<P> Effervescent tablets are tablets which are designed to dissolve in water, and release carbon dioxide . The common ingredients are citric acid and sodium bicarbonate, which react when in contact with water to produce carbon dioxide . Effervescent antacids may also contain aspirin, sodium carbonate, or tartaric acid . </P> <P> Well known brands are Alka - Seltzer in the United States, along with Eno and Andrews in the United Kingdom . </P> <P> Under the generic name algeldrate, aluminium hydroxide is used as an antacid . Aluminium hydroxide is preferred over other alternatives such as sodium bicarbonate because Al (OH), being insoluble, does not increase the pH of stomach above 7 and hence, does not trigger secretion of excess stomach acid . Brand names include Alu - Cap, Aludrox, Gaviscon, and Pepsamar . In 2016 Gaviscon was one of the biggest selling branded over-the - counter medications sold in Great Britain, with sales of £ 62 million . It reacts with excess acid in the stomach, reducing the acidity of the stomach content, which may relieve the symptoms of ulcers, heartburn or dyspepsia . Such products can cause constipation, because the aluminum ions inhibit the contractions of smooth muscle cells in the gastrointestinal tract, slowing peristalsis and lengthening the time needed for stool to pass through the colon . Some such products are formulated to minimize such effects through the inclusion of equal concentrations of magnesium hydroxide or magnesium carbonate, which have counterbalancing laxative effects . </P>

What is the ph of an antacid in water