Phitkari, commonly known as alum, has the chemical formula KAl(SO4)2·12H2O. Its IUPAC name is potassium aluminium sulfate dodecahydrate. The molecular mass of alum is 474 g/mol. It is a white crystalline double salt that is widely used in water purification, as a styptic in shaving, in the textile dyeing industry, and in food processing.
Phitkari (alum) formula: KAl(SO4)2·12H2O (potassium aluminium sulfate dodecahydrate).
Molecular mass of alum = 474 g/mol.
Alum is a double salt — contains both K+ and Al³+ cations with SO4²⁻ anions.
Contains 12 molecules of water of crystallisation.
Used in water purification: Al³+ forms Al(OH)3 colloid that coagulates impurities.
Applied as a styptic after shaving — astringent property stops minor bleeding.
Alum solution is acidic (pH ~3–4) due to hydrolysis of Al³+ ions.
Acts as a mordant in textile dyeing to fix dyes permanently onto fabrics.
Phitkari (alum) is a double salt with the formula KAl(SO4)2·12H2O.
Full IUPAC name: Potassium aluminium sulfate-12-water (or potassium aluminium dodecahydrate sulfate)
Other names:
Alum belongs to the class of compounds called alums — double sulfates of a monovalent cation (here K+) and a trivalent cation (here Al³+) with 12 molecules of water of crystallisation.
General formula of alums: M⁺M³⁺(SO4)2·12H2O
Other common alums: Chrome alum: KCr(SO4)2·12H2O; Ammonium alum: NH4Al(SO4)2·12H2O.
Molecular mass of KAl(SO4)2·12H2O:
K: 39 Al: 27 S: 2 × 32 = 64 O (in SO4)2: 2 × 4 × 16 = 128 12H2O: 12 × 18 = 216
Total = 39 + 27 + 64 + 128 + 216 = 474 g/mol
Atomic masses used: K = 39, Al = 27, S = 32, O = 16, H = 1.
Physical properties of KAl(SO4)2·12H2O:
Alum dissolves readily in hot water but has limited solubility in cold water (about 5.6 g/100 mL at 0°C, rising to 357 g/100 mL at 100°C).
Alum (phitkari) has numerous applications:
Water purification: Alum is added to turbid water. Al³+ ions hydrolyse to form Al(OH)3 colloid which coagulates suspended impurities, causing them to settle: Al³+ + 3H2O → Al(OH)3 + 3H+
Shaving and styptic: Applied to small cuts after shaving — the astringent property causes proteins to coagulate, sealing small wounds and stopping minor bleeding.
Dyeing and mordanting: Used as a mordant in the textile industry to fix dyes onto fabrics permanently.
Food additive: Used in baking powder, pickling, and to keep vegetables crisp (E522 in food labelling).
Paper sizing: Used in paper manufacturing to prevent ink from spreading.
Medical uses: Used in certain antiperspirants and as a mild antiseptic.
Leather tanning: Used in the alum tawing process for tanning hides.
Key chemical reactions of alum:
Hydrolysis in water (acidic solution): Al³+ + 3H2O ⇌ Al(OH)3 + 3H+ This produces H+ ions, making alum solution acidic.
Reaction with alkali: KAl(SO4)2 + 3NaOH → Al(OH)3↓ + K2SO4 + Na2SO4 With excess NaOH, Al(OH)3 dissolves (amphoteric behaviour): Al(OH)3 + NaOH → NaAlO2 + 2H2O
Heating alum: On heating, alum first loses water (melts at 92°C), then decomposes above 200°C to give Al2O3, K2SO4, and SO3.
Coagulation: Al³+ from alum coagulates negatively charged colloidal particles by neutralising their charge.
The chemical formula of phitkari (alum) is KAl(SO4)2·12H2O. It is potassium aluminium sulfate dodecahydrate, a double salt.
The molecular mass of alum (KAl(SO4)2·12H2O) is 474 g/mol. Calculated as: K(39) + Al(27) + 2S(64) + 8O(128) + 12H2O(216) = 474 g/mol.
When alum is added to turbid water, Al³+ ions hydrolyse to form aluminium hydroxide (Al(OH)3) colloid. This positively charged colloid coagulates and adsorbs negatively charged suspended particles (mud, bacteria), causing them to settle, leaving clear water.
Alum has astringent properties — Al³+ ions cause proteins in tissues to coagulate and contract. When applied to a small cut, this seals the wound and constricts blood vessels, stopping minor bleeding.
The IUPAC name of phitkari (alum) is potassium aluminium sulfate dodecahydrate.
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