What type of electrode is used for accurate pH determinations in chemical or clinical laboratories?
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A glass electrode that is selectively sensitive to H+ concentration but insensitive to Na+, K+, and other cations.
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What type of electrode is used for accurate pH determinations in chemical or clinical laboratories?
A glass electrode that is selectively sensitive to H+ concentration but insensitive to Na+, K+, and other cations.
What are strong acids and bases, and how do they behave in dilute aqueous solutions?
Strong acids like hydrochloric, sulfuric, and nitric acids, and strong bases like NaOH and KOH, are completely ionized in dilute aqueous solutions.
How does a pH meter work?
The signal from the glass electrode placed in a test solution is amplified and compared with the signal generated by a solution of accurately known pH.
Why are weak acids and bases of more interest to biochemists?
Because they are not completely ionized when dissolved in water and play important roles in metabolism and its regulation.
Why is pH measurement important in biochemistry?
Because pH affects the structure and activity of biological macromolecules, and a small change in pH can cause a large change in the structure and function of a protein.
How are acids and bases defined in the Brønsted-Lowry theory?
Acids are proton donors, and bases are proton acceptors.
What are equilibrium constants for ionization reactions usually called?
Ionization constants or acid dissociation constants.
What are common medical uses of pH measurements?
Measurements of the pH of blood and urine are commonly used in medical diagnoses.
What happens when a proton donor such as acetic acid loses a proton?
It becomes the corresponding proton acceptor, in this case, the acetate anion (CH3COO−).
What symbol is often used to designate acid dissociation constants?
Kₐ
What is the normal pH value of blood plasma?
7.4
What is a conjugate acid-base pair?
A proton donor and its corresponding proton acceptor, related by a reversible reaction.
What is the relationship between the strength of an acid and its ionization constant?
Stronger acids have larger ionization constants, while weaker acids have smaller ionization constants.
What condition is indicated by a blood plasma pH below 7.4 in people with severe, uncontrolled diabetes?
Acidosis
What is the reversible reaction for the conjugate acid-base pair of acetic acid?
CH3COOH ⇌ CH3COO− + H+
What is the formula for the equilibrium constant (Kₐ) of an ionization reaction?
Kₐ = [H⁺][A⁻] / [HA]
What condition is indicated by a higher than normal pH of the blood?
Alkalosis
What is pKₐ and how is it defined?
pKₐ is analogous to pH and is defined by the equation pKₐ = -log Kₐ.
Why can extreme acidosis or alkalosis be life-threatening?
Because they can cause significant changes in the structure and function of biological macromolecules.
How does the tendency to dissociate a proton relate to the strength of an acid and its pKₐ?
The stronger the tendency to dissociate a proton, the stronger the acid and the lower its pKₐ.
How do weak acids affect the pH of an aqueous solution?
Weak acids partially ionize to release a hydrogen ion, thus lowering the pH of the aqueous solution.
How can the pKₐ of any weak acid be determined?
Experimentally.
How do weak bases affect the pH of an aqueous solution?
Weak bases accept a hydrogen ion, increasing the pH.
What do titration curves reveal about weak acids?
They reveal the pKₐ of weak acids.
What is the acid dissociation constant (Ka) used for?
It expresses the extent of ionization of a weak acid or base.
What is the pKa value of dihydrogen phosphate corrected for buffer concentration and measured at physiological temperature?
6.86
How is the relative strength of a weak acid or base expressed on a logarithmic scale?
By the pKa value.
What are the three weak acids compared in the titration curves?
CH₃COOH, H₂PO₄⁻, and NH₄⁺.
What is the apparent pKa value of dihydrogen phosphate not corrected for buffer concentration?
7.2
What is the relationship between the strength of an acid and its pKa value?
The stronger the acid, the smaller its pKa.
What is the expression for the ionization constant (Ka) of a weak acid HA?
Ka = [H+][A-] / [HA]
What is the pKa value for NH₄⁺?
9.25.
