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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.

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Presentation on theme: "PH Acids, bases, pK Conjugate acid-base pairs Calculate pK from a titration curve Buffers Handerson-Hasselbalch Equation Practice some problems."— Presentation transcript:

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

2 Ionization of Water: Quantitative Treatment Concentrations of participating species in an equilibrium process are not independent but are related via the equilibrium constant: H 2 O H + + OH - K eq = [H + ][OH - ] [ H 2 O] K eq can be determined experimentally, it is –16 M at 25 C. [H 2 O] can be determined from water density, it is 55.5 M. Ionic product of water: In pure water [H + ] = [OH – ] = 10 –7 M

3 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 + ]

4 pH scale is logarithmic: 1 unit = 10-fold

5 pH of Some Common Liquids

6 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.

7 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.

8 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.

9 pK a measures acidity pK a = – log K a (strong acid large K a small pK a )

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

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

13 Weak acids have different pK a s

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

15 More Acids and Bases AcidConjugate Base HA > A + H + HA > A - + H + HA > A 2- + 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

16 Amino acids titration curves o 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 -NH3 o Thus, amino acids have 2 dissociation constants and plots with 2 stages. o Depending on the mediums pH, an amino acid can have a (+), (-) and a net 0 charge.

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27 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)/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

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31 At what point(s): glycine will be present predominantly as the species + H 3 N-CH 2 -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 + H 3 N-CH 2 -COO - ? is the net charge if Glycine -1? do the predominant species consist of a 50:50 mixture of + H 3 N-CH 2 -COOH and + H 3 N-CH 2 -COO - ? is the predominant species + H 2 N-CH 2 -COO - ? What point corresponds to the pI? Which points have the worst buffering efficiency?

32 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)

33 Blood pH Buffers Phosphate buffer (cell cytoplasm) Bicarbonate buffer (plasma) Bicarbonate buffer system is unique in the sense that – H 2 C > CO 2 (d) + H 2 O – CO 2 is a gas under normal conditions

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35 more The pH of a HCO 3 system depends on the H 2 CO 3 and HCO 3 donor/acceptor concentrations. H 2 CO 3 depends on CO 2 (d)----> depends on CO 2 (g) Thus, the pH of a bicarbonate buffer is determined by the [HCO 3 ]the solution and partial pressure of CO 2 in the gas phase

36 more pH = Pka+ log [HCO 3 ]/ pCO 2 Hypoventilation? Hyperventilation?

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


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