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| Names | |
|---|---|
| IUPAC name Caesium chloride | |
| Other names Cesium chloride | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.028.728 |
| EC Number |
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| UNII | |
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| Properties | |
| CsCl | |
| Molar mass | 168.36 g/mol |
| Appearance | white solid hygroscopic |
| Density | 3.988 g/cm3[1] |
| Melting point | 646 °C (1,195 °F; 919 K)[1] |
| Boiling point | 1,297 °C (2,367 °F; 1,570 K)[1] |
| 1910 g/L (25 °C)[1] | |
| Solubility | soluble inethanol[1] |
| Band gap | 8.35 eV (80 K)[2] |
| −56.7·10−6 cm3/mol[3] | |
Refractive index (nD) | 1.712 (0.3 μm) 1.640 (0.59 μm) 1.631 (0.75 μm) 1.626 (1 μm) 1.616 (5 μm) 1.563 (20 μm)[4] |
| Structure | |
| CsCl,cP2 | |
| Pm3m, No. 221[5] | |
a = 0.4119 nm | |
Lattice volume (V) | 0.0699 nm3 |
Formula units (Z) | 1 |
| Cubic (Cs+) Cubic (Cl−) | |
| Hazards | |
| GHS labelling: | |
  | |
| Warning | |
| H302,H341,H361,H373 | |
| P201,P202,P260,P264,P270,P281,P301+P312,P308+P313,P314,P330,P405,P501 | |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 2600 mg/kg (oral, rat)[6] |
| Related compounds | |
Otheranions | Caesium fluoride Caesium bromide Caesium iodide Caesium astatide |
Othercations | Lithium chloride Sodium chloride Potassium chloride Rubidium chloride Francium chloride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Caesium chloride orcesium chloride is theinorganic compound with the formulaCsCl. This colorless salt is an important source ofcaesiumions in a variety of niche applications. Its crystal structure forms a major structural type where each caesium ion is coordinated by 8 chloride ions. Caesium chloride dissolves in water. CsCl changes toNaCl structure on heating. Caesium chloride occurs naturally as impurities incarnallite (up to 0.002%),sylvite andkainite. Less than 20tonnes of CsCl is produced annually worldwide, mostly from a caesium-bearing mineralpollucite.[7]
Caesium chloride is widely used inisopycnic centrifugation for separating various types ofDNA. It is a reagent inanalytical chemistry, where it is used to identify ions by the color and morphology of the precipitate. When enriched inradioisotopes, such as137CsCl or131CsCl, caesium chloride is used innuclear medicine applications such as treatment ofcancer and diagnosis ofmyocardial infarction. Another form of cancer treatment was studied using conventional non-radioactive CsCl. Whereas conventional caesium chloride has a rather low toxicity to humans and animals, the radioactive form easily contaminates the environment due to the high solubility of CsCl in water. Spread of137CsCl powder from a 93-gram container in 1987 inGoiânia, Brazil, resulted in one of the worst-ever radiation spill accidents killing four, including one child, and directly affecting 249 people.
The caesium chloride structure adopts a primitive cubic lattice with a two-atom basis, where both atoms have eightfold coordination. The chloride atoms lie upon the lattice points at the corners of the cube, while the caesium atoms lie in the holes in the center of the cubes; an alternative and exactly equivalent 'setting' has the caesium ions at the corners and the chloride ion in the center. This structure is shared withCsBr andCsI and many binary metallicalloys. In contrast, the other alkaline halides have thesodium chloride (rocksalt) structure.[8] When both ions are similar in size (Cs+ionic radius 174 pm for this coordination number, Cl− 181 pm) the CsCl structure can be adopted, when they are different (Na+ionic radius 102 pm, Cl− 181 pm) thesodium chloride structure is adopted. Upon heating to above 445 °C, the normal caesium chloride structure (α-CsCl) converts to the β-CsCl form with the rocksalt structure (space groupFm3m).[5] The rocksalt structure is also observed at ambient conditions in nanometer-thin CsCl films grown onmica,LiF,KBr and NaCl substrates.[9]
