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Questions and Answers
What is the expected ratio of protonated to deprotonated histidine in blood at a pH of 7.3?
What is the expected ratio of protonated to deprotonated histidine in blood at a pH of 7.3?
What is the pKa value of the imidazole side chain of histidine?
What is the pKa value of the imidazole side chain of histidine?
What happens to the percentage of protonated histidine as pH increases from 7.0 to 8.0?
What happens to the percentage of protonated histidine as pH increases from 7.0 to 8.0?
At a pH of 8.0, what is the estimated ratio of protonated to deprotonated histidine?
At a pH of 8.0, what is the estimated ratio of protonated to deprotonated histidine?
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What can be inferred about histidine's behavior in blood at a pH of 7.3?
What can be inferred about histidine's behavior in blood at a pH of 7.3?
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What happens to the hydrogen ion concentration when the pH of a solution decreases?
What happens to the hydrogen ion concentration when the pH of a solution decreases?
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How does a buffer system function in biological systems?
How does a buffer system function in biological systems?
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Which statement about the Henderson-Hasselbalch equation is correct?
Which statement about the Henderson-Hasselbalch equation is correct?
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Why is it significant that a buffer is more effective near its pKa?
Why is it significant that a buffer is more effective near its pKa?
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Given a compound with a pKa of 4.76, how would it behave at a pH of 5.76?
Given a compound with a pKa of 4.76, how would it behave at a pH of 5.76?
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What is the role of dihydrogen phosphate (H2PO4-) in the phosphate buffer system?
What is the role of dihydrogen phosphate (H2PO4-) in the phosphate buffer system?
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Which substance in the bicarbonate buffer system forms carbonic acid when CO2 is dissolved in water?
Which substance in the bicarbonate buffer system forms carbonic acid when CO2 is dissolved in water?
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What condition occurs as a result of hyperventilation altering the balance of CO2 and O2?
What condition occurs as a result of hyperventilation altering the balance of CO2 and O2?
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How does the Henderson-Hasselbalch equation demonstrate the influence of pH on weak acids?
How does the Henderson-Hasselbalch equation demonstrate the influence of pH on weak acids?
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At which pH does aspirin (acetylsalicylic acid) have equal concentrations of its non-ionized and ionized forms?
At which pH does aspirin (acetylsalicylic acid) have equal concentrations of its non-ionized and ionized forms?
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What happens to bicarbonate (HCO3-) concentration during hyperventilation?
What happens to bicarbonate (HCO3-) concentration during hyperventilation?
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What pH range characterizes the bicarbonate buffer system's effectiveness?
What pH range characterizes the bicarbonate buffer system's effectiveness?
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What triggers the brain stem to adjust respiration rates based on blood pH?
What triggers the brain stem to adjust respiration rates based on blood pH?
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What is the expected outcome when pH decreases due to increased carbonic acid in the bicarbonate buffer system?
What is the expected outcome when pH decreases due to increased carbonic acid in the bicarbonate buffer system?
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Which factor primarily influences the distribution of weak acids in biological fluids?
Which factor primarily influences the distribution of weak acids in biological fluids?
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Study Notes
Acid-Base Chemistry and Henderson-Hasselbalch Equation
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Objectives:
- Describe the relationship between pH and hydrogen ion concentration.
- Relate pKa to the ratio of associated and dissociated compounds.
- Estimate the percentage of a compound that is dissociated or associated given pKa and pH.
- Describe the function of a buffer in a biological system.
- Explain how the bicarbonate system is a physiological buffer.
- Describe why a buffer is more effective near its pKa(s).
- Compare the ability of a compound to cross a biological membrane in the charged vs. uncharged state.
What is pH?
- Hydrogen ion (H+) concentration in an aqueous solution.
- pH = -log[H+].
- Lower pH, higher H+ concentration.
- Logarithmic scale; a change of one pH unit corresponds to a tenfold change in H+ concentration.
Why is pH Important?
- pH affects the structure and activity of biological macromolecules.
- Small changes in pH can cause large changes in structure and function.
- Blood and urine pH are used in medical diagnoses.
- Blood pH < 7.4 = acidosis; Blood pH > 7.4 = alkalosis.
- The body has a buffering capacity to maintain pH.
Conjugate Acid-Base Pair
- pH is determined by H+.
- H+ depends on solutes functioning as acids (proton donors) or bases (proton acceptors).
- Biological systems contain weak acids and their conjugate bases.
- Strength of the tendency to lose or gain a proton in H2O.
- A proton donor and its corresponding proton acceptor form a conjugate acid-base pair.
Acid Dissociation Constant (Ka)
- Ka describes the affinity of an acid for the dissociable H+.
- Keq for the reaction is called the acid dissociation constant, Ka.
- Weak acids have a low Ka.
- High HA concentration.
- Strong acids have a high Ka.
- Most in dissociated form (A-).
Acid Dissociation Constant (Ka)
- Ka values are small.
- Ka is expressed as pKa.
- Weak acids have a high pKa.
- Strong acids have a low pKa.
- Examples include ammonia (pKa 9.25) and acetic acid (pKa 4.76).