At what temperature is the apparent pKa value of dihydrogen phosphate defined?
25°C
What is the relationship between the strength of a base and the pKa of its conjugate acid?
The stronger the base, the larger the pKa of its conjugate acid.
How do you solve for [H+] in the ionization constant expression for a weak acid HA?
[H+] = Ka * ([HA] / [A-])
At what pH range are conjugate acid-base pairs effective buffers?
Between approximately 10% and 90% neutralization of the proton-donor species.
What does pure water ionize into?
Equal numbers of hydrogen ions (hydronium ions, H3O+) and hydroxide ions.
How can the pKa be determined experimentally?
It is the pH at the midpoint of the titration curve.
What is the result of taking the negative logarithm of both sides of the equation [H+] = Ka * ([HA] / [A-])?
-log[H+] = -logKa - log([HA] / [A-])
What is the equilibrium constant (Keq) expression for the ionization of water?
Keq = [H+][OH-] / [H2O]
What are the predominant ionic forms at the midpoint of titration for CH₃COOH?
CH₃COOH and CH₃COO⁻.
What substitutions are made in the Henderson-Hasselbalch equation?
Substitute pH for -log[H+] and pKa for -logKa.
What is the pKa value for H₂PO₄⁻?
6.86.
What is the ion product of water (Kw) at 25°C?
Kw = [H+][OH-] = 10^-14 M^2
What did J.B.S. Haldane and H.W. Davies experiment on to study how the body controls blood pH?
They experimented on themselves.
What is the Henderson-Hasselbalch equation?
pH = pKa - log([HA] / [A-])
What conditions are equilibrium constants Ka and pKa defined for?
Specific conditions of concentration (components at 1 M) and temperature (25°C).
How is the pH of an aqueous solution defined on a logarithmic scale?
pH = -log[H+]
How did Haldane and Davies make themselves alkaline?
By hyperventilating and ingesting sodium bicarbonate.
What results from two reversible reaction equilibria in a solution of nearly equal concentrations of a proton donor and its conjugate proton acceptor?
Buffering.
Why is the pKa value of 6.86 for dihydrogen phosphate used throughout the book?
It is corrected for buffer concentration and measured at physiological temperature, making it a closer approximation to the relevant value in warm-blooded animals.
What is the buffering region for NH₃?
Between pH 8.25 and 10.25.
Why did Haldane and Davies experience panting and violent headaches?
Because they made themselves alkaline by hyperventilating and ingesting sodium bicarbonate.
What happens when H+ or OH- is added to a buffer system?
The result is a small change in the ratio of the relative concentrations of the weak acid and its anion, leading to a small change in pH.
What is the pKa value for CH₃COOH?
4.76.
Why did Haldane and Davies decide against drinking hydrochloric acid to acidify themselves?
They calculated that it would take a gallon and a half of dilute HCl to get the desired effect, which would dissolve their teeth and burn their throats.
How is the decrease in concentration of one component of a buffer system balanced?
It is balanced exactly by an increase in the other component.
What alternative method did Haldane use to acidify himself?
He ate ammonium chloride, which breaks down in the body to release hydrochloric acid and ammonia.
Does the sum of the buffer components change when H+ or OH- is added?
No, only their ratio changes.
What was the result of Haldane's experiment with ammonium chloride?
The resulting shortness of breath mimicked that in diabetic acidosis or end-stage kidney disease.
What does the acetic acid–acetate pair buffer system absorb?
It absorbs either H+ or OH- through the reversibility of the dissociation of acetic acid.
What condition were Ernst Freudenberg and Paul György studying in infants?
Tetany, which involves muscle contractions in the hands, arms, feet, and larynx.
What does the proton donor, acetic acid (HAc), contain?
It contains a reserve of bound H+, which can be released to neutralize an addition of OH- to the system, forming H2O.
How did Freudenberg and György treat tetany in infants after reading Haldane's paper?
They gave ammonium chloride to babies with tetany, which cleared up the tetany in a few hours.
