Papers by Keyword: pH

Abstract: The degradation of concrete due to ingress of sulfate ions from the environment plays an important role in the durability of concrete constructions. Microbiologically induced concrete corrosion (MICC) damages especially sewage collection systems. The most rapid cases of deterioration always occur in areas with elevated H₂S concentrations, moisture, and oxygen in the atmosphere. During the MICC, the pH of the surface of concrete sewer pipes is reduced and it may lead to the steel depassivation and results in the corrosion of steel reinforcement. Damage due to a sulfate interaction can result in a cracking and softening, with a loss of strength of concrete. The formation of ettringite (AFt) from gypsum (forming by reaction of sulfate anion with calcium hydroxide) and C₃A via monosulfate (AFm) is the main chemical reaction of sulfate attack on concrete. Ettringite and gypsum have considerably larger volume than initial compounds, which leads to increased pressure in concrete. This paper is focused on the sulfate attack on fine-grained concrete where the effect of 0.5% sulfuric acid, simulating MICC, and a solution simulating sewage water has been investigated on changes of the pH, content of sulfates and the porosity in various types of concrete. The aim of this study is to compare the changes in different types of concrete during the sulfate attack in two kinds of medium represented the bottom part of pipelines (waste water) and the sewer crown (0.5% H₂SO₄). It was found, that after 1 year in 0.5% H₂SO₄, a visible degradation of surface occurs in all investigated types of concrete. Samples over the year in waste water became dark. Concentration of sulfates in all studied types of concrete increased six times at least after one year sulfuric acid attack and also the reduction of the pH of their aqueous leaches was determined. The solution simulating sewage water did not cause such changes.

Abstract: A series of Fe-Co nanoparticles were synthesized via sol-gel route at acidic, neutral and basic condition using rice husk as the silica source. The synthesized nanomaterials were designated as Fe-Co3, Fe-Co7 and Fe-Co9 and characterized by Fourier Transform Infrared (FTIR), Transmission Electron Microscope (TEM) and particle size analyzer. The great effect of pH was clearly evidenced from the shifting in the siloxane bond in the FTIR spectrum. TEM investigation confirmed the existence of discrete and almost sphere like nanoparticles. The particle size decreased with an increase in the pH, registering the smallest average particle size at pH 9. In brief, this study promises a fast, rapid and promising method for the conversion of silica rice husk into nanoscale bimetallic materials.

Abstract: The paper discusses the electrochemical interactions of Co-Cr alloy with different physiological solutions and organic substances, containing different ions and different pH values, which should play a key role in the materials lifetime. The physical and chemical reactions are numerous and the passivity of the Co-Cr alloy is submitted to the influence of the liquid environment. Some electrochemical investigations were carried out for understanding and predicting the passivity and the corrosion resistance of Co-Cr alloy in three types of simulated body fluids (SBF), namely Fusayama Meyer artificial saliva (pH=5), Hank’s solution (pH=7.4), Ringer's solution (pH=6.6) and citric acid as organic solution. The reason of using another type of environment (other than SBF), such as citric acid is that this medium is a powerful oxidant and has a lower pH value (pH=1.8). This acid environment can influence the state (the formation and growth) of the oxide layer on the surface of the alloy. Research has shown different behavior of the cobalt chromium alloy according to the pH, the chloride content and the oxidizing nature of the environment. Co-Cr alloy biomaterial, immersed in four aqueous media reveals that there are different levels of stabilization of the potential and different current densities depending on the pH value. The Co-Cr alloy biomaterial is very sensitive to localized corrosion (pitting) in Hank solution.

Abstract: (NH₄)₂Mo₄O₁₃ and h–MoO₃ nanocrystalline powders were synthesized by precipitation method at a varied pH range from 5.0 to 1.0. The crystal structure, morphology and optical property of samples were determined by X–ray diffractometer (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and UV–vis diffuse reflectance spectrophotometer (UV-vis DRS). At pH 2.0, 3.0 and 5.0, homogenous plate–like (NH₄)₂Mo₄O₁₃ was seen, whereas the heterogeneous hexagonal rod–shaped MoO₃ was found at very low pH of 1.0 and 1.5. Band gap energy of the synthesized (NH₄)₂Mo₄O₁₃ and h–MoO₃ were 3.38 and 3.18 eV, respectively. Photochromic properties of the products were illustrated by color difference before and after UV irradiation using CIE Lab color system. The synthesized h–MoO₃ provided a strong photochromic performance, while the (NH₄)₂Mo₄O₁₃ showed non–photochromic properties. Intercalation of H⁺ in h-MoO₃ were studied using electrochemical characterization by cyclic voltammetry (CV). The diffusion coefficient of the samples increases with decreasing pH of the solution.

Abstract: The suspensions consisted of fluid phase and solid particles have been used for over 100 years. However, these tests concerned only the particles of dimension scale of mili-or micrometer, and application of such mixtures has caused serious problems that were a consequence of sedimentation of solid particles, wall erosion, clogging of the conduits or creating of sediments. The nanotechnology development has caused a vast availability of particles with dimensions less than 100 nm. Suspensions consisting of base fluid and particles (beads, filaments, plates) with dimensions less than 100 nm were given a name of nanofluids [1].

