Acids and bases, pH and buffers

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Acids and bases, pH and buffers

• Dr. Mamoun Ahram
• Lecture 2

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Acids and bases
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Acids versus bases

• Acid a substance that produces H when dissolved
  in water (e.g., HCl, H2SO4)
• Base a substance that produces OH- when
  dissolved in water (NaOH, KOH)
• What about ammonia (NH3)?

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Brønsted-Lowry acids and bases

• The Brønsted-Lowry acid any substance able to
  give a hydrogen ion (H-a proton) to another
  molecule
• Monoprotic acid HCl, HNO3, CH3COOH
• Diprotic acid H2SO4
• Triprotic acid H3PO3
• Brønsted-Lowry base any substance that accepts a
  proton (H) from an acid
• NaOH, NH3, KOH

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Acid-base reactions

• A proton is transferred from one substance (acid)
  to another molecule
• Ammonia (NH3) acid (HA) ? ammonium ion (NH4)
  A-
• Ammonia is base
• HA is acid
• Ammonium ion (NH4) is conjuagte acid
• A- is conjugate base

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Water acid or base?

• Both
• Products hydronium ion (H3O) and hydroxide

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Amphoteric substances

• Example water
• NH3 (g) H2O(l) ? NH4(aq) OH(aq)
• HCl(g) H2O(l) ? H3O(aq) Cl-(aq)

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Acid/base strength
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Rule

• The stronger the acid, the weaker the conjugate
  base
• HCl(aq) ? H(aq) Cl-(aq)
• NaOH(aq) ? Na(aq) OH-(aq)
• HC2H3O2 (aq) ? H(aq) C2H3O2-(aq)
• NH3 (aq) H2O(l) ? NH4(aq) OH-(aq)

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Equilibrium constant

•    HA  lt--gt   H A-
• Ka gt1 vs. lt1

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Expression

• Molarity (M)
• Normality (N)
• Equivalence (N)

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Molarity of solutions

• moles grams / MW
• M moles / volume (L)
•  
• grams M x vol (L) x MW

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Exercise

• How many grams do you need to make 5M NaCl
  solution in 100 ml (MW 58.4)?
• grams 58.4 x 5 moles x 0.1 liter 29.29 g

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Normal solutions

• N n x M (where n is an integer)
• n the number of donated H
• Remember!
• The normality of a solution is NEVER less than
  the molarity

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Exercise

• What is the normality of H2SO3 solution made by
  dissolving 6.5 g into 200 mL? (MW 98)?

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But

• Molarity (and normality) is not useful for
  understanding neutralization reactions.
• 1M HCL neutralizes 1M NaOH
• But
• 1M HCl does not neutralize 1M H2SO3 
• Why? 

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Equivalents

• The amount of molar mass (g) of hydrogen ions
  that an acid will donate
• or a base will accept
• 1 mole HCl 1 mole H 1 equivalent
• 1 mole H2SO4 2 mole H 2 equivalents

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Examples

• One equivalent of Na 23.1 g
• One equivalent of Cl- - 35.5 g
• One equivalent of Mg2 (24.3)/2 12.15 g

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Exercise

• calculate the number of equivalents of40g of
  Mg216g of Al3
• Mg 40g x (1mol/24g) x (2eq/1mol) 3.3 eq
• Al3 16g x (27g/1mol) x (3eq/1mol) 1.8 eq

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Exercises

• Calculate milligrams of Ca2 in blood if total
  concentration of Ca2 is 5 mEq/L. (MW 40.1)
• (20.1 g/1000 mEq) x (1000 mg/g) x (5 mEq /L)
  100 mg/L
• What is the normality of H2SO3 made by dissolving
  6.5g in 200 ml? equivalents? (MW 98)

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Examples (calculate grams)
Physiological gram mEq/L 1 Eq Major electrolytes
? 136-145 23.1 g Na
? 98-106 - 35.5 g Cl-
? 3 (24.3)/2 12.15 g Mg2
? 4.5-6.0 (40.1/2) 20.05 g Ca2
? 3.4-5.0 39.1 g K
? 25-29 61 g HCO3-
? 2 SO3-2 and HPO43-
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Titration and equivalence point

• The concentration of acids and bases can be
  determined by titration

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Excercise

• A 25 ml solution of 0.5 M NaOH is titrated until
  neutralized into a 50 ml sample of HCl. What is
  the concentration of the HCl?
• Step 1 - Determine OH-
• Step 2 - Determine the number of moles of OH-
• Step 3 - Determine the number of moles of H
• Step 4 - Determine concentration of HCl

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A 25 ml solution of 0.5 M NaOH is titrated until
neutralized into a 50 ml sample of HCl

• Moles of base Molarity x Volume
• Moles base moles of acid
• Molarity of acid moles/volume

