Sodium chlorate is an example of an inorganic substance that is achiral, but which crystallizes in a chiral space group P2₁3. This means that individual crystals can be left-handed or right-handed. We can tell these enantimorphs apart by the way they rotate the plane of polarized light. In the figure below you will see that when viewed with white light between two polarizers, some crystals appear darker depending on which way the polarizers are turned. The crystals can be identified as d-NaClO₃ (dextrorotatory) or l-NaClO₃ (levorotatory) depending on which way they rotate the plane of polarization (to the right or left when viewed looking into the beam).

An historic paper in 1898, by F. S. Kipping and W. J. Pope [J. Chem. Soc. Trans. 73 (1898) 606] showed that when a saturated solution of sodium chlorate was left to crystallize, equal numbers of L and D crystals were formed. This seems reasonable, since there is no external chiral influence. However, when they added D-glucose to the solution, they found that 68% of the crystals were l-NaClO₃ (levorotatory). This was a remarkable result, showing the effect of a chiral reagent molecule (the D-glucose) on the process of crystallization. This result has since been used to support theories of the origins of biohomochirality (e.g., the preference in nature for L-amino acids). The result was apparently verified by Niedermaier and Schlenk in 1972 [Chem. Ber. 105 (1972) 3470].

In order to better understand the nature of this chiral interaction, our group set out to make measurements on the glucose/sodium chlorate system. On checking the results of Kipping and Pope, we were amazed to find no bias in the crystallization! Within statistical uncertainty, we find 50% of crystals formed are l-NaClO₃ and 50% are d-NaClO₃. So, what could have caused Kipping and Pope to make the observations that they did (strongly in favour of l-NaClO₃)? We tested various hypotheses, including checking the quality of the glucose used. It turns out that Kipping and Pope may have been the victim of the little-understood (at that time) effect of unintentional seeding. When repeating their experiment, nuclei of crystals from the previous trial could have survived and caused the apparent bias in the percentage of levorotatory crystals formed. Our results can be found in the ACS journal Crystal Growth and Design [1].

References:

1. A. J. Alexander, Cryst. Growth Des. 8, 2630 (2008): "Crystallization of sodium chlorate with D-glucose co-solute is not enantioselective". doi:10.1021/cg800540z