Formula: C₂₄H₂₀BK (=K[BPh₄])

The potassium ion, K⁺, is very soluble and very few precipitating agents are known. Tetraphenylborate was first synthesized in the 1950s and, in the sodium form, is very soluble in water. When K⁺ is introduced into solution, however, potassium tetraphenylborate is formed and precipitates. The solubility of K[BPh₄] is very low in water (1.8×10^-4 M), but is much higher in acetone and other organic solvents, including lipids.

The crystal structure of KBPh₄ shows that K⁺ is coordinated with the ring-shaped delocalized π (pi) bonds of four benzene rings from two adjacent KBPh₄ molecules. In the nearby image, K⁺ has been removed from the structure for purposes of clarity to reveal the red “π-cage”.

Distribution

Tetraphenylborate also forms precipitates with ions of similar size and charge: NH₄⁺, Rb⁺, Cs⁺, Ag⁺ and Tl⁺. Tetraphenylborate was originally proposed as an analytical reagent for determining K⁺ either gravimetrically or by titration; however, standard techniques of flame photometry, atomic absorption spectroscopy, and ICP-AES are all generally regarded as superior to this use. In laboratory settings, it has been used as an extractant for K in minerals, essentially weathering K-bearing minerals by reducing the K⁺ concentration in solution to nearly zero by precipitating K⁺. Tetraphenylborate has, from time to time, been proposed as an extractant for measuring quantities and fluxes of exchangeable and “fixed” K⁺ and NH₄⁺ for evaluating soil fertility. It has also been proposed as a tool in the cleanup of radioactive spill sites in which radiocesium, ¹³⁷Cs⁺, is to be immobilized and recovered.

Flaschka, H., and A.J. Barnard, Jr. 1960. Tetraphenyl boron (TPB) as an analytical reagent. Adv. Anal. Chem. Instr. 1:1-117.

Hoffmann, K., and E. Weiss. 1974. J. Organometallic Chem. 67:221-228. [crystal structure].

Scott, A., R.R. Hunziker, and J.J. Hanway. 1960. Chemical extraction of potassium from soils and micaceous minerals with solutions containing sodium tetraphenylboron: I. Preliminary experiments. Soil Sci. Soc. Am. Proc. 24:191-194.