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Another method

• MacidVacid MbaseVbase

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Note

• What if one mole of acid produces two moles of H
• Consider the charges (or normality)

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Modified equation

• Na x Va Nb x Vb
• Na normality of acid
• Va volume of acid
• Nb normality of base
• Vb volume of base
• NaOH1
• H2SO32
• H3PO43

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Exercises

• If 19.1 mL of 0.118 M HCl is required to
  neutralize 25.00 mL of a sodium hydroxide
  solution, what is the molarity of the sodium
  hydroxide?
• If 12.0 mL of 1.34 M NaOH is required to
  neutralize 25.00 mL of a sulfuric acid, H2SO4,
  solution, what is the molarity of the sulfuric
  acid?

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Ionization of water

• H3O H

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Equilibrium constant

• Keq 1.8 x 10-16 M

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Kw

• Kw is called the ion product for water

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pH
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What is pH?
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Exercise

• What is the pH of 0.01 M HCl?
• What is the pH of 0.01 N H2SO3?
• What is the pH of a solution of 1 x 10-11 HCl?

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Acid dissociation constant

• Strong acid
• Strong bases
• Weak acid
• Weak bases

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pKa
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What is pKa?
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Henderson-Hasselbalch equation
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The equation
pKa is the pH where 50 of acid is dissociated
into conjugate base
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Buffers
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Maintenance of equilibriumLe Châteliers
principle
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What is buffer?
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Titration
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Midpoint
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Buffering capacity
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Conjugate bases
Acid Conjugate base
CH3COOH CH3COONa (NaCH3COO)
H3PO4 NaH2PO4
H2PO4- (or NaH2PO4) Na2HPO4
H2CO3 NaHCO3
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How do we choose a buffer?
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Problems and solutions

• A solution of 0.1 M acetic acid and 0.2 M acetate
  ion. The pKa of acetic acid is 4.8. Hence, the pH
  of the solution is given by
• Similarly, the pKa of an acid can be calculated

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Exercise

• What is the pH of a buffer containing 0.1M HF and
  0.1M NaF? (Ka 3.5 x 10-4)
• What is the pH of a solution containing 0.1M HF
  and 0.1M NaF, when 0.02M NaOH is added to the
  solution?

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At the end point of the buffering capacity of a
buffer, it is the moles of H and OH- that are
equal
Equivalence point
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Exercise

• What is the concentration of 5 ml of acetic acid
  knowing that 44.5 ml of 0.1 N of NaOH are needed
  to reach the end of the titration of acetic acid?
  Also, calculate the normality of acetic acid.

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Polyprotic weak acids

• Example

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Hence
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Excercises

• What is the pH of a lactate buffer that contain
  75 lactic acid and 25 lactate? (pKa 3.86)
• What is the pKa of a dihydrogen phosphae buffer
  when pH of 7.2 is obtained when 100 ml of 0.1 M
  NaH2PO3 is mixed with 100 ml of 0.1 M Na2HPO3?

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Buffers in human body

• Carbonic acid-bicarbonate system (blood)
• Dihydrogen phosphate-monohydrogen phosphate
  system (intracellular)
• Proteins

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Bicarbonate buffer
CO2 H20
H2CO3
H HCO3-
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Blood buffering
Blood (instantaneously)
CO2 H20
H2CO3
H HCO3-
Lungs (within minutes)
Excretion via kidneys (hours to days)
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Arterial blood gases (ABG)
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Calculations

• The ratio of bicarbonate to carbonic acid
  determines the pH of the blood
• Normally the ratio is about 201 bicarbonate to
  carbonic acid
• Blood pH can be calculated from this equation
•  pH pK log (HCO3-/H2CO2)
• pK is the dissociation constant of the buffer,
  6.10
• H2CO3 0.03 x pCO2

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Titration curve of bicarbonate bufferNote pKa
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Why is this buffer effective?

• Even though the normal blood pH of 7.4 is outside
  the optimal buffering range of the bicarbonate
  buffer, which is 6.1, this buffer pair is
  important due to two properties
• bicarbonate is present in a relatively high
  concentration in the ECF (24mmol/L)
• the components of the buffer system are
  effectively under physiological control the CO2
  by the lungs, and the bicarbonate by the kidneys
• It is an open system (not a closed system like in
  laboratory)

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Open system

• An open system is a system that continuously
  interacts with its environment.

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ExerciseH HCO3- ? H2CO3 ? CO2 H2O

• Blood plasma contains a total carbonate (HCO3-
  and CO2) of 2.52 x 10-2 M. What is the HCO3-/CO2
  ratio and the concentration of each buffer
  component at pH 7.4?

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Exercise (continued) H HCO3- ? H2CO3 ? CO2
H2O

• What would the pH be if 10-2 M H is added and
  CO2 is eliminated (closed system)?

