Kohlrausch’s Law of Independent Ionic Migration

Kohlrausch’s law combines limiting ionic conductivities to estimate the limiting molar conductivity of an electrolyte. The relation is useful for Estimate limiting electrolyte conductivity from ionic contributions.. For CaCl₂, Λm°=λ°Ca²⁺+2λ°Cl⁻=0.01190+2·0.00763=0.02716 S·m²/mol. Check: chloride is counted twice. The final check tests a balance, direction or magnitude relevant to this formula. First map each quantity to the actual experiment, then substitute values. The result describes the modelled relationship; it does not automatically establish every mechanism in the real sample. At infinite dilution, ionic interactions vanish in the limiting model; it does not predict conductivity at an arbitrary finite concentration.

The symbols mean: Λ_m°: limiting molar conductivity of electrolyte (S·m²/mol); ν_i: stoichiometric number of ion i (dimensionless); λ_i°: limiting ionic conductivity (S·m²/mol). Follow Λm° and ionic conductivities in S·m²/mol (or another consistent unit). Concentrations, activities, volumes and times must use compatible scales. A logarithm needs a dimensionless argument, and a balance must include all contributing species. State temperature and standard state whenever they are part of the definition; otherwise the result may refer to a different chemical condition. An amount is not a mass without molar mass, and activity is not an unnormalised concentration. Say whether inputs were measured or taken from a reference.

The additivity applies at infinite dilution, where interionic effects vanish in the limiting model. Ionic conductivities must correspond to the same temperature and unit system. Do not treat finite-concentration conductivities as limiting values. Check this assumption for the actual sample, temperature and phase. Nonideality, mass transport or side reactions can make a precise calculation systematically wrong. Compare the balanced reaction and the activity or concentration scale with the model, then state the assumption that most affects the result. Near model boundaries, check activities and phase state rather than extending a dilute-solution approximation. Different conditions require a fresh model check.

Recalculate the example: For CaCl₂, Λm°=λ°Ca²⁺+2λ°Cl⁻=0.01190+2·0.00763=0.02716 S·m²/mol. Check: chloride is counted twice. Check Λm° and ionic conductivities in S·m²/mol (or another consistent unit) and substitute the value back into the relation. Also ask whether its sign, balance or magnitude is chemically plausible. If a check fails, inspect units, coefficients and reference values before blaming rounding. Substitution checks the algebra; comparing assumptions with the experiment checks whether the model is appropriate. At infinite dilution, ionic interactions vanish in the limiting model; it does not predict conductivity at an arbitrary finite concentration.

Kohlrausch’s Law of Independent Ionic Migration is related to Molar Conductivity, Ionic Strength, Faraday-Gesetz der Elektrolyse, but answers a distinct question: Estimate limiting electrolyte conductivity from ionic contributions.. Do not confuse it with a relation for a neighboring quantity or a different standard state. Name the target quantity and sample conditions first, then choose the equation containing that quantity. Similar-looking equations may use different stoichiometric bases or validity ranges. The linked entries may supply inputs for another step, but they do not replace this distinction. This keeps the interpretation auditable.