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An electrochemical cell consisting of a flow-through yttria-stabilized zirconia (YSZ) sensor and a flow-through Ag/AgCl reference electrode has been employed to measure pH of high-temperature acidic sulphate solutions relevant to the pressure acid leaching (PAL) of nickel laterites. In a previous study, this cell was used to measure pH of H2SO4, Al2(SO4)3-H2SO4 and MgSO4-Al2(SO4)3-H2SO4 solutions at 250oC. In this work, the solutions range in complexity from the binary MgSO4-H2SO4, NiSO4-H2SO4, and Al2(SO4)3-H2SO4, through the ternary MgSO4-Al2(SO4)3-H2SO4 and NiSO4-Al2(SO4)3-H2SO4, to the PAL process solutions, whereas the temperature ranges from 200oC to 250oC. The measured and theoretical pH values typically agree within less than 0.1 pH unit and 0.2 pH units in synthetic solutions and PAL solutions, respectively. This is an improvement over the results of the previous study in synthetic solutions, which show differences between theory and experiment as high as 0.4 pH units. The conversion of measured potentials into pH values is based on the new mixed-solvent electrolyte (MSE) speciation model of the OLI Systems software calibrated independently based on solubility measurements. Both Henderson’s equation and the exact definition of the diffusion potential were employed in treating the obtained experimental data. Experimental pH values calculated using the diffusion potentials evaluated by either approach are essentially the same. This finding suggests that Henderson’s equation, which is based on readily available limiting ionic mobilities, can be effectively used. Lithium chloride is found to be a suitable alternative to sodium chloride as the reference electrode solution for the measurement of pH of aluminium-containing solutions, because it did not induce precipitation of aluminium as an alunite-type compound. The experimental results indicate that the high-temperature behaviour of Ni, Co and Mn sulphates can be satisfactorily approximated with that of MgSO4. The experimental findings also support the postulation that acid should be added to a PAL process so that the solution pH is around 1 at the leach temperature, regardless of the feed composition. The cell can be used for hydrometallurgical process research and development on a laboratory scale with very satisfactory performance, provided that a well-behaved YSZ sensor is available.

Class of 2009 Abstract / OBJECTIVES: The primary objective of this experiment was to characterize the pH stability range of 20% Pluronic lecithin organogel (PLO). We intended to determine the viscosity at varying pHs. We prepared six samples of 20% PLO. METHODS: An initial rheological reading of each sample was recorded by a dynamic stress rheometer. Each sample was titrated drop-wise with citric acid or KOH in 0.5 pH increments. When the desired pH was obtained, a 0.5 mL sample was analyzed with a dynamic stress rheometer, RS-200, using Rheos software. RESULTS: Since PLO is a non-Newtonian substance, viscosity changed relative to shear stress and we were not able to examine a correlation of pH with viscosity. Instead we inputted the data into Microsoft Excel® and plotted a shear stress versus viscosity curve for each sample to identify trends. CONCLUSIONS: We were unable to achieve our primary objective of determining the viscosity characterization of 20% PLO at varying pHs due to the non-Newtonian nature of the material. Subjectively, we determined the viscosity of 20% PLO is not substantially affected by pH. Other factors such as temperature, excess liquid, and surfactant ability may influence viscosity and need to be examined in the future.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Memoria para obtener el título de Química / Chile es uno de los diez mayores productores de vino en el mundo y el cuarto exportador a nivel global, alcanzando una producción de más de mil millones de litros en el año 2016. Para la elaboración de vinos de calidad, existen diversas variables de importancia tales como la temperatura de producción, los niveles de exposición al oxígeno, el pH, el contenido de compuestos fenólicos, metales, entre otros. De hecho, el vino (particularmente el vino tinto) es un producto reconocido por su alta concentración de compuestos fenólicos y capacidad antioxidante (CA), los que se vinculan no solo a determinadas propiedades fisico-químicas y sensoriales, si no que a los potenciales efectos beneficiosos sobre la salud derivados de su consumo moderado. En este trabajo, se realizó un