Chemistry of CO2 Neutralisation - Can weight be used as an indicator of scruber life?

Sutty

I doubt this will turn out to be useful, but would be happy to be proved wrong! It will be interesting to see the real-world weight changes anyway.

To empirically check the expected weight change you could take a known weight of sorb, eg 100g, place it in a CO2 atmosphere for a while to completely react, and weigh it again. (may get a bit hot in the process)

Neil

__________________
Never forget that life is a finite resource.

jaap

OK

My post was perhaps a little hars sounding.

But your "internet chemistry" combined with claims about what is possible and not got me fired up.

Just a few things, before you reject the base catalysis step you could perhaps ponder on these reactions that you posted earlier:

2NaOH + CO2 ----> Na2CO3 (salt) + H20 + Heat
Ca(OH)2 + CO2 ----> CaCO3 + H20 + Heat.

What is you idea on the ballpark solubility of Na2CO3 compared to CaCO3?
What do you imagine happens if Na2CO3 is present thougther with an excess of Ca(OH)2 and some water?

If the strong base has no catalytic function. How do explain the color (pH) shift that occurs over time on the (used)-sorbs that contains an indicator?

Anyway, your weight method will probably give some correlation (But I'm very sceptical to the usefulness). I'm quite certain that even if it would give some useful information it would be strongly linked to the particular scrubber design, how the scrubber is treated and the temperatures.

If you goal is to eliminate the influence of the weight of the water I hope you are very careful not to get tempted to attempt drying the sorb...that could really turn ugly.

Anyway, I think one thing can be certain. If the weight goes down the scrubber is definately dangerous to use...

__________________
My initials: JAAP

diveoceanos

The point of this thread is ----> The chemical reaction causes a change in weight inside the scrubber. Is there any way we can use this parameter in any usefull way?

You doubt it. Fair enough!

As for the chemistry I would suspect that we could open a new thread to discuss buffer solutions chemistry and pH index markers. If that is of any interest but I suppose that it is not relevant with this thread and will be not interesting for many people. I can send you by PM or email if you like all the equations in the system with Pkbs and so on but I am not sure if that is what you want. But because you asked me I can tell you a few things about buffer solutions and about carbonate salts solubilities: Na2CO3 is highly soluble in water and gives a lot of bicarbonate anions some CO2 bubbles and sodium positive ions. CaCO3 on the contrary when formed is much less soluble and forms a whitish material virtualy insoluble in water. When CO2 passes through a water buffer system like the one of Ca(OH)2 and NaOH it forms H3O+ that reacts with OH- in the buffer. then the reaction of Ca(OH)2 <-----> Ca++ + 2OH- which is bidirectional (weak base) is shifted to the right. This situation continues until the total concentration of OH- drops (pH decreases) to such a point that a die changes its color (acid form which is pink or whatever other color is being attributed to the acid form of the index chemical used). NaOH provides a certain alkalic pH and it provides a relatively constant alkalic pH throughout thre *life* of the scrubber and it is completely soluble in water. It's quantity is small but this is how buffer solutions work. At the end of scrubber *life* the OH- starts to drop and the buffer solution looses its aility to maintain a constand pH, that's why the color change.

Does this make any difference to the whole issue? I don't think so.

Just to relax your concerns I am aware of how buffer solutions work and one important and necessary element for them to work is water. Therefore water is needed for the reaction inside the scrubber (#1). Any attempt to dry the sorb would definitely had awkward results! But I believe there are no kids reading this forum. I beleive that there are rebreather divers who read this! I can not understand your worries. Where are they based? How did you figured out that anyone could possibly dry out the sorb to weight it and then use it? Where did I write anything like that or where did I write anything that could possibly be perceived as such statement?

ChemicalDiver

I agree with Sotos -
The concept is sound -> Go and get the data!

We can debate whether this is useful when we have the data. It is certainly interesting enough to many people here to be worth spending 2 minutes with a kitchen balance, whether useful or not - and there is no cost involved.

It could just be that the water absorbed from the lungs from one dive to the next is quite homogeneous and typically the same for each person on each "similar" dive. You can then account for this in the calculation.
Each of us could get an additional validation that everything is going as anticipated with the scrubber. I do not see how pH indicator is any different in that also does not show you inside channeling, does it? Yet people find it useful.
I very much doubt that any water is released from the scrubber to be deposited elsewhere. So it is just the more than usual exhalation water or leaks being added, which just adds a safety margin and if it is indeed a leak, probably not an unjust one.

