# What makes Coke acidic?

What makes Coke and other soft drinks acidic? These drinks are carbonated, so a freshly-opened can should have a lot of dissolved carbonic acid, but the ingredients also lists phosphoric acid. Would flat Coke be significantly less acidic than carbonated Coke?

• This is an interesting question. Off-handedly, I'd say it was mainly the phosphoric acid (which is added intentionally for "bite"), but if someone sits down and calculates given the pressure of the carbonation and the concentration of phosphoric acid, it would be a great answer. – jonsca Jan 1 '13 at 20:59
• Interestingly, the 'other coke' is a base. – long Nov 29 '13 at 2:17
• I think coke is a Acid, because if you put a coin in a glass of coke it cleans and strips the coin. If it were a alkalis or neutral it would not do that, were as a acid would. – user6934 Jun 24 '14 at 6:27

If you look at the list of ingredients, you will find that Coca-Cola and many other brands contain phosphoric acid (food grade - don't worry). This is probably the dominant cause of acidity. Citric acid is another common component. Of course, the carbon dioxide is also acidic, but weakly, so there should be very little difference between carbonated and flat.

• Please see @Chemistry answer. My oral mucosa would agree, but of course that's not very scientific. – Brian Sep 9 '13 at 7:24
• I am of the opinion that this is the correct answer. My reasoning is that the pH of most colas is on the order of 2-4, and the pKa of carbonic acid is about 6. You'd need a hell of a lot of carbonic acid to get to that pH, while the pKa of phosphoric acid is 2.1, so it'd be quite easy to get to a pH of, say, 3. – chipbuster Sep 9 '13 at 17:11

Flat coke is less acidic than carbonated coke. I did a lab report on this topic, and found that Carbonated coke had a concentration of around 0.131M whereas flat coke had a concentration of 0.064M. I guess we can assume that the carbonic acid that's dissolved in Coke makes up a large proportion of the acidity.

• pK of carbonic acid is around 3.5, it is not a factor at pH=2.5 – Greg May 15 '17 at 6:16

Mostly Carbonic acid, probably. Because it is the only species in coke, that can escape in the form of gas.

Just shake a coke, or heat it and see how much of the acidity is gone. It is the part that is due to $\ce{CO2}$.

• Could you back this up? This seems to be a guess at the moment. I don't see the connection between "it being a gas" and "it is the largest contributor to acidicity". Phosphoric acid doesn't easily form a gas, that's all. – ManishEarth Jan 2 '13 at 18:52
• Yes, this is mostly a guess. The connection that you don't see is that: When you shake or heat coke, its acidity goes away almost totally and this change is due to CO2 being undissolved. – sencer Jan 3 '13 at 16:52