LAB #3: GRAVIMETRIC ANALYSIS OF CALCIUM CARBONATE IN ANTACID
Our stomachs secrete gastric or stomach acid (mostly HCl) to help us digest the food we eat. In particular, acids help activate the enzyme pepsin that is responsible for breaking proteins into amino acids so they can be absorbed through the small intestine wall into the blood stream. Sometimes that gastric acid can cause discomfort. Upset stomachs or heartburn are often the result of either too much acid production in the stomach or stomach acid splashing up into the esophagus. For a long time humans come up various remedies to chemically neutralize stomach acid.
Antacids are the oldest effective medications for heartburn. Chalk (calcium carbonate) has been chewed for centuries to provide some relief and is still popular in some parts of the world. Most commercially available antacids are combinations of aluminum and magnesium hydroxide. Some effervescent antacids contain sodium bicarbonate, that old household remedy for tummy aches known as “baking soda.” Some antacids are combined with an alginate [an insoluble substance that increases surface tension in liquid] to form a compound that floats on gastric fluids to protect the esophagus from acid exposure. Antacid tablets are slow acting and have less neutralizing power than a liquid form of antacid. Tablets must be chewed, and may not interact well with gastric acid. For most, the convenience of tablets far outweighs these slight disadvantages.
Such a variety of commercial antacids occupy pharmacists’ shelves that this discussion will concentrate on their basic ingredients (See accompanying Table).
Sodium bicarbonate [NaHCO3]
– Sodium bicarbonate is a weak, short-acting antacid. While generally a safe household remedy, its high sodium content is a disadvantage. Unlikely to be recommended by doctors, “bicarb” or “baking soda” is still a common component of many patent medicines. Bicarbonate has an effervescent property that explains the commercial survival of antacid/pain-killer combinations such as Alka-Seltzer™ and Bromo-Seltzer™. Bicarbonate reacts with stomach hydrochloric acid to release carbon dioxide gas (CO2) that is quickly absorbed, but sometimes elicits a satisfying belch ("burp"). An imbalance of the body’s normal pH level (systemic alkalosis) can result from overuse of bicarbonate. Those who require sodium restriction for high blood pressure or heart disease should avoid bicarbonate.
Magnesium Hydroxide [Mg(OH)2]
– Magnesium hydroxide is best known as milk of magnesia. Like magnesium citrate or magnesium sulfate, it is an effective laxative. Were it not for its tendency to cause diarrhea, magnesium hydroxide would be the most ideal antacid. To counter the diarrhea effect, most manufacturers add aluminum hydroxide, which is constipating. The combination substantially raises the price, and the addition of the less-effective aluminum hydroxide reduces the antacid benefit. Magnesium hydroxide is not absorbed by the intestine. However, its interaction with stomach acid produces magnesium chloride that can be absorbed. Magnesium has many functions in human cells, including the heart, and may have harmful effects if levels in the blood rise. This is not a problem for a person with healthy kidneys, but magnesium should be avoided if renal failure is present.
Aluminum Hydroxide [Al(OH)3]
– Compared to magnesium hydroxide, aluminum hydroxide is a weak, slow-acting antacid, and its acid-neutralizing effect varies among commercial products. Aluminum may protect the stomach lining from the damaging effects of alcohol and other irritants. Aluminum hydroxide inactivates the gastric digestive enzyme pepsin. However, the principal reason for its inclusion in commercial antacid preparations is to counteract the diarrhea effect of magnesium. Aluminum hydroxide has other uses. It binds phosphate in the gut lumen to produce insoluble aluminum phosphate, a feature that is useful in kidney failure when the serum phosphate is abnormally high. It is also useful for patients who tend to form phosphate-containing kidney stones. A very small amount of aluminum is absorbed, and brain damage might occur with its long-term use in the treatment of kidney failure. Chronic, excessive use of aluminum hydroxide may deplete the body of phosphate, causing metabolic bone disease (e.g., osteoporosis, osteomalacia) and risking spontaneous fractures, especially in the malnourished. Aluminum hydroxide may alter the absorption of certain drugs (including some used to treat cardiac disease or high blood pressure) so they should not be taken simultaneously.
Calcium Carbonate [CaCO3]
– Calcium Carbonate (chalk) is the most potent usable antacid. It can completely neutralize stomach acid. Nonetheless, it is not always the best choice for regular use.
- About one-third of the administered calcium is absorbed, and high blood calcium or calcium-containing kidney stones are slight risks.
- Phosphate bound by calcium in the gut or bone may deplete the serum phosphorus in some kidney failure patients.
- A systemic alkalosis from prolonged and aggressive use infrequently produces metabolic consequences. [Alkalosis is excess base (alkali) in the body fluids. This is the opposite of excess acid (acidosis).]
Another disadvantage of calcium carbonate may be the tendency for gastric acid secretion to rebound after calcium is given. Popular calcium antacid tablets include Tums™ and Titralac™. The dose should not exceed 3 g per day.
Peppermint flavoring – Peppermint is the most common antacid flavoring. By relaxing the lower esophageal sphincter to release gas, peppermint encourages the release of a belch after a meal, hence the popularity of after-dinner mints.
Antiflatulent – Simethicone is a surfactant, which presumably by breaking down bubbles within the gut renders gas available for absorption. Despite the lack of evidence of effectiveness, simethicone is included in some popular antacid preparations, thereby increasing their cost.
Alginic acid – Prepared from kelp (seaweed), alginate acts as a physical acid barrier for the esophagus in gastroesophageal reflux. It is not an antacid. When ingested, this tasteless and apparently harmless substance floats on gastric fluid to prevent the reflux of acid and pepsin into the esophagus. Preparations such as Gaviscon™ or Algicon™, combine alginate with antacids, and are popular heartburn remedies. There are no satisfactory clinical trials, but these preparations have little neutralizing power and are probably of little benefit in those reflux patients who have complicating esophagitis.
Most commercial antacids contain two or more components. The most common combinations are varying concentrations of sodium hydroxide and aluminum hydroxide. Some widely advertised brands, those with additional components and those containing greater concentrations of the effective ingredients, tend to cost more. There are other antacids that sacrifice efficacy in the interests of taste and acceptability. Space precludes detailed listing of all products. However, labeling is required, and armed with the above information, consumers should be able to select the product most suitable to their needs.
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