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