Development of methods for the determination of several anions using flow injection analysis - fia

Τhe development of a new simple and rapid reagent-injection method for the spectrophotometric determination of citric acid in beverages and pharmaceutical formulations is reported. The method is based on the inhibitory effect of citric acid upon the Fe(III) catalytic oxidation of 2,4-diaminophenol (DAP) by H2O2. The analyte forms stable complexes with Fe(III) thus reducing its catalytic effect. The decrease in absorbance of the oxidation product is monitored at 500 nm. The various chemical and physical variables of the FI system were optimized and a study of interfering ions was also carried out. A linear calibration graph was obtained from 3 - 1000 mg l-1 citric acid. The precision was very good (sr = 1.9 %) and the detection limit was satisfactory (cL = 0.96 mg l-1). The sampling rate was 90 injections per hour. The method was found to be adequately selective considering the ions the samples contain, and has been successfully applied to the determination of citric acid in commercial beverages and pharmaceutical formulations. The average accuracy was evaluated by comparison of the FIA results with those obtained by the Boehringer-Mannheim UV-method test combination kit for citric acid and it was also very good (er=0.96%). A new reversed flow-injection method is reported for the simultaneous determination of fluoride (F-) and monofluorophsphate (MFP) ions in toothpastes, using a single calibration curve for both analytes. The developed method is the first FI procedure reporting either the determination of MFP or the simultaneous determination of both ions. It is based on the inhibitory effect of F- ion upon the Fe(III) catalytic oxidation of 2,4-diaminophenol (DAP) by H2O2. Free F- ion forms stable complexes with Fe(III), thus reducing its catalytic effect, while MFP ion does not inhibit the reaction under flow conditions. MFP ion is determined indirectly after quantitative acid hydrolysis to free F-. The decrease in the absorbance of the oxidation product is monitored at 500 nm. The various chemical and physical variables of the FI system were optimized and a study of interfering ions was also carried out. Linear calibration graph was obtained in the range 0.3 - 9 mg l-1 F- ion. The precision, sr, was very good (0,5 % at 4.5 mg l-1 F-, n = 12) and the 3σ detection limit, cL, was satisfactory (0.09 mg l-1). The sampling rate was 120 injections h-1. The method has been successfully applied to the simultaneous determination of F- and MFP in toothpastes. The calculated relative errors, er, compared to a reference spectrophotometric method were in the range 0.94 - 1.5 %, while the obtained recoveries of known amounts of the analytes varied between 97.8 and 102.0 %. A sensitive solvent extraction flow-injection (SE-FI) method for the simultaneous spectrophotometric determination of free cyanide (CN-) and thiocyanate (SCN-) ions in human saliva and pralidoxime solutions is reported. Free CN- and SCN- ions form coloured (λmax = 540 nm) ternary complexes with copper (C(II)) and 2,2'-dipyridyl-2-quinolylhydrazone (DPQH) that are extractable into chloroform. The determination of SCN- in the presence of CN- is accomplished after on-line masking of the latter with formaldehyde through a Binary Inlet Static Mixer (BISM). Total SCN- and CN- are determined in a second run, without the use of the masking agent. The proposed method allows the determination of the analytes in the range of 0.023 - 4 mg l-1 SCN- and 0.013 - 3 mg l-1 CN-, with the 3σ detection limits being 0.007 and 0.004 mg l-1 respectively. The precision of the method (sr < 1.0 % at 1.0 mg l-1 CN- or SCN-, n = 12 in both cases) and the sampling rate were quite satisfactory (60 injections h-1). The developed method was applied to the analysis of human saliva and pralidoxime solutions and gave excellent results since the percent recoveries were in the range of 98.0-102.2 % for both analytes and the mean relative error was er = 1.7 % compared to reference spectrophotometric methods.. A new, rapid FI method has been developed for the spectrophotometric determination of monofluorophosphate (MFP) ion in toothpastes. MFP ion is hydrolyzed on-line by alkaline phosphatase (APase) immobilized on a cellulose nitrate membrane, prior to injection in the FI system. The yielded orthophosphate ion are determined spectrophotometrically (λmax = 690 nm) using a molybdenum blue based FI approach. The chemical and FI variables that affected the enzymatic reaction were studied and optimized. A study of interferences was also carried out. The proposed method is very precise (sr = 0.7 % at 1.0 x 10-4 mol l-1 MFP, n = 12), fast (sampling rate of 72 h-1) and allows the determination of MFP ion in the range of 4.0 x 10-5 to 6.0 x 10-4 mol l-1 with a satisfactory 3σ detection limit of 4.0 x 10-6 mol l-1. The application of the proposed FI method to toothpaste samples yielded accurate results (er < 2.0 %) compared to a potentiometric reference procedure