Acid-base potentiometric titration using a stainless steel electrode without oxidative treatment.

An AISI 304 stainless steel laminar electrode without oxidative treatment was investigated for the potentiometric titration of hydrochloric acid with sodium hydroxide. The proposed electrode was obtained from metalworking cuttings. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray fluorescence spectroscopy and X-ray diffraction were used to study the surface morphology and chemical composition of the electrode. The electrode showed a sensitivity of 59.18 ± 0.37 mV/pH, which was reproducible under intermediate conditions. Potentiometric titration showed a curve with deviations from pH 9.5 with respect to the glass electrode. However, this did not affect the quantification as the jumps of the curves coincided. The endpoint was 9.25 mL for both electrodes and the hydrochloric acid concentration was 0.0845 mol/L, with a deviation of 0.0004 mol/L from the standard concentration of 0.0841 mol/L. The nonartificially oxidised electrode did not show any crystalline oxide phases, whereas after oxidation it showed semicrystalline phases of iron and chromium oxides and increased the crystallinity of the steel. Despite the low content of surface oxides, stainless steel electrodes can give a Nernstian response to pH, depending on the surface characteristics of the material. This leads to the need to calibrate any electrode prior to oxidative treatment to rule out a Nernstian response without surface modification.

Quantification of total bioactive contents and evaluation of the antioxidant and antibacterial activities of crude extracts from Ephedra altissima Desf

The present work describes the in vitro biological activities of the crude extracts (petroleum ether, ethyl acetate and n-butanol) prepared from the species Ephedra altissima Desf. The estimation of total phenolic, flavonoid and tannin contents were carried out using the Folin-Ciocalteu, trichloroaluminum and acidified vanillin methods, respectively. The evaluation of the in vitro antioxidant activities were performed by three different methods namely: scavenging of the free radical ABTS, permanganate reducing antioxidant capacity, and potentiometric assay. In addition, the antibacterial activity was assessed by the agar disk diffusion assay against seven bacterial strains. The results of the phytochemical screening revealed the presence of several types of secondary metabolites. The EtOAc extract exhibited the highest content of phenols (125.62 ± 1.51 & 956;g EGA mg-1 of extract). The greatest flavonoid and tannin contents were observed for n-BuOH extract (19.18 ± 0.39 µg EQ mg-1 of extract and 8.95 ± 1.70 & 956;g EC mg-1 of extract, respectively). Moreover, the EtOAc extract revealed potent antioxidant activity in all the tested methods. Furthermore, the aqueous extract from the species E. altissima showed a good ability to reduce iron III to iron II with a value of 0.68 ± 0.3 moL eq L-1 in potentiometric assay. All the crude extracts (PE, EtOAc and n-BuOH) displayed inhibition of bacterial growth against at least three strains with values of MIC ranging from 3.125 to 50 µg mL-1. Therefore, these results suggest that Ephedra altissima could be used as an important source of natural bioactive compounds with antioxidant and antibacterial properties.

Quantification of total bioactive contents and evaluation of the antioxidant and antibacterial activities of crude extracts from Ephedra altissima Desf

The present work describes the in vitro biological activities of the crude extracts (petroleum ether, ethyl acetate and n-butanol) prepared from the species Ephedra altissima Desf. The estimation of total phenolic, flavonoid and tannin contents were carried out using the Folin-Ciocalteu, trichloroaluminum and acidified vanillin methods, respectively. The evaluation of the in vitro antioxidant activities were performed by three different methods namely: scavenging of the free radical ABTS, permanganate reducing antioxidant capacity, and potentiometric assay. In addition, the antibacterial activity was assessed by the agar disk diffusion assay against seven bacterial strains. The results of the phytochemical screening revealed the presence of several types of secondary metabolites. The EtOAc extract exhibited the highest content of phenols (125.62 ± 1.51 & 956;g EGA mg-1 of extract). The greatest flavonoid and tannin contents were observed for n-BuOH extract (19.18 ± 0.39 µg EQ mg-1 of extract and 8.95 ± 1.70 & 956;g EC mg-1 of extract, respectively). Moreover, the EtOAc extract revealed potent antioxidant activity in all the tested methods. Furthermore, the aqueous extract from the species E. altissima showed a good ability to reduce iron III to iron II with a value of 0.68 ± 0.3 moL eq L-1 in potentiometric assay. All the crude extracts (PE, EtOAc and n-BuOH) displayed inhibition of bacterial growth against at least three strains with values of MIC ranging from 3.125 to 50 µg mL-1. Therefore, these results suggest that Ephedra altissima could be used as an important source of natural bioactive compounds with antioxidant and antibacterial properties.(AU)