Look forward to what you find out. As all chemists know, the science (or art) of chemistry starts with a hypothesis and is always followed by an experiment. Conclusions and possible utility are drawn after the experiment.
I think your initiative is great and look forward to results. It could turn out to be completely useless, but wouldn't it be nice if it were just a little useful?

Best,
Alex

sunnyboy

No, we can debate it now. One thing in particular mentioned by jaap but missed is this:

BREAKTHROUGH OF THE SCRUBBER IS THE MOST DANGEROUS PART OF THE EQUATION TO THE DIVER. Breakthrough can kill you whether or not the scrubber is 1/10 used, 1/2 used or 9/10 used. Early breakthrough can kill you even if the scrubber is almost new.

Even if weighing "works", weight will not give you any indication of scrubber breakthrough - only an indication of overall amount of scrubber used, and therefore a probability of potential breakthrough (i.e. if the scrubber is more than 50% used, possibility of breakthrough is much greater than if the scrubber is new).

Weighing your scrubber "new" might tell you if you've packed it properly, but there are lots of other ways that are equally effective and as easy (like shaking it).

I mean, go for it - but don't expect a revolutionary breakthrough in scrubber duration detection.

Cheers,

-S

diveoceanos

And what?

Personally I believe that the most safe way is to be a ble to have a warning (visual or sound alarm or other) after the scrubber before the inhalation counterlung with CO2 detectors. I beleive that you should be able to get alarm at 0,5mbar and 1mbar and that's all. This is the equivalent of 500 and 1000 ppm at atmosperic pressure. There is no need to continously monitor this one with decimal point accuracy. Again three sensors and a voting logic and a means of two point calibration before the dive would be nice to have. However not necessary.

Do you see any other way to do this while diving?

Having said that this approach with weighting the scrubber, remember it is an experiment and at its best it aims at providing an extra point to monitor scrubber life. Let's wait for the results. Personally I don't expect a revolutionary breakthrough invention. But definitely I am going to enrich my knowledge even during the process.

Exactly!

Weighting your scrubber "new" gives you (for this purpose) a reference point where you can start calculating weight changes.

Thanks for the input,
Sotos

sunnyboy

I don't know where monitoring came into this. The discussion has been about weighing as a tool for predicting scrubber life.

I agree that once reliable CO2 sensors are available that will work in the rather hostile environment of an Rebreather, they will become invaluable. But that topic is separate from the idea of weighing scrubber.

Well, you sure can't weight it!

I still think it's of marginal usefulness, given the number of unknowns and the potential usefulness of even a positive result.

Even if you can get a provable, repeatable, reliable relationship between scrubber weight and scrubber life, it still won't predict breakthrough. Breakthrough is about gas kinetics, scrubber packing, flow paths and such.

Cheers,

-S

jaap

Sure, but since you started by pulling some very dodgy chemistry talk I thought it might be good point out some of the incorrect statements.

OK

He, he. My question about the solubilities and the reaction between Na2CO3 and Ca(OH)2 was mearly an attempt to make you think & and perhaps realize that there is nothing wrong with NaOH being reformed.

So I will spell it out more clearly, Its wrong to completely reject the following reaction:

Ca(OH)2 + NaCO3 -> CaCO3 + 2NaOH

The above reaction is necessary to describe what NaOH does during "steady state" operation of these sorts of scrubbers. Any notion that NaOH is used up and not reformed is simply wrong, IMHO.

Sure it does not make much difference to your case, but as I said lets get rid of the dodgy and incorrect chemistry statements, OK?

Sure no kids, but given the both incorrect and rather self confindent statements in the beginning I thought it would be prudent to add some cautioning (even if its most likely not needed). Besides sadly, its quite easy to pull any chemistry talk and most people will thing it really cool, advanced and correct just because they don't have a clue about these things.

And yes, there was some talk about science, investigating new things and such in earlier posts. Taking critisism of all kinds and from all kinds of angels, just or not is sort of a key part of the process if you want to play scientist ;-)

I wish you the best of luck with your investigations! Might even do some weighing myself just for curiosity ;-)

__________________
My initials: JAAP

diveoceanos

OK.

