Spectroscopic pH measurement for high temperatures, pressures and ionic strength

Abstract

Accurate measurement of pH is important in several diverse fields, such as process control, reaction equilibrium and kinetics, environmental research to monitor seawater chemistry and natural water quality, biomedical research and in oilfields. The International Union of Pure and Applied Chemistry (IUPAC) has issued guidelines for the standard potentiometric technique for pH measurements. These methods and calibrating standards are, however, recommended only for 278 -323°K, 0.101325 MPa and ionic strengths below 0.1 mol/kg water. 1 The reasons for these measurement constraints are the uncertainty in liquid junction potential and reference electrode stability at high temperatures, pressures and ionic strength and the lack of calibrating standards. Many chemical reactions and processes require pH monitoring and control at extreme conditions of pressure, temperature and salinity. In oilfield applications, accurate pH measurements require that they be made at downhole wellbore conditions with fluids in their native state at high temperatures (typically 323-473°K), high pressures (typically 30 to 100 MPa), and high ionic strengths (typically 0.5-5 mol/kg water). pH is a key parameter whose accurate measurement at downhole wellbore conditions is critical in understanding formation fluid water chemistry to predict corrosion and scale potential in well tubing and surface facilities.Spectroscopic measurement of pH with very high accuracy using pH-sensitive dyes is a well-established laboratory technique for ambient conditions since the early 1900's. 2,3 More recently, this technique has been shown to improve precision for seawater pH measurements. 4 Using equimolal tris buffers for total pH scales, dye equilibrium dissociation constant was characterized at 0.101325 MPa pressure as a function of temperature (293°K to 303°K) and salinity over the narrow salinity range characteristic of seawater (30 to 37 salinity range corresponding to ionic strengths of ϳ0.53 to 0.66 mol/kg water). Yao and Byrne 5 have also applied this technique for freshwater pH measurements, where potentiometric methods can prove to be problematic. Using phosphate buffers, the dye equilibrium

Influence of reaction conditions on the enantioselectivity of biocatalyzed C–C bond formations under high pressure conditions

“…Here a shift in the ee of 2-HPP towards higher excess of (S)-2-HPP was observed by decreasing pH from pH 7.5 to pH 6.5 (Berheide et al, 2010). However, it is reported in literature that an increase in pressure leads to a decrease in pH (Raghuraman et al, 2006). Thus our results revealed that the pressure effect on the enantioselectivity of BFD is a separate and contrary effect compared to the pH effect.…”

“…We showed that the tendency towards higher (R)-selectivities observed at high pressures could be enhanced when the reactions were run under alkaline conditions. In literature pressure induced pH shifts were analyzed especially for aqueous buffer systems and acid/base equilibria (Neumann et al, 1973;Morild, 1977;Marshall and Franck, 1981;Cavaille and Combes, 1996;Raghuraman et al, 2006). Raghuraman et al (2006) developed a calibration procedure for pH sensitive dyes using chemical equilibrium models.…”

Influence of the hydrostatic pressure and pH on the asymmetric 2‐hydroxyketone formation catalyzed by Pseudomonas putida benzoylformate decarboxylase and variants thereof

“…Due to the three deprotonation equilibria of phosphoric acid in water the pH of phosphate buffers significantly decreases by increasing pressure (Raghuraman et al, 2006). To exclude a pH-related effect on the enantioselectivity of BFD caused by a pressureinduced pH-shift, the formation of 2-HPP by BFD was investigated at different pH-values and benzaldehyde concentrations.…”

The Impact of Reservoir Fluid Compositional Variation and Valid Sample Acquisition on Flow Assurance Evaluation

“…Consequently, DFA pH measurements are generally much more accurate than normal laboratory pH measurements of oilfield waters. [26,27] Figure 18 shows the composition of gases from different compartments in a single field. [25] The CO2 content is seen to vary enormously in these samples.…”

“…[24] Of course, CO2 can affect corresponding water pH, and this too can be measured downhole. [26] Indeed, aqueous samples are normally cooled and flashed before laboratory pH measurements, which allows carbonate solids to precipitate and CO2 gas to evolve. Consequently, DFA pH measurements are generally much more accurate than normal laboratory pH measurements of oilfield waters.…”