Pharmaceutical Analysis · Short notes

Titrimetric Analysis: Short Notes & Cheat Sheet

Acid–base, non-aqueous, redox, precipitation, complexometric and diazotisation titrations, with indicators, standards and back titration.

Short notes

Read these before you attempt the topic's MCQs.

Match the drug's chemistry to the titration: amines to non-aqueous or nitrite, metals to EDTA, reducing agents to redox.

Choose an indicator that changes colour at the equivalence-point pH.
Strong acid with strong base: phenolphthalein or methyl orange both work. Weak acid with strong base: phenolphthalein (pH 8.3–10). Strong acid with weak base: methyl orange (3.1–4.4) or methyl red.
Weak acid with weak base gives no sharp end point and is avoided.
Primary standards must be pure, stable, non-hygroscopic and preferably of high molar mass: potassium hydrogen phthalate for bases; anhydrous sodium carbonate or borax for acids.
Back titration is used when the reaction is slow or the sample is insoluble: add excess reagent, then titrate what is left (e.g. aspirin by saponification with excess NaOH, back-titrated with HCl).

Weak bases (many drug amines) are titrated with perchloric acid in glacial acetic acid, using crystal violet. Mercuric acetate is added for halide salts.
Weak acids are titrated with sodium methoxide or tetrabutylammonium hydroxide in non-aqueous solvents.
Potassium permanganate is its own indicator and is used in sulfuric acid; not hydrochloric acid, which it would oxidise to chlorine.
Iodimetry is direct titration with iodine (e.g. ascorbic acid). Iodometry is indirect: iodine liberated by the analyte is titrated with sodium thiosulfate, adding starch near the end point.
Ceric ammonium sulfate is a stable oxidant used with ferroin indicator (e.g. ferrous salts).

Mohr's method titrates chloride with silver nitrate in neutral solution, using potassium chromate (red-brown silver chromate at the end point).
Volhard's method is a back titration in acid: excess silver nitrate is titrated with thiocyanate using ferric alum.
Fajans' method uses adsorption indicators such as fluorescein and eosin.
EDTA binds metal ions 1:1. Eriochrome black T is used at pH 10 (ammonia buffer) for calcium, magnesium and zinc, changing from wine-red to blue. Murexide is used for calcium at pH 12.
Diazotisation (nitrite titration) assays primary aromatic amines such as sulfonamides and procaine, with starch–iodide paper or a potentiometric end point.

High-yield facts in one place. Cover the right-hand column and test yourself.

Titration	Indicator
Weak acid × strong base	Phenolphthalein
Strong acid × weak base	Methyl orange / methyl red
Non-aqueous (weak base)	Crystal violet
Permanganate	Self-indicating
Iodometry / iodimetry	Starch
Ceric sulfate	Ferroin
Mohr (Cl⁻)	Potassium chromate
Volhard	Ferric ammonium sulfate
EDTA (Ca, Mg, Zn)	Eriochrome black T, pH 10
Nitrite titration	Starch–iodide paper (external)

Drug	Assay
Aspirin	Back titration after saponification
Ascorbic acid	Iodimetry
Calcium gluconate	Complexometric (EDTA)
Sulfonamides	Nitrite (diazotisation) titration
Ephedrine and other amines	Non-aqueous, perchloric acid
Ferrous sulfate	Ceric sulfate or permanganate
Sodium chloride	Mohr or Volhard

IodiMetry = Me (direct), IodOmetry = Other (indirect): Iodimetry titrates with iodine directly; iodometry titrates liberated iodine with thiosulfate.

Permanganate titrations use sulfuric acid, never hydrochloric acid.
Starch is added near the end point in iodometry; added early, it binds iodine irreversibly.
Mohr's method needs a neutral pH; Volhard's works in acid.
Perchloric acid in glacial acetic acid makes weak bases behave as stronger bases (levelling effect).

Attempt the Titrimetric Analysis MCQs while the notes are fresh. Anything you get wrong is saved to your incorrect list for the next revision round.