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A spectrophotometric enzymatic flow injection (FI) system for the determination of diethyl-p-nitrophenylphosphate (paraoxon) is proposed. The method was based on the determination of the acetic acid formed by the enzymatic reaction of the acetylcholinesterase, immobilized on glass beads, with the substrate acetylcholine. The acetic acid formed permeates through a PTFE membrane and is received by a solution (pH 7.0) containing the acid-base indicator Bromocresol Purple (B.C.P.), leading to a pH change and therefore to a color change. The variation of the absorbance of the solution is detected spectrophotometrically at 400 nm. The determination of paraoxon is related to its inhibitory action on the enzyme. Therefore the analytical signal is the difference between the signal that corresponds to the free and the one that corresponds to the inhibited enzyme, considering a fixed acetylcholine concentration. The correlation between the peak height and paraoxon concentration at a given acetylcholine concentration is linear in the range from 5.0 x 10(-7) mol L-1 to 5.0 x 10(-5) mol L-1 (r = 0.998) of paraoxon, with a relative estimated standard deviation (R.S.D.) of +/- 1.7% (n = 10) considering a solution containing 5.0 x 10(-6) mol L-1 of paraoxon and a solution containing 5.0 x 10(-3) mol L-1 of acetylcholine. Therefore, the quantitative limit detection is about 2.5 x 10(-7) of paraoxon (3 sigma). A 1,1'-trimethylene-bis(4-formylpyridinium bromide)dioxime (TMB-4) solution was used to reactivate the enzyme.

A linearization method based on modified Gran functions, and a general nonlinear regression program were used to study potentiometric titration curves of denatured ovalbumin and lysozyme in 6 mol L–1 guanidine hydrochloride medium with the aim of determining the ionizable species. With both numerical techniques it was possible to determine the sum of the carboxylic groups, the imidazol, the α-amine, and the sum of ɛ-amine, phenolic and sulfhydryl groups, if the protein is completely denatured, and assumes a randomly coiled conformation. A total of 87.8 ± 2.5 and 20.7 ± 0.6 groups per mol were determined in the ovalbumin and lysozyme, respectively. These values are very close to the 88 and 21 groups expected by aminoacid composition of both proteins, indicating that all ionizable groups were exposed to the solvent. For ovalbumin the distribution of groups was very similar to that expected by the aminoacid composition, but for lysozyme some anomalies were observed, suggesting the existence of interactions between ionizable groups, altering the dissociation constants.

A flow-injection conductimetric system procedure is proposed for the determination of paraoxon. The method is based on the inhibition of immobilized acetylcholinesterase in controlled porous glass beads (CPG) with acetylcholine as substrate. The enzymatic reaction is inhibited by a diethyl-p-nitrophenylphosphate (paraoxon) solution. A 1, 1′-trimethylene-bis(4-formylpyridinium bromide)dioxime solution was used to reactivate the enzyme. The correlation between the peak height, for a given acetylcholine concentration, is linear from 1.0×10-7 to 5.0×10-5mol l-1 of paraoxon; therefore, the quantitative limit of detection was about 1.0×10-7mol l-1 of paraoxon. For semi-quantitative purposes, the limit can be considered to be about 1.0×10-8mol l-1. The relative estimated standard deviation obtained using a 1.0×10-3mol l-1 acetylcholine solution and a 5.0×10-6mol l-1 paraoxon solution was 2.2% (n=8).

A biosensor constructed by combining an oxygen electrode with the liquid ejected by bombardier beetles (Pheropsophus aequinoctialis) immobolized on a collagen membrane is developed for hydrogen peroxide. The sensor presents a linear range of 2.0 x 10(-4)-2.0 x 10(-3) M and a reproductibility of approximately 2%. It shows a better lifetime than similar ones employing purified catalase. The selectivity is good since other enzymes are not present in the natural source employed.

Seeds of Canavalia maritima are ground after removal of the external coat and are used for the construction of a new enzymatic electrode for urea. Aliquots (10 mg of the powder) are immobilized at the surface of an ammonia gas-sensing electrode, via glutaraldehyde or, alternatively, on PVC membranes. The analytical performance of the biosensors for serum urea determination is comparable to the spectrophotometric method.

An amperometric method utilizing an enzyme electrode was developed for the determination of the glucose content of instant coffee. The electrode was constructed by combining an oxygen electrode with a β- d -glucose oxidase-immobilized collagen membrane. Samples of instant coffee were analysed and the results obtained by the proposed method and by the established AOAC method agreed well (r=0.9964). The sensor was stable for more than 2 weeks and for 320 analyses.

