A homemade colorimeter for quantitative and qualitative analysis of organophosphates

Organophosphates are widely used as pesticides, and many of these may bring risks to the human health. Therefore, the knowledge of whether or not these compounds are being used in abusive amounts is important. There are many analytical methods that can provide determination of these compounds, such as NMR and UV-Vis spectroscopy, but these methods are not practical for practical, simples and fast field analysis. In this context, we built a homemade colorimeter for low cost analysis of organophosphates. The colorimeter (C) was built using two LEDs, one emitting light through a sample, and another LED detecting the light that passed. The second LED converts the light into potential energy, measured with a multimeter¹. With this, it is possible to calculate the absorbance of the sample, which is the product of degradation of the studied pesticides(paraoxon and parathion). A calibration curve of the product has to be previously made, so the concentration can be determined. To test it’s quality, the alkalin hydrolysis of parathion was carried out on it to study the kinetics, and compare it with a comercial photometer. The rate constant calculated by the colorimeter was 0,013 cm-1, with the spectrophotometer was 0,012 cm-1, meaning an error of 8,33%. The reactions were repeated several times, maintaining the results, now we aim to use catalysts to improve the process. The colorimeter had decent results to evaluate the concentration, or to detect pesticides with colored hydrolysis products, being much more practical and cheaper (10,00US$) than the usual equipment.