How to Titrate Alkyllithiums

Contributed by Jeffrey Bode
Department of Chemistry
University of California, Santa Barbara

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Titration of n-BuLi

To 1.00 mmol dipheynylacetic acid in 8 mL dry THF at rt was added n-BuLi dropwise via syringe, taking careful note of the amount added. Upon addition of each drop, a yellow cloud formed which quickly dissipated. Towards the end of the titration, a white precipitate began to form. At the point when the yellow color persists, the titration was determined to be finished. This processes was repeat three times with three different portions of DPAA in dry THF. The concentration of n-BuLi is calculated as (mmols DPAA/ml n-BuLi). The average of three titrations gives the concentration to ▒0.05 M.

Be sure that you know how to correctly handle and measure accurately the volume of solution dispensed from a syringe! If unsure, see How To Use a Syringe.

Diphenylacetic acid

Commerically available diphenylacetic acid (DPAA) must recrystallized and dried prior to use to ensure accurate tritrations of alkyl lithiums. This is done easily by recrystallization from boiling toluene followed by drying overnight in a vacuum dessicator at 50 íC containing Dreirite or CaCl2. It should be stored in a dessicator.