User-friendly diameter-based measurement paper sensor for chloride detection in water

Chloride (Cl⁻) is an anion widely distributed in nature. It is also an essential parameter to consider when assessing the water quality for ensuring drinking water safety, preventing infrastructure damage, mitigating environmental impact, identifying groundwater contamination. This work presents the first development of a diameter-based measurement paper sensor for chloride analysis using the reaction based on a Mohr's precipitation titration. The paper sensor that has a circular shape with 3-cm diameter was pre-coated with AgNO₃ and CrO₄²⁻ forming brown precipitates of the Ag₂CrO₄. The sensor was sealed using lamination films with 3-mm diameter hole-punched inlets on the top of the lamination film for sample delivery. To detect chloride, the sensor was simply immersed into the sample. The chloride solution flows into the central sample inlet and spreads radially to undergo the displacement reaction with Ag₂CrO₄ precipitate, forming AgCl white precipitate whose diameter proportional to the chloride that can be observed within 3 min. Concentration of AgNO₃ used was found to impact the analytical figures of merit. The lower AgNO₃ concentration yields lower limit of detection, narrower linear range but higher sensitivity. The sensor was applied for chloride analysis in tap water, drinking water and industrial water and the chloride concentration obtained from the developed sensors are not significant differences from those obtained from the standard titration method at 95% confidence interval (two tailed P = 0.08) indicating that the developed sensor provides accurate analysis of chloride in water samples from various sources. The developed sensor was used by the untrained staffs for on-site of analysis chloride in tap water collected at 26 locations in SaenSuk Municipality area, Chonburi, Thailand. The results showed that the chloride level in all samples is in range of 52.2–84.7 mg L⁻¹ which is far below the acceptable range set by the Provincial Waterwork Authority of Thailand (< 250 mg L⁻¹) indicating that the tap water used in this area is safe for consumers.