Recrystallization (chemistry)
Recrystallization is a widely used technique in chemistry aimed at purifying solid substances. This process involves dissolving the substance along with its impurities in a suitable solvent at elevated temperatures. As the solution cools, the desired pure substance crystallizes out, while impurities remain dissolved. This method is particularly effective when the target substance dissolves in the chosen solvent at high temperatures but not at lower temperatures, allowing for effective separation.
Historically, recrystallization emerged as a solution to the challenges posed by impurities, which could compromise scientific experiments. The technique can be enhanced by using multiple solvents or filtration methods to further isolate the pure crystals. Although the process may require several iterations to achieve the desired purity, it remains a cornerstone in various fields, particularly in pharmaceuticals where high-purity compounds are critical. Additionally, crystallization offers insights into the structural properties of substances, as the resulting crystals maintain the same molecular structure as the original material.
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Recrystallization (chemistry)
Recrystallization is a method used to purify substances. In this context, a pure substance is one that is made up entirely of molecules that all have the same structure. It also is used to isolate chemicals in a solid form for storage or processing.
Historically, impurities were a significant problem for chemists and scientists, since they were very difficult to detect, let alone remove, and they could corrupt experiments. Chemists discovered recrystallization, which involves dissolving the impurities and overall substance, at which point the impurities can be removed. After the technique was improved and developed over the course of the twentieth century, it became a very reliable way of purifying substances.
Background
The structure of crystals was discovered in the early twentieth century. M. von Laue, W. Friedrich, and P. Knipping put crystals of copper sulfate and zinc-blende (ZnS) in the beam from an X-ray tube in April 1912. They saw discrete diffraction spots, which indicated the atoms in crystals were arranged in a space-lattice form. Physics professor William Henry Bragg and his son, William Lawrence Bragg, learned about this discovery a few months later. W.L believed that the diffraction spots indicated the diffraction of waves on the lattice plane families that make up a crystal. He created an equation, known as Bragg's law, to calculate the diffraction spot positions. He found that zinc-blende is not a simple cubic lattice, as Laue had deduced; it is a face-centered cubic lattice. He continued to diagram crystal structures, while his father designed the first X-ray spectrometer with an ionization chamber. The pair used it to record X-ray spectra. In 1913, the father and son published the crystal structure of diamond, based on the X-ray diffraction method they had developed. They were awarded the 1915 Nobel Prize in Physics for their work.
Many substances used in science, such as sugars and fats, are crystalline. Modern manufacturing, especially pharmaceuticals, has developed new types of crystals and new crystallization processes.
As a means of purifying substances, recrystallization is a long-practiced process. It can be summed up as follows:
- Select a solvent that can dissolve the substance in question at an appropriate temperature.
- Bring the solid to the correct temperature and dissolve it.
- In some cases, use decolorized carbon to mark impurities or use a second solvent.
- Cool the solution.
- Wait for crystals to form.
- If necessary, filter out impurities and decolorized carbon.
- Allow crystals to dry.
Another method has been used when no solvent that can achieve the proper reaction is available. In these cases, the substance can be melted and then cooled so it can return to its solid form. Again, the impurities should be left behind. However, this method is not as reliable at achieving complete separation of impurities from the primary substance since the process does not include an additional solvent for the impurities to remain within. The method is effective at recrystallizing silicon, which is used in computer parts.
Overview
Substances can be dissolved in cold solvents, but most solids are more prone to dissolve at higher temperatures. When attempting recrystallization, the ideal solvents are those that can dissolve the substance at high temperatures but fail to do so when cooler. This allows the substance to return to a solid state in cooler temperatures. However, this also depends on the nature of the impurity. Recrystallization is only effective if the primary substance is dissolved at a high temperature and resists dissolving at lower temperatures, while the impurities involved remain dissolved even at room temperature. The process gets its name because the resulting solid is typically in the form of crystals and may not be in the same form as it was prior to dissolution. However, the molecular structure of the substance should be the same and free of impurities. One way to test this is to find the substance's melting point. The larger the amount of impurities within a solid substance, the lower its melting point tends to be. Spectroscopy is another method of testing crystals for impurities.
In some cases, a second solvent may be applied to the materials. The first solvent dissolves both the substance and impurities. The second solvent should only be capable of dissolving the substance or impurities. This helps separate the primary substance from the impurity, keeping one dissolved while the other solidifies.
