Departamento de Química
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- Ionic interactions in phosphate aqueous solutionsPublication . Ferra, Maria Isabel Almeida; Amaro, Albertina; Mendonça, AntónioMean activity coefficients of NaCl have been calculated, by means of the Pitzer theory,in mixed solutions of NaCl and NaH2PO4 and of NaCl and Na2HPO4, with ionic strength lower than 2 mol kg-1, at 25 ºC. Electromotive force measurements, using a silver, silver chloride and a sodium ion selective electrodes, show nernstian response, in those mixed electrolyte solutions.
- Δ8 Labdanes from Halimium VerticillatumPublication . Urones, J. G.; Marcos, I. S.; Moro, R. F.; López Rodilla, Jesus Miguel; Mendonça, António José Geraldes deThree new acids in the form of methyl esters and the acetyl derivative of an alcohol have been isolated from Halimium verticillatum. Their structures have been established by spectroscopic methods and/or chemical correlations as those of methyl 15hydroxy-7-oxo-8-labden-17-oate, methyl 7a,l5-dihydroxy-glabden-17-oate, methyl 7/3,15-dihydroxy-8-labden-17-oate and 17-nor-15acetoxy-7-oxo-8-labdene.
- Application of the Pitzer theory to the evaluation of pH of the phthalate standard solutionPublication . Mendonça, António; Ferra, Maria I.The pH value of the potassium hydrogen phthalate 0.05 mol.kg-1 has been detemined, using the Bates-Guggenheim convention and lhe Pitzer model for the calculation of the chloride íon activity coefficient. A comparison has been made between lhe two approaches.
- Activity coefficients in the evaluation of food preservativesPublication . Mendonça, António; Vaz, Margarida I. P. M.; Mendonça, DinaFood preservatives are used in very low concentrations. However, usually they are used in aqueous high ionic strength solutions. The Pitzer model allows activity coefficient determination in these cases. Without the knowledge of activity coefficients, activity or ‘correct concentration’ cannot be known. In this work, the activity coefficients of sodium chloride, potassium chloride and calcium chloride in pure solutions were calculated by the Debye H¨uckel theory and the Pitzer theory. The calculated values were compared with experimental values. Activity coefficients of pure sodium benzoate in aqueous mixed solutions with potassium chloride or calcium chloride were evaluated using the Pitzer model. The same evaluation can be done to more complex systems once all Pitzer parameters values for the ionic species are known.
- Determination of Pitzer parameters for sodium benzoate at 298.15 KPublication . Mendonça, António; Vaz, Margarida; Ferra, Maria I.Sodium benzoate is a salt widely used in industry as preserver. Its activity coefficients in aqueous solutions can be estimated by Pitzer theory, once the necessary parameters are known. Electromotive force (emf) measurements on galvanic cells without liquid junction have been used for the determination o f mean activity coefficients of sodium chloride in the ternary system sodium chloride + sodium benzoate (NaB) + water. Pitzer theory has been applied to this system and those data enabled the evaluation of the specific parameters for sodium benzoate at 298.15 K by linear regression. The values obtained f o r / B(0) NaB and B(1) NaB parameters, respectively 0.177 kg mol-1 and 0.34 kg mol-1 are close to those published already for other carboxylic acid salts.
- A Introdução à Qualidade no Currículo de Química Analítica: Necessidades e MotivaçãoPublication . Mendonça, AntónioA Qualidade na sociedade moderna é uma exigência, não só às empresas mas também aos profissionais. Deste modo, as exigências colocadas aos novos licenciados vão para além dos seus conhecimentos de Qualidade adquiridos nos currículos clássicos de Química Analítica. Existem excelentes manuais de Química Analítica, como o de Valcárcel [l] ou o de Kellner et ai. [2], que colocam o tema da Qualidade numa posição importante. Infelizmente, nem sempre é possível efectuar o estudo aprofundado da Qualidade no currículo de Química Analítica, sobretudo quando os alunos não pertencem a licenciaturas em Química ou em variantes desta, dado que existem outras matérias a ser leccionadas e que não podem ser menosprezadas. Valcárcel [l] sugere diferentes conteúdos a leccionar de acordo com o nível de compreensão desejado, que ocuparão pelo menos, 20 a 30 horas, o que corresponderia na prática à ocupação quase que exclusiva de uma disciplina semestral. Neste trabalho, apresenta-se uma proposta para a inclusão no currículo de Química Analítica de , licenciaturas, não nucleares em Química, de uma Introdução à Qualidade. Partindo de casos práticos justifica-se a necessidade de normalização e daí progride-se no sentido da Qualidade em Química, figura l. Espera-se deste modo motivar os alunos para a importância da Química Analítica no âmbito da Qualidade.