What happens when the product [H+][OH-] transiently exceeds Kw?
The equilibrium quickly adjusts to restore the product to 1 x 10^-14 M^2 (at 25°C).
What is titration used for?
To determine the amount of an acid in a given solution.
At what pH is the phosphate buffer system maximally effective?
At a pH close to its pKa of 6.86.
What is typically used as the strong base in titration?
Sodium hydroxide (NaOH).
What pH range does the phosphate buffer system tend to resist changes in?
Between about 5.9 and 7.9.
Why is pH important in biological processes?
Almost every biological process is pH-dependent; a small change in pH produces a large change in the rate of the process.
How is the endpoint of a titration determined?
With an indicator dye or a pH meter.
What are buffers?
Buffers are aqueous systems that tend to resist changes in pH when small amounts of acid or base are added.
Why is the phosphate buffer system effective in biological fluids?
Because extracellular fluids and most cytoplasmic compartments in mammals have a pH in the range of 6.9 to 7.4.
What role do H+ ions play in biological reactions?
H+ ions are direct participants in many reactions, but even in reactions where there is no apparent role for H+ ions, pH changes can still affect the process.
How can the concentration of the acid in the original solution be calculated?
From the volume and concentration of NaOH added.
What does a buffer system consist of?
A buffer system consists of a weak acid (the proton donor) and its conjugate base (the proton acceptor).
What equation is used to calculate the pH of a phosphate buffer solution?
The Henderson-Hasselbalch equation.
How do enzymes and molecules involved in cellular reactions relate to pH?
Enzymes and many molecules contain ionizable groups with characteristic pKa values, which are influenced by the pH of the surrounding medium.
What is an equivalent in the context of titration?
The amount of a substance that will react with, or supply, one mole of hydrogen ions in an acid-base reaction.
What is an example of a buffer system?
A mixture of equal concentrations of acetic acid and acetate ion.
In the Henderson-Hasselbalch equation, what is the acid in the phosphate buffer system?
H2PO4- (the species that gives up a proton).
What are examples of weak acids in biological systems?
The protonated amino and carboxyl groups of amino acids and the phosphate groups of nucleotides function as weak acids.
How many equivalents are there in one mole of a monoprotic acid like HCl?
1 equivalent.
What is the buffering region in a titration curve?
The relatively flat zone extending about 1 pH unit on either side of the midpoint pH of 4.76 in the titration curve of acetic acid.
In the Henderson-Hasselbalch equation, what is the conjugate base in the phosphate buffer system?
HPO4^2- (the species that gains a proton).
How does the pH of the surrounding medium affect ionizable groups in proteins?
The ionic state of ionizable groups is determined by the pH of the surrounding medium, and when sequestered in the middle of a protein, their apparent pKa can differ significantly from their pKa in water.
How many equivalents are there in one mole of a diprotic acid like H₂SO₄?
2 equivalents.
What happens at the midpoint of the buffering region?
The concentration of the proton donor (acetic acid) exactly equals that of the proton acceptor (acetate), and the buffering power is maximal.
What is the pH of a mixture of 0.042 M NaH2PO4 and 0.058 M Na2HPO4?
The pH can be calculated using the Henderson-Hasselbalch equation.
What role do ionic interactions play in protein stability?
Ionic interactions are among the forces that stabilize a protein molecule and allow an enzyme to recognize and bind to its substrate.
What does a titration curve plot?
pH against the amount of NaOH added.
How does the pH of the acetate buffer system change when a small amount of H+ or OH- is added?
The pH changes slightly, but the change is very small compared to the pH change that would result if the same amount were added to pure water or a solution of NaCl.
What are the pKa values for histidine?
pK1 = 1.8, pK2 (imidazole) = 6.0, and pK3 = 9.2.
How do cells and organisms maintain a specific and constant cytosolic pH?
Cells and organisms maintain a specific and constant cytosolic pH, usually near pH 7, to keep biomolecules in their optimal ionic state.
