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I've been doing several experiments using soda to quantify the amounts of caffeine and benzoate. Because of our methods of analysis, we had to decarbonate our soda. The experiment offered two ways for decarbonating the soda: shaking it or heating it. Then, to make sure all the benzoate in the soda is converted to benzoic acid, we add a small amount of $\ce{HCl}$ (to protonate it.)

Several years ago, I did a similar experiment to quantify the amount of caffeine. To decarbonate the soda, I added salt ($\ce{NaCl}$.)

I'm wondering if adding salt would be inappropriate in the more recent experiment. None of the lab procedures mention adding salt and, to me, this is the easiest way to decarbonate soda. Therefore, I'm thinking there may be a specific reason the salt method was left out of the procedure.

My thought is there could be some interaction between $\ce{Na+}$ and $\ce{Cl-}$ that could "neutralize" the $\ce{HCl}$ so to speak, making the protonation of benzoate less effective.

Perhaps it would make sodium benzoate instead of benzoic acid? Perhaps they just didn't think to add salt. Any thoughts on this?

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Some salts added obviously will alter the pH value of the soda ($\ce{NaHCO3}$, for example), and others not (like the example provided by you, $\ce{NaCl}$). Changing the ionic strength by addition of salts may render your mathematical model to describe the composition of the analyte more complex.

Hence, let me I suggest an alternative to outgassing the soda: immerse the open container into an ultrasound bath. This works rapidly, however ensure the container is widely open and may accommodate the foam briefly generated. Otherwise, it may surprise you as a sticky geyser. Second hint: gently approach the active regions in the ultrasound bath to moderate the gas evolution.

(Degassing solvents with ultrasound is a common preparation for HPLC, too.)

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When the salt concentration is increased, some of the water molecules are attracted STRONGLY by the salt ions, which decreases the number of water molecules available to interact with the polar CO2 of the soda.

As a result of the less availability for solvent molecules, CO2 molecule find it difficult to remain in the aqueous medium, hence escape.

This would then be possible with a number of other salts as well, if these are having good extent of hydration. If this is so, please let us know.

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Na+ + BzCOO- ◄► BzOONa is an equilibrium. Part of the answer is that increasing [Na] will shift the reaction towards the right. I guess heating the liquid makes sense, I've no idea how consistently you can remove the excess CO2 by shaking. I've also no idea how NaCl changes the solubility of CO2. Have you considered doing simple weight change experiments to compare the three techniques? You should. You should also consider that, in general, adding anything to an analyte increases the chances that there will be interference. Since the analytical techniques (as well as the work-up processes) weren't specified (to be honest, I wouldn't be able to help any more if they were, I'm almost certainly not familiar with the chemistry behind them), there's no way for me to say whether NaCl would interfere or not. I'm sure you understand there are other chemicals present in the "soda", so that your idea of adding NaCl would require pretty stringent verification of its "non effect" on the results. I sure wouldn't go there. I've been involved in the creation of analytical techniques and the process can be quite tedious as well as lengthy. (Co-incidentially, CO2 absorbed from the atmosphere turned out to influence not only the NaOH reagent solution (virtually no storage stability), but also the result of the titration (both time and mixing speed) itself. I sure wouldn't go there. But that's what literature searches are for.

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