Based on our last post outlining the differences between pH and acidity as measures of acid strength, a short exercise will help emphasize the differences. Here we will compare two deceptively similar acids: 5% store bought white vinegar and Coca-Cola classic.
As stated before, 5% white vinegar has an acidity of 5% which means 5 grams of acetic acid (CH3COOH)
per 100 mL of vinegar. In addition, the pH usually is in the 2.5 – 2.7 range.
Now, is Coca-Cola Classic as acidic as white vinegar? A first (but incorrect) assessment would look at the fact that the average pH of Coca-Cola Classic is about 2.53. This puts it at the lower range of white vinegar. However, as discussed before, the grams of acid per liquid is what matters since the quantity of acid dictates how much it can act (and dissolve) other compounds.
The active acid in Coca-Cola Classic is phosphoric acid (H3PO4). The average amount per can of coke works out to 17mg per 100 mL of Coca-Cola. This gives an acidity of only 0.017%. Granted, since phosphoric acid has three hydrogens instead of one like acetic acid there are some chemical considerations but they are not as relevant at this low concentration.