What is the pH at the midpoint of the titration curve of acetic acid equal to?
The apparent pKa of acetic acid.
What characteristic do the intracellular and extracellular fluids of multicellular organisms share?
They have a characteristic and nearly constant pH.
At what pH is the carboxyl group of histidine entirely deprotonated?
At pH 7.3.
How is constancy of pH achieved in multicellular organisms?
Constancy of pH is achieved primarily by biological buffers, which are mixtures of weak acids and their conjugate bases.
Why does NaCl have no buffering power?
Because it is a salt of a strong acid and strong base, and does not resist changes in pH.
What is the first line of defense against changes in internal pH in multicellular organisms?
Buffer systems.
At what pH is the α-amino group of histidine fully protonated?
At pH 7.3.
What do the high concentrations of proteins in the cytoplasm of most cells contain?
Many amino acids with functional groups that are weak acids or weak bases.
Which group in histidine is partially dissociated at pH 7.3?
The imidazole group.
What is the pKa of the side chain of histidine?
6.0
What equation is used to calculate the fraction of histidine with its imidazole side chain protonated?
The Henderson-Hasselbalch equation.
In what forms can histidine exist near neutral pH?
Either the protonated form or the unprotonated form.
What is the Henderson-Hasselbalch equation?
pH = pKa + log([A-]/[HA]).
Why do proteins containing histidine residues buffer effectively near neutral pH?
Because histidine can exist in either the protonated or unprotonated form near neutral pH.
What is the pKa value used for the imidazole group in histidine?
pKa = 6.0.
What is plotted on the titration curve of acetic acid?
The pH of the mixture is plotted against the amount of NaOH added, expressed as a fraction of the total NaOH required to convert all the acetic acid to its deprotonated form, acetate (CH3COO-).
What is the role of histidine in proteins regarding pH buffering?
Histidine residues in proteins help buffer effectively near neutral pH.
What is the pH value used to calculate the fraction of protonated imidazole in histidine?
pH = 7.3.
What happens at the midpoint of the titration of acetic acid?
The concentrations of the proton donor (CH3COOH) and the proton acceptor (CH3COO-) are equal, and the pH is numerically equal to the pKa.
What is shown in Figure 2-19?
The ionization of histidine, illustrating its role as a weak acid with a pKa of 6.0.
What is the pKa value of acetic acid as shown in the titration curve?
4.76
What do conjugate acid-base pairs consist of?
A proton donor and a proton acceptor.
What is the useful region of buffering power in the titration of acetic acid?
Generally between 10% and 90% titration of the weak acid.
What are monoprotic compounds? Give examples.
Compounds that can give up only one proton, such as acetic acid and ammonium ion.
What are the predominant ionic forms at the points designated in the titration curve of acetic acid?
CH3COOH and CH3COO-
What are diprotic compounds? Give examples.
Compounds that can give up two protons, such as carbonic acid and glycine.
What is the pH value at which the buffering region is centered in the titration curve of acetic acid?
Around pH 4.76
What are triprotic compounds? Give an example.
Compounds that can give up three protons, such as phosphoric acid.
What system buffers blood plasma?
The bicarbonate system.
What does the shaded zone in the titration curve of acetic acid represent?
The useful region of buffering power.
What does the equilibrium or dissociation constant (K) represent?
It represents the tendency of an acid to lose its proton and form its conjugate base.
What are the components of the bicarbonate buffering system?
Carbonic acid (H2CO3) as proton donor and bicarbonate (HCO3-) as proton acceptor.
What must the equilibria conform to during the titration of acetic acid?
Their characteristic equilibrium constants.
How is the strength of an acid related to its tendency to lose a proton?
The stronger the acid, the greater its tendency to lose its proton.
What is the first equilibrium constant in the bicarbonate buffering system?
K1 = [H+][HCO3-] / [H2CO3]
What is the formula for the dissociation reaction of an acid (HA)?
HA ⇌ H⁺ + A⁻
Why is the bicarbonate buffer system more complex than other conjugate acid-base pairs?
