Fontes de imprecisão na determinação de S disponível por turbidimetria

Abstract

The analysis of available S levels in the soil using turbidimetric approach has low accuracy, and the causes are still unclear. Thus, the objective of this work was to identify sources of inaccuracy in the official method of Minas Gerais for determining available S (PROFERT). In the method, monocalcium phosphate solution - Ca(H2PO4)2, 500 mg/L P, 2 mol/L HOAc is used as extractor. As components of the method, the effects of cuvette, filtration, agitation, storage time of the NH4OH solution, and spectrophotometer error on analytical precision were evaluated. Among the presented results, the effect provided by the preparation of samples with NH4OH stored for 7 d led to an increase in absorbance over time; an equipment error effect was detected, from the Instrutherm Digital Mod. UV-2000 A. spectrophotometer; furthermore, it was demonstrated that the inaccuracy of liquid reagent dosing negatively impacts the method's precision. With this, it was possible to raise hypotheses about probable causes of the observed inaccuracies, which may promote more conclusive new research such as the hypothesis that the storage of NH4OH can promote ammonia volatilization, changing the pH of the solution and delaying the formation of barium sulfate, thus altering the absorbance reading.