BP106T Unit 4 – Gastrointestinal Agents & Topical Inorganic Compounds

Volumetric Analysis is the quantitative measurement of the concentration of an unknown analyte solution by reacting it stoichiometrically with a standardized solution of known concentration (titrant). A Primary Standard is a substance of extraordinary chemical purity (>= 99.9%), high equivalent weight (minimizing weighing error), complete atmospheric stability (non-hygroscopic, non-efflorescent, non-oxidizable), and high water solubility (e.g. Anhydrous $Na_2CO_3$, Potassium Hydrogen Phthalate - KHP, Potassium Dichromate $K_2Cr_2O_7$, Oxalic Acid). A Secondary Standard is a substance whose concentration must be determined by titrating against a primary standard (e.g. $NaOH, HCl, KMnO_4, Na_2S_2O_3$). Acid-base indicators are weak organic acids or bases whose ionized and unionized forms exhibit distinct visible colors.Primary Standard (jaise Oxalic acid, KHP) wo pure chemicals hote hain jo 99.9% shuddh hote hain aur hawa me moisture nahi sookhte. Phenolphthalein acid me colorless aur base me pink isliye hota hai kyunki Quinonoid theory ke mutabiq base me uska structure Benzenoid ring se badalkar colored Quinonoid ring ban jata hai.

Very weak bases ($ ext{pK}_b > 8$, e.g., Ephedrine, Diazepam, Metronidazole) and very weak acids ($ ext{pK}_a > 8$) cannot be titrated accurately in water because water acts as a competing amphiprotic base or acid, obscuring sharp end-point pH inflections. Non-Aqueous Titrations utilize organic non-aqueous solvents with low dielectric constants to enhance the apparent acidity or basicity of the solute. Solvents are classified into: (1) Aprotic (chemically inert, zero proton transfer: Benzene, Chloroform, Toluene), (2) Protogenic (acidic solvents that donate protons, forcing weak bases to accept protons: Glacial Acetic Acid, Formic Acid), (3) Protophilic (basic solvents that accept protons, enhancing weak acid strength: Dimethylformamide - DMF, Ethylenediamine), and (4) Amphiprotic (can donate or accept protons: Methanol, Ethanol).Ephedrine aur Diazepam jaisi kamzor dawaiyon ko paani me titrate nahi kiya ja sakta kyunki paani unhe compete karta hai. Isliye unhe Glacial Acetic Acid (Protogenic solvent) me ghol kar 0.1M Perchloric Acid (HClO4) se titrate karte hain. Agar dawai hydrochloride salt (B.HCl) ho, toh chloride ion ko mask karne ke liye Mercuric Acetate milaya jata hai aur Crystal Violet indicator violet se green hota hai.

Redox (Oxidation-Reduction) titrations involve the transfer of electrons between an oxidizing agent (which gains electrons and is reduced) and a reducing agent (which loses electrons and is oxidized). Permanganometry uses Potassium Permanganate ($KMnO_4$) as a self-indicating oxidizing agent in dilute $H_2SO_4$ medium ($MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$, endpoint = persistent faint pink). Cerimetry uses Ceric Ammonium Sulphate [$Ce(SO_4)_2$] in dilute $H_2SO_4$ with Ferroin indicator (color change: intense red to pale blue). Iodimetry is the direct titration of reducing analytes (e.g., Ascorbic Acid, Sodium Thiosulphate) with standard Iodine solution. Iodometry is the indirect titration where an oxidizing analyte oxidizes excess Potassium Iodide ($KI$) to liberate free Iodine ($I_2$), which is subsequently titrated against standardized Sodium Thiosulphate ($Na_2S_2O_3$) using freshly prepared Starch indicator.Permanganometry me KMnO4 self-indicator hota hai aur faint pink color deta hai. Isme HCl nahi daal sakte kyunki KMnO4 use toxic chlorine gas me badal dega. Iodimetry me direct Iodine se titration karte hain (jaise Vitamin C ka assay), jabki Iodometry me reaction se nikle huye Iodine ko Sodium Thiosulphate aur Starch indicator se naapa jata hai.