Padrão alternativo de dicromato de potássio para determinação de cor da água

Abstract

In this work was developed an alternative method for quantifying water color, using potassium dichromate as standard reagent and UV-Visible spectrophotometer to measure color, with a view to practicality and low cost, since the standard reagent, platinum-cobalt, indicated by national and international standards, it has a high cost and this solution has stability of only six months. And to make sure that it was a stable solution was made successive measurements of potassium dichromate solutions in 1.0 to 10.0 mg L-1 in 0.005 M sulfuric acid, and using the spectrophotometer UV / VIS a wavelength of 450.0 nm. And from the results of the measurements was determined a color conversion factor of potassium dichromate for the platinum-cobalt. This factor was established by doing a correlation of colors obtained in potassium dichromate to the colors displayed in the solutions of the platinum-cobalt standard at concentrations of 5.0 to 50.0 mg L-1. Because the waters color of the black river produce high values the study was also done with the concentrations to 10.0 to 50.0 mg L-1 of the potassium dichromate and from 50.0 to 250.0 mg L-1 of the platinum-cobalt, and the conversion factor of the color to highest concentrations was similar to that of the lower concentrations, a value equal to 5.2. The color values obtained by applying the conversion factor was equal to the values given by the standard method platinum-cobalt, with similar systematic error the potassium dichromate method, which showed a sensitivity (0.0067) higher that the standard method platinum-cobalt (0.0013). The repeatability (n = 10) and reproducibility (n = 6) of the method were verified by the variation coefficient of the measured samples and analyzed on the same day (<10.0%) and on different days (<10.0%) showing compliance with the values indicated by IUPAC. The validation steps confirmed the interchangeability between the Pt / Co and K2Cr2O7 methods, showing that the alternative method may be used with a tolerance of 10% at the end value of the Color, and in practice a tolerance of 20% is acceptable as indicated by the Standard Methods for the Examination of Water and Wastewater (APHA, AWWA and WEF, 1999; 2012). For this reason, the method was using applied with the raw water samples of black water rivers, with the that was proved the efficiency of the alternative method to determine the water color using cuvettes of 10, 20 and 50 mm of the fused quartz.