A comparison of different image analysis techniques for mapping spatiotemporal pH and carbon dissolution in density-driven convection of CO2 in water

Density-driven convection enhances the carbon dissolution rate, which is significant for the geological carbon storage. This process will also influence the spatiotemporal pH and carbon concentrations of the underground fluid. To illuminate the convection mechanism, it is critical to understand the evolution of those properties within the porous media. However, determining the spatiotemporal pH and concentration within porous media is always challenging. This study employed a combination of three pH indicators that can track a wide range in pH from 4 to 9.5 in a convection experiment. Furthermore, we compared three image-processing techniques: Hue, gray-difference, and angular representation of RGB color space $\mathbf{(ϕ,θ)}$ for quantifying color changes from the universal $\text{pH}$ indicator arising from the carbon convection. The characterized colors were mapped into pH by calibrating against benchmark solutions. The comparative results demonstrate that the color quantified by the Hue technique is most robust, showing invariance to fluid thickness, camera settings, and LED luminance. In the convection experiments, it produces a continuous spatial distribution of pH and concentration level in the system. In contrast, the $\mathbf{(ϕ,θ)}$ and gray-difference techniques were more sensitive to environmental variations. They also have significant limitations for $\text{pH}$ interpolation in the critical range due to their non-monotonic calibration paths. Although all methods ultimately produced similar estimates of total dissolved carbon, the Hue technique offers greater stability and universality for high-resolution, dynamic measurements of pH and carbon concentration in the convection experiments.

💡 Research Summary

This paper addresses a critical gap in the experimental study of geological carbon storage (GCS): the ability to monitor spatiotemporal pH and dissolved‑carbon concentrations during density‑driven convection (DDC) of CO₂ in water. Conventional pH electrodes provide only point measurements, while fluorescence methods suffer from weak signals and limited applicability to low‑concentration CO₂ dissolution. The authors therefore develop a non‑intrusive, image‑based approach that combines a broad‑range universal pH indicator with quantitative color analysis.

Indicator formulation
A mixture of three safe dyes—bromocresol green, bromothymol blue, and methyl red—was diluted 1:50 to produce a universal indicator spanning pH 4 to 9.5. The mixture yields a smooth color transition from red (acidic) through green to blue (basic), allowing detection of pH changes as small as 0.1 units. Thirteen benchmark solutions covering the full range were prepared in transparent micro‑plates for calibration.

Image‑processing techniques
Three color‑quantification strategies were evaluated:

1. Gray‑difference – a single scalar derived from weighted RGB values (0.299 R + 0.587 G + 0.114 B).
2. Hue (H) from HSV space – the angular position on the color wheel, normalized to