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Buffer solutions resist pH changes when small quantities of acids or bases are added. These are useful in chemical reactions in which slight pH changes could drastically alter the efficiency or products produced. Buffer solutions can be used in small scale, bench lab work or mass production chemical plants. Buffer solutions are created from a variety of acids and their conjugate bases depending on the needs of the reaction.
Choosing a Buffer Solution
Before creating a buffer solution, the acid and base used to create the solution must be selected. The first consideration is what the pH being buffer actually is. Buffer solutions can be acidic (have a pH less than 7) or alkali (have a pH greater than 7). A buffer solution using citric acid and monopotassium citrate will have a pH of 3.1. A buffer solution of boric acid and sodium hydroxide will have a pH of 12.7. The acid and base selected should not have any type of reaction with whatever compound is being buffered.
The concentration of the acid and base in the buffer solution should be equivalent. The actual proportions might be altered slightly to adjust the pH but will still be quite close. For most common buffers, the quantities for buffer solutions can be found in a table that provides correct amounts and resulting pH (see Additional Resources) . In other cases, the quantities must be calculated for the acid and base, although it is rarely necessary unless a reaction requires a more exotic acid and base combination.
If the quantities are ready available in a table, preparation of the buffer solution is straightforward. Dissolve the acid in slightly less water than the final volume specified in the table. Dissolve the conjugate base into the water as well. Check the pH. If necessary, it can be corrected by adding 6 M HCl (acid) or 6 M NaOH (base) dropwise to the solution. Once the pH is correct, add the final quantity of water to dilute the solution to the specified volume.
Two Solution Method
If the quantities needed to create the buffer solution are not readily available, create a preparation of the acid and conjugate base with equal concentrations. Place a pH monitor into one of the solutions. Slowly add the other solution while monitoring the pH. When the pH is at the desired value, stop adding the other solution. This method can waste quite a bit of one of the solutions, so be sure to try to locate the table values before using this method with more expensive reagents.