Liquid-liquid extraction of dicyanocobalamin was performed, using quaternary ammonium salts (Aliquat 336) as liquid ion exchangers. A chemical model, taking into account aggregation of the cyanide and dicyanmbalamin ammonium salts in the organic phase, results in quantitative agreement with experimental data, covering the whole concentration range of practical interest. Vitamin B12 is the moat complex of all vitamin molecules. Although total chemical synthesis was achieved once in the laboratory, it is made industrially by biosynthesis. Vitamin B12 or cyanocobalamin (Cbl) is obtained as a dilute component in a fermentation broth. Its purification to obtain a pharmaceutical grade product can be achieved by a two-stage ion-exchange process using an ion-exchange resin with substituted ammonium salts (Shafer and Holland, 1955).As part of an investigation of other ways to concentrate and purify vitamin B12, (Calvarin, 1989), this article describes a liquid-liquid extraction experiment where Cbl is extracted as an anionic cyanide complex CblCN-by an organic phase containing quaternary ammonium sdta (Aliquat 336) used as anion exchangers. The results are represented by a simple, but credible, empirical ideal associated solution model assuming aggregation of species in both phases. The species involved in this extraction are presented in Figure 1. Experimental SectionMaterials. Vitamin B12 was obtained from RhonePoulenc as a pharmaceutical grade product of 96% purity. Sodium cyanide and p-xylene were purchased as Prolabo Rsctapur products, dodecane as a Prolabo technical grade product, and l-decanol as a Fluka Purum product.The mixture of trialkylmethylammonium chlorides (trialkyl= C8-Cl0, mainly capryl), known under the name of Aliquat 336, was bought as an organic solution from Fluka. It was converted to cyanide salts before extraction experiments, by repeated contacts with concentrated aqueous sodium cyanide solutions. In the conditions of the experiments, the quaternary ammonium cations form ion pairs only with CN-and CblCN-, denoted respectively as ACN and ACblCN, and various aggregates. It does not diesolve in the aqueous phase as was experimentally verified.Analytical Methods. Water Karl-Fischer titrations were performed on a Tacussel Aquaprocesseur. Interfacial tensions were measured on a Kruas K10 ring tensiometer. Cbl and CblCN-concentrations in the aqueous phase were measured on a Hewlett-Packard 8452 diode-array spectrophotometer, between 480 and 820 nm. They have absorption peaks at 551 nm for Cbl and 580 nm for CblCN-, -0888-58851 921 2631-1705$03.OO/ 0 and therefore both concentrations can be obtained simultaneously. Cyanide concentrations in aqueous phase were measured on a Metrohm 512 potentiometer by Ag+ titration: this method does not distinguish free cyanide anions and CblCN-complex, and the result is therefore an overall cyanide concentration. The total ammonium concentration in the organic phase is the sum of the ionpair [ACN] and [ACblCN] concentrations; it is measured by Ag+ potentiometric titrati...