Colorimetric pH measurement for seawater samples using a three light-emitting diodes detector and a calibration method for temperature dependence

Spectrophotometric determination of pH and carbonate ion concentrations in seawater: Choices, constraints and consequences

Accurate and precise marine CO₂ system measurements are important for marine carbon cycle research and investigations of ocean acidification. Seawater pH is important because it can be used to characterize a wide range of chemical and biogeochemical processes. Saturation states of calcium carbonate minerals, which are directly proportional to carbonate ion concentration ([CO₃^2-]), influence biogenic calcification and rates of carbonate dissolution. Spectrophotometric pH and carbonate ion measurements can both benefit greatly from the high sensitivity, stability, consistency and processing speed made possible through automation. Spectrophotometric methods are well-suited for shipboard, underway and in situ deployments under harsh conditions. Spectrophotometric pH measurements typically have a reproducibility of 0.0004-0.001 for shipboard and laboratory measurements and 0.0014-0.004 for in situ measurements. Shipboard spectrophotometric measurements of [CO₃^2-] are becoming common on research expeditions. This review highlights the development of methods and instrumentation for spectrophotometric pH and [CO₃^2-] measurements, and discusses the pros and cons of current technology. A comprehensive summary of the analytical merits of different flow analysis instruments is given. Aspects of measurement protocols that bear on the quality of pH and [CO₃^2-] measurements, such as indicator purification, sample pretreatment, etc., are also described. Based on three decades of experience with seawater analysis, this review includes method recommendations and perspectives directly applicable or potentially applicable to pH and [CO₃^2-] analysis of seawater.

Colorimetric pH Measurement of Pressurized Groundwater Containing CO2

A gas-tight pH measurement is needed to monitor water chemistry at a CO2 geological storage site. In the CO2 reservoirs, the temperature and pressure are generally more than the critical point of CO2 (31.2°C and 7.38 MPa). In this study, a colorimetric pH measurement method was examined up to 20 MPa for future application to various CO2 reservoirs. A mixture of two color indicators, bromocresol green (BCG) and metacresol purple (mCP), was considered to be a suitable measurement method between pH 3 and 9. The uncertainty up to 20 MPa was less than 0.12 pH units without any correction of pressure effects. We demonstrated a pH measurement of formation water at the Nagaoka CO2 post-injection site. The pH measurement was successfully accomplished under a high-pressure condition (ca. 11 MPa) and without degassing of CO2.

Evaluation of the Effect on Temperature Conversion of pHT at 25°C in the Temperature Range 0 - 40°C Due to Incorrect Estimations of Salinity, Alkalinity, and Phosphate and Silicate Concentrations

Under conditions where carbonate and oceanic parameters such as alkalinity (A(T)), salinity (Sp), and phosphate (P(T)) and silicate (SiT) concentrations cannot be obtained, the use of fixed values to calculate total pH (pHT) at 25°C was demonstrated. For temperatures of 20 - 30°C, these values caused a maximum error of ±0.001 in pHT. For temperatures of 0 - 40°C, using fixed values with correct SP values caused a maximum error of approximately +0.0021.