Carbon dioxide in water, analysis of dissolved species HPLC?

[rwooduk]

Does anyone here have any analytical experience with dissolved gas, particularly CO₂?

When CO₂ is dissolved in water there can be several reactions:

What I would ideally like to do is dissolve CO₂ into water and then by some means of analysis, see how much of each species there is in the water. However, carbonic acid seems very difficult to detect, see:

http://www.chemistryviews.org/details/ezine/973843/Tracking_Carbonic_Acid.html

I could use a bicarbonate indicator for the bicarbonate but this is just a colour change and won't give an amount. Hydrogen carbonate seems to be the same as bicarbonate so I've no idea how I would differentiate between the two. And also I have no idea on the carbonate.

Would these things even exist in stable amounts of would they keep reacting and reversibly reacting?

If someone could point me in the right direction, or just give me a place to start it would really be appreciated as this really isn't my field (not a chemist).

I have access to HPLC, GC etc etc

 

[Bystander]

"Speciation" on what timescale?

 

[rwooduk]

"Speciation" on what timescale?

Thanks, I have not heard of this before. Literally as soon as I stop bubbling the CO₂ into the water I want to measure the amounts present. However, I will then be transferring the liquid (with a beaker) and pouring it into a reaction vessel. Also for analysis I'm going to have to walk over to the chemistry building with the solution in hand. I'm getting the feeling this is not going to be easy. Any idea of how I determine the "speciation" over time? Or what methods of analysis I could use? Thanks again for any help.

 

[Bystander]

I'm getting the feeling this is not going to be easy.

It's not going to be holding any more still than carbonated water (that is, not at all). Supposedly, "Mentos" will accelerate the degassing of "some" colas (see Mythbusters). Agitation/mixing/stirring will accelerate any process, be it solution/dissolution or exsolution. Total amount in solution is easy, total assay. Speciation? I've never seen anything to be regarded as believable; every method depends on "knowing/assuming" pK_a1,2 and pK_hyd. There might be spectroscopic methods, but, again, they will depend upon assumptions about absorbtion cross-sections.

 

[Borek]

Do you have to measure them all? Once you measure pH you can rather easily calculate relative concentrations of all other carbonic acid forms present (the only tricky thing being H₂CO₃ vs CO₂(aq), as they are typically treated as a single entity).

 

[rwooduk]

It's not going to be holding any more still than carbonated water (that is, not at all). Supposedly, "Mentos" will accelerate the degassing of "some" colas (see Mythbusters). Agitation/mixing/stirring will accelerate any process, be it solution/dissolution or exsolution. Total amount in solution is easy, total assay. Speciation? I've never seen anything to be regarded as believable; every method depends on "knowing/assuming" pK_a1,2 and pK_hyd. There might be spectroscopic methods, but, again, they will depend upon assumptions about absorbtion cross-sections.

Sorry could you tell me what you mean by "total assay"?

Do you have to measure them all? Once you measure pH you can rather easily calculate relative concentrations of all other carbonic acid forms present (the only tricky thing being H₂CO₃ vs CO₂(aq), as they are typically treated as a single entity).

Really we are looking for changes in amounts during reactions (once ultrasound is applied to the CO2 / water mixture). I understand calculations based on pH can be done but I'm unsure of how you would get an amount of one species in the mixture. If you have a free minute could you perhaps give an example?

Many thanks for the advice!

 

[Borek]

Start with the acid dissociation

HCO₃^- ↔ H⁺ + CO₃^2-

and the acid dissociation constant (know, given in tables):

$$K_a = \frac {[H^+][CO_3^{2-}]}{[HCO_3^-]}$$

Solve for the CO₃^2-/HCO₃^- ratio and you have pH and Ka on the right (both known).

This is just a starting point, but I am running out of time.

 

[rwooduk]

Start with the acid dissociation

HCO₃^- ↔ H⁺ + CO₃^2-

and the acid dissociation constant (know, given in tables):

$$K_a = \frac {[H^+][CO_3^{2-}]}{[HCO_3^-]}$$

Solve for the CO₃^2-/HCO₃^- ratio and you have pH and Ka on the right (both known).

This is just a starting point, but I am running out of time.

That's great I'll take it from there, thanks again!

 

[rwooduk]

If anyone has this issue in future I also came across this very useful word document:

http://www.thuisexperimenteren.nl/infopages/Carbondioxide in water equilibrium.doc

 

[rwooduk]

Ok, think I'm nearly there with this. I can assume an ideal solution and use the following:

Therefore I can get K0,K1,K2 AND Kw by inserting the above into:

The next part is where I'm a little confused, I want to use these:

To calculate these:

This is good, I can measure the pH and get [H+] and I know the K values by calculating from the above BUT 1. how did they get 9.36 from 9.33-9.35? 2. What is CT? 3. I still see 2 unknowns in 9.36 namely CT and [HCO3].

Finally, Borek kindly gave me a starter here:

What is the difference in method between what I have posted above and this? And again I'm still left with two unknowns.

Thanks for any advice, I'm a little lost.

p.s. would it be better to just use the ratios:

But again 2 unknowns :-/

 

[Borek]

C_T is total concentration of all forms (also known as the analytical concentration):

$$C_T = [H_2CO_3] + [HCO_3^-] + [CO_3^{2-}]$$

Unfortunately that's something you need to know/measure (but in most applications total concentrations of the acid involved is much easier to control/determine than every ion separately). What I was trying to tell you was that you don't need to measure everything, there is a set of equations describing the equilibrium, so it is enough to know some parameters to be able to calculate everything else as well.

 

[rwooduk]

I see, that makes sense, okay I will try to use the equilibrium conditions and see how I get on.

Thanks again for your help!