Caesium chloride is colorless in the form of large crystals and white when powdered. It readily dissolves in water with the maximum solubility increasing from 1865 g/L at 20 °C to 2705 g/L at 100 °C.[10] The crystals are highlyhygroscopic and deliquescent. Caesium chloride crystals gradually disintegrate at ambient conditions.[11] Caesium chloride does not formhydrates.[12]
| Т (°C) | 0 | 10 | 20 | 25 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| S (wt%) | 61.83 | 63.48 | 64.96 | 65.64 | 66.29 | 67.50 | 68.60 | 69.61 | 70.54 | 71.40 | 72.21 | 72.96 |
In contrast tosodium chloride andpotassium chloride, caesium chloride readily dissolves in concentrated hydrochloric acid.[14][15] Caesium chloride has also a relatively high solubility informic acid (1077 g/L at 18 °C) andhydrazine; medium solubility inmethanol (31.7 g/L at 25 °C) and low solubility inethanol (7.6 g/L at 25 °C),[12][15][16]sulfur dioxide (2.95 g/L at 25 °C),ammonia (3.8 g/L at 0 °C),acetone (0.004% at 18 °C),acetonitrile (0.083 g/L at 18 °C),[15]ethylacetate and other complexethers,butanone,acetophenone,pyridine andchlorobenzene.[17]
Despite its wideband gap of about 8.35 eV at 80 K,[2] caesium chloride weakly conducts electricity, and the conductivity is not electronic butionic. The conductivity has a value of the order 10−7 S/cm at 300 °C. It occurs through nearest-neighbor jumps of lattice vacancies, and the mobility is much higher for the Cl− than Cs+ vacancies. The conductivity increases with temperature up to about 450 °C, with an activation energy changing from 0.6 to 1.3 eV at about 260 °C. It then sharply drops by two orders of magnitude because of the phase transition from the α-CsCl to β-CsCl phase. The conductivity is also suppressed by application of pressure (about 10 times decrease at 0.4 GPa) which reduces the mobility of lattice vacancies.[18]
| Concentration, wt% | Density, kg/L | Concentration, mol/L | refractive index (at 589 nm) | Freezing point depression, °C relative to water | Viscosity, 10−3 Pa·s |
|---|---|---|---|---|---|
| 0.5 | – | 0.030 | 1.3334 | 0.10 | 1.000 |
| 1.0 | 1.0059 | 0.060 | 1.3337 | 0.20 | 0.997 |
| 2.0 | 1.0137 | 0.120 | 1.3345 | 0.40 | 0.992 |
| 3.0 | 0.182 | 1.3353 | 0.61 | 0.988 | |
| 4.0 | 1.0296 | 0.245 | 1.3361 | 0.81 | 0.984 |
| 5.0 | 0.308 | 1.3369 | 1.02 | 0.980 | |
| 6.0 | 1.0461 | 0.373 | 1.3377 | 1.22 | 0.977 |
| 7.0 | 0.438 | 1.3386 | 1.43 | 0.974 | |
| 8.0 | 1.0629 | 0.505 | 1.3394 | 1.64 | 0.971 |
| 9.0 | 0.573 | 1.3403 | 1.85 | 0.969 | |
| 10.0 | 1.0804 | 0.641 | 1.3412 | 2.06 | 0.966 |
| 12.0 | 1.0983 | 0.782 | 1.3430 | 2.51 | 0.961 |
| 14.0 | 1.1168 | 0.928 | 1.3448 | 2.97 | 0.955 |
| 16.0 | 1.1358 | 1.079 | 1.3468 | 3.46 | 0.950 |
| 18.0 | 1.1555 | 1.235 | 1.3487 | 3.96 | 0.945 |
| 20.0 | 1.1758 | 1.397 | 1.3507 | 4.49 | 0.939 |
| 22.0 | 1.1968 | 1.564 | 1.3528 | – | 0.934 |
| 24.0 | 1.2185 | 1.737 | 1.3550 | – | 0.930 |
| 26.0 | 1.917 | 1.3572 | – | 0.926 | |
| 28.0 | 2.103 | 1.3594 | – | 0.924 | |
| 30.0 | 1.2882 | 2.296 | 1.3617 | – | 0.922 |
| 32.0 | 2.497 | 1.3641 | – | 0.922 | |
| 34.0 | 2.705 | 1.3666 | – | 0.924 | |
| 36.0 | 2.921 | 1.3691 | – | 0.926 | |
| 38.0 | 3.146 | 1.3717 | – | 0.930 | |
| 40.0 | 1.4225 | 3.380 | 1.3744 | – | 0.934 |
| 42.0 | 3.624 | 1.3771 | – | 0.940 | |
| 44.0 | 3.877 | 1.3800 | – | 0.947 | |
| 46.0 | 4.142 | 1.3829 | – | 0.956 | |
| 48.0 | 4.418 | 1.3860 | – | 0.967 | |
| 50.0 | 1.5858 | 4.706 | 1.3892 | – | 0.981 |
| 60.0 | 1.7886 | 6.368 | 1.4076 | – | 1.120 |
| 64.0 | 7.163 | 1.4167 | – | 1.238 |
Caesium chloride completely dissociates upon dissolution in water, and the Cs+cations aresolvated in dilute solution. CsCl converts tocaesium sulfate upon being heated in concentrated sulfuric acid or heated withcaesium hydrogen sulfate at 550–700 °C:[21]
Caesium chloride forms a variety of double salts with other chlorides. Examples include 2CsCl·BaCl2,[22] 2CsCl·CuCl2, CsCl·2CuCl and CsCl·LiCl,[23] and withinterhalogen compounds:[24]
Caesium chloride occurs naturally as an impurity in the halide mineralscarnallite (KMgCl3·6H2O with up to 0.002% CsCl),[26]sylvite (KCl) andkainite (MgSO4·KCl·3H2O),[27] and in mineral waters. For example, the water ofBad Dürkheim spa, which was used in isolation of caesium, contained about 0.17 mg/L of CsCl.[28] None of these minerals are commercially important.
