Determination of photostability of monoammonium glycyrrhizinate

In a previous study, it was shown that monoammonium glycyrrhizinate was thermostable in the pH range 7.0–9.0. To establish the best storage conditions of this sweetener, we have determined the photodegradation of 6×10⁻⁵ M aqueous solutions of monoammonium glycyrrhizinate (λ_max:256 nm, ϵ=10600 M⁻¹ cm⁻¹). The photodegradation appeared to follow first-order kinetics and was found to be pH-dependent. The degradation rate constant was calculated to be 8.85×10⁻⁴, 5.38×10⁻⁴, 4.33×10⁻⁴, 4.66×10⁻⁴ and 5.12×10⁻⁴ min⁻¹, respectively, at pH 2.0, 4.5, 6.0, 8.0 and 10.0.     

Monitoring of Zn(II), Cd(II), Pb(II) and Cu(II) During Refining of Some Vegetable Oils Using Differential Pulse Anodic Stripping Voltammetry

In this study, a novel approach has been developed using differential pulse anodic stripping voltammetry (DPASV) for the simultaneous determination of Zn(II), Cd(II), Pb(II) and Cu(II) in edible oils at hanging mercury drop electrode. The microwave digestion of oil samples was carried out with concentrated HNO₃ and H₂O₂. KNO₃ was used as a supporting electrolyte. The experimental conditions optimized such as deposition time, stirring rate and size of mercury drop were 300 s, 600 rpm and 10 mm², respectively. The method was applied to quantify Zn(II), Cd(II), Pb(II) and Cu(II) in crude and refined hazelnut, corn, sunflower and olive oils. During refining of different vegetable oils, the removal efficiencies of Zn(II), Cd(II), Pb(II) and Cu(II) were calculated as 98.20–99.91, 98.50–99.90, 95.26–99.76 and 95.93–99.92 %, respectively. The limits of detection for Zn(II), Cd(II), Pb(II) and Cu(II) were 2.1?×?10^?8, 8.7?×?10^?10, 7.1?×?10^?9 and 3.4?×?10^?9 and the limits of quantification were 6.8?×?10^?8, 2.9?×?10^?9, 2.3?×?10^?8 and 1.1?×?10^?8 M with linear regression coefficients (R ²) of 0.9930, 0.9928, 0.9893 and 0.9931, respectively. It was observed that the above metals in crude and refined vegetable oils could be determined simultaneously by the DPASV method.     

Purification and characterisation of two pectinmethylesterase from persimmon (Diospyros kaki)

Two pectinmethylesterase isoforms, PME I and PME II, have been separated and purified from persimmon using chromatography techniques. Both isoforms presented differences in molecular weight (PME I: 51 kDa, PME II: 30 kDa), isoelectric point (PME I: 8·4, PME II: 6·9) and K_m values (PME I: 54 μg ml⁻¹, PME II: 31 μg ml⁻¹). They differed in their optimum pH and thermal stability, PME I being the more thermostable isoform. Both isoforms exhibited similar behaviour with respect to Ca²⁺ and Na⁺ concentrations and were strongly inhibited by CaCl₂ concentrations of over 80 mM and by NaCl concentrations of over 500 mM . Both isoforms were activated by low concentrations of polygalacturonic acid and were competitively inhibited by D -galacturonic acid and high concentrations of polygalacturonic acid. ©1997 SCI     

Interaction between Cyanidin 3-glucoside and Cu(II) ions

The interaction between cyanidin 3-glucoside (Cy 3-glc) and Cu(II) in a binary solvent (methanol–water, 1:1, (v/v)) and in a phosphate buffer was studied. It is demonstrated that Cy 3-glc and Cu(II) ions form a complex that is manifested by appearance of a new absorption band. In the complex, the reduction of Cu(II)–Cu(I) and oxidation of Cy 3-glc takes place. The association constant of the complex formation was calculated using the Rose–Drago method. The complex stoichiometry in the binary solvent was 1:1, and the association constant was K = (28,600 ± 2600) M⁻¹. Interaction of Cy 3-glc and Cu(II) ions has two stages “fast” and “slow”. The kinetics of these processes has been investigated. Thermodynamic data for the “fast” process: enthalpy (ΔH^∗) and entropy (ΔE^∗) of activation, were calculated.     

