The crystal structure of Rochelle salt (sodium potassium tartrate tetrahydrate NaKC ₄ H ₄ O ₆ . 4H ₂ O)

The complete crystal structure of Rochelle salt (sodium potassium tartrate tetrahydrate) has been determined by Fourier and Patterson methods. Some of the difficulties in the application of these methods are discussed. The tartrate molecule is found to lie approximately in three planes, the planes of each half of the molecule being inclined at 60 $^{\circ} $ to the plane of the carbon atoms. The tartrate molecules are bonded to sodium and potassium atoms both directly and through the medium of water molecules. If the water molecules are to preserve their customary tetrahedral 'bonding' it is necessary to suppose that one of the carboxyl groups of the molecule is also a dipole. A reversal of the continuous chain of carboxyl-water-water dipoles is a possible explanation of the peculiar dielectric properties of the salt.