Ionic Equilbria

Question 1

Describe Acids and Bases by Arrhenius

Answer 1

Acids: substances that contain hydrogen and can dissolve in water to release H into solution

HCl + H2O  H+(aq) + Cl-(aq)

Bases: Substances that dissolve in water to release hydroxide ions (OH-) into solution

NaOH + H2O  Na+(aq) + OH-(aq)

Question 2

Show the introduction of theories for acid base

Answer 2

Arrhenius’ classification – In aqueous solutions:

• *Acid** yields hydrogen ion: HA  H+ + A-
• *Base** yields hydroxyl ion: BOH  B+ + OH-
• *Bronsted-Lowry Theory (“proton theory”), 1923**
• *Acid** tends to give out proton: proton donor
• *Base** tends to accept proton: proton acceptor

Lewis Electronic Theory (“electronic” theory), 1923
Acid accepts an electron pair to form a covalent bond
Base provide an electron pair to form a covalent bond
H+ + OH-  H2O

Question 3

What is the bronsted lowry concept?

Answer 3

Acid + Base Conjugate acid + Conjugate base

Conjugate acid = molecule or ion created when the base accepts the proton (conjugate acid-base pair)

Conjugate base = molecule or ion remaining after the eacid has lost a proton

Reaction can proceed in either forward or backward direction; in each case the acid donates a proton to the base

Question 4

Describe Acid - Base Equilibrium

Answer 4

When strong acids are dissolved in water, complete dissociation occurs

When weak acids are dissolved in water, they dissociate only

Question 5

What is Sorensen’s definition for pH

Answer 5

Definition:

   pH ( “ pH+ ”) = - log [“ H+ ”] = - log [H3O+] =    

“pX”: -log10X (- “logX”) —- negative common logarithm
For pure water, pH = -log [H+] = -log (1 x 10-7) = 7

Question 6

Describe the pH scale?

Answer 6

Acids pH < 7

Bases pH > 7

Neutral pH = 7

Question 7

Describe the ionization of water?

Answer 7

• Water can be both (very very.. WEAK) base and acid
• Other H2O acts as an acid and loses H+ to become OH-
• One H2O acts as a base and gains H+
• H3O+: hydronium (solvated ion), H+ is used as a symbol of H3O+ for simplicity

Question 8

What is Kw defined as? (Kw= ionic product of water)

Answer 8

Kw = k [H2O]2 = [H3O+][OH-]

At 25C, Kw = 10-14 (mol/L)2 = [H3O+][OH-] = 10-7 · 10-7

Question 9

Describe Ionization of weak acids?

Answer 9

Define Ka for acid HA at acid-base equilibrium —

HA + H2O  H3O+ + A-
HA  H+ + A-

Ka = [H+][A-] / [HA]

Ka is the ionization constant of a weak acid HA (acidity constant, or dissociation constant of HA) Units mol/L

Question 10

What is the pKa of a weak acid?

Answer 10

Ka = [H+][A-]/[HA]

pKa = - logKa
The stronger an acid, the smaller its pKa.
Ionization constants are used to measure the strength of acids and bases and calculate the extent of ionization, such as Ka and pKa, Kb, pKb

Question 11

Describe the ionization of weak bases?

Answer 11

Define Kb for a base B at acid-base equilibrium —

NH3 + H2O  NH4+ + OH-
B + H2O  BH+ + OH-

Kb: base ionization constant (basicity constant)
Kb = [BH+][OH-]/[B]

For water solution only contains B, BH+, and OH-
[BH+] = [OH-]
[OH-]2 = Kb[B]

Question 12

What is the pKa of a weak base?

Answer 12

Define: the Ka of the base B is the Ka of its conjugate acid BH+. For its conjugate acid BH+, at equilibrium, the acidity constant of BH+ is Ka

NH3 + H2O  NH4+ + OH-
B + H2O  BH+ + OH-
BH+ Ka H+ + B
Ka = [H+][B] / [BH+]
pKa = - log Ka

The stronger a base, the higher its pKa.

Question 13

pKa and pKb

Answer 13

pKa + pKb = 14
pOH + pH = 14

Kb is less and less used in pharmacy

Question 14

What is the ionization of ampholytes?

Answer 14

Most polyprotic acid systems are amphoteric systems (Except HnA and An-)
Ionization of amphoteric electrolytes
Amphoteric electrolyte (or ampholyte): A specie can function either as an acid or a base.
Ampholytes often are molecules contain both acidic and basic functional groups, which will act as acids under certain conditions, and act as bases under certain other conditions. Examples: amino acid, proteins

Question 15

What is a zwitterion form and isoelectric point?

Answer 15

• *Zwitterion:** Amphoteric species that carry both positive and negative charges and the whole molecule is electrically neutral.
• *Isoelectric point (pI):** the pH at which the zwitterion forms and the net charge of the ions equal zero.

Question 16

What is a salt?

Answer 16

• A salt is the combination of an anion (- ion) and a cation (+ ion)
• An acid + A base => A salt
• In a solid salt, the ions are held together by the difference in charge
• If a salt dissolves in water, it usually dissociates to form ions
• The reaction of anions or cation of the salt with water to produce acidic or alkaline solution is called hydrolysis NaCl
• Solutions containing a salt of a strong acid and a strong base (SASB)
• Nacl
• Solutions containing a salt of a weak acid and a strong base (WASB)
• NH4 Cl
• Solutions containing a salt of a strong acid and a weak base (SAWB)
• NH4 Ac
• Solutions containing a salt of a weak acid and a weak base (WAWB)

Question 17

A solution with a salt of a strong acid and a strong base

Answer 17

The solution is neutral, pH = 7
Neither the cation or the anion undergoes hydrolysis

Question 18

A salt of a weak acid and a strong base

Answer 18

Basic pH

pH = ½(pKw + pKa + log[A-])

Question 19

A salt of a strong acid and a weak base

Answer 19

Acidic pH

pH = ½( pKa - log[BH+])

Question 20

A salt of a weak acid and a weak base

Answer 20

Can be acidic, neutral, or basic

Question 21

What is the importance of ionic equilibria in pharmaceutical science?

Answer 21

Many drugs and ingredients are organic weak electrolytes.
Weak electrolyte can exist in an ionized form and an unionized form
The degree of ionization directly affects drug physicochemical properties, such as solubility, hydrophobicity, stability, etc.
The degree of ionization directly affects drug dissolution, absorption, transportation, and excretion.
Ionized forms are more water-soluble and less lipophilic
Unionized forms are less water-soluble but more lipophilic, and more permeable to lipid membrane
pH partition hypothesis for drug absorption: the absorption of a weak electrolyte is determined mainly by its existing unionized form at the site of absorption, when drug are absorbed from the GI tract by passive diffusion. (page 238)

Question 22

The SUMMARY

Answer 22

1. Ionization of water
2. Ionization of strong acids or strong base (SA, SB) pH = -log[H+]
3. Ionization of weak acids (WA) pH = ½ (pKa – log [HA])
4. Ionization of weak bases (WB) pH = ½ (14 + pKa + log [B])
5. Ionization of polyprotic electrolytes
6. Ionization of ampholytes
7. Ionization of salts

• Solutions containing a salt of a strong acid and a strong base (SASB)pH= 7
• Solutions containing a salt of a weak acid and a strong base (WASB)pH = ½ ( 14+pKa+log[a-])
• Solutions containing a salt of a strong acid and a weak base (SAWB)pH = ½ ( pKa – log [BH+])
• Solutions containing a salt of a weak acid and a weak base (WAWB)