Control of pH-Responsiveness in Graphene Oxide Grafted with Poly-DEAEMA via Tailored Functionalization.

Polymer-grafted nanomaterials based on carbon allotropes and their derivatives (graphene oxide (GO), etc.) are typically prepared by successive reaction stages that depend upon the initial functionalities in the nanostructure and the polymerization type needed for grafting. However, due to the multiple variables involved in the functionalization steps, it is commonly difficult to predict the properties in the final product and to correlate the material history with its final performance. In this work, we explored the steps needed to graft the carboxylic acid moieties in GO (COOH@GO) with a pH-sensitive polymer, poly[2-(diethylamino)ethyl methacrylate] (poly[DEAEMA]), varying the reactant ratios at each stage prior to polymerization. We studied the combinatorial relationship between these variables and the behavior of the novel grafted material GO-g-poly[DEAEMA], in terms of swelling ratio vs. pH (%Q) in solid specimens and potentiometric response vs. Log[H+] in a solid-state sensor format. We first introduced N-hydroxysuccinimide (NHS)-ester moieties at the -COOH groups (GO-g-NHS) by a classical activation with N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide (EDC). Then, we substituted the NHS-ester groups by polymerizable amide-linked acrylic moieties using 2-aminoethyl methacrylate (AEMA) at different ratios to finally introduce the polymer chains via radical polymerization in an excess of DEAEMA monomer. We found correlated trends in swelling pH range, interval of maximum and minimum swelling values, response in potentiometry and potentiometric linear range vs. Log[H+] and could establish their relationship with the combinatorial stoichiometries in synthetic stages.

Técnicas de titulación ácido-base: consideraciones metrológicas / Acid-base titration techniques: metrological considerations / Técnicas de titulação ácido-base: considerações metrológicas

Resumen Se evaluaron diferentes sistemas de medición para titulaciones ácido-base, con el propósito de determinar cuál presenta mejores características metrológicas. Estos sistemas incluyen volumetrías y gravimetrías, las cuales se combinaron con diferentes métodos de detección del punto de equivalencia: (i) potenciometría, (ii) indicador ácido-base y (iii) conductividad electrolítica. Este artículo propone una nueva aproximación para la estimación de la incertidumbre de medición asociada a la detección del punto de equivalencia. Los resultados indicaron que, para la determinación del punto de equivalencia, las incertidumbres más bajas se obtienen mediante el método potenciométrico en comparación con el método conductimétrico cuyo sesgo es hasta 78 veces mayor. Sin embargo, se encontró que el método potenciométrico es fuertemente dependiente de la resolución del instrumento y/o la cantidad de agente titulante que se adiciona en la cercanía al punto de equivalencia. También se encontró que la incertidumbre disminuye cerca del 40% empleando métodos gravimétricos en comparación con los volumétricos. Al realizar la evaluación del sesgo de los sistemas de medición acorde con la guía ISO 33:2015, los resultados mostraron que, de los métodos evaluados, el método gravimétrico con detección potenciométrica presentó las mejores características metrológicas, pues se obtuvieron sesgos e incertidumbres menores.

