SEARCHING FOR THE ORIGIN OF THE DISCREPANCY BETWEEN SPECTROSCOPICALLYAND CONDUCTIMETRICALLY DETERMINED DISSOCIATION-CONSTANTS - THE CASE OF 4-HYDROXY-7-NITROBENZOFURAZAN REVISITED

The dissociation in organic solution of 4-hydroxy-7-nitrobenzofurazan (NBDOH) has been studied by UV-vis spectroscopy and conductimetry. The unusual variations of the UV-vis spectrum with phenol concentration, previously mentioned in the literature, could be attributed to the hig h sensitivity of the compound towards basic residues, still present af ter standard purification of the solvent. This neutralization step sev erely affects the dissociation measurement especially when small amoun ts of phenol are used, as it is the case with spectrophotometry. This could explain the discrepancy usually found between spectroscopically and conductimetrically obtained data. In organic solutions, water trac es acted as a base and were proved to be necessary to allow the dissoc iation of the phenol. Taking into account the initial neutralization s tep by basic residues, the dissociation constant was calculated to be 1.2 x 10(-5) from spectrophotometric measurements, in good agreement w ith that found by conductimetry.