Avaliação da capacidade tamponante - um experimento participativo

The aim of this work is to show an experiment from which students can learn some of the main characteristics of buffer solutions. A mixture of some acid-base indicators, named as Yamada's indicator, can be used to estimate pH values in an acid-base titration of a buffer, with good approximation. In the experiment it is also possible to verify the relationship between the buffer capacity and the concentrations and the molar ratio of the components of a NH3 / NH4+ buffer solution. The shortage of experiments associated with the relative small importance given to many aspects of buffer solutions, is now explored with simplicity. In the proposed experiments, students prepare buffer solutions by themselves, calculate the pH, understand how acid-base indicators act and learn how buffer solutions work through graph constructed by sharing experimental data.

Studies on formation of chromium niobates

The precise nature of the reaction between chromium chloride and potassium niobate at specific pH levels 12.0, 10.8 and 7.6 has been studied by means of electrometric techniques involving pH and conductometric titrations. The well defined breaks and inflections in the titration curves provide cogent evidence for the formation and precipitation of chromium ortho-Cr2O3.3Nb2 O5, hexa-4Cr2O3.9Nb2 O5 and meta-Cr2O3.3Nb2 O5 niobates in the vicinity of pH 7.5, 6.8 and 5.6, respectively. Analytical investigations of the precipitates have also been carried out which substantiate the results of the electrometric study.

Uma abordagem robusta de cálculo aplicada a um sistema potenciométrico de medida de concentrações hidrogeniônicas em soluções aquosas

The data analyzed in this work were generated following the methodology developed by Molina et al.(J. Electroanal. Chem., 1979) for the calibration of a potentiometric system of measurement of hydrogen-ion concentrations resulting from neutralizations, at 25 ºC, of acidic or alkaline solutions at constant ionic strength (0.1 mol.l-1) held with NaClO4. The observed data present a serious deviation in relation to the mathematical model derived from the Nernst equation, for pH values ranging from 3 to 11, where pH=-log[H+]. We show that the minimization of the sum of the absolute values of the residuals gives estimates that are not influenced by outlying values.

Compostos organolantanídeos contendo o ligante pirazinamida: síntese, caracterização e atividade catalítica na polimerização do etileno

In this work we report the synthesis of some organolanthanide compounds which were identified as LnCl2Cp(PzA)2, Ln = Nd, Sm, Eu and Tb, Cp = cyclopentadienyl and PzA = pirazinamide, by elemental analyses, complexometric titration with EDTA, thermal analyses and IR spectra. Thermal analysis and infrared spectra indicated that the coordination of the pyrazinamide to the lanthanide ions was made by the O atom of the carbonyl group and by one or both N atoms of the pyrazinamide ring. This class of compound showed catalytic activity of ca. 4.0 to 6.4 kgPE molLn-1 h-1 bar-1, in ethylene polymerization, using methylaluminoxane as cocatalyst. The resulting polyethylene presented low crystallinity (20%).

Construção e aplicação de eletrodo de aço inoxidável UNS S31254 como eletrodo indicador em titulações condutimétricas

UNS S31254 SS electrodes have been built to substitute platinum in conductimetric titrations. The electrodes were tested in both acid-basic titration (chloridric acid and sodium hydroxide) and precipitation titration (sodium chloride and argentum nitrate as titrant). The practical application was exemplified from conductimetric tritations of HF ¾ HNO3 mixtures used in metalurgical industry to passivate stainless steels. The results were compared with those obtained using commercial platinum electrodes. The equivalent volumes obtained were comparable within 3% experimental error. Its application depends on the nature of electrolyte. These results have shown that stainless steel, less expensive than platinum (about three order of magnitude), can substitute platinum electrodes in routine analyses and didactic laboratories.

Determinación de cinc a microescala en un medicamento especifico

This work presents zinc determination in certain medicines that contain zinc oxide and zinc undecylenate. The technique consists of a spectrophotometric micro-scale titration, where EDTA is used as titrant, and xylenol orange as an indicator, in a medium adjusted to pH = 6 with acetic acid and sodium acetate. After each added portion of EDTA, the absorbance value is measured at a selected wavelength, in order to detect the end-point of the spectrophotometric titration. The results already obtained are satisfactory and promote student's interest. An additional contribution intends to propose the use of micro-scale techniques.