Abstract Different measurement systems for acid-base titrations were evaluated to determine which one had the best metrological characteristics. These systems included volumetric and gravimetric titrations, which were combined with different detection systems: (i) potentiometry, (ii) acid-base indicator, and (iii) electrolytic conductivity. This article proposes a new approach for estimating the measurement uncertainty associated with the detection of the equivalence point. The results showed that, for the determination of the equivalence point, the lowest uncertainties were obtained by the potentiometric method in comparison with the conductometric method, whose bias was 78 times higher. The potentiometric method, however, was strongly dependent on the instrument resolution and/or the titrant amount that is added close to the equivalence point. It was also found that uncertainty decreases about 40% using gravimetric methods as compared to volumetric methods. When performing bias evaluation of the measurement systems according to the ISO 33:2015 guide, results showed that, among the evaluated methods, the gravimetric method with potentiometric detection presented the best metrological characteristics, with the lowest biases and uncertainties.

Resumo Foram avaliados diferentes sistemas de medida para titulações ácido-base, a fim de determinar quais possuem as melhores características metrológicas. Estes sistemas incluem titulações e gravimétricas, que foram combinadas com diferentes sistemas de detecção do ponto de equivalência: (i) potenciometria, (ii) indicador ácido-base e (iii) condutividade electrolítica. Este artigo propõe uma nova abordagem para estimar a incerteza de medição associada à detecção do ponto de equivalência. Os resultados indicaram que, para a determinação do ponto de equivalência, as incertezas mais baixas são obtidas pelo método potenciométrico enquanto que a incerteza do método condutométrico é até 78 vezes maior que a obtida pela potenciometria. Além disso, a potencimetria é altamente dependente da resolução do instrumento e/ou da quantidade de titulante para ser adicionado na proximidade com o ponto de equivalência. Também verificou-se que a incerteza diminui em torno de 40% utilizando métodos gravimétricos. De acordo à guia de norma ISO 33:2015, os resultados mostraram que os métodos de titulação ácido-base avaliados gravimetricamente com detecção potenciométrica proporcionaram as melhores características de metrologia, porque obtiveram os menores distorções e incertezas.

Label-free potentiometric biosensor based on solid-contact for determination of total phenols in honey and propolis.

We developed a label-free potentiometric biosensor using tyrosinase extracted from Musa acuminata and immobilized by covalent bond on a surface of a solid-contact transducer. The transducer was manufactured containing two layers. The first layer contained a blend of poly(vinyl) chloride carboxylated (PVC-COOH), graphite and potassium permanganate. On this layer, we deposited a second layer containing just a mixture of poly(vinyl chloride) carboxylated and graphite. On the last layer of the transducer, we immobilized the tyrosinase enzyme by reaction with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride. The solid-contact potentiometric biosensor presented at low detection limit of 7.3×10-7M and a linear range to catechol concentration between 9.3×10-7M and 8.3×10-2M. This biosensor was applied to determine the amount of total phenols in different samples of honey and propolis. The results agreed with the Folin-Ciocalteu method.

Determination of fluoride at low concentrations: validation of method using ion selective electrode to analyze the water used in the preparation of dialysis solutions / Determinação de fluoreto em baixas concentrações: validação de método com eletrodo íon seletivo para análise da água utilizada na preparação de soluções de diálise

This paper describes the development and validation of an analytical methodology for determining the concentration of fluoride in water used for the preparation of dialysis solutions, by means of potentiometry with ion selective electrode. The investigated validation parameters were: selectivity, homoscedasticity, linearity, limit of detection, limit of quantification, reliability of measurement and precision. The optimized conditions of analysis were: HOAc/-OAc/NaCl/CDTA buffer (pH = 5.0 ± 0.1), in ratio of 10:1 (sample/buffer); concentrations of the standard solutions in the analytical curve: 0.05-0.80 mg/L. The evaluated method exhibited suitable validation parameters with detection and quantification limits equal to 0.020 and 0.050 mg/L, respectively. Also, an electronic spreadsheet was developed and validated for monitoring the quality of analytical curve of the methodology that calculates the decision limit to 0.20 mg/L.(AU)

