Colloids Charge and Coagulation
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Direct answer
Every lyophobic sol survives on a surface charge, acquired by preferential adsorption of one ion or by ionisation of surface groups — an AgI sol made with excess KI adsorbs iodide and goes negative. Coagulation demands neutralising that charge, and the Hardy-Schulze rule ranks the effort: flocculating power climbs steeply with counter-ion valence, so for negative sols Al3+ > Ba2+ > Na+ and for positive sols [Fe(CN)6]4− > SO4^2− > Cl−. The coagulating value — the minimum electrolyte concentration in millimoles per litre — is the inverse ranking, and NCERT's numbers for an As2S3 sol (NaCl 52, MgCl2 0.72, AlCl3 0.093) quantify the rule vividly.
What you must remember
- Hardy-Schulze orders: negative sol (As2S3, AgI in iodide excess): Al3+ > Ba2+ > Na+; positive sol (Fe(OH)3): ferrocyanide ion > sulphate > chloride — the effective ion's charge decides, not the electrolyte's identity.
- NCERT coagulating values for As2S3: NaCl 52, MgCl2 0.72, AlCl3 0.093 mmol/L — aluminium needs roughly 560 times less sodium's dose, the numbers JEE recycles.
- Charge origin: preferential adsorption of the ion common to the particle's lattice, or ionisation of surface groups; the ion in excess during preparation donates the charge.
- Double layer and zeta potential: the slipping-plane potential; the sol destabilises as zeta potential collapses towards zero.
- Electrophoresis: charged particles migrate in an applied field — the experimental proof of sol charge; electro-osmosis is its stationary-phase mirror.
- Gold number: milligrams of protective lyophilic colloid that just fail to prevent coagulation of 10 mL of standard gold sol against 1 mL of 10% NaCl; smaller is stronger — gelatin 0.005-0.01 outranks gum arabic 0.15-0.25.
- Other coagulation routes: boiling, persistent dialysis, electrophoresis, and mixing oppositely charged sols — mutual coagulation, the working principle when alum clears muddy water.
- Optical and mechanical identity: the Tyndall effect scatters light only in colloids, and Brownian motion keeps particles suspended against gravity.
Coagulation arithmetic and NCERT numbers
Start from the sol, then reason forward. Mixing silver nitrate into excess potassium iodide deposits AgI particles bathed in iodide; each adsorbs I−, so the sol is negative and the flocculating ion must be a cation. Compare candidates through the NCERT table: Na+ carries a coagulating value of 52 mmol/L, Mg2+ of 0.72, Al3+ of 0.093 — magnesium needs 72 times less than sodium, aluminium 560 times less. Each unit of valence buys an order of magnitude. Now flip to the positive side: the ferric hydroxide sol from ferric chloride hydrolysis adsorbs H+ and demands anions, with the tetravalent ferrocyanide ion the most efficient flocculant NCERT lists. Finally the protective case: adding gelatin before the electrolyte keeps a red gold sol red, because the lyophilic coat physically blocks ion approach — the same chemistry that keeps inks and milk stable, and the reason the gold number ranks protectors inversely to their numeric value.
What JEE still asks of colloids
Surface chemistry, colloids included, was removed from the JEE Main syllabus in the 2023-24 rationalisation; the questions now live in JEE Advanced — whose revised syllabus retains colloids, electrophoresis, coagulation with the Hardy-Schulze rule and emulsions — and in NCERT-based school exams. Main-only aspirants can treat this page as concept reinforcement; Advanced candidates should hold it as scoring territory since the content rarely changes. The traps persist across both regimes. Flocculating power versus coagulating value: inverse rankings, so the ion with the smallest coagulating value is the most powerful flocculant. The gold number inversion: a smaller number means stronger protection, not weaker. And charge prediction: AgI from excess AgNO3 is positive, from excess KI negative — the side in excess supplies the adsorbed ion, a sentence that decides many one-mark questions.
Frequently asked questions
Which ion coagulates a negative sol most efficiently?
The counter-ion of highest charge — Al3+ over Ba2+ over Na+ — per the Hardy-Schulze rule, quantified by NCERT coagulating values of 0.093, 0.72 and 52 mmol/L for As2S3.
What exactly is a coagulating value?
The minimum electrolyte concentration, in millimoles per litre, needed to precipitate a sol; it falls sharply as the counter-ion's charge rises.
What does the gold number measure?
The protective power of a lyophilic colloid — the milligrams needed to just protect 10 mL of a standard gold sol; smaller values mean stronger protection, gelatin near 0.005-0.01 being the benchmark.
Why is an AgI sol negative when prepared with excess KI?
The particles preferentially adsorb the lattice ion present in excess — iodide — so the dispersed phase carries negative charge stabilised by its double layer.
What happens when two oppositely charged sols are mixed?
Their charges neutralise mutually and both coagulate — exploited when the positive aluminium hydroxide sol from alum clarifies negatively charged muddy water.