A comparative study of polyphenoloxidases from taro (Colocasia esculenta) and potato (Solanum tuberosum var. Romano)

Taro (C. esculenta) is a staple food in many tropical regions. A comparative study of crude polyphenoloxidases from taro (tPPO) and potatoes (pPPO) was carried out to provide information useful for guiding food processing operations. Crude PPO was prepared by cold acetone precipitation using ascorbic acid as antioxidant. The PPO content of taro acetone powder was 770±17 units (mg protein)⁻¹ as compared with 3848±180 units (mg protein)⁻¹ in potato acetone powder. The pH-activity optimum was pH 4.6 for tPPO and pH 6.8 for pPPO. Both enzymes retained >80% activity after incubation at pH 4.5–8 but there was rapid activity loss at pH < 4. The temperature-activity optimum (T_opt) was 30°C for tPPO and 25°C for pPPO with 75 and 27% of their respective maximum activity retained at 60°C. Both tPPO and pPPO were irreversibly inactivated by 10 min heating at 70°C. The activation enthalpy (ΔH^#) and activation entropy (ΔS^#) for tPPO heat-inactivation were 87.4 (±0.1) kJ mol⁻¹ and −56.2 (±4) J mol⁻¹ K⁻¹, respectively. For pPPO, ΔH^# was 59.1 (±0.1) kJ mol⁻¹ whilst ΔS^# was −141 (±4) J mol⁻¹ K⁻¹. The apparent substrate specificity was established from values V_max/K_m as: 4-methylcatechol>chlorogenic acid>dl-dopa>catechol>pyrogallol> dopamine>>caffeic acid for tPPO. There was no detectable activity towards caffeic acid. The substrate specificity for pPPO was: 4-methylcatechol>caffeic acid>pyrogallol>catechol>chlorogenic acid >dl-dopa>dopamine. According to the order of inhibitor effectiveness (sodium metabisulphite>ascorbic acid>NaCl≈ (EDTA), there was a significant lag-phase before increases occurred in the absorbance at 420 nm. Preincubation of PPO with inhibitors increased the extent of inhibition, indicating a direct effect on the structure of the enzyme.     

Microwave-assisted digestion of β-lactoglobulin by pronase, α-chymotrypsin and pepsin

The effects of microwave irradiation (MWI) on kinetic parameters for pronase, α-chymotrypsin and pepsin hydrolysis of bovine β-lactoglobulin (β-Lg) were evaluated. The experiments were performed under MWI or conventional heat (CH) at 40 °C. The initial velocity (V₀) of peptide bonds cleavage was measured by o-phthaldialdehyde method; the peptide profile was analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS–PAGE). Higher catalytic effectiveness (K_cat K_m⁻¹) values were obtained in the pronase and α-chymotrypsin digestions performed under MWI (7793 and 2073 min⁻¹ mM⁻¹, respectively) in comparison with the values in the respective CH digestions (1802 and 941 min⁻¹ mM⁻¹, respectively). The Michaelis–Menten constant (K_m) for either enzyme was reduced under MWI. Pepsin showed very low activity on β-Lg at pH 4.0 regardless of the heating procedure used. For two enzymes, pronase and α-chymotrypsin, differences in SDS–PAGE profiles were obtained due to the MWI applied during the enzymatic hydrolysis. The combined enzyme/MWI treatments could have a relevant application in the development of β-Lg hydrolysates.     