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Exercise (continued) H HCO3- ? H2CO3 ? CO2
H2O

• What would the pH be if 10-2 M H is added under
  physiological conditions (open system)?

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Acidosis and alkalosis

• Can be either metabolic or respiratory
• Acidosis
• Metabolic production of ketone bodies
  (starvation)
• Respiratory pulmonary (asthma emphysema)
• Alkalosis
• Metabolic administration of salts or acids
• Respiratory hyperventilation (anxiety)

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Acid-Base Imbalances

• pHlt 7.35 acidosis
• pH gt 7.45 alkalosis

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Respiratory Acidosis
H HCO3- ? H2CO3 ? CO2 H2O
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Respiratory Alkalosis
H HCO3- ? H2CO3 ? CO2 H2O
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Metabolic Acidosis
H HCO3- ? H2CO3 ? CO2 H2O
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Causes of respiratory acid-base disorders

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Causes of metabolic acid-base disorders

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Compensation

• Compensation The change in HCO3- or pCO3 that
  results from the primary event
• If underlying problem is metabolic,
  hyperventilation or hypoventilation can help
  respiratory compensation.
• If problem is respiratory, renal mechanisms can
  bring about metabolic compensation.

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Complete vs. partial compensation

• May be complete if brought back within normal
  limits
• Partial compensation if range is still outside
  norms.

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• Acid-Base Disorder Primary Change Compensatory
  Change
• Respiratory acidosis pCO2 up HCO3- up
• Respiratory alkalosis pCO2 down HCO3-
  down
• Metabolic acidosis HCO3- down
  PCO2 down
• Metabolic alkalosis HCO3- up PCO2 up

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FULLY COMPENSATED
pH pCO2 HCO3-
Resp. acidosis Normal Butlt7.40
Resp. alkalosis Normal butgt7.40
Met. Acidosis Normal butlt7.40
Met. alkalosis Normal butgt7.40
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Partially compensated
pH pCO2 HCO3-
Res.Acidosis
Res.Alkalosis
Met. Acidosis
Met.Alkalosis
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Examples
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Example 1

• Mr. X is admitted with severe attack of asthma.
  Her arterial blood gas result is as follows
• pH 7.22
• PaCO2 55
• HCO3- 25
• pH is low acidosis
• paCO2 is high in the opposite direction of the
  pH.
• HCO3- is Normal
• Respiratory Acidosis

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Example 2

• Mr. D is admitted with recurring bowel
  obstruction has been experiencing intractable
  vomiting for the last several hours. His ABG is
• pH 7.5
• PaCO2 42
• HCO3- 33
• Metabolic alkalosis

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Example 3

• Mrs. H is kidney dialysis patient who has missed
  his last 2 appointments at the dialysis centre.
  His ABG results
• pH 7.32
• PaCO2 32
• HCO3- 18
• Partially compensated metabolic Acidosis

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Example 4

• Mr. K with COPD.His ABG is
• pH 7.35
• PaCO2 48
• HCO3- 28
• Fully compensated Respiratory Acidosis

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Example 5

• Mr. S is a 53 year old man presented to ED with
  the following ABG.
• pH 7.51
• PaCO2 50
• HCO3- 40
• Metabolic alkalosis

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Practice ABGs

• pH 7.48 PaCO2 32 HCO3- 24
• pH 7.32 PaCO2 48 HCO3- 25
• pH 7.30 PaCO2 40 HCO3- 18
• pH 7.38 PaCO2 48 HCO3- 28
• pH 7.49 PaCO2 40 HCO3- 30
• pH 7.35 PaCO2 48 HCO3- 27
• pH 7.45 PaCO2 47 HCO3- 29
• pH 7.31 PaCO2 38 HCO3- 15
• pH 7.30 PaCO2 50 HCO3- 24
• 10. pH 7.48 PaCO2 40 HCO3- 30

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Answers to Practice ABGs

• Respiratory alkalosis
• Respiratory acidosis
• Metabolic acidosis
• Compensated Respiratory acidosis
• Metabolic alkalosis
• Compensated Respiratory acidosis
• Compensated Metabolic alkalosis
• Metabolic acidosis
• Respiratory acidosis
• Metabolic alkalosis

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Salivary buffers

• Salivary pH ? 6.3
• Main buffers
• Bicarbonate
• Phosphate
• Proteins
• Below pH 5.5, demineralization usually follows

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Flow rate

• H2CO3 1.3 mM/L almost constant, but HCO3-
  is not
• The greater the salivary flow, the more
  bicarbonate ions available for combining with
  free hydrogen ions
• Normal salivary flow rates 0.1 and 0.6
  mL/minute

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Buffering saliva
Salivary carbonic anhydrase