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 determi- ned 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 × 10-7–1 × 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 œrodowisku kwaœnym nadmanganian natychmiast 4 reaguje z sym-difenylokarbazydem tworz¹c kompleks, którego absorbancja by³a mierzona. Zakres liniowoœci metody wynosi³ 5 × 10-7–1 × 10-4 mol L-1, a granica wykrywalnoœci (3σ) – 6 × 10-8 mol L-1. Proponowan¹ metodê zastosowano do oznaczania Mn2+ w œciekach i ¿ywnoœci uzyskuj¹c wzglêdne odchylenie standardowe lepsze ni¿ 1,85%. Czêstotliwoœæ 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 human’s health [1,2]. Thus, determination of manganese in running and industrial waters is impor- tant. Tap water of too high content of manganese may cause staining of clothes and plumbing fixtures, and encrustation of mains. The average concentration of manga- nese in natural waters is quite low – 0.1–1.0 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 [3,7–10]. 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. In the previous work, a flow-injection system for speciation MnO– and Mn2+, 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 spectrophotometri- cal 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 (10-8–10-3 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 ERC–3522). Another pump, P2, (IKA–Schlauchpumpe, Janke & Kunkel GMBH, IKA–Labortechnik, PA–B1) 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 spectropho- tometric signal was detected in a 10 mm-in-length flow cell using a UV–VIS 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 pack- ing 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 condi- tions 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. The contact time depends on the sample flow rate and also on the reactor length. Both factors were optimised. The influence of the flow rates was investi- gated in the range: 0.2–4.0 mL min-1, which is presented in Figure 2a. The increasing peak heights were observed up to 1.2 mL min-1, and they decreased slowly afterwards. The optimum flow rate was found to be 1.2 mL min-1, which is rather slow, yet ensu- res low reagent consumption at the same time. Another factors influencing the efficiency of the reaction between MnO– and 4 S-DPC are the pH and the concentration of the carrier solution. At the pH of 2–3.2, S-DPC reacts with MnO– and produces a soluble yellow coloured complex strongly 4 Flow-injection system for spectrophotometrical determination of Mn(II) 409 absorbing at 308 nm [14]. Since after mixing the carrier and the reagent streams one obtains the pH of about 2.5, the reaction between MnO– and S-DPC proceeds unaffec- 4 ted. The concentration of the reagent solution was varied between 0.0001 and 0.005 mol L-1. In a series of measurements, 10-5 mol L-1 solution of Mn2+ was used while the concentration of S-DPC carried through the reactor 2 was changed. The results are shown in Figure 2b, where the reaction yield is represented by the peak height. Ac- cording to these results, the concentration of 0.004 mol L-1 was chosen as an optimun.

45 a) 40

35

30

25

20 Peak height, a.u. 15 -0.5 0.5 1.5 2.5 3.5 4.5

Reagent flow rate, mL min-1

45 b) 40 35 30 25

Peak height, a.u. 20 15 -0.0005 0.0005 0.0015 0.0025 0.0035 0.0045 0.0055 Sym-diphenylcarbazide, mol L-1 Figure 2. a) The influence of the reagent’s flow rate (Pump 2) on the analytical signal; [Mn2+] = 1 × 10-5 mol L-1, [S-DPC] = 0.004 mol L-1 at pH = 3.05, carrier flow rate (Pump 1) = 1.0 mL min-1, reactor length R2: 100cm; b) The influence of the Sym-diphenylcarbazide concen- tration on the analytical signal; [Mn2+] = 1 × 10-5 mol L-1, carrier flow rate = 1.0 mL min-1, reagent flow rate (pump 2) = 1.2 mL min-1, reactor length (R2): 100cm; c) The influence of the reactor length (R2) on the analytical signal; [Mn2+] = 1 × 10-5 mol L-1, carrier flow rate (Pump 1) = 1.0 mL min-1, [S-DPC] = 0.004 mol L-1 at pH = 3.05, reagent flow rate (pump 2) = 1.2 mL min-1 (Continuation on the next page) 410 M. Noroozifar and M. Khorasani-Motlagh

43 c) 38

33

28

Peak height, a.u. 23

0 18 0 50 100 150 200 250 Reactor length, cm

Figure 2. (Continuation)

Having established optimum Mn2+ concentration, pH and the flow rate, one in- vestigated the effect of the length of the reactor 2. The response of the system was studied at various reactor lengths employed: from 30 to 200 cm, with the I.D. fixed at 0.8mm. The results are presented in Figure 2c. The peak height significantly increases for lengths ranging between 30 and 100 cm and remains nearly constant afterwards. It seems that the reactor length of 100 cm provides a sufficient time for the injected amount of MnO– to completely react with S-DPC. Thus, the length of 100 cm was 4 chosen as optimum.

