Pigment Red 52 and Pigment Red 52:1: hydration states and five crystal structures from powder data

Abstract Pigment Red 52:1 (P.R.52:1, Ca(C 18 H 11 ClN 2 O 6 S)) is an industrial organic pigment with a magenta shade used for printing inks. Pigment Red 52 (P.R.52, Na 2 (C 18 H 11 ClN 2 O 6 S)) is the corresponding sodium salt. Two phases were found for P.R.52, a sesquihydrate (α-phase), and an anhydrate (β-phase). P.R.52:1 exists in at least three phases: a trihydrate (β-phase), a monohydrate (α-phase), and an anhydrate (γ-phase). The commercial phase of P.R.52:1 is the monohydrate. All structures were determined by X-ray powder diffraction. In the instance of the monohydrate of P.R.52:1, the structure solution was supported by single-crystal data of a tiny crystal. The hydration state of all phases was determined by Rietveld refinements. All crystal structures contain double layers: the polar/ionic layers contain the metal cations, sulfonate, carboxylate and keto groups as well as the water molecules, if present. The anionic double layers contain the naphthalene and chlorotolyl fragments. The unit cell volumes of the different hydration states of P.R.52:1 follow a linear trend with an expansion of 23.1 Å 3 per water molecule per formula unit. For a set of eight (including P.R:52 and P.R.52:1) similar industrial organic hydrazone pigments with Na + or Ca 2+ as counter ions, an increment of 22 ± 4 Å 3 per water molecule was found.