Titrimetric Analysis
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Direct answer
Titrimetric (volumetric) analysis determines a drug by adding a standard solution until the reaction reaches an endpoint signalled by an indicator; its four reaction families are acid-base (including non-aqueous titration of weak bases with perchloric acid in glacial acetic acid), redox (self-indicating permanganate titrations and iodometric-iodimetric titrations with starch), complexometric (EDTA for metal ions, Eriochrome Black T wine-red to blue at pH 10) and precipitation (Mohr, Volhard and Fajans). The equivalence point, where stoichiometry is exactly satisfied, is approximated by the practical endpoint, their gap — the titration error — minimised by indicator choice. Weak bases that water cannot titrate yield sharp endpoints only in non-aqueous solvent, which is why perchloric acid with crystal violet is the pharmacopoeial workhorse for pharmaceutical bases.
What you must remember
- Core terms: standard solution, titrant, endpoint versus equivalence point, titration error; requirements — rapid, definite, stoichiometric reaction with a detectable endpoint.
- Primary-standard chain: potassium hydrogen phthalate standardises perchloric acid and sodium hydroxide; sodium carbonate standardises acids; potassium dichromate standardises sodium thiosulphate.
- Indicator ranges: methyl orange about pH 3.1-4.4 (strong acid-weak base titrations), phenolphthalein about 8.3-10.0 (weak acid-strong base) — selection follows the pH jump at equivalence.
- Non-aqueous titration: weak bases titrated with perchloric acid in glacial acetic acid, crystal violet indicator, sharp endpoints because the aprotic solvent no longer competes as a proton donor — the basis of many IP amine and alkaloidal salt assays.
- Redox pair: iodimetry titrates directly with iodine; iodometry liberates iodine from potassium iodide by an oxidant and titrates it with thiosulphate — starch added near the endpoint, blue vanishing to colourless; permanganate is self-indicating and runs in sulphuric acid, never hydrochloric (chloride is oxidised).
- Complexometry: disodium EDTA titrates calcium and magnesium (water hardness, antacid content) in ammonia buffer pH 10 with Eriochrome Black T — wine-red to pure blue; masking agents hide interfering metals.
- Argentometry: Mohr (direct, chromate indicator, neutral pH only), Volhard (back-titration with thiocyanate and ferric indicator in acid), Fajans (adsorption indicators such as fluorescein).
- Back-titration: excess reagent added, residual titrated — for insoluble or slow-reacting drugs such as zinc oxide.
The confusions examiners mine
Iodimetry versus iodometry is the richest vein, and the hook is direction: iodimetry uses iodine as titrant (direct), iodometry produces iodine and measures it with thiosulphate (indirect, for oxidants such as copper or chlorine); write "iodine is the indicator" and lose the question — starch is the indicator, iodine the species. The second trap is permanganate medium: hydrochloric acid is forbidden because permanganate oxidises chloride, wasting titrant — sulphuric acid is prescribed, and naming the reason earns the mark. Third, chromate in Mohr's method works only in neutral to faintly alkaline solution — acid converts chromate to dichromate, alkali precipitates silver hydroxide — and Volhard exists because it tolerates acid. Finally, the EDTA endpoint must be told in order — wine-red metal-indicator complex, EDTA stripping the metal at equivalence, free indicator blue — and non-aqueous answers should name the standard, indicator and blank to look trained.
Frequently asked questions
Why are weakly basic drugs titrated in non-aqueous solvents?
In water their neutralisation curves are too flat for a sharp endpoint, but in glacial acetic acid with standard perchloric acid the base behaves strongly enough for a crisp crystal violet endpoint — the basis of many IP assays.
Distinguish iodimetry from iodometry.
Iodimetry titrates reducing agents directly with standard iodine, whereas iodometry liberates iodine from potassium iodide using the oxidant under test and titrates that iodine with standard thiosulphate.
Why must permanganate titrations use sulphuric, never hydrochloric, acid?
Permanganate oxidises chloride ions, consuming extra titrant and distorting the endpoint, while sulphuric acid provides acidity without an oxidisable anion.
Explain the wine-red to blue endpoint of the EDTA titration.
At pH 10, Eriochrome Black T complexes magnesium as wine-red; when EDTA has bound all metal ions it displaces the indicator, releasing its free blue form.
How do the three argentometric methods differ?
Mohr titrates chloride directly with silver nitrate and chromate indicator in neutral medium; Volhard back-titrates excess silver with thiocyanate and ferric ion in acid; Fajans uses adsorption indicators on the precipitate surface.