Smartphone-based detection for the determination of silicate in the presence of phosphate

A simple smartphone-based detection method was developed for determining silicate in the presence of phosphate using the molybdenum blue reaction. The method is based on a reaction of analyte with ammonium molybdate to form molybdosilicic heteropolyacid. Amongst other ions, phosphorus(V) interferes this determination forming molybdophosphoric heteropolyacid in the same conditions. Oxalic acid was selected to decompose the interfering compound selectively. Then, molybdenum(VI) was reduced to molybdenum(V) using ascorbic acid forming silicomolybdenum blue. The reactions were performed in a cuvette (3 cm 3 ). The smartphone and the PhotoMetrixPro® app were applied for the detection and analytical signal evaluation. Channel B of the RGB colour space model was selected for analyte determination, and the signal for the formed compound was measured after 9 min. The possibility of using smartphones with different camera specifications was also verified. Using the method, silicate was determined in the concentration range of 0.24–20.00 mg dm −3 , with a precision of less than 4.9% (CV), and a limit of detection of 0.07 mg dm −3 . The method was validated by determining silicate in the presence of phosphate in certified reference materials representing groundwater, wastewater, and lake water.