The Use of Surfactants for the Spectrophotometric Determination of Zinc Basing on Complexes with Azo Dyes

A simple and direct spectrophotometric methods for determination of zinc are based on reaction with azo dyes: 1-(2-pyridylazo)-2-naphtol (PAN) and Chromothrop 2B in the presence benzyldimethyldodecylammonium bromide (BDDABr) have been developed. Optimum concentrations of PAN, Chromothrop 2B, BDDABr and pH ensuring maximum absorbance were defined. The effect of foreign ions was elucidated. The complex Zn-PAN-BDDABr shows maximum absorbance at 597nm with the molar absorptivity value 8.55⋅10 L molcm, and complex ZnChromothrop 2B-BDDABr has maximum absorbance at 595nm with the molar absorptivity value 4.26⋅10 L molcm. The Beer’s law is obeyed for zinc concentrations in the range 0.24-0.80 μgmL for the complex Zn-Chromothrop 2BBDDABr and 0.36-0.80 μgmL for the complex Zn-PAN-BDDABr. The method basing on the complex Zn-PAN-BDDABr has been applied for the determination of zinc in milk, milk substituting preparations, insulin and blood serum. INTRODUCTION. The very sensitive and selectivity methods for the spectrophotometric determination of many metals and also zinc are based on the reaction with azo-dyes [1-3]. The most popular of these chromogenic reagents are 1-(2-pyridylazo)-2-naphtol (PAN) and Chromothrop 2B. PAN belongs to group of pyridylazo dyes and Chromothrop 2B is example of o-hydroxyarylazo compounds. Metal-PAN complex does not dissolve in aqueous solution, so it is necessary to do a solvent extraction. This complex had been extracted with non-polar solvents such as chloroform or benzene, which was found to be carcinogenic. Application of surface active substances, which play the role of a protective colloids gives possibility to increase solubility of a Zn-PAN and Zn-Chromothrop 2B complexes and creates better conditions for determination [4-7]. The harmful organic solvents used in the extractionspectrophotometric methods have been eliminated. An advantage of these methods is a direct spectrophotometric measurement of absorbance of analysed solution.. Application of three component systems, and also more often use of four-component systems, causes the spectrophotometric methods to exhibit high sensitivity-molar absorptivity usually larger than 1⋅10 L molcmand can compete with other extractionspectrophotometric and spectrophotometric methods [8-13]. EXPERIMENTAL. Reagents. All the reagents were of analytical grade and were used without further purification. Double-distilled water was used in all experiments. Zinc stock solution (1mg mL) was prepared by dissolving 0.43987 g of ZnSO4⋅7H2O (PPH”POCh” S.A.) in water with addition of 0.5 mL of concentrated H2SO4 and dilution up to 100 mL. Working solutions of 10 μg mL and 20 μg mL were obtained by diluting the stock solution. 1-(2-pyridylazo)-2-naphtol (PAN) solution in methanol (1⋅10 mol L) was obtained by dissolving 0.0250 g of PAN (PPH”POCh” S.A.) in methanol and dilution up to 100 mL. Chromothrop 2B solution (1⋅10mol L) was obtained by dissolving 0.12835 g of Chromothrop 2B (PPH”POCh” S.A.) in water and dilution up to 250 mL. Triton X-100 (1⋅10 mol L) was obtained by dissolving 3.2343 g of Triton X-100 (BDH) in water and dilution up to 500 mL. Benzyldimethyldodecylammonium bromide (BDDABr) solution (1⋅10mol L) was obtained by dissolving 3.8445 g of BDDABr (Fluka) in water and dilution up to 100 mL. Borate buffer pH=8, pH=7.6 [14]. Control of blood serum Cormay HP and samples of human blood serum (Medical Laboratory). Insulin Actrapid, Mixtard (Polfa, Tarchomin, Poland)Milk Nan 1 (Nestle) and milk substituting preparations Isomil (Abbot Laboratories B.V.), Nutrison (Ovita Nutricia). Apparatus. An Jasco (Japan) model