Conjugate Acid-Base Pair (Examples)
- Monoprotic: Acetic acid (CH3COOH), Acetate (CH3COO-), Ammonium ion.
- Diprotic: Carbonic acid, Glycine.
- Triprotic: Phosphoric acid.
Titration Curve
- Titration is used to determine the amount of acid.
- Acid is titrated with a solution of a strong base (NaOH).
- Plot of pH against amount of NaOH added, revealing the pKa of the weak acid.
Titration Curve (Continued)
- As NaOH is added, OH- combines with H+ to form water (H2O).
- CH3COOH dissociates further, resulting in more acetate (CH3COO-).
- At midpoint, the concentration of the proton donor equals the concentration of the proton acceptor (pH = pKa).
Titration Curve (Additional Points)
- Three weak acids with different dissociation constants (acetic acid, dihydrogen phosphate, ammonium ion).
- Their highest Ka corresponds to their lowest pKa and strong acids have lower pKa.
Buffers
- Resist changes in pH when small amounts of acid (H+) or base (OH-) are added.
- Buffer systems consist of a weak acid and its conjugate base.
- Effective within ±1 pH unit of the pKa of the weak acid.
Acetic Acid-Acetate as a Buffer System
- Nearly equal concentrations of conjugate proton donor and acceptor.
- Small change in the ratio of weak acid and its anion results in small change in pH.
Henderson-Hasselbalch Equation
- pH = pKa + log ([A-]/[HA]).
- Relates pH, pKa, and the ratio of proton acceptor (A-) to proton donor (HA).
- Shows that the pKa of a weak acid is equal to the pH of the solution at the midpoint of its titration.
Henderson-Hasselbalch Equation (Continued)
- Knowing the pKa of an acid and the pH, one can predict if the acid is largely protonated or largely deprotonated.
- If pH > pKa, the ratio [A-]/[HA] > 1 and the acid is largely deprotonated.
- If pH < pKa, the ratio [A-]/[HA] < 1 and the acid is largely protonated.
Amino Acids Buffer Cells and Tissues
- Intracellular and extracellular fluids have a characteristic and nearly constant pH.
- Defense mechanism against changes in pH = buffer systems.
- Cytoplasm of most cells contains high concentrations of proteins.
- Functional groups (weak acids/bases) buffer effectively near neutral pH (e.g., histidine).
Ionization of Histidine in the Blood
- The pKa value of the imidazole side chain of histidine is 6.0.
- Blood pH is 7.3. The ratio of protonated to deprotonated histidine is estimated to be 10:1.
Phosphate Buffer System
- Dihydrogen phosphate (H2PO4-) acts as the proton donor.
- Hydrogen phosphate (HPO42-) acts as the proton acceptor.
- A buffer system at physiological pH, near neutral pH (approximately 6.86).
- Active in biological fluids, effective in extracellular and intracellular environments, at pH ranges 6.9–7.4.
Bicarbonate Buffer System
- Carbonic acid (H2CO3) is the proton donor.
- Bicarbonate (HCO3-) is the proton acceptor.
- CO2(aq) is in equilibrium with CO2(g).
- pH is ultimately determined by dissolved bicarbonate (HCO3-) and CO2 in biological fluids.
Bicarbonate Buffer System (Continued)
- Effective near pH 7.4.
- Involves three reversible reactions.
- CO2 in lungs and bicarbonate in the blood are involved in maintaining blood pH (controlled by brain stem).
- Detection of changes in CO2 and pH.
Bicarbonate Buffer System (Further Points)
- Hyperventilation upsets the balance of O2 and CO2, favoring CO2 removal, raising blood pH, and leading to alkalosis.
- Homes remedies for mild alkalosis include breathing into a paper bag, increasing CO2 in the blood.
Influence of pH on Ionizable Drugs (General Principles)
- Many drugs are weak acids that exist in solution as both nonionized/lipid-soluble and ionized/lipid-insoluble forms.
- Distribution across membranes is affected by pKa and pH of the environment.
- The pKa represents the pH at which half of the drug is in its ionized form.
- Higher pH → increased dissociation.
- Lower pH → decreased dissociation.
Dissociation of Aspirin
- Aspirin (acetylsalicylic acid) has a pKa of 3.5.
- At pH 3.5, aspirin is 1:1 associated:dissociated.
- Stomach pH is ~1.5.
- Aspirin in stomach is largely associated (protonated).
- Duodenum pH is ~ 6.5.
- Aspirin in duodenum is largely dissociated (deprotonated).
- A 10-fold change in H+ concentration corresponds to a one-unit change in pH.
- A 100- or 1000-fold change corresponds to two- or three-unit pH changes.
Estimating % Dissociated
- Given the pH and pKa, and knowing the ratio relationship of protonated to deprotonated forms, one can estimate the percentage dissociated.
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Description
This quiz covers key concepts in acid-base chemistry, including the Henderson-Hasselbalch equation. You'll learn about the relationship between pH, hydrogen ion concentration, and how buffers function in biological systems. Additionally, it explains the significance of the bicarbonate system as a physiological buffer.