Analytical application of food dye Sunset Yellow for the rapid kinetic determination of traces of copper(II) by spectrophotometry

A sensitive and rapid kinetic method for trace determination of Cu(II) was developed and validated, based on its catalytic effect on the oxidation of disodium-6-hydroxy-5-[(4-sulphophenyl)azo]-2-naphtalenesulphonic acid (wide used, food colour “Sunset Yellow FCF”, E110, in text selected as SY) by hydrogen peroxide in borate buffer at pH 10.5. The reaction was monitored spectrophotometrically by measuring the decrease in the absorbance of SY at 478.4 nm. The optimum operating conditions regarding concentration of reagents, pH and temperature were established. The calibration curve was linear up to 318 ng mL⁻¹ of Cu(II) and the limit of detection (3σ/S) is 5.0 ng mL⁻¹, and limit of quantification (10σ/S) is 16.67 ng mL⁻¹. The proposed kinetic procedure was successfully applied to monitoring of the concentration of Cu(II) in fruit, wine and milk samples from different areas. The results obtained by the proposed kinetic procedure were compared by those obtained by ICP-OES method, and shown good agreement. The proposed kinetic method could be used for monitoring of quality of drinks or fruit depending on Cu(II) concentration, because of its important role as nutritional element.     

Quality changes in sardines (Sardina pilchardus) stored in ice and at ambient temperature

The keeping times of sardines were 21–27 h (average 23 h) at ambient temperature (T_amb) and 8–11 days (average 9.5 days) in ice. The muscle pH increased from 6.15 to 6.85 atT_amband from 6.24 to 6.55 in ice. Trimethylamine–nitrogen (TMA-N) and total volatile basic nitrogen (TVB-N) accumulated rapidly atT_ambto reach 11.2 mg 100 g⁻¹of fish and 57.3 mg 100 g⁻¹of fish, respectively at spoilage. In ice, the rate was rapid for two trials (2.72 mg TMA-N 100 g⁻¹of fish per day and 5.20 mg TVB-N 100 g⁻¹of fish per day) and slow for three trials (0.21 mg TMA-N 100 g⁻¹of fish per day and 3.7 mg TVB-N 100 g⁻¹of fish per day). Likewise, the histamine development rate was rapid during one trial (5.4 per day atT_amband 0.313 per day in ice), slow (average rate 3.503 per day during four trials atT_amb, 0.305 per day during one trial in ice) and zero during the other three ice-trials. Bacteria multiplied rapidly atT_amb; psychrotrophs multiplied slowly, and represented 10–68% of mesophiles at rejection point.In ice, bacteria multiplied slowly after lag phases of up to 120 h, but reached levels of rejection similar to those observed forT_ambstorage, H₂S-producing bacteria represented 0.5–7% of the initial fish flora and 3–10.1% at rejection point. They grew with a shorter lag phase as was the situation with the total flora.     

Growth and viability of Lactobacillus acidophilus NRRL B-4495, Lactobacillus casei NRRL B-1922 and Lactobacillus plantarum NRRL B-4496 in milk supplemented with cysteine,ascorbic acid and tocopherols

The effect of three reducing agents (RAs), l-cysteine, ascorbic acid, and tocopherols, on the growth of Lactobacillus acidophilus, Lactobacillus casei, or Lactobacillus plantarum during milk fermentation was evaluated. pH, redox potential, and Lactobacillus counts were determined until pH ≈ 4.6. Further, the study aimed to optimise the concentration of the RAs by formulating the fermented milks with the same RA at different concentrations (0–250 mg L⁻¹ for l-cysteine or ascorbic acid and 0–15 mg L⁻¹ for tocopherols) using a Box–Behnken experimental design. After 45 days of refrigerated storage, the viability of each Lactobacillus species was maximised. We observed that the effect of RA on Lactobacillus is species dependent; ascorbic acid and tocopherols reduced the fermentation time (29%–43%), whereas l-cysteine enhanced the Lactobacillus counts (≥1 log₁₀ cfu mL⁻¹). Lactobacillus species differ in terms of oxygen sensitivity.     

