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Chapter 2: Problem 39

Biochemical Connections You need to carry out an enzymatic reaction at \(\mathrm{pH} 7.5 .\) A friend suggests a weak acid with a \(\mathrm{p} K_{\mathrm{a}}\) of 3.9 as the basis of a buffer. Will this substance and its conjugate base make a suitable buffer? Why or why not?

Short Answer

Expert verified
No, the weak acid with pKa of 3.9 is not suitable for buffering at pH 7.5.

Step by step solution

01

Understanding pH and pKa

The pH of a solution indicates its acidity, while the pKa of a weak acid represents the pH at which the acid is half dissociated. To form an effective buffer, the pKa should be close to the desired pH of the solution.
02

Check the difference between pH and pKa

The difference between the provided pH (7.5) and the pKa of the suggested weak acid (3.9) is significant. In general, a buffer works best within ±1 pH unit of its pKa.
03

Determine Buffer Suitability

Since the pKa of the weak acid (3.9) is far from the desired pH (7.5), this substance and its conjugate base will not form an effective buffer for the reaction. A pKa closer to 7.5 is necessary for optimal buffering capacity.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

pH
The pH of a solution measures its acidity or basicity. It's a scale that ranges from 0 to 14. Solutions with a pH less than 7 are considered acidic, while those with a pH greater than 7 are basic or alkaline. A pH of 7 is neutral, like pure water. The pH value is crucial for many chemical reactions, including enzymatic reactions. It can influence the structure and activity of molecules, making it essential to maintain the correct pH for specific processes.
The formula to determine pH is \( \text{pH} = -\log[\text{H}^+] \), where \( \text{H}^+ \) represents the concentration of hydrogen ions in the solution.
pKa
The pKa value indicates the strength of a weak acid. It is the pH at which half of the acid molecules have dissociated into their ions. In simpler terms, pKa is a measure of how easily a weak acid gives up a proton. The lower the pKa, the stronger the acid. This value is crucial for creating buffer solutions. A buffer solution works best when the pKa of the weak acid is close to the desired pH. This helps to maintain the pH with minimal changes even when small amounts of acids or bases are added.
Weak Acid
A weak acid only partially dissociates in water. Unlike strong acids, which almost completely ionize, weak acids maintain a balance between their ionized and non-ionized forms. Common examples include acetic acid and citric acid. Weak acids are essential in buffer solutions because they help stabilize the pH. Their partially dissociated state means they can react with added acids or bases to minimize changes in pH. This makes them perfect for maintaining steady conditions in various biological and chemical processes.
Buffer Capacity
Buffer capacity refers to the ability of a buffer solution to resist changes in pH when acids or bases are added. It's an important property for maintaining a stable pH in solutions. The effectiveness of a buffer depends on the concentration of the acid and its conjugate base. A higher concentration increases the buffer capacity, allowing the solution to neutralize more added acid or base. Additionally, an effective buffer works best within a range of ±1 pH unit of its pKa value. If the pKa is too far from the desired pH, the buffer's capacity will be compromised.
Enzymatic Reaction
Enzymatic reactions are biochemical processes catalyzed by enzymes. Enzymes are proteins that speed up chemical reactions without being consumed in the process. They work best at specific pH and temperature conditions. For example, human enzymes generally function optimally around pH 7.4, the pH of blood. Maintaining the correct pH for enzymatic reactions is crucial because changes in pH can alter the enzyme's shape and function. This is why buffers are often used in biochemical experiments to ensure the pH remains stable, allowing enzymes to work efficiently.

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Most popular questions from this chapter

Mathematical Define buffering capacity. How do the following buffers differ in buffering capacity? How do they differ in pH? Buffer a: \(0.01 \mathrm{M} \mathrm{Na}_{2} \mathrm{HPO}_{4}\) and \(0.01 \mathrm{M} \mathrm{NaH}_{2} \mathrm{PO}_{4}\) Buffer b: \(0.10 \mathrm{M} \mathrm{Na}_{2} \mathrm{HPO}_{4}\) and \(0.10 \mathrm{MNaH}_{2} \mathrm{PO}_{4}\) Buffer \(c: 1.0 M \mathrm{Na}_{2} \mathrm{HPO}_{4}\) and \(1.0 \mathrm{M} \mathrm{NaH}_{2} \mathrm{PO}_{4}\)

Recall Define the following: (a) Acid dissociation constant (b) Acid strength (c) Amphipathic (d) Buffering capacity (e) Equivalence point (f) Hydrophilic (g) Hydrophobic (h) Nonpolar (i) Polar (j) Titration

Reflect and Apply Many of the buffers used these days, such as HEPES and PIPES, were developed because they have desirable characteristics, such as resisting pH change with dilution. Why would resisting pH change with dilution be advantageous?

Mathematical A catalog in the lab has a recipe for preparing 1 L of a TRIS buffer at \(0.0500 \mathrm{M}\) and with \(\mathrm{pH} 8.0\) : dissolve \(2.02 \mathrm{g}\) of TRIS (free base, \(\mathrm{MW}=121.1 \mathrm{g} / \mathrm{mol}\) ) and \(5.25 \mathrm{g}\) of TRIS hydrochloride (the acidic form, \(\mathrm{MW}=157.6 \mathrm{g} / \mathrm{mol}\) ) in a total volume of \(1 \mathrm{L}\). Verify that this recipe is correct.

Reflect and Apply Both RNA and DNA have negatively charged phosphate groups as part of their structure. Would you expect ions that bind to nucleic acids to be positively or negatively charged? Why?
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