Abstract: The degradation of concrete due to ingress of sulfate ions from the environment plays an important role in the durability of concrete constructions, especially in sewage collection systems where concrete sewer pipes are exposed to sulfates from waste water and from biogenic activity of bacteria. During this process the pH of the surface of concrete sewer pipes is reduced and it may lead to the steel depassivation and results in the corrosion of steel reinforcement. Damage due to sulfate interaction can result in the cracking and softening, with loss of strength of concrete. This paper is focused on the sulfate attack on fine-grained concrete where the effect of one-year contact of 0.5% H₂SO₄, and 5% Na₂SO₄ on changes of pH and content of sulfates in 7 types of concrete has been analyzed. It was found that after one year of sulfate attack on concrete, significant growth of content of sulfates is observed in the lowermost layer of the samples. Samples treated by 5% Na₂SO₄ contain slightly more sulfates in the upper layers than samples treated by sulfuric acid. The reduction in pH of aqueous leaches occurred in all layers of the samples. However, even in the lower layers of the samples, the reduction of pH below 9.5 did not turn up (except for SRS sample), and thus the conditions for the depassivation of reinforcement were not met.

Abstract: Spectrophotometric determination of hydroxymethylfurfural (HMF) based on p-toluidine as a reagent has been studied. The absorption intensities were measured at a wavelength range of 250 to 500 nm to record the absorption spectra before and after reaction with the HMF. The solution response was obtained at pH 3. The concentration of HMF was 5 × 10^-3 M, whereas for p-toluidine was 8 × 10^-3 M .The relative standard deviation (R.S.D.) of reproducibility was 3.33% when concentration of p-toluidine 3 × 10^-3 M was used .The photostability was found to be good with 1.9 %. M was used . The photostability was found to be good with 1.9 %.

Abstract: This paper presents the results of the combined study of experiments and modeling of the pitting corrosion behavior of low carbon steel. The effects of pH are elucidated via experiments on low carbon steel exposed to various corrosive media. The corrosion rates for the steel samples immersed in various corrosive media were determined by polarization experiments via a gamry potentiostat. The microscopic observations of the surfaces reveal clear evidence of corrosion pits that increase in size with increasing exposure duration. The observed pit size distribution and the evolution of pit size are modeled using statistical models. The implications of the results are used for the application of low carbon steels in corrosive environment.

Abstract: The use of fly ash as an alternative material for cement substitute in concrete mix already a common practice nowadays. However, as a waste material, fly ash varies in quality and condition, as shown by variation of its fineness, specific gravity, Loss on Ignition (LOI) and also on its chemical composition. By measuring the acidity (pH) of the fly ash in water solution, percentage of mass retained on 45 μm sieve, and superplasticizer demand of the fly ash, we can develop a quick estimation of the quality of fly ash. This study aims to investigate a quick method to estimate the quality of fly ash by measuring the physical and chemical pointers, as indicator for its properties and the effect on the setting time and compressive strength of mortar. Fly ash content was varied from 0-70% of the total mass of cementitious materials to make HVFA mortar. Fly ashes were obtained from four power plants in Indonesia. Tests conducted were material characterization, setting time, temperature rise, and compressive strength of mortar at different ages. Different fly ash quality can be shown by the fast pointers; namely pH, superplasticizer demand and % retained on 45 μm sieve. Setting time and strength development were affected by the different properties of fly ash.

Abstract: Jarosite formation is undesirable in bioleaching processes as it depletes the needed ferric reagent for the oxidation of most sulfide minerals. Although it creates kinetic barriers thereby retarding the leach rates of most minerals, jarosite serves as support for the attachment of bioleaching microbes, facilitating biooxidation rate. Microbial ferrous-oxidation by mesophilic microbe was studied in a recently reported novel packed-column bioreactor with a view to investigate the potential of using solution pH to manage jarosite accumulation in the bioreactor in addition to establishing a base case data for the bioreactor. Experiments were conducted in the bioreactor packed with glass balls (15mm diameter) at constant temperature of 38.6 °C, residence time of 18 hours, airflow rate of 20 mLs^-1 and pH values of 1.3, 1.5 and 1.7. The results showed that the amount of jarosite accumulation is proportional to the solution pH, and to the duration of operation of the bioreactor. Jarosite precipitation concentrations of 4.95, 5.89 and 7.08 gL^-1 were obtained after 10 days of continuous operation at solution pH values of 1.3, 1.5 and 1.7 respectively, while after 15 days the precipitations concentrations increased to 5.50, 7.90, 9.98 gL^-1 respectively. The results also showed that 33% and 52% precipitate reduction could be achieved by gradual decrease in the bioreactor solution pH to 1.5 and 1.3 after being continuously operated for 10 days at pH 1.7 respectively after an addition of 5 days. A maximum ferrous oxidation rate (), 6.85 mmol.L^-1.h^-1 and the affinity kinetic constants (,), of 0.001 and 0.006 for Hansford and Monod models respectively. Although a directly relationship exist between jarosite formation and pH, the results of this study may be relevant in bioleach heaps or at least in column bioreactors to manage/control jarosite accumulation thereby improving the leach kinetics of mineral sulfides.