Thermodynamic Properties Of D,L-methionine In KCl Aqueous Solution

Methionine is one of the essential amino acids for animals which is widely used in feed,medicine and other industries.In this paper,using hydrochloric acid acidification D,L-methionine potassium production D,L-methionine technology as the background,thermodynamic properties of D,L-methionine in KCl solution were studied which not only provides the basis for the industrial process optimization and equipment designs,but also supplies thermodynamics data of amino acid in the electrolyte solution which provides reference for biochemical research.Firstly,the dissociation constants of methionine in aqueous solutions with 0.25-4.0 mol/kg KCl at 25℃,30℃ and 35℃ were measured by changing the pH value.The dissociation constants of methionine in KCl solutions decrease with increasing temperature,which means dissociation is endothermic.The enthalpy change of protonation was calculated and increases with the increase of ionic strength.Through electromotive force method,the activity coefficients of 0.1-1mol/kg KCl in 0.025-0.20mol/kg methionine aqueous solutions at 25℃ were measured.The activity coefficients of methionine in KCl aqueous solutions were also calculated.The DH model,SIT model and Pitzer model were applied to fit dissociation constants and activity coefficients,which show that the Pitzer model fits better.Secondly,at 25℃-35℃ the laser method was used to measure the solubilities of D,L-methionine in different KCl solutions and the pH-related solubilities in 1.0mol/kg and 2.0mol/kg KCl solutions.The salting-out coefficients of D,L-methionine in KCl solution were calculated,which showed that KC1 had a salting-out effect on D,L-methionine.The activity coefficient correlation model and empirical models(Van’t Hoff model and Apelbalt model)were adopted to fit the temperature-dependent solubilities of D,L-methionine in KCl solutions.The results show that the empirical model fits better;the pH-dependent solubility model was used for D,L-methionine in KCl solution under the influence of pH,and the corresponding model parameters were obtained.Third,the dissolution enthalpy,entropy and Gibbs free energy of D,L-methionine in KCl solution were obtained on the basis of solubility and dissociation constant data,which shows that the dissolution is an entropy-driven and entropy-increasing process instead of spontaneous.The Gibbs free energy of D,L-methionine transfer in pure water and KCl aqueous solutions was calculated showed that D,L-methionine would be unstable in high-concentration KCl solutions.Finally,the supersolubilities of D,L-methionine in KCl solution were measured by laser method.The results show that the mesoscale stability zone narrows with the increase of temperature and stirring speed under the experimental conditions.Compared with the pure water solution,the addition of KCl narrows the width of the metastable zone,and with the increase of KCl concentration,the width of the metastable zone first narrows,and then slightly widens.The use of a larger agitator also narrows the width of the mesoscale zone.