análisis para evaluar la contribución relativa de diversos constituyentes del vino a la capacidad antioxidante. Las variables en estudio fueron: pH, hierro y concentración de ácido galico (AG) o catequina (CAT) o ácido cafeico (CAF) según correspondiera, y los resultados fueron analizados mediante un diseño Screening Factorial 23 con el objetivo de obtener la capacidad antioxidante por el método Ensayo de DPPH. Para cada solución modelo donde los compuestos fenólicos fueron AG y CAT, se observó que solo estos generaron una contribución significativa y positiva a la capacidad antioxidante, resultando no significativas las variables: pH, hierro y sus interacciones. En cambio, para la solución modelo conteniendo CAF se obtuvo que la concentración de Fe (III), fue la única variable significativa, con contribución positiva a la CA, probablemente debido a la formación de ácido caftárico, el cual podría coordinar al Fe (III). De acuerdo a estos últimos resultados, se realizó un cuarto experimento, donde se utilizó Fe (II) en vez de Fe (III), en las mismas condiciones del experimento anterior, y se obtuvo un resultado diferente: las variables pH, CAF y la interacción pH-Fe(II) resultaron significativas, la primera con contribución negativa y las dos restantes con contribución positiva. El pH podría afectar la cupla redox Fe(II)/Fe(III) y esto influir en la coordinación del ácido caftárico. Para comprobar el estudio anterior, se realizó otro experimento para evaluar la CA, pero en vino tinto real. Para este efecto, se utilizó un tratamiento con resina de intercambio catiónico para analizar un mismo vino a diferentes pH y concentración de hierro, de esta forma se obtuvo: un vino a pH 2,3 y ≈0 mg L-1 de hierro (Vino CR) y un vino a pH 3,4 y ≈1 mg L-1 de hierro (Vino SR). Mediante el método ORAC-FL, la CA inicial de ambos vinos disminuye en el tiempo, lo contrario se observó por el método DPPH, la CA aumentaba, probablemente por efecto de reacciones de copigmentación en el vino. Para estudiar el efecto del hierro se fortificó ambos vinos a una concentración de 5 mg L-1 de hierro (III), se obtuvo, inicialmente, que por ORAC-FL la capacidad antioxidante aumentó en comparación al vino sin dopar y, al transcurrir 21 dias ocurría lo mismo. Por DPPH la capacidad antioxidante de los vinos fortificados fue menor frente a los no fortificados y esto se mantuvo en el tiempo. Respecto al estudio del efecto de pH en el vino, los resultados demostraron que el pH no tenía mayor incidencia en la capacidad antioxidante por el método ORAC-FL, probablemente debido a que este implica el uso de buffer a pH 7,4. Mediante DPPH se observó un aumento de la CA para vinos SR y CR a medida que aumentaba el pH en el rango 3,0>pH>4,0 y, en el tiempo, sólo se observó un aumento de la CA en los vinos SR / Chile is one of the ten wine-producing countries in the world and the fourth of wineexporting countries, reaching a production of over a billion liters in 2016. In the production of quality wines there are many important variables such as oxigen exposure, pH, phenolic content, metals, among others. In fact, wine (red wine, particularly), is known for its high phenolic content and antioxidant capacity that are not just related to physicochemical and sensory properties but also to the potential beneficial effects on health derived from moderate consumption. In this research an analysis to evaluate the relative contribution of various constituents of the wine to the antioxidant capacity was performed. The variables under study were: pH, iron and gallic acid (AG) or catechin (CAT) or caffeic acid (CAF), and the results were analyzed through an Screening factorial 23 design with the goal to obtain the antioxidant capacity by DPPH. In the model wine solution with AG and CAT was observed that only these factors generated a significant and positive contribution to the antioxidant capacity resulting in non significant variables: pH, iron and their interactions. However, for the solution containing CAF, the Fe (III) concentration was the only significant variable, with a positive contribution to CA, probably due to the formation of caftaric acid, which could coordinate Fe (III). According to the latter results, a fourth experiment was performed, where Fe (II) was used instead of Fe (III), under the same conditions of the previous experiment, and a different result was obtained: pH, CAF and the pH-Fe (II) interaction were significant, the first with negative contribution and the remaining two with a positive contribution. The pH could affect the