Food, snacks, and beverages contain varying amounts of different acids which contribute to their tastes significantly. Acids are found among the ingredients of juices and soda drinks giving them their pungent acidic taste. For instance, coca-flavored drinks possess carbonic and phosphoric acids. On the other hand, apple juice as well as lemon-flavored soda such as Sprite, 7 up and mist contains citric acid.
Acids can be classified, according to the number of ionizable hydrogen atoms they possess, into:
a) Aprotic Acids: Acids that lose one H atom during their neutralization reactions with a base (e.g. NaOH) such as hydrochloric acid (HCl) and nitric acid (HNO3).
HCl + NaOH → NaCl + H2O
HNO3 + NaOH → NaNO3 + H2O
b) Polyprotic Acids: Acids that lose more than H atom during their neutralization reactions with a base (e.g. NaOH). They can be sub-classified into
Diprotic: such as sulphuric acid (H2SO4)
H2SO4 + 2 NaOH → Na2SO4 + 2 H2O
Triprotic: such as phosphoric acid (H3PO4) and citric acid C3H5O(COOH)3
H3PO4 + 3 NaOH → Na3PO4 + 3 H2O
C3H5O(COOH)3 + 3 NaOH → Na3C3H5O(COO)3
The concentration of citric acid in a sample of apple juice could be estimated through titration of the sample with a standard solution of sodium hydroxide of known concentration. A complete neutralization reaction occurs when a certain volume of the titrant (NaOH) is sufficient to neutralize the entire amount of citric acid in the sample converting it to sodium citrate salt.
Phenolphthalein is used as an indicator in this experiment to determine the endpoint where complete neutralization occurs. Phenolphthalein changes according to the pH of the medium. It is colorless in an acidic medium, while pink-colored in a basic medium. Phenolphthalein is added in a few drops into the flask containing the sample containing citric acid. Then the titrant (NaOH) is withdrawn from the burette while shaking the flask. The sample will remain colorless till complete neutralization occurs as shown in Figure 1a. Near the endpoint, a pale pink color will appear in the sample flask then disappear immediately with shaking. After complete neutralization, the first excess drop of NaOH will turn the phenolphthalein into its pink-colored form. The volume of the titrant is recorded at the appearance of the first persistent pale pink color as shown in Figure 1b.
Note: If the excess volume of NaOH was withdrawn from the burette beyond the endpoint, the color of the sample in the flask will turn into an intense pink color due to the high basicity as shown in Figure 1c.