Covalent Immobilization of a α-Amylase onto Poly(methyl methacrylate-2-hydroxyethyl methacrylate) Microspheres and the Effect of Ca2+ Ions on the Enzyme Activity

α-Amylase was covalently immobilized onto poly(methyl methacrylate-2-hydroxyethyl methacrylate) microspheres, which were activated by using either epichlorohydrin (ECH) or cyanuric chloride (C₃N₃Cl₃). The properties of the immobilized enzyme were investigated and compared with those of the free enzyme. For the assays carried out at 25 °C and pH 6.9, the relative activities were found to be 73.0% and 90.8% for epichlorohydrin and cyanuric chloride bound enzymes, respectively. Upon immobilization, the maximum activities were obtained at lower pH values and higher temperatures as compared with the free enzyme. Kinetic parameters were calculated as 2.51 g/L, 28.54 g/L and 15.50 g/L for K_m and 1.67 × 10⁻³ gL⁻¹ min⁻¹ 2.89 × 10⁻⁴ gL⁻¹ min⁻¹ and 1.89 × 10⁻³ gL⁻¹ min⁻¹ for V_max for free, epichlorohydrin and cyanuric chloride bound enzymes, respectively. Enzyme activities were found to be ca. 32.7% for ECH and 41.1% for C₃N₃Cl₃ activated matrices after storage for one month. On the other hand the free enzyme lost its activity completely within 20 days. Immobilization, storage stability and repeated use capability experiments carried out in the presence of Ca²⁺ ions demonstrated higher stability in the presence of these ions. The enzymes immobilized in the presence of Ca²⁺ ions retained 90.6% and 90.8% of the original activities even after 30 days in the case of ECH and C₃N₃Cl₃ activations, respectively. In repeated batch experiments, i.e., 20 uses of the enzyme in 3 days; in the absence of Ca²⁺ ions retentions of 79.2% and 77.1% of the original enzyme activities were observed for ECH and C₃N₃Cl₃ immobilized enzymes, respectively, whereas, in the case of addition of Ca²⁺ ions to the assay medium, these values were enhanced to 95.3% and 92.2%.     

Biochemical changes throughout the ripening of a traditional Spanish goat cheese variety (Babia-Laciana)

The physico-chemical characteristics, proteolysis (classical nitrogen fractions, caseins and their degradation products and free amino acids), and lipolysis (fat acidity and free fatty acids) were studied throughout the ripening of three batches of Babia-Laciana cheese, a Spanish traditional variety made from raw goats’ milk. The main compositional characteristics of this cheese at the end of the ripening are its high content of total solids (TS) (78.0±2.4 g 100 g⁻¹ of cheese) and fat (61.1±1.2 g 100 g⁻¹ of TS), the presence of residual lactose (1.6±0.8 g 100 g⁻¹ of TS) and its low content of sodium chloride (1.1±0.7 g 100 g⁻¹ of TS) and ash (2.8±0.5 g 100 g⁻¹ of TS). Its pH values (4.44±0.72) are extraordinarily low. The evolution and final values of the different nitrogen fractions show that this cheese undergoes a very slight proteolysis, a fact which was corroborated when the caseins and their degradation products were quantified: β-casein did not undergo any modification throughout ripening, while only 21% of the α_s-caseins were degraded. Free amino acids content increased by a factor of about 7 throughout ripening, resulting in a high content of γ-amino butyric acid and a low content of glutamic acid at the end of the process. Fat acidity increased very slightly, approximately 4.5 times, during ripening, reaching final values of 3.5±2.2 mg KOH g⁻¹ of fat. The total free fatty acids content showed a similar evolution to fat acidity. At the end of the ripening process, the main free fatty acid was C_18:1, followed by C₁₆ and C₁₀.     