Utilização de um eletrodo de grafite-epóxi recoberto com [Zn(FEN)3][tetratris(4-clorofenil) borato]2 sensível a zinco(II) em meio 1,10-fenantrolina como eletrodo indicador em titulações potenciométricas de precipitação

The construction and analytical evaluation of a coated graphite-epoxy electrode sensitive to the zinc-1,10-phenantroline complex based on the [Zn(fen)3][tetrakis(4-chlorophenyl)borate]2 incorporated into a poly(vinylchloride) (PVC) matrix are described. A thin membrane film of this ion-pair, dibutylphthalate (DBPh) and PVC were deposited directly onto an electrically conductive graphite-epoxy support located inside a Perspex® tube. The best PVC polymeric membrane contains 65% (m/m) DBPh, 30% (m/m) PVC and 5% (m/m) of the ion-pair. This electrode shows a response of 19.5 mV dec-1 over the zinc(II) concentration range of 1.0 x 10-5 to 1.0 x 10-3 mol L-1 in 1,10-phenantroline medium, at pH 6.0. The response time was less than 20 seconds and the lifetime of this electrode was more than four months (over 1200 determinations by each polymeric membrane). It was successfully used as an indicator electrode in the potentiometric precipitation titration of zinc(II) ions.

O emprego de quitosana quimicamente modificada com anidrido succínico na adsorção de azul de metileno

The adsorption capacity of alpha-chitosan and its modified form with succinic anhydride was compared with the traditional adsorbent active carbon by using the dye methylene blue, employed in the textile industry. The isotherms for both biopolymers were classified as SSA systems in the Giles model, more specifically in L class and subgroup 3. The dye concentration in the supernatant in the adsorption assay was determined through electronic spectroscopy. By calorimetric titration thermodynamic data of the interaction between methyene blue and the chemically modified chitosan at the solid/liquid interface were obtained. The enthalpy of the dye/chitosan interaction gave 2.47 ± 0.02 kJ mol-1 with an equilibrium constant of 7350 ± 10 and for the carbon/dye interaction this constant gave 5951 ± 8. The spontaneity of these adsorptions are reflected by the free Gibbs energies of -22.1 ± 0.4 and -21.5 ± 0.2 kJ mol-1, respectively, found for these systems. This new adsorbent derived from a natural polysaccharide is as efficient as activated carbon. However 97% of the bonded dye can be eluted by sodium chloride solution, while this same operation elutes only 42% from carbon. Chitosan is efficient in dye removal with the additional advantage of being cheap, non-toxic, biocompatible and biodegradable.

Titulação potenciométrica aplicada na determinação de ácidos graxos livres de óleos e gorduras comestíveis

Free fatty acids are a measure of evaluating fats and oils, submitted to abusive conditions, besides being a quality characteristic of edible vegetable oils. The official method of determination (AOCS Ca 5a-40, 2004) is based on titration, using phenolphthalein as an indicator. For crude and degummed oils, the titration end point is difficult to be observed due to the high level of pigments, resulting in dark solutions. In this case and others, such as self-life studies, in which sample quantities are limited, potentiometric titration may be a very good alternative.

Complexation of enalapril maleate with beta-cyclodextrin: NMR spectroscopic study in solution

A detailed NMR (¹H , COSY, ROESY) spectroscopic study of complexation of enalapril maleate with beta-cyclodextrin was carried out. The ¹H NMR spectrum of enalapril maleate confirmed the existence of cis-trans equilibrium in solution, possibly due to hindered rotation along the amide bond. The cis-trans ratio remained almost the same in the presence of beta-cyclodextrin but in one case it was found significantly different which suggests a catalytic role of beta-cyclodextrin in the isomerization. ¹H NMR titration studies confirmed the formation of an enalapril-beta-cyclodextrin inclusion complex as evidenced by chemical shift variations in the proton resonances of both the host and the guest. The stoichiometry of the complex was determined to be 2:1 (guest: host). The mode of penetration of the guest into the beta-cyclodextrin cavity as well as the structure of the complex were established using ROESY spectroscopy.

