Using Sodium Bismuthate As an Oxidant and Sym-Diphenylcarbazide As a Complexing Agent

Chem. Anal. (Warsaw), 49, 405 (2004) Flow-Injection System for the Spectrophotometric Determination of Mn (II) Using Sodium Bismuthate as an Oxidant and Sym-diphenylcarbazide as a Complexing Agent by M. Noroozifar* and M. Khorasani-Motlagh Department of Chemistry, Sistan and Baluchestan University, Zahedan P.O. Box 98165-181, Iran Key words: flow-injection, manganese determination, sodium bismuthate reactor, sym-diphenylcarbazide Flow-injection spectrophotometric determination of manganese in aqueous solution, using a solid sodium bismuthate reactor has been presented. Manganese (Mn2+) can be determined spectrophotometrically at 308 nm after its on-line oxidation to MnO at the room 4 temperature. In acidic media permanganate immediately reacts with sym-diphenylcarbazide to form a complex, the absorbance of which is measured. The linear range of the method was 5 × 10-71 × 10-4 mol L-1 with a detection limit (3σ) of 6 × 10-8 mol L-1. The proposed procedure was applied to the determination of Mn2+ in effluents and food, with a relative standard deviation better than 1.85%. The sampling frequency as high as 110 h-1 could be achieved. Zaproponowano spektrofotometryczn¹ metodê oznaczania manganu w wodnych roztworach w uk³adzie przep³ywowo-wstrzykowym z zastosowaniem reaktora z bizmutanem sodu. Mangan (Mn2+) oznaczano spektrofotometrycznie po uprzednim utlenieniu on-line, w temperaturze pokojowej, do MnO. W rodowisku kwanym nadmanganian natychmiast 4 reaguje z sym-difenylokarbazydem tworz¹c kompleks, którego absorbancja by³a mierzona. Zakres liniowoci metody wynosi³ 5 × 10-71 × 10-4 mol L-1, a granica wykrywalnoci (3σ) 6 × 10-8 mol L-1. Proponowan¹ metodê zastosowano do oznaczania Mn2+ w ciekach i ¿ywnoci uzyskuj¹c wzglêdne odchylenie standardowe lepsze ni¿ 1,85%. Czêstotliwoæ próbkowania mog³a osi¹gn¹æ 110 h-1. * Corresponding author. E-mail: [email protected] 406 M. Noroozifar and M. Khorasani-Motlagh Trace Mn2+ is necessary for plants and animals, however if its concentration exceeds certain limit manganese becomes harmful and influences humans health [1,2]. Thus, determination of manganese in running and industrial waters is important. Tap water of too high content of manganese may cause staining of clothes and plumbing fixtures, and encrustation of mains. The average concentration of manganese in natural waters is quite low 0.11.0 mg L-1 [3], however in certain reservoirs occasionally becomes as high as 10 mg L-1. According to the World Health Organiza- tion (WHO), the approved level of manganese is 0.5 mg L-1 for humans, and 0.1 mg L-1 to avoid staining problems [4]. Solid-phase reactors coupled with FIA mode are more advantageous than homo- geneous systems [5,6] and their applications to the determination of inorganic and organic samples has been reported [3,710]. However, currently used solid-phase reactors employing oxidants, such as PbO [3] and NaBiO [11], which are mainly 2 3 used in the spectrophotometrical analysis at present, are not in spectrophotometry with a reagent analysis. Only few reagents, such as dimethyldistearylammonium [12], neoterazolium [13] (after extraction with organic solvents) and sym-diphenylcarbazide [14] (aqueous solution) have been used for the spectrophotometrical determination of manganese in the off-line mode. Mn2+ was oxidised to MnO by sodium bismuthate, and the obtai- 4 ned permanganate was determined spectrophotometrically at 526 nm [11]. The main disadvantage of this method was its poor detection limit, 0.38 mg L-1. In order to improve the detection limit, we have proposed the use of sym-diphenylcarbazide to complex Mn(VII). The absorbance of the formed complex was measured at 308 nm. Such a procedure allowed one to improve the detection limit by a factor of 40 [14]. Furthermore, the use of flow injection (FI) mode makes this method even more attrac- tive. Flow injection requires only very small sample to be used, which eliminates many of the stringent clean particles often necessary for standard manganese determi- nations. FI represents the most advanced form of solution manipulation available to analytical chemists. It allows one to mix and transport the reagents and the products directly to the point of measurement. 2+ In the previous work, a flow-injection system for speciation MnO and Mn , 4 based on the off-line Mn2+ oxidation by sodium bismuthate and formation of the MnO 4 -diphenylcarbazide complex was described [15]. In this paper the on-line oxidation mode was introduced instead of the off-line one, as well as MnO was complexed 4 with sym-diphenylcarbazide. The flow-injection sample loading and spectrophotometrical detection were applied. Experimental conditions were appropriately optimised. Concentration of manganese is some real samples was determined and the results were compared with those obtained by GFAAS. Flow-injection system for spectrophotometrical determination of Mn(II) 407 EXPERIMENTAL Reagents and Solutions All solutions were prepared using analytical grade reagents and doubly distilled water, unless specified otherwise. 