The effect of gel composition on the adsorption of invertase on poly(acrylamide/maleic acid) hydrogels

The effects of external stimuli such as pH of solution, temperature, substrate concentration of solution and storage stability on the invertase adsorption capacity of poly(acrylamide/maleic acid) [P(AAm/MA)] hydrogels, synthesized by gamma irradiation of ternary mixtures of AAm/MA/Water, were investigated. The adsorption capacities of the hydrogels were found to increase from 4.0 to 13.3 mg invertase/g dry gel with increasing amount of MA in the gel system, while P(AAm) gel adsorbed only 3.1 mg invertase/g dry gel. Kinetic parameters were calculated as 20.6 mM for K_m and 6.44×10⁻⁵ mol/dm³ min for V_max for free enzyme and in the range of 23.6–57.7 mM for K_m and 8.62×10⁻⁵–1.05×10⁻⁴ mol/dm³ min for V_max, depending on the amount of MA in the hydrogel. Enzyme activities were found to increase from 50.0 to 74.0% with increasing amount of MA in the gel system and retained their activities for one month storage. The enzyme activities, after storage at 4°C for one month, were found to be 21.0 and 50.0–74.0% of the initial activity values for free and adsorbed enzyme, respectively. The optimal pH values for free and adsorbed enzymes were determined as 4.56 and 4.56–5.00. The optimum temperature for free and adsorbed enzymes was 55°C. Adsorption studies show that, not only gel composition but also the stimuli, temperature and pH of the solution, play important roles on the invertase adsorption capacity of poly(AAm/MA) hydrogels.     

Formation and dynamic interfacial adsorption of glycinin/chitosan soluble complex at acidic pH: Relationship to mixed emulsion stability

The formation and interfacial adsorption of glycinin/chitosan (CS) soluble complex were investigated at acidic pH. The stability of the mixed emulsion stabilized by the complex was also evaluated at pH 4.5. Turbidimetric analysis, isothermal titration calorimetry (ITC) and dynamic light scattering were used to characterize the dynamic formation of the complex. The results showed that soluble complexes were formed mainly at pHs between 4.0 and 6.0, depending on CS/Glycinin mixing ratio. At pH 4.5, soluble complex was formed and saturated at mixing ratio = 0.1, showing a maximum size distribution at 164.2 nm. We found that the glycinin/CS soluble complex showed improved interfacial adsorption than glycinin at pH 4.5. In detail, dynamic interfacial adsorption data showed the coefficient of diffusion (K_diff), unfolding (K₁) and rearrangement (K₂) for soluble complex (K_diff, K₁ and K₂: 0.58 mNm⁻¹ s^−0.5, 2.23 E−4 s⁻¹ and 5.78 E−4 s⁻¹) were higher than those of the glycinin (K_diff, K₁ and K₂: 0.32 mNm⁻¹ s^−0.5, 1.72 E−4 s⁻¹ and 4.63 E−4 s⁻¹). The droplet size and confocal observation of the mixed emulsion fabricated with glycinin/CS soluble complex displayed improved stability at mixing ratios of 0.1 to/and 0.2, suggesting the synergistic effect of the two molecules. We concluded that interfacial and emulsifying properties of glycinin could be improved by formation of glycinin/CS soluble complex at acidic pH.     

Study of heavy metals in some cultivated and uncultivated mushrooms of Turkish origin

Twenty-four different species of uncultivated mushrooms of Turkish origin and one cultivated mushroom (Agaricus bisporus) growing in soil composts with 16 different compositions were analysed spectrometrically for their heavy metal (Pb, Cd, Hg, Fe, Cu, Mn, and Zn) contents. The highest Pb levels were 2.35 mg kg⁻¹ for the species Agaricus bitorquis and 7.00 mg kg⁻¹ for the species of Hypholoma fasciculare growing in the vicinity of the road. The highest Cd level was 3.42 mg kg⁻¹ for the species of Hydnum repandum. In the wild mushrooms the highest Fe content was 93.6 mg kg⁻¹ for the species of Bovista plumbea and the highest Cu content was 51.0 mg kg⁻¹ for the species Tricholoma terreum. The highest Mn content was 35.9 mg kg⁻¹ for the species Laccaria laccata and the highest Zn content was 31.6 mg kg⁻¹ for the species of Agaricus bitorquis.     

