Partition coefficient

Key takeaways

Partition coefficient belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Partition coefficient to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Partition coefficient from memory before moving on to harder problems.

Reference excerpt

In the physical sciences, a partition coefficient (P) or distribution coefficient (D) is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. This ratio is therefore a comparison of the solubilities of the solute in these two liquids. The partition coefficient generally refers to the concentration ratio of un-ionized species of compound, whereas the distribution coefficient refers to the concentration ratio of all species of the compound (ionized plus un-ionized). In the chemical and pharmaceutical sciences, both phases usually are solvents. Most commonly, one of the solvents is water, while the second is hydrophobic, such as 1-octanol. Hence the partition coefficient measures how hydrophilic ("water-loving") or hydrophobic ("water-fearing") a chemical substance is. Partition coefficients are useful in estimating the distribution of drugs within the body. Hydrophobic drugs with high octanol-water partition coefficients are mainly distributed to hydrophobic areas such as lipid bilayers of cells. Conversely, hydrophilic drugs (low octanol/water partition coefficients) are found primarily in aqueous regions such as blood serum. If one of the solvents is a gas and the other a liquid, a gas/liquid partition coefficient can be determined. For example, the blood/gas partition coefficient of a general anesthetic measures how easily the anesthetic passes from gas to blood. Partition coefficients can also be defined when one of the phases is solid, for instance, when one phase is a molten metal and the second is a solid metal, or when both phases are solids. The partitioning of a substance into a solid results in a solid solution. Partition coefficients can be measured experimentally in various ways (by shake-flask, HPLC, etc.) or estimated by calculation based on a variety of methods (fragment-based, atom-based, etc.). If a substance is present as several chemical species in the partition system due to association or dissociation, each species is assigned its own Kow value. A related value, D, does not distinguish between different species, only indicating the concentration ratio of the substance between the two phases.

Nomenclature Despite formal recommendation to the contrary, the term partition coefficient remains the predominantly used term in the scientific literature. In contrast, the IUPAC recommends that the title term no longer be used, rather, that it be replaced with more specific terms. For example, partition constant, defined as

where KD is the process equilibrium constant, [A] represents the concentration of solute A being tested, and "org" and "aq" refer to the organic and aqueous phases respectively. The IUPAC further recommends "partition ratio" for cases where transfer activity coefficients can be determined, and "distribution ratio" for the ratio of total analytical concentrations of a solute between phases, regardless of chemical form.

Partition coefficient and log P The partition coefficient, abbreviated P, is defined as a particular ratio of the concentrations of a solute between the two solvents (a biphase of liquid phases), specifically for un-ionized solutes, and the logarithm of the ratio is thus log P. When one of the solvents is water and the other is a non-polar solvent, then the log P value is a measure of lipophilicity or hydrophobicity. The defined precedent is for the lipophilic and hydrophilic phase types to always be in the numerator and denominator respectively; for example, in a biphasic system of n-octanol (hereafter simply "octanol") and water:

log ⁡ P oct/wat = log 10 ⁡ ( [ solute ] octanol un-ionized [ solute ] water un-ionized ) . {\displaystyle \log P_{\text{oct/wat}}=\log _{10}\left({\frac {{\big [}{\text{solute}}{\big ]}_{\text{octanol}}^{\text{un-ionized}}}{{\big [}{\text{solute}}{\big ]}_{\text{water}}^{\text{un-ionized}}}}\right).}

To a first approximation, the non-polar phase in such experiments is usually dominated by the un-ionized form of the solute, which is electrically neutral, though this may not be true for the aqueous phase. To measure the partition coefficient of ionizable solutes, the pH of the aqueous phase is adjusted such that the predominant form of the compound in solution is the un-ionized, or its measurement at another pH of interest requires consideration of all species, un-ionized and ionized (see following). A corresponding partition coefficient for ionizable compounds, abbreviated log P I, is derived for cases where there are dominant ionized forms of the molecule, such that one must consider partition of all forms, ionized and un-ionized, between the two phases (as well as the interaction of the two equilibria, partition and ionization). M is used to indicate the number of ionized forms; for the I-th form (I = 1, 2, ... , M) the logarithm of the corresponding partition coefficient, log ⁡ P oct/wat I {\displaystyle \log P_{\text{oct/wat}}^{I}} , is defined in the same manner as for the un-ionized form. For instance, for an octanol–water partition, it is

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Illustrations

Worked examples

Example 1 — a first encounter with Partition coefficient

Start with the simplest possible case. Write down what Partition coefficient claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Partition coefficient before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Partition coefficient ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Partition coefficient

In research: Partition coefficient appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Partition coefficient in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Partition coefficient is common in secondary-school and first-year university syllabi. It links to neighbouring topics Equilibrium chemistry, Medicinal chemistry, Physical chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Partition coefficient outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Partition coefficient in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Partition coefficient means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
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Frequently asked questions

What is Partition coefficient in simple terms?

In the physical sciences, a partition coefficient (P) or distribution coefficient (D) is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. This ratio is therefore a comparison of the solubilities of the solute in these two liquids.

Why does Partition coefficient matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Partition coefficient?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Partition coefficient.