An improved molybdenum blue spectrophotometry using a soluble membrane filter and CaCO3-column was proposed for determining arsenic in drinking water supplied from ground water in the presence of phosphate. A 100 mL sample solution containing 0.5 – 10 μg arsenic was passed through a CaCO3-column to remove phosphate, arsenate (As(V)). Arsenite (As(III)) which was not retained on the column was oxidized to As(V). As(V) was converted into a heteropolymolybdenum blue anion. The blue anion was collected on a membrane filter as an ion-associate with n-dodecyltrimethylammonium ion by filtration. The filter was dissolved in 2 mL of 2-methoxyethanol. The absorbance of the solution was measured at 810 nm against a reagent blank. Total inorganic arsenic was determined by reducing As(V) to As(III) before the column treatment. The RSDs for 10 μg L−1 of As(III) and As(V) were 2.9%. Phosphate 0.2 mg L−1 (as P) and iron 0.1 mg L−1 did not interfere with the determination of 10 μg L−1 arsenic. The proposed method was successfully applied to ground waters.

Sensitive visual colorimetry was proposed for the on-site determination of arsenic in drinking water supplied from groundwater. The method was based on advanced molybdenum blue (MB) colorimetry combined with an enrichment technique using a membrane filter. A portable device equipped with two attachments of a CaCO₃-cartridge was used to eliminate interference from phosphate and a tiny membrane holder to retain MB was proposed for on-site analysis. Ten mL of a sample solution containing 0.1–0.4 μg arsenic was passed through the CaCO₃-cartridge. The arsenic in the sample solution was converted into MB and collected on a membrane filter as an ion-associate with a cationic surfactant. The arsenic was determined visually from the color intensity of the filter using a color chart made from the standard solution. The sensitivity is sufficient to detect the concentration of 10 μg L⁻¹ adopted by the WHO standard. The speciation analysis of As(III) and As(V) was also successfully performed. The proposed method was applied to several groundwater samples, and the analytical results were well comparable with those obtained by HG-AAS.

We present a novel fiber optic sensor for real-time sensing of silica scale formation in geothermal water. The sensor is fabricated by removing the cladding of a multimode fiber to expose the core to detect the scale-formation-induced refractive index change. A simple experimental setup was constructed to measure the transmittance response using white light as a source and a spectroscopy detector. A field test was performed on geothermal water containing 980 mg/L dissolved silica at 93 °C in Sumikawa Geothermal Power Plant, Japan. The transmittance response of the fiber sensor decreased due to the formation of silica scale on the fiber core from geothermal water. An application of this sensor in the evaluation of scale inhibitors was demonstrated. In geothermal water containing a pH modifier, the change of transmittance response decreased with pH decrease. The effectiveness of a polyelectrolyte inhibitor in prevention of silica scale formation was easily detectable using the fiber sensor in geothermal water.