Log Kow Environmental Calculator

Calculate log Kow, bioconcentration factor, and environmental fate of organic chemicals.

Benzene: 2.13, DDT: 6.91, Ethanol: -0.31
Typical soil: 0.01-0.05
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What Log Kow Predicts

The octanol–water partition coefficient measures how a compound distributes between a fatty phase and water. Octanol is used because it approximates lipid membranes, so log Kow predicts whether a chemical will accumulate in tissue or wash away in water.

log KowBehaviourEnvironmental fateExample
< 1Very water solubleMobile, rapidly dilutedEthanol (−0.31)
1–3Moderately solubleSome mobilityAspirin (1.19)
3–5LipophilicSorbs to soil and sedimentNaphthalene (3.30)
> 5Strongly lipophilicBioaccumulatesDDT (6.91), PCBs

The threshold that matters in regulation is log Kow above 5, at which bioaccumulation potential becomes a formal concern under most chemical assessment frameworks. Compounds above this partition into fat faster than organisms can excrete them, so concentration climbs through the food chain.

Why It Also Matters for Drugs

The same property governs whether a drug crosses membranes. Lipinski's rule of five sets log P below 5 for oral bioavailability — too hydrophilic and the molecule cannot cross the gut wall; too lipophilic and it will not dissolve, and tends to accumulate in fat.

Note that log P and log Kow refer to the neutral species. For ionisable compounds, log D accounts for the fraction ionised at a given pH, which for acids and bases can differ from log P by several units at physiological pH.

Worked Examples

Example 1: DDT: logKow=6.91, fish BCF
logBCF=0.85×6.91-0.70=5.17
Result: BCF=150,000 — extreme bioaccumulation
Explains DDT food chain magnification
Example 2: Atrazine: logKow=2.61, soil foc=0.02
Koc=10^(0.81×2.61+0.10)=232
Result: Kd=232×0.02=4.64 L/kg
Moderate soil binding, leaches to groundwater
Example 3: The regulatory threshold
Compound with log Kow = 5.2
Result: Flagged as bioaccumulative
Above 5, partitioning into fat outpaces excretion. Most chemical assessment frameworks trigger further evaluation at this point.
Example 4: Why log D differs from log P
Weak acid, log P = 3.5, pKa = 4.0, at pH 7.4
Result: log D ≈ 0.1
At pH 7.4 the acid is over 99.9% ionised, and the charged form barely partitions into octanol. The effective lipophilicity is far lower than log P suggests.

Common Mistakes

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Ignoring ionisation

log Kow describes the neutral molecule. For acids and bases, log D at the relevant pH is the meaningful quantity and can be several units lower.

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Assuming high log Kow means high toxicity

It predicts bioaccumulation and membrane partitioning, not intrinsic toxicity. A compound can accumulate and be relatively harmless, or clear quickly and be highly toxic.

⚠️
Using single-value estimates for regulation

Estimation methods disagree, sometimes by more than a log unit. Regulatory assessment requires measured values for compounds near decision thresholds.

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Forgetting that persistence matters too

Bioaccumulation potential is only one criterion. A lipophilic compound that degrades quickly poses far less risk than one that persists for decades.

Frequently Asked Questions

Why does logKow predict environmental fate?
High logKow = lipophilic = partitions into organic matter (fat, soil organic carbon, plant waxes). Low logKow = hydrophilic = stays in water. This drives bioaccumulation, soil sorption, membrane permeation, and absorption.
Persistent organic pollutants (POPs)?
PCBs, DDT, dioxins: very high logKow (5-8) AND slow degradation. Result: global contamination, food chain magnification. Stockholm Convention bans 12 'dirty dozen' POPs. Measurement: logKow is a key screening criterion.
Why is octanol used as the reference phase?
Its polarity and hydrogen bonding approximate lipid membranes reasonably well, making the partition coefficient a useful predictor of biological and environmental partitioning.
What log Kow indicates bioaccumulation?
Above 5 is the common regulatory threshold, where partitioning into fat outpaces excretion and concentrations increase up the food chain.
What is the difference between log P and log D?
log P applies to the neutral species; log D accounts for ionisation at a specified pH. For ionisable drugs they can differ by several units.
Does high log Kow mean a compound is toxic?
No. It predicts accumulation and membrane crossing, not intrinsic toxicity. Risk depends on accumulation, persistence and potency together.

Formula Explorer connections

Interpretation: This formula connects molecular properties, material structure or environmental transport to a macroscopic behavior or exposure estimate. Assumption: Use parameters measured for the same material, solvent, temperature and environment. Empirical correlations may not transfer outside their calibration range.

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