Design improvement of gas-liquid equilibrium phase-diagram experiment for binary systems
[Objective]The gas-liquid equilibrium phase-diagram experiment for binary systems is a fundamental experiment in physical chemistry,essential for understanding phase equilibrium.It is a classic experiment conducted in university physical chemistry courses.However,the experimental setup features hidden theoretical flaws in its design,contradicting fundamental principles of thermodynamics.Owing to the formation of gas-phase condensate,the actual number of phases in the device is three,failing to meet the condition of two-phase equilibrium.Additionally,no thermal equilibrium exists between the system and the environment.This paper presents an in-depth analysis of the scientific issues present in traditional experimental apparatus from the perspective of physical and chemical theory.[Methods]We conduct a theoretical analysis of the composition of gas-phase condensate in traditional experimental apparatus and reveal that true"phase equilibrium"is not achieved when the system contains three phases.Furthermore,we investigate the impact of condensation temperature on the composition of gas-phase condensate.The experimental results demonstrate that when the temperature of the heated liquid phase remains nearly constant at a specific value,the concentration of n-propanol in the gas-phase condensate is higher at a condensate water temperature of 5℃ than at a temperature of 15℃.This highlights that the boiling point does not correspond to a specific composition ratio in the traditional experimental setup.[Results]Furthermore,this paper proposes an improved in situ testing experiment setup for gas-liquid equilibrium phase diagrams of two-component systems,which holds significant importance for experimental teaching and academic research.This new device model utilizes a four-way valve group to conduct in situ sampling and testing of gas-phase substances in the system,eliminating the need for gas-phase condensation.Consequently,the new setup enables in situ monitoring of gas-phase substances throughout the experiment,unlike traditional devices that require the destruction of gas-phase substances to form a new liquid phase.This ensures adherence to the basic condition of phase number in the phase rule.Additionally,the designed quantitative ring volume is adjustable.A volume of 3-5 mL is expected to result in a significant reduction of the lower limit of detection concentration,thereby greatly enhancing detection accuracy.Furthermore,in both the gas and liquid phases of the system,the temperature remains constant,fulfilling the essential requirement of constant temperature(T)in the phase rule.This is in contrast with traditional boiling point meters,which often exhibit large temperature gradients within the system.Additionally,the system features an auto-adjustment function for pressure via a balloon,ensuring that internal pressure remains unchanged and preventing material loss.This enables the in situ detection of gas-phase substances under constant pressure.[Conclusions]The simple structure of the novel experimental setup allows for large-scale preparation.The improved method presented in this paper achieves the expected goal,providing new ideas for the development of the instrument for gas-liquid equilibrium phase-diagram experiments for binary systems.
two componentsphase diagramgas-liquid phase equilibriumphase rule
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