Cesium Chloride
Properties
| State | Solid at room temperature |
| Color | White crystalline |
| Solubility | Very soluble in water (1910 g/L at 20 °C) |
| Melting Point | 646 °C |
| Boiling Point | 1297 °C |
About Cesium Chloride
CsCl is the textbook example of an ionic solid that doesn't crystallize in the rock-salt structure. The radius ratio for Cs+/Cl- comes out to about 0.93, well above the 0.732 cutoff where 6-coordinate octahedral packing stops being the stable arrangement, so each cesium sits at the center of a cube of eight chlorides and vice versa. That 8:8 coordination is its own structural archetype — the CsCl structure type — and it shows up again in CsBr, CsI, and the high-pressure phases of NH4Cl. The single property that makes CsCl indispensable in molecular biology is the density of its saturated aqueous solution: at 20 °C you can pack about 1910 g into a liter of water, and the resulting solution sits around 1.92 g/mL. That density window happens to bracket the buoyant densities of double-stranded DNA (around 1.70 g/mL), single-stranded DNA, RNA, and most viruses. Spin a CsCl solution at 100,000 g for 24 hours in an analytical ultracentrifuge and the cesium and chloride ions redistribute into a smooth concentration gradient — and any DNA in the tube floats to the position where its density matches the local solution density. That's the experiment Meselson and Stahl ran in 1958 to prove DNA replication is semiconservative, and it's still the standard prep for CsCl-purified plasmid DNA when you can't tolerate the trace endotoxin from kit-based preps.
Where you'll encounter it
If you've ever opened the rotor of a Beckman Optima ultracentrifuge in a virology lab and seen the thin white band near the middle of a polyallomer tube, that's a CsCl gradient with virus particles concentrated at their isopycnic point. The technique still gets used for purifying adeno-associated virus for gene therapy, where the AAV2 and AAV9 serotypes band at slightly different densities (1.41 vs 1.39 g/mL) so a single CsCl gradient can resolve full from empty capsids. CsCl is also the alkali-halide of choice for IR spectroscopy windows below 700 cm-1, where KBr cuts off — you press the sample into a CsCl pellet for far-IR work on metal-ligand stretches and lattice modes.
Common Uses
- Isopycnic density-gradient ultracentrifugation for plasmid DNA, viral particle, and ribosome purification
- Far-IR spectroscopy windows and pellets for measurements below the 700 cm-1 KBr cutoff
- Reference compound for the 8:8 CsCl-type structure in undergraduate solid-state chemistry teaching
- Starting material for CsI scintillator crystals used in gamma-ray detectors and PET imaging
- Electrolyte component in cesium-ion-selective electrodes and reference half-cells
- Source of Cs+ for ion-exchange studies on zeolites and clay minerals
- Mass-spectrometry calibration salt for high-mass ESI tuning
- Minor component in some flux mixtures for high-temperature welding and brazing
Safety Information
CsCl carries a GHS Acute Toxicity Category 4 (oral) classification with an LD50 around 2300 mg/kg in rats, plus mild skin and eye irritation classifications. The systemic toxicity is mostly potassium-channel related — Cs+ is similar enough in size to K+ that it gets transported through K+ channels and competes with potassium in cardiac and neuronal tissue, so very large doses can cause arrhythmias. Standard lab quantities are not a serious concern; the practical issue is hygroscopy and dust. Wear gloves and safety glasses, and don't generate dust clouds when weighing out the kilogram quantities you sometimes need for ultracentrifugation prep. There's no OSHA PEL for cesium chloride specifically.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.