- Activity coefficients of sodium benzoate and potassium benzoate in water at 298.15 KPublication . Mendonça, António; Cardoso, Cristina M. P.; Juusola, Pekka M.Industrial additives, like sodium benzoate and potassium benzoate, are used in low concentrations in aqueous solutions. However, in general, the ionic strength of the aqueous media is high. The Pitzer model allows the determination of activity coefficients in aqueous media. In this work, Pitzer parameters for sodium benzoate were recalculated (β(0)=0.162 mol−1 kg and β(1)=0.38 mol−1 kg) and Pitzer parameters for potassium benzoate were determined (β(0)=0.157 mol−1 kg and β(1)=0.27 mol−1 kg) from cell potential difference (cpd) measurements on galvanic cells without liquid junction. From these values, activity coefficients of potassium benzoate in water and in aqueous potassium chloride solutions were calculated by the Pitzer model. The activity coefficients were also calculated by modified Guggenheim equations with parameters obtained in this work and the results of the two models were compared.
- Activity coefficients of dipotassium phthalate and potassium hydrogen phthalate in water at 298.15KPublication . Mendonça, António; Juusola, Pekka M.Pitzer model parameters and modified Guggenheim equation parameters for dipotassium phthalate (K2PH) and potassium hydrogen phthalate (KHPh) were determined from cell potential difference (cpd) measurements on galvanic cells without liquid junction. Mean activity coefficients of these salts in water at 298.15 K were calculated by the Pitzer model and modified Guggenheim equations. The Pitzer parameters values obtained for K2Ph where: Beta 0(K2Ph)+steta(Cl,Ph) = 0.25 ± 0.01 mol−1 kg; Beta 1 (K2Ph)= 0.86 ± 0.08 mol−1 kg; C(K2Ph)= −0.010 ± 0.002 mol−2 kg2 and ΨK,Cl,Ph = 0.014 ± 0.005 mol−2 kg2. The Pitzer parameters values obtained for KHPh were: Beta0 (KHPh + steta(Cl,Ph)= 0.084 ± 0.013 mol−1 kg, = 0 mol−1 kg. The results were also compared to the values obtained from Pitzer parameters published in literature. The parameter values for the modified Guggenheim equations (bK,Ph = −0.041 mol−1 kg and bK,HPh = −0.020 mol−1 kg) obtained in this study were used in the mean activity coefficients calculations. The modified Guggenheim equation allows us to obtain similar mean activity coefficient values, as the Pitzer equation, up to 1.5 mol kg−1 ionic strength for K2Ph and 0.5 mol kg−1 for KHPh.
- Synthesis and characterization of novel fluorescent N-glycoconjugatesPublication . Esteves, Ana P.; Rodrigues, Lígia; Silva, Marília Elisabete Tavares Ferreira; Gupta, Sanjay; Campos, Ana Maria Ferreira Oliveira; Machalický, Oldřich; Mendonça, AntónioNovel fluorescent N-glycoconjugates containing D-glucose, glycine and coumarin or naphthalenetriazole derivatives were prepared by peptide synthesis type methods. The fluorescence properties (spectra, quantum yields) of the compounds were evaluated.
- Activity coefficients of aqueous Hydrochloric acid solutions up to a Molality of 6.0 mol kg-1 at temperatures from (5 to 45) °CPublication . Juusola, Pekka M.; Partanen, Jaakko; Vahteristo, Kari P.; Mendonça, António; Minkkinen, Pentti O.Simple three parameter activity coefficient equation originating from the DebyeHückel theory was used for the calculation of activity coefficients of aqueous hydrochloric acid at temperatures 5-45 °C up to a molality of 6 moI kg-I. N ew data from galvanic ce1ls without a liquid junction were used in the parameter estimations of this equation. These data consist of sets of measurements in the molality range 0.3-6.5 moI kg-I at temperatures 5, 15, 25, 35, and 45°C. It was observed that alI estimated parameters folIow very simple equations with respect to temperature. They are either constant or depend linearly on the temperature. The activity coefficient equation was tested with the celI potential difference data of Hamed and Ehlers (1933), ElIis (1916), Ákerlõf and Teare (1938), and Faita and Mussini (1964). It was observed that the most important data in these sets can be predicted welI by means of the activity coefficient equation. The activity coefficients obtained using the new equation were also compared to those obtained by means of the two-parameter Hückel and Pitzer equations presented by Partanen and Covington (2002) up to molality 2.0 moI kg-t, to those obtained by means of the Pitzer equations presented by Saluja et ai. (1986) and Holmes et ai. (1987), and to those obtained by means of the equation presented by Hamer and Wu (1972) for the temperature of 25°C.