Buffering agents keep solutions at a constant pH level by absorbing additional hydronium or hydroxide ions. Natural buffers exist in biological systems, and salt mixtures of weak acids or alkalis can be used as buffering agents. They are used in industry and products such as detergents, food, brewing, shampoo, and swabs. Blood pH is maintained by carbonic acid and hydrogen carbonate ions.
In acid-base chemistry, a buffering agent is a substance that can be used to keep a solution at a constant pH, or acidity-alkalinity level. Acids in aqueous solution provide hydronium ions (H3O+), while alkalis provide hydroxide ions (OH-). The pH of the solution is a measure of the relative concentrations of these ions. A solution containing a buffering agent can, to some extent, absorb the additional hydronium or hydroxide ions that are introduced when an acid or base is added without any alteration of the H3O+/OH- ratio and therefore without change in pH. Natural buffers are found in biological systems, and buffering agents have many uses in chemistry and industry.
When an acid is dissolved in water, it will ionize, to a greater or lesser extent, into H+ ions and negatively charged anions. H+ ions combine with water to form hydronium ions (H3O+) and the negative anion is known as the conjugate base. For example, aqueous hydrochloric acid forms hydronium ions and the conjugate base is the chloride ion: HCl + H2O → H3O+ + Cl-. The conjugate base can react with hydronium ions to reform the acid, but in this case, the chloride ion is a weak base, so the hydrochloric acid in water consists almost entirely of hydroxide ions and chloride ions, making it a strong acid. In a weak acid, however, the reverse reaction is significant because the conjugate base is stronger and therefore the concentration of hydronium ions is lower.
A mixture containing a salt of a weak acid together with the acid from which it is derived can often be used as an acid buffering agent; the salt ensures an abundant supply of the conjugate base of the acid. Strong acids and their salts are not useful as buffering agents, because an acidic buffering agent requires large amounts of acid to be present in a non-ionized form and because the pH, in any case, usually must be maintained at a level nearly neutral or only moderately acidic or alkaline. Similarly, an alkali buffering agent usually involves a salt of a weak alkali along with the alkali itself.
A simple example of an acidic buffer solution is an aqueous solution of acetic acid and sodium acetate. Acetic acid is a weak acid, so most of it will not be ionized. By adding an acid, the additional hydronium ions will react with the acetate ions in the sodium acetate, forming more acetic acid, which will remain mostly unionized and therefore won’t have much effect on pH. When an alkali is added, the additional hydroxide ions (OH-) will react with the acetic acid to form acetate ions and water, and with the smaller amount of hydronium ions to form more water, again with little effect on pH.
Living organisms employ buffering agents in a number of roles. For example, blood pH should be kept near 7.4, slightly on the alkaline side of neutral. Since a wide variety of chemicals of varying acidity and alkalinity can enter the bloodstream from ingested food, a buffering agent is needed to ensure this value is maintained. This is achieved by a combination of carbonic acid (H2CO3) and hydrogen carbonate (HCO3-) ions.
Buffering agents are widely used in industry and many common products. They are used in detergents, food and brewing to ensure that the pH remains within the range required by the yeasts responsible for fermentation. Shampoo works best when it is slightly acidic and generally contains a buffering agent to prevent loss of acidity during use. Swabs are also heavily used in biology and biochemistry. Good’s buffers, developed in the 1960s by NE Good, are a group of buffering agents carefully designed not to interfere with biological reactions.
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