Kinetics of autoxidation of sulphur(IV) oxospecies in aqueous ethanol

The kinetics of autoxidation of sulphur(IV) oxospecies (S(IV)) in ethanol + water (3 + 1 v/v) solvent are reported. Oxidation only proceeds in the presence of metal ion and amine. When Mn(II) and glycine are added, kinetically significant complexes MnGly and MnGly₂ are formed and the initial rate of oxidation is of first order with respect to complex and S(IV) and of zero order with respect to oxygen. The overall reaction appears to be autocatalytic and is accompanied by the formation of SO⁻₂ radicals, probably a by-product of reaction. The implications of this reaction to food systems containing S(IV) are considered.     

Multiple forms of polygalacturonase from mango (Mangifera indica L. cv Alphonso) fruit

Polygalacturonase (PG) from mango pulp revealed three isoforms (I, II, III) upon ion exchange and gel filtration chromatography, each having an abundance of 68%, 6% and 26%, and molecular weights (M_r) 40, 51 and 45 kDa, respectively. The pH optimum for the isoforms was between 3 and 4. PG-I was stable over a wide pH range (4–7.5) unlike PG II and III, which were stable at pH 4 and 5, respectively. The optimum temperature was around 40 °C for all the three isoforms. Their apparent K_m for pectic acid was in the range 0.22–0.25 mg ml⁻¹. The V_max for PG I, II and III was 5.7, 3.6 and 4.4 μmol GalA equivalent h⁻¹, respectively. Cd²⁺, Cu²⁺ and Fe²⁺ and EDTA inhibited whereas GalA, Gal, Fuc, Rha and Ara stimulated PG-I activity, in particular. The major endogenous substrates for mango PG were identified to be two rhamnogalacturonans varying in their sugar ratio. These results are discussed in the light of pectin dissolution in vivo in ripening mango.     

Purification,properties and heat inactivation of pectin methylesterase from apple (cv Golden Delicious)

Pectin methylesterase from apple (cv Golden Delicious) was extracted and purified by affinity chromatography on a CNBr‐Sepharose®‐PMEI column. A single pectin methylesterase peak was observed. Isoelectric points were higher than 9. Kinetic parameters of the enzyme were determined as K_m = 0.098 mg ml⁻¹ and V_max = 3.86 µmol min⁻¹ ml⁻¹ of enzyme. The optimum pH of the enzyme was above 7.5 and its optimum temperature was 63 °C. The purified PME required the presence of NaCl for optimum activity, and the sodium chloride optimum concentration increased with decreasing pH (from 0.13 M at pH 7 to 0.75 M at pH 4). The heat stability of purified PME was investigated without and with glycerol (50%), and thermal resistance parameters (D and Z values) were calculated showing that glycerol improved the heat resistance of apple PME. © 2000 Society of Chemical Industry     

Purification and characterisation of trypsins from the pyloric caeca of mandarin fish (Siniperca chuatsi)

