# Why does the pH of soft drinks get lower as they go flat?

I am doing an investigation about the chemistry of soft drinks and as part of this I observed the effect of time on the pH of soft drinks fully expecting the pH to increase as the carbonic acid, a weak acid, decomposed and CO2 escaped. Instead, time after time, the pH decreased, not by much though in fact it hardly made a difference to the acidity of the drinks but it the pH still decreased every time.

So I did a control experiment with soda water and found that the pH did increase when the CO2 escaped the drink. I originally did it with Coke and Pepsi, which aside from carbonic acid also have phosphoric acid (Pepsi has citric acid as well) so the increase in acidity probably has to do with an interaction between the the weak carbonic acid and the strong phosphoric/citric acid. Can anyone help me to explain this?

• What sort of soft drinks and what sort of soda water have you measured ? – Maurice Jan 18 at 19:45

I think you said it... phosphoric acid. Carbonic acid is "buffering" phosphoric acid... test seltzer water (composed of water and carbon dioxide with no other additives).

Another way of thinking about this is to consider carbonic acid (nominally a weak acid) to actually be a base. Looking at the pKa (the pH at which half of the acid is dissociated) is the best way to describe this.

The pKa of Carbonic acid is 6.35

The pKa Phosphoric acid is 2.16

pH is basically a measure of free (dissociated) H+ in water... so as CO2 is removed, it is not available to act like a buffer and bind with H+... so the free H+ increases.

• Thanks a lot. I'll have to read up on buffers I guess. We are in the middle of an acids and bases unit at the time of our summer break and are going to learn about buffers in the first few weeks of the new year, which isn't very practical as the draft for this paper is due after the first week. – 1111 Jul 23 '16 at 0:02
• I am a little fuzzy about this. While the pKa of CO2 seems high enough, for it to act as a base either CO2 or H2CO3 would have to accept a proton, so what counts is if the pKa of the resulting cation exceeds the soda pH. Do we know that? – Oscar Lanzi Jan 18 at 22:48
• @OscarLanzi : you're right, the pH cannot possibly decrease if an acid ($CO_2$) is removed (and nothing else is being added or removed, e.g. the solution is not evaporating). – user6376297 Jan 19 at 19:24

The observation is interesting. If you have two acids (in your case H2CO3 and H3PO4) and one of them (H2CO3 as CO2) leaves solution pH must increase.

If you just mix Na2HPO4/NaH2PO4/CO2/water and let CO2 evaporate pH will increase or (if solution is very acidic) stay unchanged.

Can you please write the method of measurement ("pH paper" or "pH electrode"), conditions ("wait for 1 h at 60C" or "shake vigorously at 20 C for 5 minutes") and the results ("pH dropped from 5 to 4.5 then to 4.0" or "pH was 6, didn't change for an hour, then dropped to 4.2 in 1 minute"). Is this diet or normal coke? Do you see the same effect in plastic bottle and in a glass cup?

Oxidation of sugar is a plausible cause, but I need more info to come up with a better hypothesis.

• pH electrode. I did this at a range of temperatures ranging from ice cold to 40°C (10°C intervals) with two trials for each temperature for both Coke and Pepsi, leaving the drinks alone for 50 minutes whilst controlling the temperature of the ice/water baths. To make sure these results were legitimate and not just faulty apparatus I did an informal experiment where I used a magnetic stirrer to shake the drinks and the pH still went down. Basically, over the course of 50 minutes, I typically saw falls from 3.0 pH to ~2.75 pH, numerically slight falls but significant. I used normal Coke+Pepsi. – 1111 Jul 22 '16 at 23:34
• Continuing on from last comment (reached character max) --- Coke and Pepsi both aluminium canned. If it also helps, I live in South East Asia so the sugar in the drink would most likely have been glucose and not high fructose corn syrup, they use the cheapest ingredient regionally. Also when doing research I came across a science fair paper saying that it didn't matter whether the drink was bottled, canned, etc... Not the most reliable source but... – 1111 Jul 22 '16 at 23:34
• And when I did the magnetic stirrer experiment it was at room temp. With the pH electrodes, they did observe distilled water to be a little bit alkali (6.15) and slowly drifting towards a little acidic (~6.9) but my chem teacher said it should've been around around pH 6. They were kept in the same beaker for weeks and I only bothered to change the water when mould started developing. Same calibration solution used for weeks and weeks as well. Even if the apparatus were not well kept, they aren't fully responsible for this observation. – 1111 Jul 22 '16 at 23:38
• And the sugar is probably sucrose, not glucose. – 1111 Jul 22 '16 at 23:44
• @1111 " (6.15) and slowly drifting towards a little acidic (~6.9)" Wait... 6.1 is slightly acidic. 6.9 is almost neutral. 8 is slightly alkali. But earlier you give correct numbers... from 3.0 pH to ~2.75 pH... Hmm, I can't explain this behavior and it looks strange to me... Good luck searching for the reason. – sixtytrees Jul 23 '16 at 2:20

You can just use the three reversible reactions involved in the amount of carbon dioxide dissolved in a fizzy drink, use Le Chatelier's principle and solve for the changes in pH: \begin{align} \ce{CO2(g) &<=> CO2 (aq)}\tag{1}\\ \ce{CO2(aq) + H2O(l) &<=> H2CO3(aq)}\tag{2}\\ \ce{H2CO3(aq) &<=> H+(aq) + HCO3-(aq)}\tag{3} \end{align}