When impurities are not fully separated from the primary substance, scientists employ forms of filtration. Often, purifying a substance is a gradual process. It may require multiple instances of recrystallization and filtration, with the substance becoming more pure each time.
If filtering is necessary, several factors need to be considered. Filtration is often carried out with paper, specifically designed for exposure to liquids. Filter paper is typically placed over a container—usually a beaker—and the paper is sometimes shaped like a funnel to help speed up the process. The paper should initially be exposed to a sample of the solvent, enough to dampen it. Then the substance should be poured onto the filter paper, which should collect the pure crystals and allow the rest to fall below. Another liquid—not the solvent—should be prepared and then poured through the filter to wash the crystals. Another method is vacuum filtration, which consists of sucking the crystals through a funnel. The suction should help separate the liquid from the crystals.
Another risk of the recrystallization process is the potential contamination threat that filter paper brings. If crystals are left to dry on filter paper, they sometimes pull tiny fibers from the paper when they are removed. These can ruin test results, which is why the crystals should be moved to a dish and allowed to air dry in that location.
After dissolving, it is ideal to cool the solution slowly. If the substance recrystallizes too quickly, it is more likely to simply reclaim some of the impurities that were just removed. In some cases, crystals do not form at all. If that happens, more extreme temperature decreases (such as chilling the solution in ice water) are used to help trigger the crystallization. Once crystals begin to form, however, the substance is usually returned to room temperature to moderate the crystals' growth. Too much remaining solvent can also inhibit crystal growth, so it should remain at a high temperature long enough to evaporate some of the solvent before cooling the substance. The crystals may also fail to grow because they lack a surface to grow on. Scratching the glass of a container can provide that surface.
Bibliography
Authier, Andre. "100th Anniversary of the First Crystal Structure Determinations." Oxford University Press Blog, 20 Aug. 2013, https://blog.oup.com/2013/08/100th-anniversary-first-crystal-structure-determinations-bragg/. Accessed 7 Mar. 2018.
Essam, Nourhan. "Recrystallization: Definition, Principle, Purpose, Steps, Applications and More." PraxiLabs, 5 Nov. 2024, praxilabs.com/en/blog/2022/11/07/recrystallization/. Accessed 19 Nov. 2024.
"Filtering." Wired Chemist, www.wiredchemist.com/chemistry/instructional/laboratory-tutorials/filtering. Accessed 2 Mar. 2018.
Gao, Zhenguo, et al. "Recent Developments in the Crystallization Process: Toward the Pharmaceutical Industry." Engineering, vol. 3, no. 3, 2017, pp. 343 – 353. Science Direct, doi: doi.org/10.1016/J.ENG.2017.03.022. Accessed 7 Mar. 2018.
Harwood, Laurence M. and Christopher J. Moody. Experimental Organic Chemistry: Principles and Practice. Oxford, 1989.
"Recrystallization." University of Alberta, www.chem.ualberta.ca/~orglabtutorials/Techniques%20Extra%20Info/Recrystallization.html. Accessed 4 Mar. 2018.
"Recrystallization." University of Massachusetts. people.chem.umass.edu/samal/267/owl/owlrecryst.pdf. Accessed 4 Mar. 2018.
"Recrystallization." University of Pittsburgh, www.pitt.edu/~ceder/add‗info/recrystallization.html. Accessed 4 Mar. 2018.
"Recrystallization." University of Toronto, www.chem.utoronto.ca/coursenotes/CHM249/Recrystallization.pdf. Accessed 3 Mar. 2018.
"Recrystallization." Wired Chemist, www.wiredchemist.com/chemistry/instructional/laboratory-tutorials/recrystallization. Accessed 2 Mar. 2018.
"Recrystallization Technique." Designer Drug, www.designer-drug.com/pte/12.162.180.114/dcd/chemistry/equipment/recrystallization.html. Accessed 4 Mar. 2018.
"What Is Recrystallization?" Innovate Us, www.innovateus.net/science/what-recrystallization. Accessed 4 Mar. 2018.
Xiongxiong, Gao, Zeng Weidong, Zhao Qingyang, Zhang Saifei, Li Mingbing, and Zhu Zhishou. "Acquisition of Recrystallization Information Using Optical Metallography in a Metastable Beta Titanium Alloy." Journal of Alloys and Compounds, vol. 727, 2017, pp. 346-352. ScienceDirect, doi: https://doi.org/10.1016/j.jallcom.2017.08.141. Accessed 4 Mar. 2018.