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Rubidium Chloride

RbCl salt
Molar Mass
120.918 g/mol
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Properties

StateSolid (hygroscopic)
ColorWhite crystalline
SolubilityVery soluble in water (910 g/L at 20 °C); slightly soluble in methanol
Melting Point718 °C
Boiling Point1390 °C

About Rubidium Chloride

Rubidium chloride is a white ionic salt (RbCl, 120.918 g/mol) that crystallizes in the rock-salt (NaCl-type) structure at ambient pressure, transitions to the CsCl-type structure above about 0.5 GPa, and dissolves in water to about 910 g/L at 20 C. It is the rubidium analog of NaCl and KCl with the predictable property gradient — larger cation, weaker hydration, lower lattice energy, higher solubility. The chemistry that makes RbCl particularly useful in research is that Rb+ (1.52 Angstrom ionic radius, octahedral) is just slightly larger than K+ (1.38) but with notably weaker hydration and slightly different selectivity through potassium channels, so substituting RbCl for KCl in electrophysiology buffers is a classic probe for K+ channel selectivity-filter behavior. Three application areas dominate practical use. In molecular biology, the 'RbCl method' for preparing chemically competent E. coli (RbCl plus MnCl2 plus CaCl2 plus potassium acetate, then heat shock at 42 C) is the workhorse for routine plasmid transformations and reliably hits 10^7 to 10^8 colonies per microgram of DNA. In nuclear medicine, a Sr-82/Rb-82 generator elutes Rb-82 chloride solution intravenously for myocardial perfusion PET imaging — Rb+ is taken up by the Na/K-ATPase as a K+ analog, distributing in the heart muscle in proportion to coronary blood flow with imaging in under 90 seconds because Rb-82 has a 75-second half-life. In materials science, RbCl single crystals are transparent across a broad range of the infrared spectrum and serve as window material for gas-cell IR spectrometry.

Where you'll encounter it

If you have ever transformed E. coli for a routine cloning prep and used a homemade competent-cell stock from the lab freezer, the recipe almost certainly relied on the Hanahan-style RbCl protocol — the Rb+ neutralizes the lipopolysaccharide negative charge on the outer membrane and lets the supercoiled plasmid slip through during the 42 C heat shock, hitting transformation efficiencies an order of magnitude higher than the simpler CaCl2-only Mandel-Higa method. In a nuclear cardiology suite at a major hospital, the Sr-82/Rb-82 generator sits in a lead pig the size of a small refrigerator and gets eluted directly into the patient's IV line — the 75-second Rb-82 half-life means the entire imaging study finishes before the radioisotope has decayed away, giving a roughly 1.5 mSv effective dose per scan and avoiding the cyclotron logistics of FDG. RbCl optical windows are visible inside the sample chamber of older Perkin-Elmer FTIR instruments configured for far-IR work below 400 cm-1 where KBr starts to lose transparency.

Common Uses

  • Hanahan-method preparation of chemically competent E. coli for plasmid transformation
  • Potassium-channel selectivity probe in patch-clamp electrophysiology
  • Sr-82/Rb-82 generator output for myocardial perfusion PET cardiac imaging
  • Single-crystal infrared window material for FTIR gas-cell spectroscopy
  • Specialty electrolyte and analytical-chemistry calibration standard

Safety Information

GHS: Acute toxicity Category 4 (oral), Skin and eye irritant. Oral LD50 in rats is around 4.4 g/kg — moderate acute toxicity, but chronic exposure displaces tissue K+ and can produce arrhythmia, fatigue, and ataxia at sustained intakes above 1 mg Rb/kg/day because rubidium has a longer biological half-life than potassium and accumulates in muscle and cardiac tissue. Standard lab PPE (nitrile gloves, splash goggles, lab coat) is sufficient for routine handling. OSHA does not list rubidium specifically; ACGIH treats soluble rubidium salts under general nuisance-dust limits. Hygroscopic — store in a tightly closed bottle.

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.

Constituent Elements

Rb — Rubidium Cl — Chlorine

Frequently Asked Questions

What is the molar mass of rubidium chloride?
RbCl has a molar mass of 120.918 g/mol, from Rb (85.468) plus Cl (35.450). Compare to KCl at 74.55 g/mol and CsCl at 168.36 g/mol — the down-the-group mass jump tracks the increasing atomic number, while the lattice constant goes from 6.29 Angstroms (KCl) to 6.59 (RbCl) to 4.12 (CsCl, in its CsCl-type structure rather than rock-salt) reflecting the room-temperature structural change between rubidium and cesium chlorides.
Why use RbCl instead of CaCl2 to make competent E. coli?
The Hanahan RbCl protocol (RbCl, MnCl2, CaCl2, potassium acetate, plus glycerol, with two ice-water buffer washes followed by a 42 C heat shock in the presence of plasmid DNA) routinely delivers transformation efficiencies of 10^7 to 10^8 cfu per microgram of pUC19 — roughly 10-fold higher than the simpler Mandel-Higa CaCl2 method. The Rb+ neutralizes negative phosphate charges on the LPS outer membrane more effectively than Ca2+, the MnCl2 stabilizes the membrane, and the brief heat shock creates transient pores. For most subcloning the simpler CaCl2 method is fine, but for low-yield ligations or library construction the RbCl recipe is worth the extra steps.
How does Rb-82 PET cardiac imaging actually work?
A strontium-82/rubidium-82 generator (Sr-82 has a 25.5-day half-life, parked in a lead-shielded column) is eluted with saline directly into a patient's IV at the start of the scan. Rb-82 (75-second half-life, positron emitter) reaches the myocardium within seconds and is actively pumped into cardiomyocytes by the Na+/K+-ATPase as a K+ analog, distributing in the heart wall in proportion to coronary blood flow. The PET scanner detects the 511 keV annihilation photons from positron annihilation and reconstructs perfusion maps in under three minutes. Because the half-life is so short, a stress and rest study can be done back-to-back in 30 minutes, and the patient effective dose is roughly 1.5 mSv — much lower than a Tc-99m sestamibi SPECT study.