Partition Coefficient Calculator

Enter the concentration of a substance in one medium and the concentration of the same substance in another medium (both at equilibrium) to determine the partition coefficient.

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Partition Coefficient Formula

The partition coefficient describes how a solute distributes between two media after equilibrium is reached. In this calculator, it is found by dividing the concentration of the substance in medium 1 by the concentration in medium 2.

K_p = \frac{C_1}{C_2}

The ratio is most useful when both concentrations are measured using the same concentration basis and the same units. If one value is entered in grams per liter and the other in milligrams per liter, convert them first before interpreting the result.

How to Use the Calculator

1. Enter the concentration of the substance in medium 1.
2. Enter the concentration of the substance in medium 2.
3. Leave the partition coefficient field blank to solve for it, or enter the coefficient and one concentration to solve for the missing concentration.
4. Make sure the concentrations represent equilibrium values, not initial values before mixing or extraction.

If you need to solve for a concentration instead of the coefficient, the same relationship can be rearranged as follows:

C_1 = K_p \times C_2

C_2 = \frac{C_1}{K_p}

How to Interpret the Result

• A larger coefficient means the substance is more concentrated in medium 1 at equilibrium.
• A smaller coefficient means the substance is more concentrated in medium 2.
• A value close to equal distribution means the solute has little preference between the two media.
• If the order of the media is reversed, the numerical value becomes the reciprocal of the original coefficient.

K_{2/1} = \frac{1}{K_{1/2}}

Example

Suppose a solute has an equilibrium concentration of 1.2 g/L in hexane and 0.30 g/L in water. The partition coefficient is:

K_p = \frac{1.2}{0.30} = 4

This means the solute is four times as concentrated in the hexane phase as in the water phase under the stated conditions.

Why Equilibrium Matters

A partition coefficient is defined using concentrations measured after the two phases have finished redistributing the solute. If the system has not reached equilibrium, the ratio can be misleading. Temperature, solvent choice, mixing time, and the chemical form of the solute all affect the final distribution.

This is especially important for weak acids, weak bases, and other ionizable compounds. Their apparent distribution can change with pH because the neutral and ionized forms do not partition the same way.

Partition Coefficient vs. Distribution Ratio

The partition coefficient usually refers to the inherent equilibrium ratio for a substance between two phases. In practice, some experiments report a broader distribution ratio that includes all forms of the substance present in a phase. For neutral compounds these values may be very similar, but for ionizable compounds they can differ significantly.

Partition Coefficient vs. logP

In chemistry, biochemistry, and pharmaceutical work, the logarithm of the partition coefficient is often reported instead of the raw ratio. This calculator returns the partition coefficient itself, not its logarithm.

\log P = \log_{10}(K_p)

Common Applications

• Liquid-liquid extraction: estimating how efficiently a solute moves into an extracting solvent.
• Environmental chemistry: evaluating how compounds distribute between water and organic phases.
• Pharmaceutical science: assessing lipophilicity and membrane-partition behavior.
• Separation processes: comparing solvent systems for purification and recovery.
• Analytical chemistry: predicting sample preparation and extraction performance.

Common Mistakes

• Using concentrations that were not measured at equilibrium.
• Mixing units between the two concentration fields.
• Reversing the order of the media and then comparing the value to a reference reported in the opposite order.
• Assuming the coefficient stays constant when temperature or pH changes.
• Confusing the raw partition coefficient with its logarithmic form.

Quick Answers

Is the partition coefficient unitless?

It is treated as unitless when both concentrations are expressed on the same basis and in the same units.

Can the coefficient be less than one?

Yes. That simply indicates the substance prefers medium 2 over medium 1 for the order used in the calculation.

Why does the value change when I swap the media?

The ratio is directional. Switching the numerator and denominator changes the result to its reciprocal.