A Voltammetry Method For Determination The Strength Of Br(?)nsted-lowry Acids And Bases In Aprotic Solvent

The acid-base dissociation constant?pK_a and pK_b? is one of the important physicochemical parameters.Many organic compounds possess acidic or basic functional groups which have a strong impact on their physicochemical and biological properties.Until now,the pK_a and pK_b value of many compounds in aqueous have been obtaind.However,the determination of the acid-base dissociation constant in a non-aqueous solvent is less reported.Aprotic solvents,including dimethylsulfoxide,N,N dimethylamide and acetonitrile,have been widely applied in the fields of organic synthesis and drug analysis.It is a significant research work to develop a simple and effective determination method of the acid-base dissociation constant in a aprotic solvent.The redox behaviors of hydroquinone and p-benzoquinone are a typical proton-coupled electron transfer reaction?PCET?.The small concentrations of Br???nsted acids caused a new cathodic peaks in the cyclic voltammograms of quinone derivatives in acetonitrile.In this paper,the voltammetric method was used to study the factors affecting the peak potential of the new cathode.The results showed that the height of the new wave increases with the Br???nsted acid concentration increasing,and the peak potential is dependent on the pK_a of Br???nsted acid,but not the quinones concentration.On this basis,we proposed a new mechanism of voltammetric determination the pK_a of Br???nsted acids in acetonitrile and established the relationship between the pK_a of Br???nsted acid and the peak potential of the new peak.In this paper,we also developed a new method for indirectly determining the weak Br???nsted acid-base dissociation constant in aprotic solvents.Obiously,the conjugate base?A^-? of weak acids?HA?is a strong base and the conjugate acid?RNH₃⁺? of weak base?RNH₂? is a strong acid,and the pK_a and pK_b of the conjugate acid and base could be determined by the above mentioned voltammetric method.The pK_a and pK_b of weak acid and weak bases were derived by the autoprotolysis constant(pK_auto) of acetonitrile.The acid-base dissociation constant values obtained by our proposed method are basically consistent with those of the literature.Moreover,we also determined the dissociation constant of sulfadiazine drug in acetonitrile.In this paper,a method for the electrochemical determination of the organic acid-base dissociation constant in aprotic solvent was established,which has the advantages of independence of reference electrode potential drift,fast analysis speed,small sample amounts,and coverd a wide range of pK_a and pK_b.