On industrial scale, CsCl is produced from the mineralpollucite, which is powdered and treated with hydrochloric acid at elevated temperature. The extract is treated withantimony chloride,iodine monochloride, or cerium(IV) chloride to give the poorly soluble double salt, e.g.:[29]
Treatment of the double salt withhydrogen sulfide gives CsCl:[29]
High-purity CsCl is also produced from recrystallized (and) by thermal decomposition:[30]
Only about 20tonnes of caesium compounds, with a major contribution from CsCl, were being produced annually around the 1970s[31] and 2000s worldwide.[32] Caesium chloride enriched with caesium-137 forradiation therapy applications is produced at a single facilityMayak in theUral Region of Russia[33] and is sold internationally through a UK dealer. The salt is synthesized at 200 °C because of its hygroscopic nature and sealed in a thimble-shaped steel container which is then enclosed into another steel casing. The sealing is required to protect the salt from moisture.[34]
In the laboratory, CsCl can be obtained by treatingcaesium hydroxide,carbonate, caesium bicarbonate, or caesium sulfide with hydrochloric acid:
Caesium chloride is the main precursor to caesium metal by high-temperature reduction:[31]
A similar reaction – heating CsCl with calcium in vacuum in presence ofphosphorus – was first reported in 1905 by the French chemist M. L. Hackspill[35] and is still used industrially.[31]
Caesium hydroxide is obtained byelectrolysis of aqueous caesium chloride solution:[36]
Caesium chloride is widely used incentrifugation in a technique known asisopycnic centrifugation. Centripetal and diffusive forces establish a density gradient that allow separation of mixtures on the basis of their molecular density. This technique allows separation of DNA of different densities (e.g. DNA fragments with differing A-T or G-C content).[31] This application requires a solution with high density and yet relatively low viscosity, and CsCl suits it because of its high solubility in water, high density owing to the large mass of Cs, as well as low viscosity and high stability of CsCl solutions.[29]
Caesium chloride is rarely used in organic chemistry. It can act as aphase transfer catalyst reagent in selected reactions. One of these reactions is the synthesis ofglutamic acid derivatives
where TBAB is tetrabutylammonium bromide (interphase catalyst) and CPME is a cyclopentyl methyl ether (solvent).[37]
Another reaction is substitution oftetranitromethane[38]
where DMF isdimethylformamide (solvent).
Caesium chloride is a reagent in traditionalanalytical chemistry used for detecting inorganic ions via the color and morphology of the precipitates. Quantitative concentration measurement of some of these ions, e.g. Mg2+, withinductively coupled plasma mass spectrometry, is used to evaluate the hardness of water.[39]
| Ion | Accompanying reagents | Residue | Morphology | Detection limit (μg) |
|---|---|---|---|---|
| AsO33− | KI | Cs2[AsI5] or Cs3[AsI6] | Red hexagons | 0.01 |
| Au3+ | AgCl,HCl | Cs2Ag[AuCl6] | Gray-black crosses, four and six-beamed stars | 0.01 |
| Au3+ | NH4SCN | Cs[Au(SCN)4] | Orange-yellow needles | 0.4 |
| Bi3+ | KI,HCl | Cs2[BiI5] or 2.5H2O | Red hexagons | 0.13 |
| Cu2+ | (CH3COO)2Pb, CH3COOH, KNO2 | Cs2Pb[Cu(NO2)6] | Small black cubes | 0.01 |
| In3+ | — | Cs3[InCl6] | Small octahedra | 0.02 |
| [IrCl6]3− | — | Cs2[IrCl6] | Small dark-red octahedra | – |
| Mg2+ | Na2HPO4 | CsMgPO4 or 6H2O | Small tetrahedra | – |
| Pb2+ | KI | Cs[PbI3] | Yellow-green needles | 0.01 |
| Pd2+ | NaBr | Cs2[PdBr4] | Dark-red needles and prisms | – |
| [ReCl4]− | — | Cs[ReCl4] | Dark-red rhombs, bipyramids | 0.2 |
| [ReCl6]2− | — | Cs2[ReCl6] | Small yellow-green octahedra | 0.5 |
| ReO4− | — | CsReO4 | Tetragonal bipyramids | 0.13 |
| Rh3+ | KNO2 | Cs3[Rh(NO2)6] | Yellow cubes | 0.1 |
| Ru3+ | — | Cs3[RuCl6] | Pink needles | – |
| [RuCl6]2− | — | Cs2[RuCl6] | Small dark-red crystals | 0.8 |
| Sb3+ | — | Cs2[SbCl5]·nH2O | Hexagons | 0.16 |
| Sb3+ | NaI | or | Red hexagons | 0.1 |
| Sn4+ | — | Cs2[SnCl6] | Small octahedra | 0.2 |
| TeO33− | HCl | Cs2[TeCl6] | Light yellow octahedra | 0.3 |
| Tl3+ | NaI | Orange-red hexagons or rectangles | 0.06 |
It is also used for detection of the following ions:
