Digital Image Analysis for the Colorimetric Determination of Aluminum, Total Iron, Nitrite and Soluble Phosphorus in Waters

A webcam was used to determine Al(III), total Fe, NO2-, and H2PO4- in waters by digital image colorimetry. Least squares regression was used to obtain a quadratic equation based on the red, green, blue, hue, saturation, and brightness color values, which allowed the evaluation of the wavelength of maximum absorption of the analytes. To determine the concentrations, the red, green, and blue intensities were converted into absorbances. For Al(III) and NO2- determination, the best results were attained when the absorbances relative to the green color were used. For total Fe and H2PO4- determination, the combined absorbances of green and blue and red and blue color led to higher sensitivity (slope) and coefficients of determination. The limits of detection for Al(III), total Fe, NO2-, and H2PO4- determinations were 2.59 x 10(-7), 3.58 x 10(-7), 1.09 x 10(-7), and 1.65 x 10(-7) mol L-1, respectively. The values for total Fe, NO2-, and H2PO4- were lower than comparative spectrophotometric techniques. The limits of detection were compatible with the Brazilian legislation for potable water. The developed procedure was successfully used for the determination of the wavelength of maximum absorption and the concentration of the analytes in water and wastewater. The reported procedure is simple, versatile, and low cost and demonstrates that a webcam is a suitable detector for determining Al(III), total Fe, NO2-, and H2PO4- in water with adequate accuracy and precision.