# pH Acids, bases, pK Conjugate acid-base pairs

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pH Acids, bases, pK Conjugate acid-base pairs
Calculate pK from a titration curve Buffers Handerson-Hasselbalch Equation Practice some problems

Ionization of Water: Quantitative Treatment
Concentrations of participating species in an equilibrium process are not independent but are related via the equilibrium constant: [H+]•[OH-] H2O H+ + OH- Keq = ———— [H2O] Keq can be determined experimentally, it is 1.8•10–16 M at 25C. [H2O] can be determined from water density, it is 55.5 M. Ionic product of water: In pure water [H+] = [OH–] = 10–7 M

What is pH? pH is defined as the negative logarithm of the hydrogen ion concentration Simplifies equations The pH and pOH must always add to 14 In neutral solution, [H+] = [OH–] and the pH is 7 pH can be negative ([H+] = 6 M) pH = -log[H+]

pH scale is logarithmic:
1 unit = 10-fold

pH of Some Common Liquids
FIGURE 2-15 The pH of some aqueous fluids.

Titration The process of gradually adding known amounts of reagent to a solution with which the reagent reacts while monitoring the results is called a titration.

Equilibrium constant=ionization constant= dissociation constant
Each acid has a characteristic tendency to lose its protons in an aqueous solution. The stronger the acid the greater the tendency. The tendency of any acid (HA) to lose a proton and form its conjugate base (A+) is defined by the equilibrium constant K for the reversible rxn.

Equilibrium constant=ionization constant= dissociation constant
HA-----> H+A K= [H][A]/[HA] The relative strengths of weak acids and bases are expressed as their dissociation constant, which expresses the tendency to ionize.

pKa measures acidity pKa = –log Ka (strong acid  large Ka  small pKa) FIGURE 2-16 Conjugate acid-base pairs consist of a proton donor and a proton acceptor. Some compounds, such as acetic acid and ammonium ion, are monoprotic; they can give up only one proton. Others are diprotic (carbonic acid and glycine) or triprotic (phosphoric acid). The dissociation reactions for each pair are shown where they occur along a pH gradient. The equilibrium or dissociation constant (Ka) and its negative logarithm, the pKa, are shown for each reaction. *For an explanation of apparent discrepancies in pKa values for carbonic acid (H2CO3), see p. 63.

Buffers are mixtures of weak acids and their anions (conjugate base)
Buffers resist change in pH At pH = pKa, there is a 50:50 mixture of acid and anion forms of the compound Buffering capacity of acid/anion system is greatest at pH = pKa Buffering capacity is lost when the pH differs from pKa by more than 1 pH unit

FIGURE 2-17 The titration curve of acetic acid
FIGURE 2-17 The titration curve of acetic acid. After addition of each increment of NaOH to the acetic acid solution, the pH of the mixture is measured. This value is plotted against the amount of NaOH added, expressed as a fraction of the total NaOH required to convert all the acetic acid (CH3COOH) to its deprotonated form, acetate (CH3COO–). The points so obtained yield the titration curve. Shown in the boxes are the predominant ionic forms at the points designated. At the midpoint of the titration, the concentrations of the proton donor and proton acceptor are equal, and the pH is numerically equal to the pKa. The shaded zone is the useful region of buffering power, generally between 10% and 90% titration of the weak acid.

If you change the labeling of axis, what happens to the titration curve?

Weak acids have different pKas
FIGURE 2–18 Comparison of the titration curves of three weak acids. Shown here are the titration curves for CH3COOH, H2PO-4, and NH+4 . The predominant ionic forms at designated points in the titration are given in boxes. The regions of buffering capacity are indicated at the right. Conjugate acid-base pairs are effective buffers between approximately 10% and 90% neutralization of the proton-donor species.

Henderson–Hasselbalch Equation: Derivation
HA H+ + A-

More Acids and Bases Acid Conjugate Base
HA > A + H+ HA > A- + H+ HA > A2- + H+ Note: In some cases, the conjugate base has a – charge but in others it does not! IMPORTANT POINT: The conjugate base ALWAYS has one less + charge than the acid

Amino acids titration curves
An amino acid can act as a base or an acid. Such substances are called to be amphoteric, and are referred to as ampholyte. A crystalline amino acid dissolved in water is ionized, and can act as a weak acid or base. 2 titratable groups: -COOH and -NH3 Thus, amino acids have 2 dissociation constants and plots with 2 stages. Depending on the medium’s pH, an amino acid can have a (+), (-) and a net “0” charge.

