Carbonic measurement Aminex HPX87

[nasirpour]

Hi
I would like to measure carbonic acid using an Aminex HPX87H and UV detector, so anybody can propose a method to measure?
thank you

[tom jupille]

That combination would not be my first choice, because while carbonate ion does absorb somewhat (lambda max around 217 nm), it's not a particularly strong chromophore so I would not expect good sensitivity. High-purity sulfuric acid would be the eluant.

Suppressed conductivity using octanesulfonic acid seems to be the generally preferred procedure.

[nasirpour]

Hi Tom
Thank you for the proposition, I was analysis beet molasses for organic acids. I measure normally OA using this column (Aminex HPX 87H) with water+ sulfuric acid (pH=2.2), flow rate 0.6 ml/min at 210 nm. I have a peak at around 5.9 min (the first peak) the exact time for sodium carbonate. However, I injected 10 micro liter of 1:10 diluted of molasses the absorbance of that peak is very high (10 times more than 10% sodium carbonate). So I was wondering was it carbonic acid? and also its concentration compared to 10% sodium carbonate 10 times higher!!!, So the client ask us to measure the carbonic acid of that sample, based on the explained result, it should be contained an enormous concentration of carbonic acid. So I'm looking for a procedure for that acid.
Thank you

[tom jupille]

So I was wondering was it carbonic acid?

That sounds pretty close to the "total exclusion" time for such a system, which means that it is non-specific and *any* sufficiently ionized and hydrophilic compound will elute there. My guess is that your peak consists of a lot more than carbonate!

Would an ion-selective eletrode work with diluted molasses???

[Markus Laeubli, Metrohm]

At which Retention time is your first organic acid?

I do not have experience with the Aminex column, but on our ion-exclusion columns carbonic acid elutes not as the first peak. On the Metrosep Organic Acids - 250/7.8 column dead time is around 7 min. and carbonate eluetes around 19 minutes.

Therefore I agree with Tom. What you see is the 'injection peak' and that is strongly depending on the matrix.

To determine Carbonate it is strongly recommended to use conductivity detetction after inverse suppression. Using an anion suppressor loaded with e.g. Li+ and regenerated with H+. Under these conditions carbonic acid is converted to carbonate and delivers a stable signal.

Be aware that you only get the total carbonate (HCO3 + CO3). If you need to have the results for hydrogen carbonate and carbonate then old titration is still the techique of choice.

[nasirpour]

Thank you Markus and Tom,
My first organic acid come out at 6.7 (Oxalic acid) and so based on your important experiment it is not the carbonic. So I will try to find the Li2CO3 and test it.
Just for my information to measure the carbonic acid I should do a simple titration?
Thank again

[Markus Laeubli, Metrohm]

The determination of carbonate and bicarbonate is a simple acid/base titration. for details check with your metrohm representative.

[Markus Laeubli, Metrohm]

Dear Tom

There are different pH electrodes available for different matrices. Sugars and other components in molasses will have an influence to pH measurement, but as a titration is required the absolute accuracy of the measured pH is not that important.