BUFFER SOLUTIONS RESIST CHANGES IN pH

Certain solutions, called buffered solutions, resist changes in pH like a stubborn child resists eating her Brussels sprouts steadfastly at first but choking them down reluctantly if enough pressure is applied (such as the threat of no dessert). Although buffered solutions maintain their pH very well when relatively small amounts of acid or base cire added to them or the solution is diluted, they can withstand the addition of only a certain amount of acid or base before becoming overwhelmed. 

A buffered solution resists changes in pH when acids or bases are added or when dilution occurs. The buffer is a mixture of an acid and its conjugate base. There must be comparable amounts of the conjugate acid and base (say, within a factor of 10) to exert significant buffering. 

To make the buffer solution, a solution of acetic acid and a solution of sodium acetate are combined. To understand how this buffer solution resists changes in pH, first recall what happens when a strong acid is added to plain water, as in Figure 10.10b. The pH of the solution quickly decreases because the concentration of hydronium ions increases. Add a strong base to plain water, and you quickly increase the pH by decreasing the relative concentration of hydronium ions, as in Figure 10.10c. 

In treating buffered solutions in this chapter, we will start by considering the equilibrium calculations. We will then use these results to show how buffering works. That is, we will answer the question How does a buffered solution resist changes in pH when an acid or base is added ... 

Describe the components of a buffer solution, and explain how a buffer solution resists changes in pH. 

How do buffers work A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. The mixture of ions and molecules in a buffer solution resists changes in pH by reacting with any hydrogen ions or hydroxide ions added to the buffered solution. 

By definition, a buffer solution resists changes in pH with dilution or with addition of acids or bases. Generally, buffer solutions are prepared from a conjugate acid/base pair, such as acetic acid/sodium acetate or ammonium chloride/ammonia. Chemists use buffers to maintain the pH of solutions at a relatively constant and predetermined level. You will find many references to buffers throughout this text. 

Thus, a buffer solution resists changes in pH. When we add a modest amount of a strong base or a strong acid to a buffer solution, the pH changes very little. 

A buffer solution resists changes in pH when it is diluted or when acid or base is added. 

A buffer solution resists changes in pH when diluted or when small amounts of acid or base are added. Two common types of buffer solutions are (1) a weak acid mixed with a salt of its conjugate base and (2) a weak base mixed with a salt of its conjugate acid. 

In this way, the buffer solution resists changes in pH. The amount of H or OH that a buffer system can absorb without allowing significant pH changes to occur is called its Iniffer capacity. 

To what extent can a buffer solution resist change in pH A simple example will be cited. If 10 ml of 0.1 N HCl is added to 990 ml of pure water (pH 7.0), the pH of water drops 4 units and becomes 3. Similarly, if 10 ml of 0.1 N NaOH is added to 990 ml of pure water, the pH Increases by 4 points and becomes 11. However, if 10 ml of 0.1 NHCl is added to 990 ml of a buffer consisting 0.1 N acetic acid and 0.1 M sodium acetate (pH 4.76), the drop in pH is only 0.01 points. The pH changes merely to 4.75. Similarfy, addition of 10 ml of 0.1 JVNaOH ta990 ml of above buffer solution elicits a rise of merely 0.01 units on the pH scale. The pH becomes 4.77. We thus see that buffer solutions resist changes in pH to a very significant extent (we will consider the same example quantitatively a little later). 

---