TITRATION OF COCA-COLA®

Introduction

The acid in soft drinks (aka "sodas" or "pops") provides a taste that is both sweet and sour, and does not interfere with other tastes in the drink. Phosphoric acid is the acid that is present in all soft drinks, but the percentage of phosphoric acid may vary (they don’t give out the secret recipes to just anyone!). 

Pepsi contains citric acid and phophoric acid. Coca-cola only contains phosphoric acid. This procedure to determine the amount of h. It is also important that we do not use diet soft drinks since the artificial sweeteners that they contain have acidic functional groups that will also interfere with the titration.

Soft drinks are also carbonated beverages. The carbonation can produce some carbonic acid in the beverage, which would affect the results. Therefore, to ensure that you are only titrating the phosphoric acid you will use de-carbonated soda. This can easily be obtained by gently heating the soda, or as you may already know, by rapidly shaking the bottle. 

You should remember from previous titrations that the titration is complete when you reach the equivalence point. The equivalence point is when starting material has completely reacted. Usually this is not visually apparent without special aid. In past experiments you probably used an indicator that changed color very close to the equivalence point to signal when the reaction is complete. This visual change is the endpoint. In this experiment, the Cola is dark brown and would mask any color changes thus preventing the use of such indicators.

Phosphoric acid is a weak, triprotic acid. It dissociates as shown in Equations 1-3.

Show image of phophoric acid

Since each dissociation of an H+ occurs in a separate reaction, each reaction has its own equilibrium constant, and therefore its own equivalence point. Figure 10.1 shows what a typical titration curve for a Cola sample looks like. You should notice that although there are three equivalence points, only two can be seen on the graph. Since glass electrodes measure H3O+ ions and above pH 10.5 the acid content is very low, the electrode begins to become bombarded with the high concentration of other ions that have been added during the titration (mainly Na+ in this case). This is referred to as the alkaline error, and makes the pH appear lower than it really is. For this reason we will stop the titration at a pH of 10. In so doing we will not be able to observe the third equivalence point of phosphoric acid.