Acid-Base Principles

Question 1

If you are given concentrations for [H⁺] & [Cl^-], how do you calculate the pH of the HCl solution?

Answer 1

pH = -log [H+]

Question 2

What is physiological pH?

Answer 2

7.4 - slightly alkaline

Question 3

In the dissociation of weak acid HB: HB ⇆ H+ + B-
the dissociated H+ & conjugate base will reach an equilibrium with the weak acid HB according to WHAT?

Answer 3

The dissociation constant measures the propensity of, in this case, a weak acid to dissociate into an acid (proton) and its conjugate base.

Question 4

How do you calculate the dissociation constant, K_a?

Answer 4

K_a = [H⁺][B^-] / [AB]

The dissociated acid and base will reach an equilibium according to K_a:

HB ⇆ H⁺ + B^-

Question 5

How does the dissociation constant, K_a, relate to the Henderson-Hasselbalch equation?

Answer 5

The Henderson-Hasselbalch equation rearranges the formula for the dissociation constant for calculation of pKa, which can then be used to calculate the pH of a solution.

pKa = - log₁₀ [H+] [B-] / [HB]

Ie., pKa is the negative log (base 10) of the dissociation constant

Since pH = -log [H⁺] then:

pH = pKa + log₁₀ [B^-] / [A⁺]

NB: If the acid and base are in equal concentrations, then pH = pKa because the log of 1 = 0

Question 6

A solution of a weak acid is most effective as a buffer when its pH is ??? its pKa value?

A. higher than

B. lower than

C. equal to

Answer 6

C. equal to … or at least close to

That is, if [BASE] = [ACID], then their ratio will be 1. The log of 1 = 0, so pH = pKa.

Question 7

What is the MOST ABUNDANT BUFFER in the mammalian body system?

Answer 7

PROTEIN

protein^n- ⇆ protein ⁽ⁿ⁺¹⁾ + H+

pKa various

The most abundant buffer system in cell cytoplasm and in blood plasma. Since proteins are composed of amino acids joined in a linear string by peptide bonds, they always possess at least one free amino group (-NH₂) and one free carboxylic acid group (COOH^-) at opposite ends of the strand, and they often possess additional free amino groups and carboxylic acid groups because those groups are present on the R-group side chains of a variety of amino acids. These groups act as weak acids and bases respectively, and, as a result, can combine with H+ ions, reducing the concentration of free H+ ions in the solution, a buffering function.

Question 8

Which buffer has a pKa that is closest to physiological pH and thus makes it an ideal buffer?

Answer 8

H₂PO₄^- ⇆ HPO₄^2- + H⁺

pKa 6.8 (closest to physiological pH 7.4 - ideal for buffer)

Phosphoric acid (H₂PO₄^- ) is illustrative of a weak acid that has more than one dissociable proton.

The pH increases in the buffer solution in a series of steps (gets more alkaline), first when it dissociates into dihydrogen phosphate (pKa 2.1), second when it dissociates to monohydrogen phosphate ion (pKa 6.8) and finally to phosphate ion, with a very alkaline pKa of 12.3.
Phosphoric acid-monohydrogen phosphate is an important acid-base pair for buffering the blood because the pKa is very close to pH.

Question 9

What is the MOST IMPORTANT BUFFER system in the body, and why?

Answer 9

H₂CO₃ ⇆ HCO₃^- + H⁺

pKa 6.1

Carbonic acid in blood is most important buffer system in the body because it also dissociates into H₂O and CO₂.

The concentration of carbonic acid is directly related to PCO₂ in the blood. Since we know from Henderson Hasselbach equation that pH is dependent on pKa + log [BASE] / [ACID], then pH must also be dependent on PCO₂ in the blood. Hence both pH and PCO₂ can be changed by the rate & depth of breathing.

The bicarbonate concentration of the buffer system can also be regulated through EXCRETION via kidney. There is also more carbonic acid and bicarbonate in blood plasma than other buffers, so it also has great physical capacity for buffering.

Question 10

How does carbonic acid work as a buffer in blood plasma? Start with how carbonic acid is formed in the blood by bicarbonate ions.

Answer 10

H₂CO₃ ⇆ HCO₃^- + H⁺

Bicarbonate ions absorb H+ ions to form carbonic acid, which can be transported to the lungs where the reaction is reversed, the H+ ions are converted to water molecules and CO₂ is excreted. It is an extracellular buffer.

Question 11

Can carbonic acid buffer against acidity that comes as a result of hypoventilation?

Why not?

Answer 11

No, it cannot buffer against the acidity of excess CO₂, which occurs with hypoventilation; this buffer system is dependent on a functioning respiratory system to excrete the carbon dioxide.

The main role of this system is to buffer against the acids produced by fat & protein metabolism or ones produced in oxygen deficiency or starvation. Ie., metabolic acidosis.

Question 12

How does Haemoglobin (Hb) in red blood cells help support the buffering action of carbonic acid?

Answer 12

Hb supports the carbonic acid-bicarbonate (H₂CO₃ → HCO₃ + H+) buffer system:

After carbonic anhydrase in the RBC catalyses the formation of carbonic acid from carbon dioxide (which diffuses into red blood cells from tissue) and water, Hb binds to dissociated proton while conjugate base, bicarbonate, diffuses out across RBC membrane into plasma in exchange for chloride ion, helping to buffer plasma’s acidic effects due to CO₂ concentration.

Question 13

Aside from supporting the carbonic acid (H₂CO₃) bicarbonate (HCO₃^-) + proton buffering reaction, how else does haemoglobin act as a buffer?

Answer 13

1) Hb’s binding to dissociated proton reduces concentration of free hydrogen ion in RBC cytoplasm, resisting pH change in blood.
2) Bohr effect results in hydrogen ions being released from haemoglobinic acid (HbH) & converted to water molecules, to be expired from lungs as water vapour.

Occurs when RBC arrives in lungs, where it is under high oxygen tension that causes the reversal of Hb + H+ → HbH reaction. The high level of O₂ tension also causes reversal of CO₂ + H₂O → H₂CO₃ reaction, so instead more CO₂ and H₂O produced, both expired from the lungs, further reducing acidifying effects of CO₂.