Ion Concentration from Conductivity Calculator

Ion Concentration from Conductivity Calculator

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Ion Concentration from Conductivity

This calculator estimates the molar concentration of a single-salt electrolyte solution based on its measured electrical conductivity (κ). It uses Kohlrausch's Law of Independent Migration of Ions, which states that the total molar conductivity of an electrolyte at infinite dilution (Λ°m) is the sum of the contributions from its individual ions.

Calculation Steps

  1. Calculate Limiting Molar Conductivity of the Salt (Λ°m):
    Λ°m = (ν+ ⋅ λ°+) + (ν- ⋅ λ°-)
    Where ν is the stoichiometric number of each ion (e.g., for MgCl₂, ν+=1, ν-=2) and λ° is the limiting molar ionic conductivity for that specific ion.
  2. Calculate Concentration (c):
    Assuming the solution is dilute enough that the molar conductivity (Λm) is approximately equal to the limiting molar conductivity (Λ°m), we can rearrange the standard conductivity equation:
    c = κ / Λ°m
    This calculation requires all units to be consistent. The calculator converts all inputs to SI base units (S/m, S·m²/mol) to solve for concentration in mol/m³, which is then converted back to the more common mol/L (M).

Frequently Asked Questions

Where can I find values for limiting molar ionic conductivity (λ°)?

Limiting molar ionic conductivities (λ°) are experimentally determined constants for individual ions in a specific solvent (usually water) at a standard temperature (typically 25°C). These values must be looked up in a chemistry textbook, a scientific handbook (like the CRC Handbook of Chemistry and Physics), or a reliable online database.

What are the limitations of this calculation?

This calculation is an approximation that works best for dilute solutions of strong electrolytes. It relies on the assumption that Λm ≈ Λ°m (that molar conductivity is constant and doesn't change with concentration). In reality, for strong electrolytes, molar conductivity (Λm) decreases slightly as concentration increases (as described by Kohlrausch's √c law). For weak electrolytes, this assumption is not valid at all, as their molar conductivity changes dramatically with concentration due to changes in the degree of dissociation.

Why does the calculator use SI units (mol/m³) internally?

To use the formula c = κ / Λm correctly, the units must be consistent. The standard SI units are:
• Conductivity (κ): S/m
• Molar Conductivity (Λm): S·m²/mol
• Concentration (c): mol/m³
Using these units ensures the calculation is dimensionally correct: (S/m) / (S·m²/mol) = mol/m³. This calculator converts all inputs (like S/cm, S·cm²/mol) to these base SI units, performs the calculation, and then converts the final mol/m³ result back to the more common mol/L for display.