Caesium chloride

Caesium chloride is the chemical compound with the formula CsCl. This colorless solid is an important source of caesium ions in a variety of applications. CsCl is also well known as a structural type.

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Preparation

Cesium chloride can be prepared by the reaction of caesium hydroxide or caesium carbonate with hydrochloric acid: the resulting salt is purified by recrystallization.

Crystal structure

The caesium chloride structure is composed of interlocking simple cubic lattices of anions and cations. It is the case that in a cubic 1:1 solid where one atom type is much larger than the other that the caesium chloride type lattice is obtained, it can be thought of as a combination of basketballs and golf balls packed in a cubic manner with the golf balls in the gaps between the basketballs. If the two atom types are similar in size (imagine field hockey balls packed with tennis balls) then in the cubic lattice the structure will be like that of sodium chloride.

Uses

Caesium chloride is used in the preparation of electrically conducting glasses.^[1]

Radioisotopes of caesium chloride are used in nuclear medicine, including treatment of cancer. In the production of radioactive sources it is normal to choose a chemical form of the radioisotope which will not be dispersed with ease in the environment as a result of an accident where the source is smashed open. For instance, radiothermal generators (RTGs) often use strontium titanate because it is insoluble in water. But for teletherapy sources, the radioactive density (Ci in a given volume) needs to be very high. As a result it is not possible with any of the insoluble caesium compounds to create the source. It is normal to use a thimble-shaped can of radioactive caesium chloride to provide the active source in a teletherapy type radiotherapy unit. In Brazil, such a source was stolen by scrap metal workers from a disused radiotherapy clinic, and serious injuries and deaths occurred as a result of external gamma exposure and internal exposure (the source was smashed open, releasing the water soluble caesium chloride). See the Goiânia accident for further details.

Caesium chloride is also widely used in the centrifugation of DNA, in a technique known as isopycnic centrifugation. In this method, a caesium chloride solution is centrifuged, allowing centrifugal and diffusive forces to establish a concentration gradient (and thus a density gradient) within the centrifuge tube. When DNA is centrifuged in this solution, fragments of DNA will migrate down the tube until they reach a zone where the density of the DNA is equal to the density of the solution. At this point, the DNA will stop migrating. This allows separation of DNA of different densities (e.g. DNA fragments with differing A-T or G-C content).

Caesium chloride (non-radioactive) is also promoted as an alternative cancer therapy.^[2][3][4][5] These claims are not supported by scientific evidence.

References