Acid–Base Properties of Monomethylgermanium(IV) in Aqueous Solutions

Abstract Monomethylgermanium (MMGe), the dominant organogermanium species in aquatic systems, exists predominantly as the neutral mononuclear species (CH 3 )Ge(OH) 3 under environmentally relevant conditions. MMGe displays conservative behavior in freshwater and marine environments, an observation consistent with high chemical stability, yet its fundamental acid–base properties remain poorly characterized. To address this gap, we investigated the protolytic properties of (CH 3 )Ge(OH) 3 at 298.2(2) K across a range of ionic strengths (0.1–1.0 mol·L −1 ) and five supporting electrolytes (LiCl, NaCl, KCl, CsCl, TMACl) by glass-electrode potentiometry. The first deprotonation constant (p K a = –log K 1 ) of (CH 3 )Ge(OH) 3 was consistently determined across experimental conditions with values ranging from 11.07(4) to 11.22(3), except in 1 mol·L −1 TMACl, where an anomalously high value (11.62(5)) was attributed to specific ion effects. 1 H and 13 C NMR titrations in 1 mol·L −1 KCl confirmed a single protonation equilibrium with p K a = 11.22(1), in excellent agreement with the potentiometric results. No evidence was found for the formation of a doubly deprotonated species under the investigated p[H] range (< 13). These findings contribute fundamental thermodynamic data for MMGe and enhance our understanding of its chemical behavior in natural waters.