FIGURE 3-9 Nonionic and zwitterionic forms of amino acids
FIGURE 3-9 Nonionic and zwitterionic forms of amino acids. The nonionic form does not occur in significant amounts in aqueous solutions. The zwitterion predominates at neutral pH. A zwitterion can act as either an acid (proton donor) or a base (proton acceptor).

FIGURE 3-11 Effect of the chemical environment on pKa
FIGURE 3-11 Effect of the chemical environment on pKa. The pKa values for the ionizable groups in glycine are lower than those for simple, methyl-substituted amino and carboxyl groups. These downward perturbations of pKa are due to intramolecular interactions. Similar effects can be caused by chemical groups that happen to be positioned nearby—for example, in the active site of an enzyme.

FIGURE 3-12a Titration curves for (a) glutamate and (b) histidine
FIGURE 3-12a Titration curves for (a) glutamate and (b) histidine. The pKa of the R group is designated here as pKR.

FIGURE 3-12b Titration curves for (a) glutamate and (b) histidine
FIGURE 3-12b Titration curves for (a) glutamate and (b) histidine. The pKa of the R group is designated here as pKR.

pI Isoelectric pH or inflection point: The midpoint of the titration curve; the pH at which a molecule has a zero charge (zwitterion or dipolar form) For a simple amino acid with only an a-carboxyl and an a-amino group, the pI is determined pI=(pK1+pK2)/2 However, for an amino acid with three or more ionizable groups, you must avoid the trap of thinking that pI is the average of pKa values: pI=(pKn+pKn+1)/2 pKn and pKn+1 are the two pKa values that describe the ionizaton of the species with a zero net charge; that is the the first ionization that adds a proton to the neutral species and gives it a net charge of -1 and the first ionization that remove a proton from the neutral species and gives it a net charge of -1

FIGURE 3-10 Titration of an amino acid
FIGURE 3-10 Titration of an amino acid. Shown here is the titration curve of 0.1 M glycine at 25°C. The ionic species predominating at key points in the titration are shown above the graph. The shaded boxes, centered at about pK1 = 2.34 and pK2 = 9.60, indicate the regions of greatest buffering power. Note that 1 equivalent of OH– = 0.1 M NaOH added.

At what point(s): glycine will be present predominantly as the species +H3N-CH2-COOH? is the average net charge of glycine +1? is the pH is equal to the pKa of the carboxyl group? does glycine have its maximum buffering capacity? is the average net charge zero? is the predominant species +H3N-CH2-COO- ? is the net charge if Glycine -1? do the predominant species consist of a 50:50 mixture of +H3N-CH2-COOH and +H3N-CH2-COO- ? is the predominant species +H2N-CH2-COO- ? What point corresponds to the pI? Which points have the worst buffering efficiency?

Acidosis and alkalosis
Blood pH< >acidosis Blood pH< > alkalosis Respiratory and metabolic Respiratory A change in acid-base status induced by altered respiration Metabolic A change in acid-base status induced by metabolic problems (diabetes, alcoholism, poisoning)

Blood pH 7.35-7.45 Buffers Phosphate buffer (cell cytoplasm)
Bicarbonate buffer (plasma) Bicarbonate buffer system is unique in the sense that H2C03----> CO2(d) + H2O CO2 is a gas under normal conditions

FIGURE 2-20 The bicarbonate buffer system
FIGURE 2-20 The bicarbonate buffer system. CO2 in the air space of the lungs is in equilibrium with the bicarbonate buffer in the blood plasma passing through the lung capillaries. Because the concentration of dissolved CO2 can be adjusted rapidly through changes in the rate of breathing, the bicarbonate buffer system of the blood is in near equilibrium with a large potential reservoir of CO2.

more The pH of a HCO3 system depends on the H2CO3 and HCO3 donor/acceptor concentrations. H2CO3 depends on CO2(d)----> depends on CO2(g) Thus, the pH of a bicarbonate buffer is determined by the [HCO3 ]the solution and partial pressure of CO2 in the gas phase

more pH = Pka+ log [HCO3]/ pCO2 Hypoventilation? Hyperventilation?

What did we learn? pH pK Handerson-H Titration curves Acidosis
Alkalosis

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