ph acids, bases, pk conjugate acid-base pairs calculate pk from a titration curve buffers...
Post on 31-Mar-2015
237 Views
Preview:
TRANSCRIPT
pH
Acids, bases, pKConjugate acid-base pairsCalculate pK from a titration curveBuffersHanderson-Hasselbalch EquationPractice 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:
H2O H+ + OH- Keq = ————[H+]•[OH-]
[H2O]
Keq can be determined experimentally, it is 1.8•10–16 M at 25C.[H2O] can be determined from water density, it is 55.5 M.
• Ionic product of water:
•In pure water [H+] = [OH–] = 10–7 M
214-2 M101]OH][H[]OH[ eqw KK
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+]
214- M101]OH][H[ wK
14]OHlog[]Hlog[ -
14pOHpH
pH scale is logarithmic: 1 unit = 10-fold
pH of Some Common Liquids
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)
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
If you change the labeling of axis, what happens to the titration curve?
Weak acids have different pKas
Henderson–Hasselbalch Equation:Derivation
HA H+ + A-
HA][
]A[logppH
-
aK
HA][
]A][H[ -
aK
]A[
HA][][H
-a+ K
A-][
HA][log-log]log[H- aK
More Acids and BasesAcid Conjugate BaseHA+--------------------> 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 curveso An amino acid can act as a base or an acid. Such
substances are called to be amphoteric, and are referred to as ampholyte.
o A crystalline amino acid dissolved in water is ionized, and can act as a weak acid or base.
o 2 titratable groups: -COOH and -NH3o Thus, amino acids have 2 dissociation constants and
plots with 2 stages.o Depending on the medium’s pH, an amino acid can
have a (+), (-) and a net “0” charge.
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 -carboxyl and an -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)/2pKn 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
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< 7.4 ---->acidosisBlood pH< 7.4 -----> alkalosisRespiratory 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
7.35-7.45Buffers
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
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?
pHpKHanderson-HTitration curves AcidosisAlkalosis
top related