The pH Dependent Thermal Stability of the Naphthalene Sulfonic Acids 1, 5-NDS and 2-NSA

Abstract

The overall goal of our experimental studies is to investigate physicochemical factors that affect the stability of the naphthalene sulfonic acids under geothermal conditions. These organic compounds are heavily used in the geothermal industry as tracer chemicals, thus, understanding their thermal stability is crucial to their successful utilization. In our experiments, 1,5-naphthalene disulfonic acid (1,5-NDS), the least thermally stable of the NDS molecules, and 2- naphthalene sulfonic acid (2-NSA), the most thermally stable, were used. The objective was to investigate the effect of pH and ionic strength on the thermal stability and to determine the breakdown products. The experiments were conducted using glass ampoules sealed by oxy-acetylene torch and placed in cold-seal autoclaves. Temperature used were 200°C and 300°C, pH was varied and ionic strength (I) ranged between 0.001 M and 0.05 M. The duration of the experiments was 40 hours. Tracers were analyzed by high performance liquid chromatography (HPLC) using fluorescence detection and standard methods. The pH dependence of the thermal stability of 1,5-naphthalene disulfonate and 2-naphthalene sulfonate have been investigated. The results show that: 1,5-NDS is not stable at 300°C and 200°C, pH = 3.0 – 7.4, I less than 0.001 M, = 0.05 M; the low thermal stability of 1,5-NDS can be explained by two effects: steric hindrance and kinetics; 2-NSA is stable at 200°C pH = 4.0 – 7.4, I = 0.05 M; 2-NSA is stable at 200°C pH = 5.0 – 7.4, I less than 0.001 M; the stability of 2-NSA at 300°C is independent of ionic strength above pH = 4.2; 2-NSA is thermally stable at 300°C and pH = 4.2 – 7.4, I = 0.05 M; 1-NSA is a breakdown product of 1,5-NDS decomposition after 40 hours at pH 5.2 – 7.2, I = 0.05 M and 0.001 M, at 300°C. Further experiments are underway to confirm and expand our knowledge of the thermal stability and kinetics of breakdown of these tracers.