Fe (II) / Fe (III) redox couple and this may influence the caftaric acid coordination. To verify the previous study, another experiment was conducted to evaluate CA, but now in real red wine. For this purpose, a cation exchange resin treatment was used to obtain the same red wine at different pH and iron concentration: CR Wine (pH 2.3, ≈0 mg L-1 iron), SR Wine (pH 3,4; ≈1 mg L-1 iron). By ORAC-FL method, the initial CA of both wines decreased over time, on the other hand by the DPPH method was observed, the CA increased, possibly due to co-pigmentation reactions in wine. To study the iron effect, both wines were fortified at a concentration of 5 mg L-1 of iron (III). Initially by ORAC-FL it was found that the antioxidant capacity increased compared to non-fortified wine, after 21 days the same result was obtained. By DPPH method the antioxidant capacity of fortified wines was lower compared to nonfortified wines and this was maintained over time too. Regarding the pH study effect in wine, the results showed that the pH had no higher incidence in the antioxidant capacity by the ORAC-FL, probably because this method involves the use of buffer at pH 7.4. DPPH increased the antioxidant capacity for SR and CR wines as the pH increased in the range 3.0> pH> 4.0 and, over time, only an increase in AC was observed in SR wines

This thesis reports the design and development of novel voltammetric pH sensors for buffered, low-buffered and unbuffered media. pH sensors in stagnant and hydrodynamic environments were designed and developed for performing measurements using square wave voltammetry. Chapter 1 introduces the motivation of this project, the current development of electrochemical sensors, and the basic theory and techniques of electrochemistry concerned within the thesis. The existing development of carbon-based electrochemical sensors and the application of screen-printing technology in sensor fabrication are highlighted. Chapter 2 introduces the screen-printing technology and the fundamental methods of numerical simulation. In addition, reagents, equipment and software packages used in the thesis are listed in this chapter. In Chapter 3, a novel design of quinone derivative-based pH probes is presented for the application in stagnant weakly buffered media (< 1mM), based on previous studies of quinone compounds in buffered media. The results from the weakly buffered system is consistent with the results in buffered systems. To further extend the application of this design in unbuffered media, a numerical model of a pH-sensitive redox particle immobilised on an electrode was developed, which predicted that the accumulation of hydrogen ions near the electrode is the possible limiting factor for the performance of this design in unbuffered media. To develop a pH-monitoring technology for unbuffered media, Chapter 4 reports on the design, fabrication and testing of different electropolymerised-phenol derivative modified electrodes, which overcome the limitation of hydrogen ions accumulation. The results revealed that 2-(methylthio)phenol graphite resin electrodes have high accuracy (ca. 1% error) in unbuffered media, benchmarked by a commercial glass pH meter. This is the first detailed study on the v application of the economical and scalable technology in pH sensing in unbuffered environment. Chapter 5 presents a unique design of electrochemical pH sensors, free from the need to use a glass reference electrode. This design integrates a pH indicator and an internal reference electrode. Different designs of ferrocene screen-printed electrodes were tested as the internal reference electrode. The nafion-coated ferrocene screen-printed electrode showed stable peak potential in a wide pH range (pH 1 - 12) with good durability (stable in 500+ cycles of test). It was then cross connected with an alizarin electrode, forming the pH sensor free from a glass reference electrode. Chapter 6 describes novel designs of hydrodynamic pH sensors. The design of a microfluidic pH sensor modified by poly-sodium salicylate was firstly demonstrated. The sensor showed a Nernstian response in a wide pH range and, in hydrodynamic conditions, provided improved accuracy in unbuffered media compared to the stagnant state. For more convenient measurements, a novel rocking disc electrode was studied for pH sensing, modified with alizarin and poly-salicylic acid as pH indicators. The electrodes modified by both chemicals showed a Nernstian response in buffered media and the highest accuracy in unbuffered media was reached at 50 rpm.