Este trabalho descreve o desenvolvimento e validação de metodologia analítica para determinar a concentração de fluoreto em água empregada para a preparação de soluções de diálise, por meio de potenciometria com eletrodo íon seletivo. Os parâmetros de validação investigados foram: seletividade, homoscedasticidade, linearidade, limite de detecção, limite de quantificação, veracidade de medição e precisão. As condições otimizadas de análise foram: tampão HOAc/-OAc/NaCl/CDTA (pH = 5,0 ± 0,1), na proporção 10:1 (amostra/tampão); concentrações das soluções-padrão da curva analítica: 0,05 a 0,80 mg/L. O método avaliado exibiu parâmetros de validação adequados com limites de detecção e de quantificação, respectivamente, de 0,020 e 0,050 mg/L. Ademais, foi também desenvolvida e validada uma planilha eletrônica para efetuar o monitoramento da qualidade da curva analítica do método que calcula o limite de decisão para 0,20 mg/L.(AU)

Measuring the antioxidant capacity of blood plasma using potentiometry.

The use of potentiometry to measure plasma antioxidant capacity to contribute to oxidative stress evaluation is presented. In this assay, plasma (n=60) diluted (0.3 to 1 ml) in phosphate buffer, pH 7.4, NaCl 9%, was submitted to potentiometry. A platinum wire was the working electrode and saturated calomel the reference. The results are presented as the difference between sample and buffer potential (ΔE). ΔE presented a good inverse correlation with added increasing concentrations of ascorbate (2.5-75 µmol/L; R=-0.99), urate (9.0-150 µmol/L; R=-0.99), and bilirubin (0.78-13 µmol/L; R=-0.99). Increase in the antioxidant capacity decreased ΔE. Depletion of the antioxidant capacity by tert-butylhydroperoxide (6.5-50 µmol/L) presented a direct correlation (0.97) with ΔE. Furthermore, ΔE presented an inverse correlation (R=-0.99) with increased antioxidant capacity of plasma (FRAP) induced by the addition of ascorbate (2.5-75 µmol/L). The response of the potentiometric method proved be adequate for measuring the plasma antioxidant depletion induced by acute exhaustive exercise in rats (control, n=15; exercised, n=15). This exercise decreased the concentration of urate (p<0.05), decreased FRAP (p<0.5), increased TBARS (p<0.5), and decreased the potentiometer sensor response (p=6.5×10⁻³). These results demonstrate the adequacy of potentiometry for evaluating the antioxidant capacity of blood plasma samples.

Consumo infantil de bebidas lácteas: sólidos solúveis totais (Brix) e pH / Child's consumption of drinks milk: total soluble solids (Brix) and pH

O objetivo deste estudo foi o de avaliar a quantidade de Sólidos Solúveis Totais (SST) e o valor do pH em bebidas lácteas (iogurtes e achocolatados), comercializadas em Teresina e consumidas por crianças. Foram avaliadas 20 bebidas lácteas por meio do experimento casualizado com 3 repetições para cada amostra. As leituras do grau Brix foram feitas por refratometria, utilizando-se o refratômetro de Abbé, e o pH foi avaliado por potenciometria. O teor médio de SST variou de 11,0 Brix a 22,0 Brix. Com relação ao pH, os valores médios mínimo e máximo para as bebidas lácteas foram 3,79 e 6,71, respectivamente. Por meio dos resultados obtidos, pôde-se concluir que a elevada concentração de SST, verificada nas bebidas lácteas, associada a um baixo pH, pode contribuir para o desenvolvimento de lesões de cárie e erosão dental, caso esses alimentos sejam consumidos, em excesso, pelas crianças.

