps do you have a distribution of your estimates over some time period so we could figure out the standard error?

[karimwassef]

The scientific probe by Apex is what I use. Reading is in the hundreds 8.33
But I assume an error of +- 0.05 which is wild, I know.

I calibrate monthly though and I use two probes (one regular) in case on drifts. They're never the same value, usually off by 0.1. One I use for the feedback and one I use to stop anything running off the rails.

[orcafood]

What kind of variation on your kH estimate do you get? Does it vary by a few points between estimates or a full degree?

[karimwassef]

My purpose isn't to lock in to a particular kH value. This method is intended to get close, but then maintain it over time. I'm really reducing variability.

Since the errors in my system are relatively constant, maintaining my pH aligns with maintaining my dKH stability... so it's all about precision, not accuracy

[orcafood]

So what kind of values do you get?

A value means nothing without error. Scientific papers require error for good reason.

[karimwassef]

Sure. But if my error is constant and I can calibrate it out, it doesn't impact me much. The thread I posted showed the measured vs. calculated Alk chart. Got to dig it back up.

Overall, I can keep my dKH between 7.5 and 8.5 by keeping pH between 8.34 and 8.35

[orcafood]

Is your alkalinity approximation steady over time? Or does it jump around each subsequent calculation? Over the span of an hour what would be the max and min alkalinity approximation you get?