107 - Acids, Bases, and Buffers

Arrhenius Acids

Substance with a hydrogen atom and can easily donate hydrogen ions or protons (H+). Produces H⁺ or OH^-

Bronsted-Lowry Acid

Substance that can transfer protons to another substance

Lewis Acid

Can accept electron pairs

Henderson-Hasslebach Equation: pH of Acid

pH = pKa + log [A^-]/[HA]

Henderson-Hasslebach Equation: pH of Base

pH = pKa + log [B]/[BH⁺]

Henderson-Hasslebach Equation: Percentage Ionized

% = [ ionized / (ionized + unionized) ] x 100%

Henderson-Hasslebach Equation: % ionized - given only pH and pKa (Acid)

% = [ (10^{(pH - pKa)}) / ( (10^{(pH - pKa)} +1) ] x 100%

Henderson-Hasslebach Equation: % ionized - given only pH and pKa (Base)

% = [ 1 / 1 + 10^{(pH - pKa)} ] x 100%

Phenolphthalein - Equation

Hind + OH^- ⇌ ind^- + H₂O

Phenolphthalein - Color at pH 4-7

Colorless

Phenolphthalein - Color at pH 8-10

Pink

Phenolphthalein - Color at pH 11-14

Colorless

Methyl Orange (Equation)

Ind + H⁺ ⇌ Hind

Methyl Orange - Color at pH 4.4

Yellow

Methyl Orange - Color at pH 3.2

Pink

Boric Acid, NF - %

99.5–100.5% H₃BO₃

Boric Acid, NF - Physical state

Solid

Boric Acid, NF - Pharmaceutical use

• Mild Antiseptic

• Isotonicity adjustments

• Buffer

Boric Acid, NF - When reacted with H₂O

H₃BO₃ + 2H₂O ⇌ B[(H₂O)₄]^- +H₃O⁺

Sulfuric Acid, NF - %

 95–98% w/w H₂SO₄

Sulfuric Acid, NF - Distinctive characteristic

Dehydrating

Sulfuric Acid, NF - Pharmaceutical use

Sulfonating/Sulfating Agent

Sulfuric Acid, NF - Equations (Diprotic: Having 2 H atoms to be released in water)

• H₂SO₄ + H₂O ⇌ HSO₄⁻+H₃O⁺

• HSO₄⁻ + H₂O ⇌ SO₄^2- +H₃O⁺

Phosphoric Acid, NF - %

• 85–88% w/w H₃PO₄

• Diluted H₃PO₄, NF: 9.5-10.5% w/v 

Phosphoric Acid, NF: Equations (Polyprotic)

• H₃PO₄ + H₂O ⇌ H₃O⁺ + H₂PO₄^- 

  • pKa = 2.12

• H₂PO₄^- + H₂O ⇌ H₃O⁺ + HPO₄^2- 

  • pKa = 7.21

• HPO₄^2- + H₂O ⇌ H₃O⁺ + PO₄^3- 

  • pKa = 12.32

Phosphoric Acid, NF: Pharmaceutical use

• Buffers

• Salt formation of weak bases

Hydrochloric Acid, NF - %

• 36.5–38% w/w HCl

• Diluted HCl, NF: 9.5-10.5% w/v

Hydrochloric Acid, NF - Pharmaceutical use

• Gastric acidifier

• Salt formation of weak bases

• Technical grade: Muriatic acid (cleaning agent and drain clog remover)

Nitric Acid, NF - %

69–71% w/w HNO₃

Nitric Acid, NF - Pharmaceutical use

• Aqua Regia - Noble metal dissolver

  • Components: 3 HCl : 1 HNO₃

Strong Ammonia Solution, NF - %

27–31% w/w NH₃

Strong Ammonia Solution, NF - Pharmaceutical use

• Alkalinifier

• CNS stimulant

Strong Ammonia SOlution, NF - Equation

• NH₃ + H₂O  ⇌ NH₄OH 

  • pKa = 9.2

Sodium Carbonate - Formula

Na₂CO₃

Sodium Carbonate - Pharmaceutical Use

• Soda Ash - Alkalinifier

• Na salt of weakly acidic drugs

• HCO₃^-: Electrolyte and Antacid

Calcium Hydroxide - Formula

Ca(OH)₂

Calcium Hydroxide - Also known as

Slaked lime

Calcium Hydroxide - Distinct characteristic

Easily absorbs CO² from air

Calcium Hydroxide - Pharmaceutical use

• Soap production

• Gas trap (+ NaOH, soda lime)

Potassium Hydroxide - Formula

KOH

Potassium Hydroxide - Characteristics

• Hygroscopic

• Deliquescent

• Easily absorbs CO₂

Potassium Hydroxide - Pharmaceutical use

• Saponifying agent

• Alkalinifier

Sodium Hydroxide - Characteristics

• Hygroscopic, deliquescent

• Easily absorbs CO2

Sodium hydroxide - Pharmaceutical use

• Caustic soda

• Saponifying agent

• Alkalinifier

Buffer Systems

• Pairs of related chemical compounds capable of resisting large changes in the pH of a solution

• Should not participate in any chemical reaction with other species in the solution

Buffer System - Composition

Weak acid and its salt or weak base and its salt

Buffer System - Functions

• Buffer systems for pharmaceutical analysis

• Maintain pH limits in drug preparations

• Maintains physiological pH

Buffer System - General Method of Preparation

1. Dry crystalline substances at 110-120°C.

2. Prepare separate solutions of components using carbon dioxide-free water.

3. Combine solutions at specific volumes to reach target pH. Table available in the USP.

Common Buffers: Hydrochloric Acid (Components)

• HCl

• KCl

Common Buffers: Hydrochloric Acid (pH Range)

1.2 - 2.2

Common Buffers: Acid Phthalate (Components)

• KHC₆H₄(COO)₂

• HCL

Common Buffers: Acid Phthalate (pH Range)

2.2 - 4.0

Common Buffers: Acetate (Components)

• CH₃COOH

• CH₃COONa

Common Buffers: Acetate (pH Range)

4.1 - 5.5

Common Buffers: Neutralized Phthalate (Components)

• KHC₆H₄(COO)₂

• NaOH

Common Buffers: Neutralized Phthalate (pH Range)

4.2 - 5.8

Common Buffers: Phosphate (Components)

• KH₂PO₄

• NaOH

Common Buffers: Phosphate (pH Range)

5.8 - 8.0

Common Buffers: Alkaline Borate (Components)

• H₃BO₃

• KCl

• NaOH