Passage 2: Acid-Base Regulation

Understanding acid-base regulation is often reduced to pigeonholing clinical

states into categories of disorders based on arterial blood sampling. An earlier

ambition to quantitatively explain disorders by measuring the production and

elimination of acid has not become standard clinical practice. Seeking back to

classical physical chemistry, we propose that in any compartment, the requirement

of electroneutrality leads to a strong relationship between charged moieties.

Figure 1 shows the relationship between [H+] of a mixture and the mean [H+] of two

mixtures. Figure 2 shows the equations related to the water dissociation constant.

Strong Relationships in Acid-Base Chemistry – Modeling Protons Based on

Predictable Concentrations of Strong Ions, Total Weak Acid Concentrations, and

pCO2. Adapted from Ring & Kellum (2016).

Which of the following is true of a buffer solution?

A) The conjugate base of a weak acid will have a significantly lower pH than the

weak acid itself

B) The conjugate acid of a weak base will partially dissociate in water

C) The buffer capacity is highest when the concentrations of the conjugate

acid and base are equal

D) The conjugate base of a strong acid is a strong base

Click to reveal answer

Correct answer: C. The point of a buffer solution is to resist certain

changes to the pH environment, whether that is in a biological system or inorganic.

In order to do this, the conjugate base will react if the environment is acidic, and the

acid will break down if the environment is basic in order to maintain the pH level.

However, this works best if both have an equal concentration since then the buffer

solution can resist pH changes. For example, as seen in the Henderson–Hasselbalch

equation, when the conjugate base and the acid have equal concentrations, the

pH will equal the pKa, which means that the pH is resisting change and adhering

to a steady pKa. This will be disrupted, however, if any of the two species have an

unequal concentration. Thus, answer choice C is the correct answer.

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Day 12 MCAT Practice Question