Technical Study On Preparation Of Soluble Trivalent Manganese Salt By Electrode Reaction

By consulting the literature,the following information was obtained.Compared with divalent manganese salts and tetravalent manganese salts,the characteristic of soluble trivalent manganese salt was few studied.And it owned extensive application prospects because of its active redox properties.Trivalent manganese salt have been widely used in chemiluminescence reaction,as well as in organic compounds analysis and in manganese redox flow battery.Trivalent manganese aqueous solution was unstable and prone to disproportionation in weak acid and neutral condition,and trivalent manganese existed in the form of precipitation under alkaline conditions.So the stability and concentration of trivalent manganese solution was not high.This greatly restricted application range of soluble trivalent manganese.Based on these information,the method and the corresponding optimum technological conditions for preparing high stability and high concentration soluble trivalent manganese was found in this thesis.The electrosynthesis of solution-phase Mn（Ⅲ）was carried out in a two-compartment cell.Sulfuric acid,pyrophosphate and acetic acid was selected as acid media respectively in electrolysis process.The acidic medium formed corresponding complex with Mn³⁺ and provided a strong acid environment which inhibited the disproportionation reaction.The divalent manganese salt corresponding to the acidic medium was selected as the manganese source,because this way avoided the interference of hetero atoms.First,optimum electrolytic process condition was successfully selected through electrochemical technique.Soluble trivalent manganese product at anode was qualitatively identified by ultraviolet and visible spectrophotometry.The stability and conductivity of soluble trivalent manganese solution was optimized,and the sulfuric acid concentration was major influence factor and the 5mol/L H₂SO₄ was selected as the rational concentration of sulfuric acid.Then the single-factor experiment was carried out on different processes,and the main process conditions were chosen for orthogonal testing.Under the optimal process conditions,the prepared soluble trivalent manganese concentration exceeded 0.3mol/L,and its half-life at room temperature was more than 1 year.Trivalent manganese sulfate-rich solid was firstly prepared from high concentration soluble trivalent manganese solution by cooling crystallization.Microscopic analysis of solids was carried out.The cyclic voltammetry（CV）was tested by electrochemical workstation,and it was confirmed that the potential of the Mn（II）/Mn（Ⅲ）pair was 1.4849 V vs.NHE and the redox process of Mn（II）/Mn（Ⅲ）was quasi-reversible process on the graphite electrode.The electrochemical kinetics parameter was calculated（electrolyte was 5M H₂SO₄ and 0.4M MnSO₄）,just like electron transfer coefficient,diffusion coefficient was 5.3057×10-6 cm2/s,and exchange current density was 1.5662×10-4 A/cm².The best diphosphate concentration（3mol/L）was also selected by the stability and concentration of soluble trivalent manganese in the solution.The optimum process condition was also determined by single factor experiments and orthogonal experiments.The concentration of prepared soluble trivalent manganese was more than 0.1 mol/L,andthe stability was good in the short term（the half life was more than 15 days）.The electrochemical kinetics parameter was calculated（electrolyte was 3M pyrophosphatic acid and 0.3M Mn H₄P₂O₈）,diffusion coefficient of Mn（II）was 8.347×10-6cm²/s and redox process of Mn（Ⅲ）/Mn（II）was the quasi-reversible process（1.315 V vs.NHE）.The same method was used in the acetic acid medium.The optimum electrolytic acetic acid concentration was 8 mol/L.The maximum prepared soluble trivalent manganese concentration was less than 0.1 mol/L.The stability of manganese（Ⅲ）acetate solution was not high because of manganese（Ⅲ）acetate’s strong organic properties.But manganese（Ⅲ）acetate solid was easily prepared.The solid rich in manganese（Ⅲ）acetate was subjected to microscopic tests（such as XPS and SEM）.By comparing the concentration and stability of soluble trivalent manganese in different acidic media,the concentration and stability of soluble trivalent manganese in sulfuric acid medium was much higher than the other two media.Soluble solids rich in manganese（Ⅲ）sulfate salt and manganese（Ⅲ）acetate salt was prepared.Due to the high concentration and stability of the prepared soluble three-part manganese solution,the foundation for the further application of trivalent manganese has been laid.