Because one of its components, carbonic acid (H2CO3), is formed from dissolved carbon dioxide (CO2) and water (H2O) in a reversible reaction.
What does the negative logarithm of the equilibrium constant (K) represent?
It represents the pKₐ value for each reaction.
What is the reversible reaction that forms carbonic acid in the bicarbonate buffer system?
CO2(aq) + H2O ⇌ H2CO3
What is the equilibrium constant for the formation of carbonic acid from dissolved CO2 and water?
K2 = [H2CO3] / ([CO2(aq)][H2O])
Under normal conditions, what phase is carbon dioxide in?
Carbon dioxide is a gas under normal conditions.
What is the equilibrium constant for CO2 dissolved in an aqueous solution being in equilibrium with CO2 in the gas phase?
Ka = [CO2(aq)] / [CO2(g)]
What is the ratio of [His] to [HisH⁺] at pH 7.3?
20 to 1
How do you convert the ratio of [His] to [HisH⁺] to the fraction of total histidine in the unprotonated form at pH 7.3?
The fraction is 20/21, or about 95.2%
What percentage of histidine is protonated at pH 7.3?
About 5%
What is the pH when the given concentrations of acid and conjugate base are substituted into the equation?
The pH is 7.0.
What types of molecules contain ionizable groups that contribute buffering power to the cytoplasm?
Nucleotides such as ATP and many metabolites of low molecular weight
What happens to the pH when more conjugate base than acid is present?
The pH is above the pKa (6.86), indicating the acid is more than 50% titrated.
What do organic acids buffer in plant cells?
The vacuoles
How much NaOH is added to the buffer in the given problem?
1.0 mL of 10.0 M NaOH.
What buffers urine?
Ammonia
How many moles of NaH2PO4 are in a liter of the buffer?
0.042 mol.
What are two especially important biological buffers?
The phosphate and bicarbonate systems
What is the amount of NaOH added in moles?
0.010 mol.
What does the phosphate buffer system consist of in the cytoplasm of all cells?
H₂PO₄⁻ as proton donor and HPO₄²⁻ as proton acceptor
What are the new concentrations of NaH2PO4 and Na2HPO4 after adding NaOH?
0.032 mol of NaH2PO4 and 0.068 mol of Na2HPO4.
What is the new pH after adding NaOH to the buffer?
The new pH is 7.2.
What is the formula used to calculate the new pH after adding NaOH?
pH = pKa + log([HPO4^2-]/[H2PO4^-]).
What happens to acetic acid as NaOH is added during titration?
The remaining nondissociated acetic acid is gradually converted into acetate.
At what pH does the endpoint of acetic acid titration occur?
At about pH 7.0.
What are the two equilibria that coexist throughout the titration of acetic acid?
The equilibria described by Equations 2-5 and 2-6.
What does Figure 2-17 compare?
The titration curves of three weak acids with very different ionization constants: acetic acid, dihydrogen phosphate, and ammonium ion.
Why are the titration curves of the three weak acids displaced along the pH axis?
Because the three acids have different strengths.
Which weak acid is the strongest among acetic acid, dihydrogen phosphate, and ammonium ion?
Acetic acid, with the highest Ka (lowest pKa).
At what pH is acetic acid half dissociated?
At pH 4.76.
What does the titration curve of a weak acid and its anion show?
That a weak acid and its anion—a conjugate acid-base pair—can act as a buffer.
What happens to NaOH when it is added to water?
NaOH dissociates completely into Na+ and OH-.
What is the concentration of OH- when 1.0 mL of 10.0 M NaOH is added to a liter of pure water?
0.010 mol/L or 1.0 × 10^-2 M.
How is pOH defined?
pOH is the negative logarithm of the concentration of OH-.
What is the pOH of the solution when 1.0 mL of 10.0 M NaOH is added to a liter of pure water?
pOH = 2.0.
What is the relationship between pH and pOH in all solutions?
pH + pOH = 14.