| Ion | Accompanying reagents | Detection | Detection limit (μg/mL) |
|---|---|---|---|
| Al3+ | K2SO4 | Colorless crystals form in neutral media after evaporation | 0.01 |
| Ga3+ | KHSO4 | Colorless crystals form upon heating | 0.5 |
| Cr3+ | KHSO4 | Pale-violet crystals precipitate in slightly acidic media | 0.06 |
TheAmerican Cancer Society states that "available scientific evidence does not support claims that non-radioactive cesium chloride supplements have any effect on tumors."[40] TheFood and Drug Administration has warned about safety risks, including significant heart toxicity and death, associated with the use of caesium chloride in naturopathic medicine.[41][42]
Caesium chloride composed ofradioisotopes such as137CsCl and131CsCl,[43] is used innuclear medicine, including treatment ofcancer (brachytherapy) and diagnosis ofmyocardial infarction.[44][45] In the production ofradioactive sources, it is normal to choose a chemical form of the radioisotope which would not be readily dispersed in the environment in the event of an accident. For instance, radiothermal generators (RTGs) often usestrontium titanate, which is insoluble in water. Forteletherapy sources, however, the radioactive density (Ci in a given volume) needs to be very high, which is not possible with known insoluble caesium compounds. A thimble-shaped container of radioactive caesium chloride provides the active source.
Caesium chloride is used in the preparation of electrically conductingglasses[43][46] and screens of cathode ray tubes.[31] In conjunction with rare gases CsCl is used inexcimer lamps[47][48] andexcimer lasers. Other uses include activation of electrodes in welding;[49] manufacture of mineral water, beer[50] anddrilling muds;[51] and high-temperature solders.[52] High-quality CsCl single crystals have a wide transparency range from UV to the infrared and therefore had been used for cuvettes, prisms and windows in optical spectrometers;[31] this use was discontinued with the development of less hygroscopic materials.
CsCl is a potent inhibitor of HCN channels, which carry the h-current in excitable cells such as neurons.[53] Therefore, it can be useful in electrophysiology experiments in neuroscience.
Caesium chloride has a low toxicity to humans and animals.[54] Itsmedian lethal dose (LD50) in mice is 2300 mg per kilogram of body weight for oral administration and 910 mg/kg for intravenous injection.[55] The mild toxicity of CsCl is related to its ability to lower the concentration of potassium in the body and partly substitute it in biochemical processes.[56] When taken in large quantities, however, can cause a significant imbalance in potassium and lead tohypokalemia,arrhythmia, and acutecardiac arrest.[57] However, caesium chloride powder can irritate themucous membranes and causeasthma.[51]
Because of its high solubility in water, caesium chloride is highly mobile and can even diffuse through concrete. This is a drawback for its radioactive form which urges a search for less chemically mobile radioisotope materials. Commercial sources of radioactive caesium chloride are well sealed in a double steel enclosure.[34] However, in theGoiânia accident inBrazil, such a source containing about 93 grams of137CsCl, was stolen from an abandoned hospital and forced open by two scavengers. The blue glow emitted in the dark by the radioactive caesium chloride attracted the thieves and their relatives who were unaware of the associated dangers and spread the powder. This resulted in one of the worst radiation spill accidents in which 4 people died within a month from the exposure, 20 showed signs ofradiation sickness, 249 people were contaminated with radioactive caesium chloride, and about a thousand received a dose exceeding a yearly amount of background radiation. More than 110,000 people overwhelmed the local hospitals, and several city blocks had to be demolished in the cleanup operations. In the first days of the contamination, stomach disorders and nausea due to radiation sickness were experienced by several people, but only after several days one person associated the symptoms with the powder and brought a sample to the authorities.[58][59]
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