An average adult produces between 2 and 3L of gastric juice daily. Gastric juice is a thin, acidic digestive fluid secreted by glands present in the stomach. It contains among other substances, hydrochloric acid. The  $\text{pH}$  of gastric juice is about 1.5. The purpose of the highly acidic medium within the stomach is to digest food and to activate certain digestive enzymes. Eating stimulates  ${\mathrm{H}}^{+}$  ion secretion. However, if the acid content is excessively high then the substances, which remove the excess acid and raise the  $\text{pH}$  to appropriate level in stomach are called antacids.
 Some common commercial antacid preparations: 
 Commercial Name                           Active ingredients
 Alka – 2                                              $CaC{O}_3$
 Alka-Seltzer                                       Asprin, ${\mathrm{NaHCO}}_3$ , citric acid
 Milk of Magnesia                              $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$
  Rolaids                                              Dihydroxy aluminum sodium  carbonate
The reactions of active ingredients with stomach acid produce ${\mathrm{CO}}_2$  causing the person to belch. The fizzing that takes place when an alka-seltzer tablet dissolves in water is caused by  ${\mathrm{CO}}_2$  which is released by the reaction between citric acid and  ${\mathrm{NaHCO}}_3$. In recent years omeprazole and lansoprazole are also marketed as antacids. These prevent formation of acid in the stomach.