# Conductivity Calculator - Cell Constant, Molar Conductivity, Kohlrausch's Law & Titration

> Calculate cell constants, molar conductivity, Λm° by Kohlrausch's law, transport numbers and weak acid Ka, plus conductometric titration curves and TDS.

- URL: https://qilingtools.com/en/chemistry/conductivity
- Category: Chemistry Tools (https://qilingtools.com/en/chemistry)
- Tags: [Chemistry](https://qilingtools.com/en/tags/chemistry)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

Electrolytic conductivity is a basic quantity in physical chemistry labs and water analysis. A conductivity meter measures conductance G (or resistance R); multiplying by the cell constant K_cell = l/A gives the conductivity κ. The cell constant is calibrated with a KCl standard of known conductivity — at 25 °C 0.01 mol/L KCl has κ = 1413 μS/cm, so a reading of 1.413 mS means K_cell = 1.000 cm⁻¹. Molar conductivity is Λm = κ/c, or Λm = 1000κ/c in S·cm²/mol with κ in S/cm and c in mol/L. Strong electrolytes follow Kohlrausch's square-root law Λm = Λm° − K√c, so a straight-line extrapolation of dilute data gives the limiting molar conductivity: with the built-in CRC Handbook KCl data up to 0.01 mol/L the intercept is 149.6, against the literature 149.86 S·cm²/mol. By the law of independent migration of ions Λm° is the sum of the ionic contributions, e.g. Λm°(HAc) = λ°(H⁺) + λ°(CH₃COO⁻) = 349.6 + 40.9 = 390.5 S·cm²/mol, which also equals Λm°(HCl) + Λm°(NaAc) − Λm°(NaCl); the transport number is t₊ = λ₊/Λm°. For a weak electrolyte the degree of dissociation is α = Λm/Λm° and Ostwald's dilution law gives Ka = cα²/(1 − α): 0.01 mol/L acetic acid with κ = 163 μS/cm has Λm = 16.3, α = 0.0417 and Ka ≈ 1.8 × 10⁻⁵. The calculator also computes conductometric titration curves (strong acid–strong base, weak acid–strong base, mixed acids and AgNO₃ precipitation, with dilution correction) from ionic conductivities and the charge balance, finds the end point of your own data from two intersecting lines, and converts water EC to 25 °C, TDS and resistivity (ultrapure water 18.2 MΩ·cm).

## How to use

1. Choose a mode: cell constant, molar conductivity, Kohlrausch's law, weak electrolyte Ka, conductometric titration or water quality
2. Enter meter readings, concentrations and temperatures, or load the built-in KCl/NaCl literature data and simulated examples
3. Read the worked steps, fitted lines and charts, copy results or export PNG, SVG and CSV

## Features

- Cell constant calibration with 0.001–1 mol/L KCl standards from conductance or resistance readings
- Linear temperature compensation to 25 °C with an adjustable coefficient (default 2%/°C)
- Molar conductivity with S·cm²/mol and S·m²/mol unit conversion
- Kohlrausch Λm vs √c regression to Λm°, with KCl and NaCl literature data built in
- Limiting ionic conductivities of 16 ions at 25 °C; build any electrolyte to get Λm° and transport numbers
- Degree of dissociation and Ka of weak electrolytes from one measurement or an Ostwald plot
- Conductometric titration curves for strong, weak and mixed acids and precipitation, with per-ion contributions and two-line end-point detection
- Water quality: EC₂₅, TDS, resistivity and ISO 3696 laboratory water grade

## FAQ

### How do I determine the cell constant of a conductivity cell?

Immerse the cell in a KCl standard of known conductivity, read the conductance G (or resistance R = 1/G) and compute K_cell = κ_standard/G. Common 25 °C values are 147 μS/cm for 0.001 mol/L, 1413 μS/cm for 0.01 mol/L, 12.88 mS/cm for 0.1 mol/L and 111.9 mS/cm for 1 mol/L KCl. If you calibrate away from 25 °C, convert the standard's conductivity to the measurement temperature first (about 1.9%/°C for KCl). Choose a standard close to your samples' conductivity.

### What is the difference between conductivity and molar conductivity?

Conductivity κ is the conductance of a 1 cm cube of solution, in S/cm, and rises with concentration. Molar conductivity Λm = κ/c is the conductance of one mole of electrolyte between electrodes 1 cm apart, in S·cm²/mol, and reflects how well each ion carries current. Because of ion–ion interactions Λm of a strong electrolyte falls slowly with concentration, while for a weak electrolyte it falls steeply as the degree of dissociation decreases.

### How is the dissociation constant of acetic acid found from conductivity?

Measure κ of an acetic acid solution and compute Λm = 1000κ/c. The limiting value from independent ionic migration is Λm°(HAc) = 349.6 + 40.9 = 390.5 S·cm²/mol. Then α = Λm/Λm° and Ka = cα²/(1 − α). For 0.01 mol/L with κ = 163 μS/cm, α ≈ 0.042 and Ka ≈ 1.8 × 10⁻⁵. With a concentration series, plot 1/Λm against cΛm: the intercept is 1/Λm° and the slope gives Ka.

### Why is a conductometric titration curve V-shaped?

When NaOH titrates HCl, the solution initially conducts mainly through the very mobile H⁺ ion (λ° = 349.6). Added NaOH replaces H⁺ with the slower Na⁺ (50.1), so conductivity falls linearly; after the equivalence point excess OH⁻ (199.1) makes it rise again, and the intersection of the two lines is the end point. A weak acid starts low and rises slowly, and the KCl + AgNO₃ precipitation titration is almost flat before the end point. Conductometric titration works for coloured, turbid or very dilute solutions.

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