Evaluation of the method Under the optimum conditions, the calibration was performed. For this purpose the peak heights (H) were plotted vs Mn(II) concentration, C (mol L-1) and the obtai- Mn ned dependence (H = 8.95 + 3.02 × 106 C , r = 0.999, n = 8) was linear in the con- Mn centration range of 5 × 10-7–1 × 10-4 mol L-1. Detection limit was calculated as 3 times the standard deviation of the blank signal (3σ) and was found to be 6 × 10-8 mol L-1. The precision of the method was determined in 7 repetitive analyses of standard solu- tions under optimum conditions. The RSD was < 1.85%.

Interference study

The interferences of common ions in the determination of 1 × 10-5 mol L-1 Mn(II) were investigated. At the 5% level, tolerable interferent-to-analyte concentration ra- tios were over 250 for Na+, K+, Ca2+, Mg2+, HPO2–, SO2–, CO2–, NO– and acetate, 4 4 3 3 95 for Cl–, Br–, Co2+ and Zn2+, 45 for Fe3+, 25 for Cd2+, Cu2+, Fe2+ and Al3+, and 1 for Flow-injection system for spectrophotometrical determination of Mn(II) 411

Hg2+ and Cr O2–. In the analyses of real samples, the only element suspected to inter- 2 7 fere was Fe, the concentration of which in mine watres exceeds the tolerance limit. However, the tolerance limit for Fe can be increased from 45 to 200 by adding 5 mL of 0.1 mol L-1 EDTA to the stock solution of the carrier.

Analytical application In order to evaluate analytical applicability of the method, real samples (Zahedan City and Hyrmand river waters) were spiked with the standard solution of Mn2+ (Table 1). The precision and accuracy of the method was proved to be satisfactory. Moreover, effluent samples were spiked with standard Mn2+ solutions to estimate the recovery. It ranged between 98.0 and 104.0, which was reasonable.

Table 1. Determination of Mn2+ in water samples spiked with the standard Mn2+ solution. Sample origin: water from the municipal supply system in Zahedan city, Hyrmand river water*

Mn2+ added Mn2+ founded RSD, % Recovery Samples mol L-1 mol L-1 n = 7 % City water 1 5 × 10-6 4.9 × 10-6 1.83 98 City water 2 1 × 10-5 1.04 × 10-5 1.67 104 Hyrmand river 1 5 × 10-6 5.1 × 10-6 1.75 102 Hyrmand river 2 1 × 10-5 9.9 × 10-6 1.55 99

* In the South of Iran.

Real samples (effluents, mine water from Kahrezack Pyrolozit Mine in the South Tehran, Iran) were analysed by the proposed FIA system. The accuracy was evaluated by comparing the results for real samples obtained with our method with those ob- tained with the standard procedure (atomic absorption spectrometry with standard addition method). Our approach occurred to be more advantageous, as seen from Table 2.

Table 2. Analyses of effluent mine waters from Kahrezack Pyrolozit Mine in the south Tehran, Iran with the proposed and the standard procedures

Concentration of Mn2+ mol L-1 Concentration of Mn2+ mol L-1 RSD, % (standard procedure)* (FIA method) (FIA method, n = 6) 2.17 × 10-4 2.09 × 10-4 1.85

4.04 × 10-4 3.98 × 10-4 1.77

1.08 × 10-3 1.1 × 10-3 1.47

2.09 × 10-3 2.00 × 10-3 1.33 * Atomic absorption spectrometry. 412 M. Noroozifar and M. Khorasani-Motlagh

Furthermore, its applicability to the analysis of manganese in food samples was investigated. Sample pretreatment procedure was performed using the reference method described by Kalra [16]. The samples were simultaneously analysed by the graphite furnace atomic absorption spectrometry. Determination results for the samples of tea, haricot beans, lentil and apple are given in Table 3.

Table 3. Comparison of the results obtained for natural food samples using the proposed FIA method and AAS standard method

Mn concentration, µg g-1 Sample Relative error, % GF-AAS method proposed method n = 4 n = 7 Tea 370.3 373.7 0.92

Haricot bean 61.8 61.0 –1.29

Lentil 52.1 52.3 0.38

Apple 39.3 38.5 –2.03

Acknowledgement

The financial support of the Sistan & Baluchestan University is acknowledged.

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Received March 2003 Accepted October 2003