Two trypsins of anionic form (trypsin A) and cationic form (trypsin B) from the pyloric caeca of mandarin fish (Siniperca chuatsi) were highly purified by a series of chromatographies, including DEAE-Sephacel, Sephacryl S-200 HR, Q-Sepharose or SP-Sepharose. Purified trypsins revealed a single band on native-PAGE. The molecular weights of trypsin A and B were 21 kDa and 21.5 kDa, respectively, as estimated by SDS–PAGE, both under reducing and non-reducing conditions. Zymography analysis showed that both trypsins were active in degrading casein. Trypsin A and B exhibited maximal activity at 35 °C and 40 °C, respectively, and shared the same optimal pH of 8.5, using Boc-Phe-Ser-Arg-MCA as substrate. The two trypsins were stable up to 45 °C and in the pH range from 4.5 to 11.0. Trypsin inhibitors are effective on these two enzymes and their susceptibilities were similar. Both trypsins were activated by metal ions such as Ca²⁺ and Mg²⁺ and inactivated by Fe²⁺, Zn²⁺, Mn²⁺, Cu²⁺, Al³⁺, Ba²⁺ and Co²⁺ to different degrees. Apparent K_m values of trypsin A and B were 2.18 μM and 1.88 μM, and K_cat values were 81.6 S⁻¹ and 111.3 S⁻¹ for Boc-Phe-Ser-Arg-MCA, respectively. Immunoblotting analysis using anti-common carp trypsin A positively cross-reacted with the two enzymes, suggesting their similarity. The N-terminal amino acid sequence of trypsin B was determined as IVGGYECEAH, which is highly homologous with trypsins from other species of fish.     

Analytical applications of 2,6-diacetylpyridine bis-4-phenyl-3-thiosemicarbazone and determination of Cu(II) in food samples

New synthesized reagent 2,6-diacetylpyridine bis-4-phenyl-3-thiosemicarbazone (2,6-DAPBPTSC) is proposed as a sensitive and selective analytical reagent for the spectrophotometric determination of copper(II) at pH 3.0 to form a yellowish orange colored 1:1 chelate complex. The maximum absorbance was measured at 370 nm. This method obeys Beer’s law in the concentration range 0.63–6.30 g ml⁻¹ and the correlation coefficient of Cu(II)–2,6-DAPBPTSC complex is 0.942, which indicates an adequate linearity between the two variables with good molar absorptivity and Sandell’s sensitivity, 0.847 × 10⁴ l mol⁻¹ cm⁻¹ and 0.0075 g cm⁻², respectively. The instability constant of complex calculated from Asmus′ method is 1.415 × 10⁻⁴ at room temperature. The precision and accuracy of the method is checked with calculation of relative standard deviation (n = 5), 0.777% and the detection limit value is 0.0056 g ml⁻¹. The interfering effect of various cations and anions has also been studied. The method was successfully applied for the determination of Cu(II) in food samples. The performance of present method was evaluated in terms of Student ‘t’ test and Variance ‘f’ test, which is indicates the significance of present method is an inter comparison of the experimental values, using atomic absorption spectrometer (AAS).     

Kinetic speciation of Co(II), Ni(II), Cu(II), and Zn(II) in model solutions and freshwaters: lability and the d electron configuration

The kinetic speciation of Co(II), Ni(II), Cu(II), and Zn(II) in model solutions of a well-characterized fulvic acid (Laurentian fulvic acid), freshwater samples from the Rideau River (Ottawa, Ontario), and freshwater samples from the Sudbury (Ontario) area were investigated by the competing ligand exchange method using Chelex 100 as the competing ligand and by inductively coupled plasma-mass spectrometry to measure the dissociation kinetics. The metal species were quantitatively characterized by the rate coefficient for the first-order dissociation of metal complex to free metal ion. This technique can be applied to almost all elements and represents an important advance in our ability to investigate the kinetic availability of metal species in the freshwater environment. The order of the lability of the metal complexes, Co(II) > Ni(II) > Cu(II) < Zn(II), follows the reverse order of the ligand field stabilization energy with the exception of Cu(II); the behavior of Cu(II) is also due to the Jahn-Teller effect, which shortens the equatorial bonds and lengthens the axial bonds of a tetragonally distorted Cu(II)-L6 complex. This study has demonstrated a relationship between the lability of metal-DOM complexes of the 3d transition metals in freshwaters and their d electron configuration. This is the first time that the importance of the d electron configuration on the lability of metal complexes in the freshwater environment has been demonstrated. The slow complexation kinetics of both Ni(II) and Cu(II) suggestthatthe usual equilibrium assumption for freshwaters may be invalid.