Let me put it in an other way. Maybe I am wrong since I don't have the time to open any textbooks.

Step by step:

#1. CO2 in water.

CO2 + H20 <-----> HCO3- and (H+) <-------> CO3-- and (H+)


#2. NaOH in water.

NaOH + H20 ------> Na+ and (OH-) (NaOH Maintains a relatively constant alkalic pH throughout the life of the scrubber)


#3 Ca(OH)2 in water

Ca(OH)2 <-------> Ca++ and 2(OH-) This is a weak base but provides a source of Ca++ as well as OH-


The neutralisation of CO2 is taking place in an aquatic solution where the following reaction is taking place:

#4 Neutralisation of CO2 and formation of CaCO3

(H+) from CO2 solution reacts with (OH-) from the base system in the scrubber.

H+ and OH- gives H2O

Ca++ and CO3-- gives CaCO3. Therefore more and more HCO3- is sifted to the right and form CO3-- untill all CO2 is being neutralised. See Note below why this reaction is shifted to right:

NOTE: It is a common principle that when there is a removal of the products by means of volatile gases or insoluble substances (CaCO3) the reaction is shifted towards the side of the products. Therefore the neutralisation of CO2 is complete as long as there is a source of OH-.

END OF SCRUBBER LIFE:

The source of OH- has been exhausted. A little more CO2 in the system and there is no more capacity to neutralise it. CO2 as it disolves in the water gives an excess of H+. There is a lot CaCO3 formed, still an aquatic solution of Na+ some Ca++ maybe an excess of H+ some HCO3-. NaOH is never reformed from the time it disolves in the water solution. It is present in the form of Na+ and OH- untill the capacity of the particular pellet to neutralise CO2 is full.

NOTE2: The scrubber It is not an homogenous solution. There is a formation of a solution on the surface of the pellets where all the reactions are taking place.

CHANGE OF COLOR.

There is a dye inside the pellets. This dye has a form of

AH <---------------> A- and H+

This dye may exist in two forms AH which is the acid form and A- which is the alkali form. The concentration of H+ determines which form predominates. It happens that AH form has a different color than A- . AH is pink! To see that take some used pink scrubber and pure some caustic soda on it. You will see the pink disappearing.

Whereever there is change of color the system of alkalis may no longer provide OH- to neutralise CO2, hence the CO2 moves to the next layer where it reacts.


Na2CO3 actually does not exist in aquatic solutions. Sodium ions are always present and their source is NaOH.

Are there any chemists to evaluate this approach? Any critisism welcome!

diveoceanos

I had a second look at the scrubber quantitative situation. This can be used as a guidance and might also be proved to be wrong. Here we go:

Sofnolime contains more than 70% of its weight in Ca(OH)2.

Therefore for a 2,5 Kg sorb (axial scrubber of Meg) there is more than 1750g of Ca(OH)2.

The mole of Ca(OH)2 is 74g. So we have in the scrubber 23 moles of Ca(OH)2.

Each mole of Ca(OH)2 can neutralise 1 mole of CO2. Since the mole of CO2 is 44g, theoretically we would expect that 23 X 44g of CO2 would be added to to weight of the scrubber if the efficiency of the scrubber is 100% (taking into account only the Ca(OH)2 which is the predominating base).

That is approximately 1 Kg of CO2.

Let's suppose that on average a diver produces 0,9 litres (1,77g) of CO2 per minute.

Theoretically again the scrubber would be good for 570 minutes under these CO2 production rates.

However since the scrubber is in the form of pellets one would expect that its efficiency is not 100%. ISC clearly states that the scrubber is good for 3,5 hours. We then assumpt that the scrubber efficiency is 210 minutes divided by 570 minutes (approximately 35%).

With 35% efficiency i would expect that 35% of 1Kg of CO2 could be neutralised in the Meg's axial scrubber. The is approximately 365g.

I would therefore assumpt that at the end of the scrubber life it will weight 365g more compare to its initial weight.

Assumption #1: All the water that is being produced inside the scrubber remains inside the scrubber

Assumption #2: No water from other sources is entering the scrubber.

Please note that the above are just assumptions and need to be modified according to actual findings from real experiments.

proposal: Check the scrubber weight after 3,5 hours of use at more or the less oxygen consumption of 1 LPM and compare it with the calculated expected weight increase of 14,5%. (365g over 2500 g).