Sarah Fiol

The use of buffers to measure the pH of seawater

The pH of seawater can be measured in the field using potentiometric and spectrophotometric methods. The use of pH standards or buffers is an important aspect of the calibration of both methods in a laboratory on a common concentration scale. The buffers can also be used to monitor the performance of pH meter and spectrophotometer during a cruise. A procedure is described for the determination of the pH of seawater, where the proton concentration is expressed as moles kg-H2O−1 using seawater buffers. The buffers are prepared in synthetic seawater in the laboratory by the methods outlined by Bates and coworkers. We have prepared four buffers (Bis, Tris, Morpholine and 2-Aminopyridine) that cover a pH range from 6.8 to 8.8. The emf values of the buffers were measured with a H2, Pt/AgCl, Ag electrode system after their preparation and bottling for use at sea. The measured emf values were found to be in good agreement (±0.05 mV) with the original measurements of Bates and coworkers from 0 to 45°C. The measured pH of these buffers are in good agreement (±0.001 pH units) with the values calculated from the equations of Dickson on the total pH scale based on Bates et al. Studies are underway to access the long term stability of these buffers. We have also used these buffers to calibrate systems used to make potentiometric and spectrophotometric measurements of pH on seawater relative to the H2, Pt/Ag, AgCl electrode from 5 to 45°C.

Modeling oxyanion adsorption on ferralic soil, part 1: Parameter validation with phosphate ion

Surface complexation models have proved to be valuable tools for predicting processes that occur at the solid-solution interface. Use of such models has become more widespread and nowadays more complex systems are studied, in an attempt to explain processes such as the competition between different species for mineral surfaces and the effect of the presence of organic matter. The aim of the present study was to analyze the mobility of phosphate in ferralic soils. The charge distribution model parameters for phosphate-goethite adsorption were used to predict phosphate mobility on samples from 2 horizons of a ferralic soil containing large amounts of iron oxides. The soil reactivity was attributed to the iron oxides, and some specific parameters were determined by means of phosphate adsorption-desorption experiments and included in the model. Adsorption of phosphate in the upper horizon, which contained more organic carbon and phosphate than the deeper one, was modeled by using the information obtained for the soil and the charge distribution model parameters derived for phosphate-goethite interaction with no need of further optimization. In contrast, some extra fitting parameters were required to improve the modeling of the phosphate adsorption in the deeper horizon.

The protonation constants of glycine in artificial seawater at 25 °C

The ionization constants of glycine in artificial seawater containing NaCl, KCl, CaCl2, MgCl2 and Na2SO4 were potentiometrically determined at 25 ° C by using a commercial glass electrode. Measurements were carried out at different salinities, and thus ionic strengths, to simulate seawater and estuarine waters. The experimental data were fitted using different equations as functions of the salinity based on Millero (1979). Comparisons were made between the parameters obtained by changing the variables and also the parameters yielded by the Pitzer interaction model, which was previously applied by Millero to seawater systems. After considering all the proposed models we found the function in S12 to give the best fit and the smallest standard deviation of the parameters, although the extrapolated pK is not as good as the one obtained by the use of Pitzers model. Some of the models we used yielded extrapolated pKs closer to the values in the literature (2.36 and 9.78) but higher standard deviations in the parameters.

Interactions Between Ionic Pesticides and Model Systems for Soil Fractions

The extensive use of herbicides in agriculture and their potentially toxic effects have promoted studies investigating the physical, chemical and biological processes that determine the mobility, bioavailability and degradation of these compounds in soils (Blasioli et al., 2011). Knowledge of these processes will enable prediction of the transport and fate of herbicides in soils and aquatic systems, and thus enable measures to be taken to limit their environmental impact. Retention is considered the main cause of the deactivation of herbicides in soils, and is important from the point of view of inhibiting the toxic properties of herbicides and of restricting their transport into aquatic systems (Jones & Bryan, 1980). Although not unique, adsorption reactions (i.e. accumulation of chemical species at the solid-solution interface) are the main cause of the retention of organic contaminants in soils, and their extent will depend on the physicochemical properties of both the adsorbent (soil) and the adsorbate (herbicide). The chemical characteristics of organic compounds are largely responsible for their behaviour in soil, and the differences in adsorption of different herbicides in the same soil are attributed to their distinct chemical properties. Although herbicides are very diverse, two groups can be distinguished in order to interpret their interactions with soil components: those involving chemical forces and those involving physical forces. The first group comprises ionic or ionizable hydrophilic compounds, while the second group comprises non polar hydrophobic compounds. Bipyridinium cations, such as paraquat (1,1’–dimethyl–4,4’–bipyridinium ion), are the best known members of the ionizable herbicides as they have been extensively used in agriculture and are consequently widely distributed in soils and waters. Paraquat (PQ) is applied as a dichloride or dibromide salt, which when dissolved in water releases the organic cation PQ2+, which can be adsorbed on the soil surface, either by replacing inorganic cations or by an ionic interaction mechanism with negatively charged sites on the soil surface, in which the electrostatic effect will be determinant (Narine & Guy, 1982). PQ adsorbs on humic substances and the degree of adsorption increases as the pH increases, as

Cu2+-Soil Fulvic Acid Binding: Effect of pH and Concentration

We studied the effect of pH on the binding of Cu2+ to a soil fulvic acid in solutions containing dissolved organic carbon at concentrations within ranges typical of freshwaters and soil solutions. ...

Acid–Base Equilibria of Cysteine in Artificial SeaWater: Effect of Ionic Strength on the Basis of SpecificInteraction Theory†

Artificial sea water, containing NaCl, KCl, nCaCl n 2 n, MgCl n 2 n and nNa n 2 nSO n 4 n, was used as a standard medium in npotentiometric equilibrium studies of the sulfur-containing amino nacid cysteine: the dependence of activity coefficients on ionic nstrength, and thus salinities, is discussed according to ndifferent models based on the Specific Interaction Theory.