LogKow (octanol-water partition coefficient)

Take a chemical, shake it with equal parts octanol and water, and measure how much ends up in each. The ratio is Kow, and the logarithm of that ratio is LogKow. Because it is logarithmic, each whole-number step is a tenfold change in preference for the oily phase.

Why octanol? It is a convenient laboratory stand-in for the fatty membranes and tissues of organisms. Chemicals with a high LogKow tend to be poorly soluble in water and readily taken up into fat, while those with a low value stay in the water and are less likely to accumulate.

LogKow is a screening tool, not proof. It works best for neutral organic chemicals. It is a poor guide for metals, for ionizing compounds whose behavior depends on pH, and for substances that are rapidly metabolized. Regulators use it where measured bioconcentration data are missing, and a measured value generally carries more weight.

On an SDS it is one data point. Read it together with BCF, persistence and aquatic toxicity. Different calculation methods or measurement conditions can give different numbers, so the SDS should say whether the figure is measured or estimated.