100 mg L-1 stock standard solution of Mn2+ was prepared by dissolving 0.3107 g of MnSO ·H O 4 2 (May & Baker Ltd, Dagenbam, England) in 1 L of water. Working standard solutions (10-810-3 mol L-1) were prepared by appropriate dilutions of the stock solution with water. 85% H PO (Merck) was diluted 3 4 with water to the concentration of 0.125 mol L-1 to obtain the carrier solution for the oxidant in reactor 1. 4 × 10-3 mol L-1 sym-diphenycarbazide (S-DPC; May & Baker Ltd, Dagenbam, England) solution was prepared daily by dissolving 0.6046 g of the compound in 150 mL of ethanol (Merck) and diluting the contents up to 250 mL with phosphate buffer. Phosphate buffer solution of pH = 3.05 was prepared by dissolving 9.9062 g of NaH PO (Merck) and 0.68 mL of H PO (85%, Merck) in 500 mL of water. 2 4 3 4 Apparatus A schematic diagram of the flow-injection system used in this work (Fig. 1) consisted of two pumps. A variable flow rate liquid chromatography pump, P1, (Bruker, Germany, Model LC22) served for deliver- ing the sample with a carrier stream of H PO at the fixed flow-rate of 1.0 mL min-1. The sample 3 4 was previously degassed using a degasser (Erma, TOKYO, Model ERC3522). Another pump, P2, (IKASchlauchpumpe, Janke & Kunkel GMBH, IKALabortechnik, PAB1) was used to deliver S-DPC compound at the fixed flow-rate of 1.2 mL min-1. Particular elements of the system were connected with 0.8 mm I.D. polyethylene tubes. The six-channel injection valve (Rheodyne, Model 7125) allowed the sample to be directly loaded into the 150 µL loop and subsequently injected into the carrier stream. The spectrophotometric signal was detected in a 10 mm-in-length flow cell using a UVVIS spectrophotometric detector (Knauer). The whole procedure was computer-controlled via Chromstar software. Only the injection valve was operated manually. IV R1 CS1 P1 R2 D CS2 P2 308 W W Figure 1. Schematic diagram of the flow-injection system utilised for determination of Mn2+, CS1: oxidant stream, H PO 0.125 mol L-1, CS2 reagent stream, 0.004 mol L-1 sym-diphenylcarbazide of 3 4 pH = 3.05, P1 HPLC pump, P2 peristaltic pump, IV injection valve, R1 solid-phase sodium bismuthate reactor, R2 reaction reactor, D detector, W waste 408 M. Noroozifar and M. Khorasani-Motlagh Preparation of the solid-phase reactor Pretreatment of the solid-phase reactor was performed according to the procedure previously described [11]. Each reactor had to be conditioned for at least 30 min before use. Conditioning involved pumping through doubly distilled water for 15 min followed by pumping the carrier (H PO ) for another 15 min 3 4 at flow rates of 1 mL min-1. The lifetime of each reactor was established by a day-by-day comparison of peak areas for the same standards. Reactor was considered to lose its oxidation capacity and was immediately replaced with a new one if peak areas started to decrease, or if the initially yellow colour of the packing changed. The latter effect appeared after pumping several samples. At the front end of the reactor the packing started to become brown and then black, which was due to the total reduction of sodium bismuthate, its hydrolysis to insoluble bismuthyl and precipitation on the surface of silica gel beads. When two-third of the reactor packing became black, it had to be replaced, which occurred usually each 250 runs within a 48 h period of time. The preparation and the exchange of the new packing was very easy. RESULTS AND DISCUSSION Preliminary tests showed that the DPC-permanganate complex is stable enough to be used for the spectrophotometrical determination of MnO. Parameters affecting 4 the performance of the FI system were investigated in order to determine the conditions providing a linear relationship between the signal and the concentration over a wide concentration range, with optimum sensitivity, precision and sampling rate. Optimisation of the variables In the case of reactor 1 the performance of the FI system depended mainly on the efficiency of the oxidation reaction occurring at the solid-liquid interface. Apart from the reactor packing, its length and diameter were important, which was discussed before [11]. The sample flow rate was fixed at 1.0 mL min-1. Similarly for reactor 2 the performance of the FI system depends on the efficiency of the reaction between MnO and S-DPC, and the stability of the reaction product. 4 The contact time between the sample zone containing MnO and the carrier solution 4 (S-DPC) inside the reactor 2 was crucial for the reaction to proceed sufficiently close to the completeness.

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