Solução tampão que mimetiza as características ácido-base do tampão SMP em resposta à acidez de solos brasileiros

Abstract

In Brazil, the SMP buffer has been used since the 60's as the main method to estimate the potential acidity and/or the soil lime requirement. However, SMP contains pnitrophenol and potassium chromate, substances with high potential for environmental contamination and able to promote the development of methemoglobin and carcinomas in lab handlers as a result of continued exposure. This work aimed to develop a buffer solution free of hazardous substances which can mimic the acid-base characteristics of the SMP buffer, with the composition used in the states of RS and SC (SMP-RS/SC), in response to the acidity of Brazilian soils. To develop the new buffer, imidazole and 2-(Nmorpholino) ethanessulfonic acid monohydrate (MES) were used as substitutes for pnitrophenol and potassium chromate, respectively. The titration of the buffer SMP-RS/SC with HCl was carried out and these data were used as the starting point for the new buffer reproduce the potentiometric behavior of SMP-RS/SC buffer. To achieve this, multiparametric regression analysis was used, taking as adjustable parameters the pKa values and the concentrations of the components of the new buffer. The final composition for the new buffer, called Santa Maria buffer (TSM) was obtained with: 37.5 mM of triethanolamine; 17.3 mM of imidazole; 30.7 mM of MES; 20.7 mM of calcium acetate and 721 mM of calcium chloride; with initial pH adjusted to 7.5. For comparison between TSM and SMP-RS/SC, we utilized 42 soil samples collected in different physiographic regions of Brazil. Linear regression was performed with pH-SMP and pHTSM values, and also, between the lime requirement by SMP and TSM, reaching determination coefficient (R2) greater than 0.99 in both cases. The TSM had equivalent sensitivity to the SMP-RS/SC in differentiating the acidity degree of soils, and can be used replacing this, without the need for change in routine laboratory and interpretations already developed to estimate the potential acidity or need limestone of soil. At temperature from 22 to 27 °C, the TSM has a long shelf life and can be stored in plasticpolypropylene or glass containers protected from light, for a period of 120 to 150 days. The replacement of SMP-RS/SC buffer by TSM is a viable alternative for soil testing laboratories to avoid the generation of hazardous wastes and to reducing the health risks for the lab handlers.