Chemistry Chapter 8: Acid-Base Titrations

Chapter 8: Acid-Base Titrations

• Acid-base titrations involve using a solution of known concentration (titrant) to determine the concentration of an unknown solution (analyte or API).
• The reaction that occurs is a neutralization reaction.
• Acidimetry: Quantitative determination of basic drugs
• Alkalimetry: Quantitative determination of acidic drugs

Titration Curves for Strong Acids and Bases

• Strong acids and bases completely dissociate in water (e.g., HCl, HClO4, NaOH, KOH).
• Consider only one equilibrium: Kw = [H3O+][OH-]
• Know the stoichiometric ratio of the acid-base reaction (S.R.).
• Example: HCl + NaOH → HOH + Na+ + Cl- S.R. = 1:1

pH Before Titration Begins

• Calculate the pH of a solution of a known concentration before a titration
• Example: 100.0 mL of 0.1000 M HCl. [H3O+] = 1.000 x 10-¹M. pH = 1.0000; POH = 13.0000

pH at Equivalence Point

• The pH at the equivalence point in a strong acid-strong base titration is 7.0000.
• Autodissociation of water governs the pH at the equivalence point.
• Example: HO-H + HO-H ←→ H3O⁺ + OH⁻; Kw = [H3O⁺][OH⁻] = 10⁻¹⁴

pH After Equivalence Point

• Calculate the concentration of excess base(OH–) to determine pH after equivalence point
• Calculate [OH−] using the formula: [OH−] = (Cbase Vbase − Cacid Vacid)/(Vbase + Vacid)

Acid-Base Color Indicators

• Indicators are organic weak acids or bases that display distinct color changes.
• The indicator's acid and conjugate base forms have distinctly different colors, helping track the reaction progress.
• Example: HIn + OH⁻ ⇌ In− + HOH Note: The ratio [HIn]/[In⁻] determines the solution's color

Titration of Weak Acid and Strong Base

• Initial point and buffer region calculations
• midpoint pH = pKa
• Equivalence point calculations for titration of a weak acid(HA) with a strong base(OH).

Titration of Weak Base and Strong Acid

• The equivalence point for a weak base and strong acid titration occurs at a pH less than 7.
• The salt formed is acidic.

Polyfunctional Acids/Bases Equilibria

• Polyfunctional acids dissociate in multiple steps, each with a specific acid dissociation constant (Ka).
• The given example uses phosphoric acid, H3PO4 (Ka1, Ka2, Ka3)
• Calculations of pH are distinct for each step.
• Examples: H2S + HOH → H3O⁺ + HS⁻; HS⁻ + HOH → H3O⁺ + S²⁻

Titrations of polyprotic amino acids

• Amino acids are polyprotic – they have multiple ionization steps.
• calculations of their pH
• Calculation of isoelectric point.

Direct/Indirect Titrations

• direct titrations,

• residual titrations (or back titrations)

• example calculations for each using appropriate formulas.