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pH Greater (ph + greater)
Selected AbstractsIdentification of hydroxyl copper toxicity to barley (Hordeum vulgare) root elongation in solution cultureENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2009Xuedong Wang Abstract The effect of pH on the acute toxicity of Cu to barley (Hordeum vulgare) root elongation was investigated in solution culture. The results showed that the median effective concentrations (EC50s; i.e., the concentration that reduced root elongation by 50% based on free Cu2+ activity) were not significantly different in the low-pH range from 4.5 to 6.5, but in the high-pH range from 7.0 to 8.0, a significant effect of pH on EC50s was found. The nonlinear relationship between EC50 and H+ activity in the present study indicated that the increased toxicity with increasing pH in solution may not be caused by decreasing H+ competition. When we take account of CuOH+ activities, a good linear relationship (r2 > 0.97) between the ratio of CuOH+ activity to free Cu2+ activity and acute Cu toxicity to barley root elongation was achieved, which indicated that the observed toxicity in the high-pH range may be caused by CuOH+ plus free Cu2+ in solution. Linear-regression analysis suggested CuOH+ had a greater binding affinity than Cu2+ at the biotic ligand sites. The logistic dose,response curve showed that expressing the Cu dose as Cu2+ + 2.92·CuOH+ improved the data fit significantly compared to consideration of the free Cu2+ activity only. Thus, our results suggest CuOH+ was highly toxic to barley root elongation. The enhanced toxicity of CuOH+ therefore needs to be considered when modeling the effect of pH on Cu toxicity to barley for exposures having pH greater than 6.5. [source] Cd, Cu, Pb, and Zn coprecipitates in Fe oxide formed at different pH: Aging effects on metal solubility and extractability by citrateENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 1 2001Carmen Enid Martínez Abstract Coprecipitates of heavy metals with Fe oxides may form in contaminated soil, water, and sediment systems, particularly when oxidation-reduction processes are occurring. Once formed, coprecipitates with ferrihydrite could limit heavy metal mobility, solubility, toxicity, and bioavailability in geochemical systems. In this study, Cd, Cu, Pb, and Zn were coprecipitated with Fe by titration to pHs 6 and 7. Metal solubility was monitored during the coprecipitation process to pH 7 and after aging of the product. In the coprecipitate formed at pH 6, metal solubility was determined after the system reached pH 6 and at subsequent time intervals. We also reacted the coprecipitates with citrate and evaluated potential metal availability at increased aging times. The pH of coprecipitate formation had little effect on the long-term solubility of Cu and Zn, whereas soluble Cd was greater in the coprecipitate formed at pH 6. Soluble percentages of metals were low at both pH 6 and 7 for Cu and Pb but averaged 1.5 to 3% for Zn and greater than 40% for Cd. Hysteresis was observed in the coprecipitation curves for Zn and Cd, revealing reduced solubility after adsorption or coprecipitation. Lead and Cu failed to show hysteresis, with strong retention in the solid phase at pH greater than 6. The citrate-extractable fraction was greatest for Cd and Zn, less for Cu, and least for Pb. Citrate-extractable metal was higher for the coprecipitate formed at pH 7 than at pH 6, suggesting increased potential availability from coprecipitates formed at higher pH. [source] Human enamel dissolution in citric acid as a function of pH in the range 2.30,pH,6.30 , a nanoindentation studyEUROPEAN JOURNAL OF ORAL SCIENCES, Issue 3 2003Michele E. Barbour The objective of this study was to investigate the dissolution of human enamel in citric acid solutions over a wide range of pH. The in vitro conditions are considered to be relevant to soft drink-induced enamel erosion. Nanoindentation was used to investigate changes in the nanomechanical properties of polished enamel surfaces after exposure to citric acid solutions. Solutions used had 38.1 mmol l,1 citric acid and pH greater than 2.3 but less than 6.3 (2.30 pH 6.30). Samples were exposed to rapidly stirred, constant composition solutions for 120 s. Statistically significant changes in enamel hardness and reduced elastic modulus were observed after exposure to all solutions. There was an approximately linear dependence of enamel hardness on solution pH for 2.90 pH 6.30. Below pH 2.90, enamel is thought to have reached the lowest possible hardness value. The reduction in enamel dissolution caused by an increase in pH of a soft drink is likely to be small. Product modification to reduce the erosive potential of drinks may require additional methods such as addition of calcium salts. [source] Nickel peroxide: A more probable intermediate in the Ni(II)-catalyzed decomposition of peroxomonosulfateINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2007P. Thendral The Ni(II) ion catalyzed thermal decomposition of peroxomonosulfate (PMS) was studied in the pH range 3.42,5.89. The rate is first order in [PMS] and Ni(II) ion concentrations. At pH greater than or equal to 5.23, the reaction becomes zero order in [PMS] and this changeover in the order of the reaction occurs at a higher concentration of nickel ions. The first-order kinetics in PMS can be explained as a rate-limiting step and is the transformation of nickel peroxomonosulfate into nickel peroxide. This peroxide intermediate reacts rapidly with another PMS to give oxygen and Ni(II). The formation of nickel peroxide is associated with a small negative or nearly zero entropy of activation. The zero-order kinetics in [PMS] can be explained by the fact that the hydrolysis of aquated nickel(II) ions into hydroxocompounds is the rate-limiting step. The turnover number is 2 at pH 3.42 and increases with pH. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 320,237, 2007 [source] Synthesis and characterization of metal sulfide clusters for toxicological studiesENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 4 2002Karl C. Bowles Abstract Zinc sulfide clusters were synthesized and characterized at low micromolar concentrations to assess the effect of metal-sulfide ligands on metal toxicity to aquatic organisms in oxic environments. Recommended preparation times are greater than 2 h initial reaction of equimolar sodium sulfide and zinc nitrate, followed by oxic aeration for 3 d. Ionic strength, pH, and anoxic stabilization time were found to be relatively unimportant in controlling the final yield. Adsorptive losses of zinc sulfide (ZnS) clusters to surfaces, however, were significant for a variety of vessel materials and membrane filters. Ionic strength and pH were found to be important factors controlling the extent of adsorptive losses with minimal loss for pHs greater than 9 and for soft waters. The Ag(I), Cu(II), and Hg(II) as metal sulfides completely suppress the analysis of sulfide, whereas Pb(II), Mn(II), and Co(II) partially suppress the analysis of sulfide by the methylene blue technique. Ultraviolet and fluorescence spectra are shown for synthesized ZnS clusters. [source] EFFECT OF FLAXSEED GUM ON QUALITY AND STABILITY OF A MODEL SALAD DRESSING,JOURNAL OF FOOD QUALITY, Issue 4 2000S. STEWART ABSTRACT Flaxseed gum was evaluated for its application as a stabilizer for salad dressing. It was found that for stabilization to occur the concentration of gum had to be higher than 0.45% (w/w). Solvent quality affected the stability of model oil-in-water emulsions. Flaxseed gum stabilized emulsions were stable at pHs greater than 2.8. Lower values caused the polysaccharide to have a compact configuration or caused cleavage of the polymer creating instability. Larger mean droplet size and creaming were observed when the pH was too low or the gum concentration was not sufficient for coverage. Salt addition greatly affected the rheological properties of the polysaccharide; however, the electrolyte improved the emulsifying capabilities of the egg yolk protein. Aging weakened the polymer network structure as shown by a decrease in Newtonian viscosity with a corresponding increase in phase angle confirming the transformation to a more fluid system. [source] | ||||||||||