Determination of the Fe(II)/Fe(III) ratio in iron gall inks by potentiometry: A preliminary study

Ancient iron gall ink manuscripts can be treated by immersion in water to dissolve excess iron compounds known to be involved in the degradation of such manuscripts. In this study, redox potential measurements were performed so as to follow the dissolution of iron gall inks from original degraded manuscripts over a period of time. Due to the complexity of the system, potentiometry was also applied to understand the interactions between the main components of iron gall inks, namely Fe(II) and Fe(III) sulphates, gallic acid and gum arabic. All these components were electrochemically active but the redox potential was mainly governed by the Fe(III)/Fe(II) redox couple. It could be demonstrated that one gallic acid molecule could readily reduce, in the experimental conditions considered here, up to four Fe(III) cations.