determining alkalinity via pH and co2*

[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]

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?