How Do Buffer Solutions Work?
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Acidity and Alkalinity
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A buffer solution is an aqueous solution that will change very little if a small amount of acid or base is added to it. Buffer solutions are used to keep pH nearly constant in a wide variety of chemical applications. The way a buffer solution works is based on the concept of pH. And pH refers to an aspect of chemistry concerning the hydrogen ion concentration, a factor that represents the degree of acidity of a substance. The pH scale---a measure of acidity---is assigned numbers ranging from 1 to 14. A pH of 1 indicates an extremely strong acid, a pH of 7 is considered neutral and a pH of 14 is the opposite of acidic---extremely basic or alkaline.
Logarithmic Scale
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A critical feature of the pH scale is that it is logarithmic. The numbers, from 14 to 1, may suggest changes in acid levels in increments of 1 unit, but this is misleading. The difference is actually by powers of 10. For example, a pH of 6 is 10 times more acidic than a pH of 5. And a pH of 4 is 100 times more acidic than a pH of 6. Three is 1,000 times more acidic than 6.
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Changing Acidity and Alkalinity
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A container of pure water is theoretically neutral, having a pH of 7. Adding an acidic substance to the water, even in fairly small amounts, will shift the pH, lowering it with each additional amount of acid added. Likewise, adding a basic or alkaline substance will shift the pH to a higher number. The water will just be at the chemical mercy, so to speak, of the acidic or alkaline substance.
Buffered Solution
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A buffer is a chemical substance---actually a combination of two substances---that resists radical shifts in the pH of the solution. An example is acetic acid (the acid in vinegar) and what's called its conjugate base, the salt sodium acetate. If these two compounds are added to the vessel of pure water, the resulting solution becomes a buffered solution. Any subsequent addition of an acid to the solution, within reason, will alter the pH either not at all or very little. This is because the buffer compounds react with the hydrogen ions.
Chemical Reaction
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The hydrogen ion has a counterpart that figures prominently at the higher end of the pH scale, the hydroxyl ion. A chemical buffer works by reacting with the hydrogen or hydroxyl ions that result from adding an acid (a substance high in hydrogen ions) or a base (a substance high in hydroxyl ions) to the solution. The buffer chemically binds these ions into a form in the compound that keeps them from being free in the solution to affect the pH.
Swamping the Buffer
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Buffers, as effective as they are, can be overwhelmed. The addition of large amounts of an acid or a base to a buffered solution can result in what a chemist would call "swamping the buffer."
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