Fajans–Paneth–Hahn Law

The Fajans–Paneth–Hahn Law (also Fajans precipitation rule, Fajans-Peneth precipitation and adsorption rule, Hahn law of precipitation and adsorption, Fajans Law), in chemistry, is a rule governing how a small amount of one substance (tracer) is carried down to a precipitate of another substance present in much larger amount (carrier) by co-precipitation or adsorption.

The rule states that:

The amount carried down is strongly affected by presence of complexing species regardless if it occurs by formation of mixed crystals or adsorption.

The law is named after chemists Kazimierz Fajans, Friedrich Paneth and Otto Hahn.

The Fajans-Paneth-Hahn law is essential for understanding the behaviour of minute amounts of substances (e.g., carrier-free radioisotopes) in solutions. Note that the tracer is precipitated from the solution even when present at concentration far below its solubility limit. The law is also applied for separation of tracer substances by co-precipitation.

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