V-530 spectrophotometer UV-VIS was used for all absorbance measurements with a 10 mm glass cell. An Elwro (Poland) model N-517 pHmeter was used for pH measurements. The spectrometer ICP-OES made by Spectro Analytical Instruments (Germany) was used with the following parameters: 27.12 MHz, power 1.1 kW, nebulizer-concentric Meinhard. Mineralization was conducted in a UniClever BM-1z microwave mineralizer, Plasmotronika (Poland). Procedure. Zn-PAN-BDDABr: To the flask of 25 mL capacity were introduced 2mL of the BDDABr solution, 1.3 mL PAN solution, 3.5 mL borate buffer (pH=7.6) and 5-20 μg zinc. The flasks were filled up with water to the same volume. After 30 min, absorbance was measured at 597 nm in 10 mm cuvettes, against the blank and the calibration graph was plotted. Zn-Chromothrop 2B-BDDABr: To the flask of 25 mL capacity were introduced 2.5 mL of the Chromothrop 2B solution, 4 mL borate buffer (pH=8), 2-20 μg zinc and 1mL BDDABr solution. The flasks were filled up with water to the same volume. After 15 min, absorbance was measured at 595 nm in 10 mm cuvettes, against the blank and the calibration graph was plotted. RESULTS AND DISCUSSIN. Absorption spectrum and effect of surfactants. Zinc reacts with PAN forming of an orange coloured water-insoluble neutral chelate. The application cationic surface active agent – benzyldimethyldodecylammonium bromide or nonionic surfactant – Triton X-100 gives possibility to increase to solubility of the Zn-PAN complex and improves conditions for its determination. Figure 1 presents the absorption spectra for the zinc chelate with PAN in the presence of BDDABr and Triton X-100.The complexes Zn-PAN-BDDABr and Zn-PAN-BDDABr Tritoin X-100 show the absorption maximum at the same wavelenght λmax=597 nm (curve1,2), and the ternary Zn-PAN-Triton X-100 system at 555 nm (curve 3) A large bathochromic effect of maximum absorption of studied system (curve 1) comparing with the complex Zn-PAN-Triton X-100 (curve 3, ∆λ=42 nm) was observed. In the presence BDDABr there is a significant hyperchromic effect. Introduction of Triton X-100 to Zn-PAN-BDDABr system distinctly lowers of absorbance. In consideration of more intensity of absorption of complex wavelenght λ=597 nm was choosen. Figure 2 presents the absorption spectra for the zinc complex with Chromothrop 2B in the presence of surfactants. Fig.1 Absorption spectra, pH=7.6, l=1cm 1. Zn-PAN-BDDABr complex measured against blank λ=597nm 2. Zn-PAN-BDDABr-Triton X-100 complex measured against blank λ=597nm 3. Zn-PAN-Triton X-100 complex measured against blank λ=555nm Fig.2 Absorption spectra, pH=8, l=1cm 1. Zn-Chromothrop 2B complex measured against blank λ=515nm, λ=580nm 2. Zn-Chromothrop 2B-BDDABr complex measured against blank λ= 595nm 3. Zn-Chromothrop 2B-Triton X-100 complex measured against blank λ=519nm, λ=585nm 4. Zn-Chromothrop 2B-BDDABr-Triton X-100 complex measured against blank λ=528nm Media and the reaction time. The complexation occurs in a weak alkaline medium with the optimum at pH=7.6 for the Zn-PAN-BDDABr complex and with the optimum at pH=8 for the Zn-Chromothrop 2B-BDDABr complex. In both cases in further experiments the borate buffer of these value pH was used. Absorbance of the complexes is constant after 30 min (Zn-PAN-BDDABr) and 15 min (Zn-Chromothrop 2BBDDABr) at room temperature and remains constant for 1.5 h. Optimum concentrations of PAN, Chromothrop 2B, BDDABr and order of reagents addition were defined. The conditions required for the studied systems are given in Table 1. Table1. The conditions of determination of the studied complexes. Molar excess System λ [nm] pH Time [min] Order of reagent addition PAN Chr.2B BDDABr Zn-PAN-BDDABr 597 7.6 30 BDDABr-PAN-