Desenvolvimento de um sistema para determinação entalpimétrica por infravermelho.

Abstract

In the present study was set up a system for entalpimetrica analysis using infrared camera as a temperature sensor. The first tests were designed to evaluate the efficiency of this system in monitoring the temperature of a thermometric titration neutralization. Therefore, we used the system for determining the concentration of an aliquot of 20 mL of NaOH solution (1,012 molL-1) using, as a titrant, a standardized HCl solution, with a concentration of 1.007 molL-1. The next step was to determine the enthalpy of neutralization solutions using 50 and 20 mL of HCl solution (1.007 molL-1) and NaOH (1.012 molL-1). The results of the determination of the enthalpy of neutralization five aliquots of 50 mL of NaOH solution (1.012 molL-1) 50 mL of HCl solution (1.007 molL-1) were - 61.81 ± 4.45 kJ mol-1 (PT-100 sensor) - 59.98 ± 2.23 kJ mol-1 (mercury thermometer) and - 57.08 ± 0.26 kJ mol-1 (infrared camera). In relation to 20 mL of solution, the determination of the enthalpy of neutralization six aliquots of 20 mL NaOH (1.012 molL-1) with 20 mL of HCl solution (1.007 molL-1) using the camera resulted in a value - 58.34 ± 1.96 kJ mol-1. In enthalpy, determined the calibration curve and the coefficient of determination (R2) for the temperatures of the reactions values. The R2 values were 0.941 (PT-100), 0.9933 (thermometer) and 0.9999 (camera), thus making the camera most suitable equipment for the determination of temperature changes of the reactions to 50 mL of NaOH solution (0.25, 0.5, 0.75 and 1 molL-1) with a solution of 50 mL HCl (1 molL-1). The third stage was the miniaturization of entalpimétrica analysis system developed. Initially, using a multichannel pipettor, was performed adding 0.1 mL of HCl solution (1.007 molL-1) at an equal volume of NaOH solution (1.012 molL-1) and after the heat capacity were calculated system and enthalpy. The results determined an average value of enthalpy of neutralization - 59.61 ± 2.94 kJ mol-1, and an error - 3.77 kJ mol-1. We also determined the calibration curve and R2 coefficient related to temperature values resulting from the addition of 0.15 mL of HCl solution (1 molL-1) at an equal volume of NaOH (0.25, 0.5, 0.75 and 1 molL-1) using single-channel and multichannel pipettors. The R2 values were 0.90 (PT-100), and 0.9921 (camera) with a single channel pipettor and 0.959 (PT-100), and 0.9748 (camera) for multichannel pipettor. Given these values, the camera was more suitable for entalpimetricas analyzes, both with single-channel pipettor as multichannel. In the last step we developed an automated system for entalpimetrica analysis using infrared camera as a temperature sensor. In an ELISA plate line (8 wells) was added a volume of 0.1 mL of HCl solution (1.007 molL-1) at an equal volume of NaOH solution (1.012 molL-1) and the 7 calculated heat capacity system and enthalpy. When analyzed manually, the value of the enthalpy was - 60.96 kJ mol-1 and using the program, the enthalpy has become - 57.65 kJ mol- 1. As a result, it can be concluded that the infrared camera is the best device for determining the enthalpy of the chemical neutralization reactions. Also a program to analyze temperature values and calculate the enthalpy is necessary because the chance of error is smaller than and calculations are done manually.