Adsorção de fenol sobre carvão ativado em meio alcalino

The present study describes phenol adsorption on commercial active carbon (CAF) under alkaline conditions in the concentration range of 0.01 to 2.08 mmol L-1. Surface characterization has been performed by means of surface area measurements, IR spectroscopy and Boehm titration. The effect of temperature on the adsorption equilibrium isotherm was investigated at 23, 30, 40, 50 and 60 °C. The results showed that adsorption capacity decreased with increasing temperature. The adsorption kinetics and the role of surface characteristics on the adsorption of phenol also discussed.

TitGer: uma planilha eletrônica para simulação de titulação de mistura de compostos polipróticos

The aim of this paper is to present a simple way of treating the general equation for acid-base titrations based on the concept of degree of dissociation, and to propose a new spreadsheet approach for simulating the titration of mixtures of polyprotic compounds. The general expression, without any approximation, is calculated a simple iteration method, making number manipulation easy and painless. The user-friendly spreadsheet was developed by using MS-Excel and Visual-Basic-for-Excel. Several graphs are drawn for helping visualizing the titration behavior. A Monte Carlo function for error simulation was also implemented. Two examples for titration of alkalinity and McIlvaine buffer are presented.

Método titrimétrico para determinar fosfito em amostras agroindustriais

A new titrimetric method for the determination of phosphite in fertilizer samples, based on reaction of H3PO3 with standard iodine solution in neutral media, is proposed. Diluted samples containing ca. 0.4% m/v P2O5 are heated and titrated with 0.05 mol L-1 iodine standard until the solution becomes faint yellow. Back titration is also feasible: a slight excess of titrant is added followed by starch indicator and titration is completed taking as the end point the change in color from blue to colorless. The influence of chemical composition and pH of buffers, temperature and foreign species on waiting time and end-point detection were investigated. For the Na2HPO4/NaH2PO4 buffer (pH 6.8) at 70 °C, the titration time was 10 min, corresponding to about 127 mg iodine, 200 mg KI and 174 mg Na2HPO4 and 176 mg NaH2PO4 consumed per determination. Accuracy was checked for phosphite determination in seven fertilizer samples. Results obtained by the proposed procedure were in agreement with those obtained by spectrophotometry at 95% confidence level. The R.S.D. (n=10) for direct and back titration was 0.4% and 1.3% respectively.

Emprego do sal di-sódico de edta como padrão no preparo de soluções

The hydrated sodium salt of EDTA, Na2H2Y·2H2O, cannot be used as a primary standard for titrations due to uncertainties in the water content. An alkalimetric titration of the homogenized solid in the presence of a small excess of BaCl2·2H2O allows one to titrate quantitatively the released two hydrogen cations with end-point indication by phenolphthalein or potentiometry. This leads one to calculate the average molar mass of the reagent and its water content, allowing to use it to prepare EDTA standard solutions. One titrated sample led to the formula Na2H2Y·1.876 H2O, and 370.01 g.mol-1 for the average molar mass.

Titulações espectrofotométricas de sistemas ácido-base utilizando extrato de flores contendo antocianinas

Considering the attraction of the students' attention by the changes in the colors of vegetable crude extracts caused by the variation of the pH of the medium, the use of these different colors in order to demonstrate principles of spectrophotometric acid-base titrations using the crude extracts as indicators is proposed. The experimental setup consisted of a simple spectrophotometer, a homemade flow cell and a pump to propel the fluids along the system. Students should be stimulated to choose the best wavelength to monitor the changes in color during the titration. Since the pH of the equivalence point depends on the system titrated, the wavelength must be properly chosen to follow these changes, demonstrating the importance of the correct choice of the indicator. When compared with the potentiometric results, errors as low as 2% could be found using Rhododendron simsii (azalea) or Tibouchina granulosa (Glory tree, quaresmeira) as sources of the crude extracts.