16.2
Buffers
A solution containing appreciable amounts of a weak conjugate acid-base pair is called a buffer solution, or a buffer. Buffer solutions resist a chang…
Adding a small amount of acid or base to a solution can cause a significant decrease or increase in the pH. However, many chemical and biochemical processes need a stable pH to function. Buffers can prevent a drastic change in the pH of a solution when their buffering capacity is not exceeded.
Buffers contain a weak acid and its conjugate base or a weak base and its conjugate acid. For example, human blood maintains its pH near 7.4 with a buffer composed of carbonic acid, a weak acid, and bicarbonate ions, its conjugate base.
Conjugate acid-base pairs form buffers as they do not neutralize their conjugate acid or base. For example, acetic acid and acetate cannot react. However, if acetic acid, a weak acid, and ammonia, a weak base, are added together, they will react to form a salt—ammonium acetate.
In a buffer, the weak acid neutralizes any added base by reacting with the hydroxide ions produced, whereas its conjugate base neutralizes any added acid by reacting with any hydronium ions. A similar mechanism works in the case of a weak base and its conjugate acid.
Two beakers, X and Y, contain the same volume of different solutions, each with a pH of 7.2. The solution in beaker X is not buffered. In contrast, the solution in beaker Y contains an acetic acid-acetate buffer.
If hydrochloric acid is added to beaker X, the pH of the solution suddenly drops due to the increased concentration of hydronium ions.
In contrast, the solution in beaker Y shows a nearly constant pH when hydrochloric acid is added to it because one of the buffer components, acetate, reacts with hydrochloric acid to produce a chloride ion and acetic acid.
Similarly, if sodium hydroxide is added to beaker X, the pH of the solution will suddenly increase due to the increased concentration of hydroxide ions.
On the other hand, the solution in beaker Y shows a minimal change in pH when sodium hydroxide is added to it, as one of the buffer components, acetic acid, reacts with sodium hydroxide and produces sodium acetate and a water molecule.
The buffer can prevent a drastic change in the pH of a solution as long as the concentration of the conjugate acid-base pair in a solution is higher than the added strong acid or base.
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Q1: What is a buffer solution and why is it important?
A buffer solution contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Buffers resist pH changes when small amounts of strong acid or base are added, which is critical for chemical and biochemical processes. For example, human blood maintains pH near 7.4 using a carbonic acid-bicarbonate buffer system.
Q2: How does a buffer prevent drastic pH changes?
When acid is added, the conjugate base component neutralizes hydronium ions. When base is added, the weak acid component neutralizes hydroxide ions. This conversion of strong acids and bases into weak conjugate pairs causes minimal pH change. The buffer works as long as the conjugate acid-base pair concentration exceeds the added strong acid or base.
Q3: Why don't conjugate acid-base pairs neutralize each other?
Conjugate acid-base pairs, such as acetic acid and acetate, do not react with each other because they are in equilibrium. However, when a weak acid like acetic acid is mixed with a weak base like ammonia, they will react to form a salt. This distinction is fundamental to buffer formation and stability.
Q4: What happens when you add strong acid to a buffered versus unbuffered solution?
In an unbuffered solution, adding strong acid causes a sharp pH drop due to increased hydronium ion concentration. In a buffered solution, the acetate component reacts with the added acid, producing acetic acid and chloride ions, resulting in minimal pH change. This demonstrates the protective effect of buffers.
Q5: What are common examples of buffer solutions?
Common buffers include acetic acid with sodium acetate, and ammonia with ammonium chloride. The acetic acid-acetate buffer consists of a weak acid and its salt, while the ammonia-ammonium chloride buffer consists of a weak base and its salt. Both effectively resist pH changes in their respective pH ranges.
Q6: What limits a buffer's ability to resist pH change?
A buffer can prevent drastic pH changes only when the concentration of the conjugate acid-base pair is higher than the concentration of added strong acid or base. Once this buffering capacity is exceeded, the buffer loses its protective effect and pH changes become significant.
Q7: How does adding strong base to a buffer solution work?
When strong base like sodium hydroxide is added to a buffer, the weak acid component reacts with it, producing sodium acetate and water. This reaction shifts the acid ionization equilibrium to the right, restoring hydronium ion concentration and maintaining relatively stable pH despite the base addition.