Agradecimentos: This work was supported by SIP-IPN (Research Grants 20170545, 20181764) and CONACyT (2015-02-1042; 2016-01-2482). The support of COFAA-IPN by the SIBE and BEIFI programs is also acknowledged, as well as the University of Quindio for the financial support through project 869 and the program of "assistance to research groups". E. Cedeno gratefully acknowledges the support of the SECITI for granting a postdoctoral fellowship under the agreement SECITI/077/2017 Ver menos

In this work, we describe the application of a micro-spatial thermal lens spectroscopy setup (thermal lens microscope, TLM) with coaxial counter-propagating pump, and probe laser beams, and an integrated passive optical Fabry-Perot, to quantify the Cr-VI concentration in water during a photocatalytic reaction in-situ. A series of test samples was analyzed using the 1.5 diphenil carbazide colorimetric method. A calibration curve was obtained by plotting of the TLM signal as a function of the concentration of Cr(VI) in a range between 0 and 10 mu g/L (1 mu g/L = 1 ppb, part per billion), with a detection limit of 53 ng/L (1 ng/L = 1 ppt, part per trillion). A solution of 10 mu g/L Cr(VI) in distillated water was placed into a cell in contact with an iron-incorporated titanium dioxide film, which was previously grown onto a 1 mm thick glass microscope slide by the sol-gel dip-coating technique. The TLM signal was registered as a function of the photocatalysis time measured from the beginning of the process, radiating the film with UV-violet light. The Cr(VI) concentration was determined with the calibration curve and after the first 50 minutes a reduction of 95 % of Cr(VI) was observed, being the chemical reaction kinetic described by a potential time decreasing function Ver menos