Potassium-sodium tartrate (Sodium potassium tartrate, ferrotic salt, E337) is a double sodium-potassium salt of tartaric acid.
Chemical formula NaKC4H4O6•4H2O.
It is named after the French apothecary Seignet, who received it around 1655.
For the first time (1920), ferrotic salt was found to have peculiar electrical properties — spontaneous polarization in a certain temperature range. Later, substances with such properties were called ferroelectrics.