Hardy-Schulze Rule: Which Ion Coagulates a Charged Sol Best
A lyophobic colloid stays dispersed because its particles carry the same charge and repel one another. Adding an electrolyte supplies oppositely charged ions that neutralise this charge, and the sol coagulates. Exam questions ask which ion does this most efficiently, and the answer follows from two facts: which charge the sol carries, and the Hardy-Schulze rule.
Coagulation Value and the Hardy-Schulze Rule is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Only the ion of opposite charge does the coagulating
Arsenious sulphide (As₂S₃) sol is negatively charged, so it is the positive ions (cations) of an added salt that neutralise it; the anions have the same sign as the sol and play essentially no part. For a positively charged sol such as hydrated ferric oxide, Fe₂O₃.xH₂O, it is the anions that coagulate. The first step in any such question is therefore to decide which ion in each option is of opposite charge to the sol, and to discard the rest.
Hardy-Schulze rule: higher charge, lower coagulation value
The Hardy-Schulze rule says that the coagulating power of the effective ion rises steeply with its charge (valence). The coagulation value (flocculation value) is the minimum concentration of electrolyte, in millimoles per litre, needed to coagulate a fixed amount of sol in a fixed time, so coagulating power and coagulation value run in opposite directions: the most powerful ion has the SMALLEST coagulation value. For a negative sol the order of coagulating power is Al³⁺ > Ba²⁺ > Na⁺, so coagulation values run Na⁺ > Ba²⁺ > Al³⁺. For a positive sol the order of power is PO₄ 3- > SO₄ 2- > Cl⁻, with coagulation values in the reverse order.
Working through a typical option set
Suppose the options are SO₄ 2-, PO₄ 3-, Al³⁺ and Ba²⁺ and the sol is As₂S₃ (negative). Cross out the two anions, because they carry the same sign as the sol. Compare the two cations by charge, 3+ against 2+: Al³⁺ has the greater coagulating power and therefore the LEAST coagulation value. The trap is picking PO₄ 3- because it has the largest charge on the list; charge only matters for the ion of opposite sign to the sol.
OlympIQ exam lens
- ✓Identify the sign of the sol first: As₂S₃ is negative, hydrated Fe₂O₃ is positive
- ✓Only the ion of opposite charge to the sol coagulates it; ignore ions of the same sign
- ✓Hardy-Schulze: coagulating power rises steeply with the charge of the effective ion
- ✓Coagulation value is inversely related to coagulating power, so the highest-charge effective ion has the lowest coagulation value
- ✓Negative sol: Al³⁺ > Ba²⁺ > Na⁺ in power; positive sol: PO₄ 3- > SO₄ 2- > Cl⁻ in power
Do not choose the ion with the highest charge in the whole option list. For a negative sol like As₂S₃ a 3- anion such as phosphate has the same sign as the sol and does not coagulate it; the answer is the most highly charged CATION, and the strongest coagulant has the smallest coagulation value, not the largest.
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