André Fernando Oliveira

Asynchronous merging zones system: spectrophotometric determination of Fe(II) and Fe(III) in pharmaceutical products

An asynchronous merging zones was proposed with simultaneous introduction of the sample and of the modifier reagent, ascorbic acid in the studied case, for sequential determination of Fe(II) and Fe(III) in pharmaceutical products. The sample and reagent attained a merging point at different times owing to the use of a delay reactor in the reagent channel. By inserting a large sample and controlling the dispersion in the flow system, Fe(II) and Fe(III) were sequentially measured in the front and in the rear of the sample zone, respectively. The results obtained for Fe redox speciation in pharmaceutical products are in agreement at a 95% confidence level with those obtained with a manual spectrophotometric procedure. The analytical frequency with the proposed flow analysis system was 54 samples h(-1), i.e. 108 determinations h(-1).

Evaluation of a high sensitivity PbO2 pH-sensor

In this paper, a new high sensitivity potentiometric pH-electrode with a response in the acid region is proposed. It consists of a PbO(2)-paraffin matrix deposited on graphite. Its simple construction and studies of some variables at 0.5molL(-1) ionic strength are reported. A direct relationship between the electric potential difference and the solution pH was observed for pH values ranging from 1.2 to 7.5. A slope greater than 100mV/decade and a conditional electric potential of about 1250mV were obtained. The results presented a high correlation with those from a conventional glass pH-electrode in complex matrix samples.

Flow injection spectrophotometric determination of reducing sugars using a focalized coiled reactor in a domestic microwave oven.

A flow injection (FI) spectrophotometric procedure for determining reducing sugars content in sugar cane juices using a focalized PTFE coiled reactor positioned at the output antenna of a domestic microwave oven at 700 W is proposed. In this system, sample solution converge to 1.0 mol l(-1) NaOH and 5.2 mmol l(-1) K(3)Fe(CN)(6) solutions previously mixed and the decrease of hexacyanoferrate(III) concentration was monitored at 420 nm. Under best analytical conditions, there was a direct relationship between absorbance decrease and reducing sugar content (fructose plus glucose concentrations) in the concentration range from 50 to 1200 micromol l(-1) with a detection limit of 15 micromol l(-1). The relative standard deviations (rds) were less than 1.4% for ten injection of 400 and 800 micromol l(-1) fructose solution and the analytical frequency was 70 h(-1). A paired t-test showed that all results obtained for sugar cane juices using this FI procedure and the Somogyi-Nelson batch procedure agree at the 95% confidence level.

Focused-microwave-assisted reaction in flow injection spectrophotometry: a new liquid–vapor separation chamber for determination of reducing sugars in wine

The usefulness of an unsegmented liquid/vapor phases flow injection system for determining reducing sugars in wines using a focalized poly(tetrafluoroethylene) (PTFE) coiled reactor positioned at the output antenna of a domestic microwave oven at 700 W is demonstrated. In this system, sample or reference solution is mixed to 2.5 mol l(-1) sodium hydroxide and 6.8 mmol l(-1) potassium hexacyanoferrate(III) solutions previously merged in a confluence point and the reducing sugars (glucose plus fructose) were oxidized by potassium hexacyanoferrate(III) solution into the PTFE coiled reactor positioned in the microwave oven cavity. After phases separation at a proposed separation chamber (SC), the decrease of K(3)Fe(CN)(6) concentration in the liquid phase was measured at 420 nm. Using this SC, a favorable dispersion profile and a decrease of hydrodynamic pressure were observed. The flow procedure presented similar sensitivities to glucose and fructose in the experimental conditions adopted, allowing the determination of the reducing sugars content (fructose plus glucose concentrations) in the concentration range from 40 to 400 mumol l(-1) with a detection limit of 9 mumol l(-1). The RSD was 1.9% for a 120 mumol l(-1) fructose solution (n=12) and the analytical frequency was 54 h(-1). The results obtained for reducing sugars in several wines using the proposed flow system and those obtained using the Somogyi-Nelson batch procedure were in agreement at the 95% confidence level.

Estudo da adsorção de brometo de etídeo em resina XAD-7

The adsorption of ethidium bromide on XAD-7 resin was studied. The Freundlich model was the most representative isotherm model to describe the sorption behavior. A solid-liquid equilibrium model was proposed to explain the resin mass influence on the sorption. The equilibrium constant value estimated was 2.31. The results showed an ethidium bromide ion-pair physical adsorption, with adsorption enthalpy equals to -19.33 kJ/mol. A pK2 value equals to 4.69 ± 0.01 was estimated by two distinct methods. The results will be applied to the ethidium bromide preconcentration aiming its decomposition.

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.

Turbidimetric determination of orthophosphate in digested plant material by flow-injection analysis

A simple, fast, and low-cost flow-injection system is proposed for the determination of orthophosphate in digested plant material. The determination is based on the precipitation reaction of orthophosphate with 0.05 mol L−1 zinc solution in 0.05 mol L−1 triethanolamine buffer (TEA/TEAH+, pH 6.0) solution containing 0.025% (w/v) poly-(vinylalcohol). The zinc orthophosphate suspension produced was directly measured at 410 nm. Under the best experimental conditions, orthophosphate was determinated in the concentration range from 5 to 60 mg P L−1, and the relative standard deviations were less than 1.6% for 10 and 20 mg P L−1 solutions (n = 9). The analytical frequency was 180 h−1. The results obtained using the proposed FIA procedure and those obtained using a standard spectrophotometric procedure agree at the 95% confidence level.

Coated-carbon rod ion-selective electrode for the determination of niobium in citric medium

Abstract The construction and analytical evaluation of a coated-graph i te niobium ion-selective electrode, based on the ion-pair-between [NbO(Citr)2]3− anion and tricaprylylmethylanunoni um cation (Aliquat 336S) in poly (vinylchloride) matrix, are described. The influence of membrane composition, organic acid concentration and foreign ions were investigated. The electrode shows a Nernstian response over the niobium concentration range 2.0 × 10−5 − 7.9 × 10−3 M, with good selectivity, precision and a useful lifetime of six months (over 800 determinations) in citric acid. Application of this sensor for niobium determination in columbite is reported.

Desenvolvimento de um titulador baseado na contagem de gotas

TitDrop II, a practice low-cost titrator based on drop counting is proposed. A microcomputer counts drops in an opto-switch and it receives pH values from a commercial pHmeter with RS-232 serial port. The volume of drops must kept constant and the amount of base in each drop is obtained by standardization in the titrator. A linear behavior between height of reservoir of titrant and drop frequency was observed, but there is no influence of low height on drop volume. Acetic acidity in samples of vinager was compared with volumetric titration, and deviation between -2.6 to 3.9% was observed.