A simple and low cost method was developed and validated for the determination of trace mercury (II) ions in dental-unit wastewater and fertilizer samples. The method was based upon the reaction of mercury (II) ions with the novel reagent 6-hydroxy-3-(2-oxoindolin-3-ylideneamino)-2-thioxo-2H-1,3-thiazin-4(3H)-one, the formed complex shows an absorption maximum at 505 nm (lambda(max)) in Britton-Robinson (B-R) buffer (pH 4-6). The corrected absorbance of the formed complex at lambda(max) was obtained employing beta-correction spectrophotometric method. Beer's-Lambert law and Ringbom's plots of the colored Hg-reagent complex were obeyed in the concentration range of 0.2-2.0 and 0.32-0.96 mu g mL(-1) mercury (II) ions, respectively with a relative standard deviation in the range of 2.1 +/- 1.3%. The limits of detection (LOD) and quantification (LOQ) of the procedure were 0.026 and 0.086 mu g mL(-1) Hg2+, respectively. The proposed method was applied for the analysis of mercury (II) in dental-unit wastewater and fertilizer samples. The validation of the method was tested by comparison with the data obtained by the inductively coupled plasma-mass spectrometry (ICP-MS). The statistical treatment of data in terms of Student's t-tests and variance ratio f-tests has revealed no significance differences. (C) 2010 Elsevier B.V. All rights reserved.