page_title: Recrystallization (chemistry)
section_title: Single perfect crystals (for X-ray analysis)
hierarchical_section_title: Recrystallization (chemistry) / Chemistry / Single perfect crystals (for X-ray analysis)
caption_attribution_description: X-ray_crystals_-_slow_evaporation_2_solvent: Solvent added (clear) to compound (orange) to give compound solution (orange) Second solvent added (blue) to compound solution (orange) to give mixed solvent system (green) Vessel sealed but a small hole allows solvent vapour (clear) to slowly evaporation overtime to give crystals (orange) and a non-saturated solution (blue-green). i.e. → Solvent added (clear) to compound (orange) to give compound solution (orange) → Second solvent added (blue) to compound solution (orange) to give mixed solvent system (green) → Vessel sealed but a small hole allows solvent vapour (clear) to slowly evaporate over time to give crystals (orange) and a non-saturated mixed solvent solution (blue-green).
context_page_description: In chemistry, recrystallization is a technique used to purify chemicals. By dissolving both impurities and a compound in an appropriate solvent, either the desired compound or impurities can be removed from the solution, leaving the other behind. It is named for the crystals often formed when the compound precipitates out. Alternatively, recrystallization can refer to the natural growth of larger ice crystals at the expense of smaller ones.
context_section_description: Growing crystals for X-ray crystallography can be quite difficult. For X-ray analysis, single perfect crystals are required. Typically a small amount (5–100 mg) of pure compound is used, and crystals are allowed to grow very slowly. Several techniques can be used to grow these perfect crystals:
  Slow evaporation of a single solvent - typically the compound is dissolved in a suitable solvent and the solvent is allowed to slowly evaporate. Once the solution is saturated crystals can form.
  Slow evaporation of a multi-solvent system - the same as above, however as the solvent composition changes due to evaporation of the more volatile solvent. The compound is more soluble in the volatile solvent, and so the compound becomes increasingly insoluble in solution and crystallizes.
  Slow diffusion - similar to the above. However, a second solvent is allowed to evaporate from one container into a container holding the compound solution (gas-diffusion). As the solvent composition changes due to an increase in solvent that has gas-diffused into solution, the compound become increasingly insoluble in solution and crystallizes.
  Interface/slow mixing (often performed in an NMR tube). Similar to the above, but instead of one solvent gas-diffusing into another, the two solvents mix (diffuse) by liquid-liquid diffusion. Typically a second solvent is "layered" carefully on top of the solution containing the compound. Over time the two solution mix. As the solvent composition changes due to diffusion, the compound becomes increasingly insoluble in solution and crystallizes, usually at the interface. Additionally, it is better to use a denser solvent as the lower layer, and/or a hotter solvent as the upper layer because this results in the slower mixing of the solvents.
  Specialized equipment can be used in the shape of a "H" to perform the above, where one of the vertical line of the "H" is a tube containing a solution of the compound, and the other vertical line of the "H" is a tube containing a solvent which the compound is not soluble in, and the horizontal line of the "H" is a tube which joins the two vertical tubes, which also has a fine glass sinter that restricts the mixing of the two solvents.
  Once single perfect crystals have been obtained, it is recommended that the crystals are kept in a sealed vessel with some of the liquid of crystallisation to prevent the crystal from 'drying out'. Single perfect crystals may contain solvent of crystallisation in the crystal lattice. Loss of this internal solvent from the crystals can result in the crystal lattice breaking down, and the crystals turning to powder.
caption: → Solvent added (clear) to compound (orange) to give compound solution (orange) → Second solvent added (blue) to compound solution (orange) to give mixed solvent system (green) → Vessel sealed but a small hole allows solvent vapour (clear) to slowly evaporate over time to give crystals (orange) and a saturated mixed solvent solution (blue-green).
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