Antacids
The walls of the human stomach contain cells which
secrete gastric juices containing hydrochloric acid. The
normal pH of gastric juices is in the 1.0 – 3.0 range.
The purposes of this acidic solution are:
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to suppress growth of harmful bacteria, and
to help in digestion by hydrolysing proteins to
amino acids.
Over-eating or eating certain types of food, or stress
(worrying) stimulates excess acid production,
causing indigestion.
Antacid -
a remedy for excess
stomach acidity
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Antacids are bases - usually, metal oxides,
hydroxides, carbonates or hydrogen
carbonates (bicarbonates).
they neutralize excess acid in the stomach
to adjust the stomach pH to the desired
level (they relieve indigestion and allow
damage done by excess acid to the stomach
lining to repair itself).
Active ingredients in ‘over-thecounter’ antacids include:
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aluminium hydroxide Al(OH)3
Magnesium hydroxide Mg(OH)2
calcium carbonate CaCO3
Magnesium trisilicate MgSi3O8 and
sodium hydrogen carbonate NaHCO3
Action of antacids
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1. Magnesium oxide
MgO (s) + 2 HCl (aq)  MgCl2 (aq) + H2O (l)
2. Magnesium hydroxide
Mg(OH)2 (aq) + 2 HCl (aq)  MgCl2 (aq) +2 H2O (l)
3. Aluminium hydroxide
Al(OH)3 (s) + 3 HCl (aq)  AlCl3 (aq) + 3 H2O (l)
4. Calcium carbonate
CaCO3 (s) + 2HCl (aq)  CaCl2 (aq) + H2O (l) + CO2 (g)
5. Sodium hydrogen carbonate
NaHCO3 (aq) + HCl (aq)  NaCl (aq) + H2O (l) + CO2 (g)
6. Magnesium trisilicate
Mg2Si3O8 (s) + 4 HCl (aq)  3 SiO2 (s) + 2 H2O (l) + 2 MgCl2 (aq)
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Antacids are often combined with
chemicals called alginates that
produce a neutralizing layer that
prevents acid reflux.
Similarly anti–foaming agents such as
dimethicone are added that reduce the
surface tension of gas bubbles,
causing them to coalesce (come
together), producing a defoaming
action.
Side effects of antacids
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Aluminum hydroxide may cause constipation or
irregularity.
Aluminum ions can also prevent uptake of
phosphate ions, due to precipitation of aluminum
phosphate. Also, they bind to certain drugs because
of their large charge density due to their small ionic
radius and high charge.
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Magnesium hydroxide has laxative properties.
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Calcium carbonate may result in kidney stones and
sodium ions may lead to hypertension.
 Example
Two solid antacid products containing
the same mass of different active
ingredients are on sale for the same
price.
One contains sodium bicarbonate, the
other calcium carbonate as the active
ingredient.
Deduce which one is a better buy and
explain your reasoning.