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In this work, a flow injection analysis (FIA) method was introduced for the determination of trace amounts of thiourea in tap water. This method is based upon the inhibition effect of thiourea on the reaction between meta-cresol purple (MCP) and potassium bromate catalyzed by bromide ions in a sulfuric acid medium. In the presence of thiourea, an induction period appears in the reaction system, and as a result, the absorbance of MCP increases at 525 nm in the FIA manifold. The chemical and FIA variables are studied and optimized using the univariate and Simplex optimization methods. Under the optimum conditions, thiourea can be determined in the range of 0.100–13.0 μg mL −1 . The limit of detection (3 σ ) for thiourea was found to be 0.0310 μg mL −1 . The relative standard deviations (RSDs) for six replicate determinations of 0.500, 5.00, and 12.0 μg mL −1 of thiourea were 4.0%, 1.8%, and 1.2%, respectively. The proposed method was also applied for the determination of thiourea in orange juice and orange peel samples with recoveries in the range of 98.0–101%. The analytical speed of the method was calculated to be about 120 sample per hour. [ABSTRACT FROM AUTHOR]

• Determination of nitroaniline isomers in binary mixtures by four spectrophotometric methods. • Similar precision and accuracy of CC and RD methods compared to PLS and PCR methods. • Selection of best pair of wavelengths and divisor in proposed methods was studied. • No need to computer programs or sophisticated equipment in suggested methods. In the present work, four simple and economic spectrophotometric methods, namely constant center (CC), ratio difference (RD), H-point standard addition method (HPSAM), and ratio H-point standard addition method (RHPSAM) were employed for the simultaneous determination of meta and para -nitroaniline with severely overlapping spectra without unitizing the preliminary physical separation techniques. In each method, the parameters affecting the resolution of the spectra were optimized to determine the cited isomers in a binary mixture with a high accuracy. Under the optimized conditions, the results obtained showed that from cited methods used, the CC and RD methods have the least errors in determination of the concentration of highly spectral overlapping species. The mean percentage recovery values for the binary mixture of meta- and para -nitroaniline were found to be 95.98 and 98.78 using the RD method, and 101.85 and 100.23 using the CC method, respectively. For comparison of the powerful multivariate methods including the principal component regression (PCR) and partial least squares (PLS) were also applied. The mean percentage recovery values for the binary mixtures of the cited isomers were found to be 101.62 and 101.47 using the PCR method, and 101.00 and 101.36 using the PLS method, respectively. A statistical comparison of the methods demonstrated that there was no significant difference between the RD, CC, PCR, and PLS methods and the reported HPLC method regarding both accuracy and precision. The proposed methods have an admissible precision and accuracy for determination of the two isomers in their binary mixtures in tap water as a real sample. [ABSTRACT FROM AUTHOR]