Desenvolvimento e validação de métodos para quantificação do teor de óxido de cálcio em cimento portland utilizando espectrofotometria de absorção no ultravioleta visível.

Abstract

The objective of this work was to develop and validate two analytical methods, the Conventional and the β-Correction, for the determination of the content of Calcium Oxide in Portland cement, through the Visible Ultraviolet Absorption Spectrophotometry (UV-VIS), using Calcon as a metallochrome indicator.The development of the methods consisted in defining, first, the ideal conditions for the preparation of the Calcium-Calcon complex solutions, which were: pH, Calcon:Calcium molar ratio, complex stability and orderof addition of the reagents. After that, both methods were submitted to several tests whose objective was to validate them according to the National Institute of Metrology, Quality and Technology (INMETRO) and the Association of Accredited Laboratories ofPortugal (RELACRE).It has been found that the maximum and minimum absorption wavelength of the Calcium-Calcon complex is equal to, respectively, 462 nm and 642 nm. Also, it was found that the solutions submitted to the UV-VIS tests should have a pH above12, Calcon: Calcium molar ratio equal to 1:1 and that, in the preparation of the solutions, the Calcium aliquot should not be added followed by solution, or conversely.In reference of thevalidation of the methods, theβ-Correction Method was not selective for the determination of Calcium in the presence of other ions contained in samples of Portland cement, which compromised its application to the purpose proposed in this work. In contrast, the Conventional Method can be considered selective, linear, precise, accurate and robust for determination of CaO in Portland cement. However, it has been found that the solution to be tested should have a theoretical concentration of Ca2+of 1.63 to 4.80 µg / ml. In addition, it hasbeen found that the method is very sensitive to pH variation, which has taken special care to keep the working value of this parameter constant.