The main objective of this study is to proceed an evaluation of the amount of total soluble solids (TSS) and the pH value in lactic drinks (yogurt and chocolate milk) commercialized in Teresina and consumed by children. Twenty lactic drinks were evaluated in this experiment with 3 repetitions of study case in each sample. The Brix degrees measurement was made by refractometry using an Abbé refractometer and the pH was evaluated using potentiometry. The medium content of TSS was in a range between 11,0 ºBrix and 22,0 ºBrix. Meanwhile the minimum maximum pH the medium values were 3,79 and 6,71. From this results what can be concluded is that a high concentration of TSS can be verified in lactic drinks, associated with a low pH may contribute to the development of cavity lesions and dental erosion in case of the excess consumption of this products by children.

Caracterizacióndde ácidos húmicos y fúlvicos de un mollisol bajo dos coberturas difreentes / Charaterization of humic and fulvic acids from a mollisol under two diferent coverlands

Para evaluar y comparar algunas propiedades de los ácidos húmicos (AH) y fúlvicos (AF) como el grado de condensación y polimerización, el contenido total de carbono, hidrógeno y nitrógeno, y sus constantes de acidez, se analizó la fracción menor a 50 µm de un Mollisol del horizonte Ap, bajo caña de azúcar, y se comparó con un horizonte A, bajo bosque. Para tal fin se hicieron extracciones sucesivas con tetraborato de sodio 0,05 M, pirofosfato de sodio 0,025 M e hidróxido de sodio 0,1M. Las sustancias húmicas se purificaron mediante tratamientos con ácidos inorgánicos en solución al 1% de HCl y HF, resinas de absorción Amberlita XAD-16 e intercambio catiónico R101H+, diálisis y liofilización, y se caracterizaron por análisis elemental, relación E4/E6 y potenciometría ácido-base de acuerdo con las funciones de Gran modificadas. La relación E4/E6 permitió establecer mayores tamaños moleculares y grado de condensación de los ácidos húmicos en relación con los ácidos fúlvicos. Por potenciometría ácido-base se determinaron, en los ácidos húmicos y ácidos fúlvicos, las concentraciones de grupos funcionales ácidos y sus respectivas constantes de acidez. Dos de éstas, tituladas entre pH 4,0 y 6,1 corresponden a grupos carboxílicos, las otras dos entre pH 7,2 y 8,7 son el resultado de grupos -OH fenólicos fuertes y carboxílicos débiles. En el horizonte Ap, bajo caña de azúcar, los resultados demostraron que las prácticas de uso y manejo de este cultivo han llevado a la homogeneización de las sustancias húmicas, mientras que en el horizonte A bajo bosque existen ácidos húmicos y fúlvicos diferenciados por tamaño molecular, aromaticidad y grado de humificación.

In order to determine and to compare some physicochemical properties of humic (HA) and fulvic (FA) acids such as degree of condensation and polymerization, the total C,H,N content and pKa values were analyzed in the smaller fraction (< 50 m) from Ap horizon under sugarcane crop in opposition to A horizon under forest, both Mollisol order. To carry out this aim, successive extractions with sodium tetraborate 0.05 M, sodium pyrophosphate 0.025 M, and sodium hydroxide 0.1 M were made. The purification of these humic substances by treatment with inorganic acids 1% solutions of HCl and HF, Amberlite XAD-16 adsorption resin and R 101H+ cationic exchange, dialysis and freeze driying were done. The humic and fulvic acids were characterized by elemental analysis, E4/E6 relation and acid-base potentiometry. The ratio E4/E6 allows to establish that molecular size and degree of condensation in humic acids were bigger than in fulvic acids. By means of acid-base potentiometry, the concentrations of functional acid groups and their acid constants in the humic and fulvic acids were determined. Four constants were found; first and second between pH 4.0 and 6.1 correspond to differents kinds of carboxyl groups and another two between pH 7.2 and 8.7 come from strong O-H phenolic and weak carboxyl groups. In the Ap horizon under sugarcane the characterization showed that the handling and land use practices promote the homogeneity of humic substances meanwhile in the A horizon under forest; the humic and fulvic acids are different due to their degree of condensation molecular size and humification index.