In acidic aqueous solution, a buffer is formed by the dissociation of the acid:
HA ⇄ H+ + A-
Hence, when acid is added, the excess H+ reacts with the A- to form more HA, lowering the pH and minimising the effect of the addition of acid. When alkali is added, the OH- reacts with the dissociated H+ to form water, which reduces the effect of the alkali by restoring the pH to normal levels. Alka Seltzer acts as a buffer because the citrate ions in solution (C6H5O73-) are able to react with H+ when acid is added, to form citric acid, C6H8O7. This neutralises the acid, increasing the pH. The excess HCO3- ions also react with H+ to form CO2 and water, hence lowering the pH and reducing the acidity of the solution. The HCO3- ions also act as a buffer when alkali is added, because they react with OH- to create water and CO32- ions, increasing the pH and reducing the effect of the addiction of alkali. (Shakhashiri B, 1989)
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The sodium citrate then dissociates in aqueous solution to form citrate ions that are able to form a buffer solution. This allows the Alka Seltzer to act as an antacid by neutralising the excess acid in the stomach, as H+ is used to form citric acid. The excess HCO3- ions in the aqueous mixture also act as a buffer which helps to reduce the acidity, further functioning as an antacid. (Shakhashiri B, 1989.) The aspirin in Alka Seltzer acts as an analgesic and also as an anti-inflammatory drug. It is absorbed by the gastro-intestinal tract and is then converted into its active ingredient, salicylic acid, which works to relive pain. (Medicines.ie, 2014) Hence, Alka Seltzer is an efficient