Walace A. de Oliveira

Fiber-optic pH sensor based on Poly(o-methoxyaniline)

Abstract This work describes the development of optical sensors for the measurement of pH. They work by measuring the diffuse reflectance of a conductive polymer, in the visible range of the spectrum. The pH sensitive material was poly(o-methoxyaniline) doped by p-toluene sulfonic acid (PoAnis/TSA), which was immobilized either on or within a polymeric structure. Two optodes were constructed. The first was assembled using a bifurcated bundle of borosilicate optical fibers. The sensing phase was a polymeric composite made of PoAnis/TSA and cellulose acetate, and was fixed at the common end of the bifurcate bundle. This optode allowed measurements in the pH range from 4.9 to 10.5 with a precision of ±0.01 pH. The response time for 90% change was 5.0 min for a full pH variation from acid to basic, and 22 min for the reversal change. The sensor response depends on the ionic strength of the solution and on the supporting electrolyte used to adjust the ionic strength. The second optode was built using two cables of optical fibers, each one having a single plastic fiber with 1.0 mm diameter. The two cables were fixed in a Perspex block, together with the pH sensitive layer. The sensing phase of this optode consisted of a film of PoAnis/TSA adsorbed on polyethylene. This optode yielded measurements in a working range from pH 2.0–10.0 and with relatively faster response time for 90% change for a variation from basic to acid (5.0 min). It was verified that pH sensors based on poly(o-methoxyaniline) offer a wide dynamic range but suffer from long response times and interferences from both cations and anions.

Comparison of merging zones, injection of reagent and single-line manifolds for enthalpimetric flow injection analysis

Abstract A flow enthalpimeter suitable for all flow injection manifolds is described. The apparatus includes the injection valve immersed in an equilibration water bath which improves sensitivity and sampling rate. Detection limits for quantitation of hydrochloric acid range from 0.4 to 0.07 mM at 80–90 samples per hour.

MOLAR EXCESS GIBBS FREE ENERGIES OF MIXTURES OF ETHANOLAMINES AND WATER

Abstract Thermodynamic information about the vapor—liquid equilibria of systems containing water and ethanolamines is needed in the design and optimization of important industrial processes. This knowledge is also useful in the investigation of some phenomena such as the thermal conversion of ethanolamines or their complexation reactions with metals. Physico-chemical investigations involving aqueous solutions of ethanolamines are scarce, and sometimes contradictory, in the literature. It has been stated [1] that the system diethanolamine—water obeys Raoults law. However, there have been studies [2] indicating departure from ideal behavior for this system. The purpose of this work was to determine the molar excess Gibbs free energies of the systems formed by water and the ethanolamines.

Determination of calcium and magnesium in limestone and dolomite by enthalpimetric flow-injection analysis

A flow-enthalpimetric method for the determination of calcium and magnesium in limestone and dolomite is described. Calcium is determined by measuring the heat evolved from the substitution reaction with Mg-EDTA. The amount of magnesium is calculated from the calcium content and the enthalpimetric signal for the reaction of the sample solution with disodium-EDTA, which gives the total concentration of calcium and magnesium. Analysis of reference materials and other samples indicates an error not exceeding 0.3% of CaO and MgO, and a sampling rate of 100 samples per hour.

Uncertainty in the sensitivity of thermistor ebullioscopes

Boiling‐temperature elevations may be estimated from the ebullioscope sensitivity. Equations to calculate the sensitivity of twin‐thermistor ebullioscopes are given. Thermistor calibrations were performed and the data fitted to several equations. The standard errors in the parameters of these equations were calculated and the resulting uncertainty in the sensitivity derived. It is shown that data of moderate accuracy are sufficient to yield sensitivity values with a reliability of 0.3%. Differences in boiling temperatures obtained by this method are capable of a precision which is among the highest reported for ebulliometry.

Determination of ethanol using flow injection enthalpimetry.

A flow injection enthalpimetric method for the determination of ethanol in fuel-grade ethanol, based on the heat of dilution of ethanol, is described. The accuracy and precision were 0.05% of ethanol; interferences by several substances are discussed. A similar procedure was also applied to the determination of ethanol in wines and spirits after distillation, yielding a precision of 0.03–0.1% of ethanol. The sampling rate in both procedures is 110 h–1.

Determination of carbohydrates by direct injection enthalpimetry

An analytical method for the determination of carbohydrates is described, which is based on the enthalpimetric signal obtained at the beginning of the reaction with periodate. The apparatus is described and relevant aspects of the procedure are discussed. Single carbohydrates can be determined in a few minutes with a precision of 1%. Analysis of mixtures of glucose and fructose is reported.

Determination of water in ethanol and acetone by direct injection enthalpimetry based on the heat of dilution.

Two analytical procedures based on the magnitude of the heat of dilution and direct injection enthalpimetry have been developed for determination of water in ethanol and acetone. The results obtained by means of the heat of dilution were compared with the values given by pycnometry, and gave a relative difference in the range 0.2-3%. The precision depends on the slope of the calibration curve, which varies with the procedure and the concentration of the sample.

Determination of iron in iron ores using enthalpimetric flow injection analysis

A method is described in which a dissolved sample of iron ore is injected into a flow manifold, after passing through a silver reductor column. The sample then merges with a dichromate solution and generates an enthalpimetric signal due to the oxidation of iron(II). The precision of the proposed method is approximately 0.3% and the sampling rate is 85 samples per hour. Interference from vanadium was negligible and titanium did not interfere. The results of the analysis of samples of iron ores by the proposed method agree with those obtained by titrimetric analysis.

Ebulliometric behaviour of aqueous solutions of urea

Boiling-temperature elevations for aqueous urea solutions in the molality range 0 to 0.38 mol·kg−1 were measured with a thermistor ebulliometer. Osmotic and activity coefficients were calculated and show behaviour very close to ideality. The boiling-temperature elevations were used to calculate the dimerization constant of urea: (2.6±0.3) × 10−2 kg·mol−1 at 371.5 K and infinite dilution. The results are discussed in terms of urea association through dipole-dipole interactions and also of the “structure-breaking” effect.