What is the final pH of the solution when 1.0 mL of 10.0 M NaOH is added to a liter of pure water at pH 7.0?
The final pH is 12.
How does the addition of NaOH affect the pH of a buffered solution compared to pure water?
In pure water, the pH increases from 7 to 12, while in a buffered solution, it increases from 7.0 to just 7.2.
Why is pH + pOH equal to 14?
Because -log(10^-14) = -log[H+] + -log[OH-], which simplifies to 14 = pH + pOH.
What is the Henderson-Hasselbalch equation?
pH = pKa + log([A-]/[HA])
What does the Henderson-Hasselbalch equation help to deduce?
Important quantitative relationships in the titration curve of weak acids.
At what point is the pKa of a weak acid equal to the pH of the solution?
At the midpoint of its titration, where [HA] = [A-].
How can the Henderson-Hasselbalch equation be used to calculate pKa?
Given pH and the molar ratio of proton donor and acceptor.
How can the Henderson-Hasselbalch equation be used to calculate pH?
Given pKa and the molar ratio of proton donor and acceptor.
How can the Henderson-Hasselbalch equation be used to calculate the molar ratio of proton donor and acceptor?
Given pH and pKa.
Why is the pKa of a weak acid equal to the pH at the midpoint of its titration?
Because at that point, [HA] = [A-], making the log term zero.
What role do weak acids or bases play in cells and tissues?
They buffer cells and tissues against pH changes.
On what does the pH of a bicarbonate buffer system depend?
The concentrations of H₂CO₃ and HCO₃⁻, the proton donor and acceptor components.
What determines the concentration of H₂CO₃ in a bicarbonate buffer system?
The concentration of dissolved CO₂, which in turn depends on the concentration of CO₂ in the gas phase or the partial pressure of CO₂ (pCO₂).
What ultimately determines the pH of a bicarbonate buffer exposed to a gas phase?
The concentration of HCO₃⁻ in the aqueous phase and by pCO₂ in the gas phase.
Why is the bicarbonate buffer system an effective physiological buffer near pH 7.4?
Because the H₂CO₃ of blood plasma is in equilibrium with a large reserve capacity of CO₂(g) in the air space of the lungs.
What are the three reversible equilibria involved in the bicarbonate buffer system?
The equilibria between gaseous CO₂ in the lungs and bicarbonate (HCO₃⁻) in the blood plasma.
What is the role of CO₂ in the bicarbonate buffer system?
CO₂ in the gas phase affects the concentration of H₂CO₃ in the aqueous phase, influencing the pH.
What is the partial pressure of CO₂ denoted as?
pCO₂.
What is the relationship between H₂CO₃ and HCO₃⁻ in the bicarbonate buffer system?
H₂CO₃ acts as the proton donor and HCO₃⁻ acts as the proton acceptor.
What happens to acetic acid at the beginning of the titration before any NaOH is added?
The acetic acid is already slightly ionized, to an extent that can be calculated from its ionization constant.
What does the added OH- from NaOH combine with during the titration?
The added OH- combines with the free H+ in the solution to form H2O.
What happens to HAc as free H+ is removed during the titration?
HAc dissociates further to satisfy its own equilibrium constant.
What is the net result as the titration of acetic acid with NaOH proceeds?
More and more HAc ionizes, forming Ac-, as NaOH is added.
What occurs at the midpoint of the titration of acetic acid with NaOH?
At the midpoint, exactly 0.5 equivalent of NaOH has been added per equivalent of the acid, and one-half of the original acetic acid has undergone dissociation.
At the midpoint of the titration, what is the relationship between the concentration of the proton donor and the proton acceptor?
The concentration of the proton donor [HAc] equals that of the proton acceptor [Ac-].
What is the pH of the equimolar solution of acetic acid and acetate at the midpoint of the titration?
The pH is exactly equal to the pKa of acetic acid.
What is the pKa of acetic acid?
The pKa of acetic acid is 4.76.