A mercury-sensitive chemically modified electrode (CME) based on modified silica gel-containing carbon paste was developed. The functional group attached to the silica gel surface was 3-(2-thiobenzimidazolyl)propyl, which is albe to complex mercury ions. This electrode was applied to the determination of mercury(II) ions in aqueous solution. The mercury was chemically preconcentrated on the CME prior to voltammetric determination by anodic stripping in the differential-pulse mode. A calibration graph covering the concentration range from 0.08 to 2 mg 1 −1 was constructed. The precision for six determination of 0.122 and 0.312 mg 1 −1 Hg(II) was 3.2 and 2.9% (relative standard deviation), respectively. The detection limit for a 5-min preconcentration period was 0.013 mg 1 −1 . A study for foreign ions was also made.

The feasibility of the use of a Batch Injection Analysis (BIA) system for potentiometric determination of acetylsalicylic acid in tablets, with a membrane ion selective electrode, was investigated. There is no significant difference between the results obtained by the proposed method and those obtained by the Standard British Pharmacopaeia method12 at the 95% confidence level. Values of 4% and 2.5% for R.S.D. were obtained by the application of the BIA-potentiometric method and for the injections respectively. About 90 determinations per hour can be performed with the proposed BIA potentiometric method. A possibilidade do uso do sistema de Análise por Injeção em Batelada (BIA) para a determinação potencimétrica de ácido acetilsalicílico em comprimidos, com eletrodo íon seletivo de membrana, foi investigado. Não há uma diferença significativa entre os resultados obtidos pelo método do proposto e pelo método padrão da Farmacopéia Britânica, ao nível de confiaça de 95%. Valores de 4% e 2.5% para o desvio padrão relativo foram obtidos pela aplicação do método BIA- potenciométrico e para as injeções respectivamente. Cerca de 90 determinações por hora podem ser realizadas pela aplicação do método descrito.

An indirect voltammetric procedure for measuring free cyanide ions, using a chemically modified carbon-paste mercury-sensitive electrode, is described. The method is based on competitive HgII-complex formation reactions between an organic ligand at the electrode surface and CN− ions in solution. A linear decrease of the peak current of mercury in a solution of a defined Hg2+ concentration was observed, when the cyanide concentration was increased from 8.0 to 49.0 mg/L (0.3-1.9 × 10−6M). The relative standard deviation for five independent measurements was 8%.

The acidity and ester index are suggested, as simple and cheap methods, adequate to be employed in the study of the authenticity of copaíba oil by small drugstores and export firms. The acidity index permits both the detection and the evaluation of the content of adulteration of copaíba oil. The ester index permits to know if the adulterant either contain ester compounds,as the case of fatty oils, or contain only non saponifiable compounds as is the case of ethyl alchool or mineral oil. The results of the aplication of the methods to eight commercial samples, of copaiba oil, where three samples were adultered, are coherent with those obtained by more sophisticated instrumental methods.

The determination of the acidity of oils by catalytic thermometric titrimetry using paraformaldehyde as the thermometric end-point indicator was investigated. The sample solvent was a 1:1 (v/v) mixture of toluene and 2-propanol and the titrant was 0.1 mol L-1 aqueous sodium hydroxide. Paraformaldehyde, being insoluble in the sample solvent, does not present the inconvenience of other indicators that change the properties of the solvent due to composition changes. The titration can therefore be done effectively in the same medium as the standard potentiometric and visual titration methods. The results of the application of the method to both non-refined and refined oils are presented herein. The proposed method has advantages in relation to the potentiometric method in terms of speed and simplicity. Investigou-se a determinação da acidez de óleos graxos pela técnica da titulação termométrica catalítica empregando-se paraformaldeido como indicador termométrico de ponto final. Empregou-se como solvente da amostra uma mistura de tolueno e 2-propanol 1:1 (v/v) e como titulante uma solução de hidróxodo de sódio 0.1 mol L-1. O paraformaldeido, sendo insolúvel no solvente, não apresenta o inconveniente, que ocorre com outros indicadores de ponto final, de alterar a composição do solvente da amostra mudando suas propriedades, inclusive a solubilidade da amostra no mesmo. Este fato permite que a titulação seja realizada no mesmo meio em que é feita a titulação potenciométrica ou com indicador visual. Os resultados da aplicação do método à amostras de óleos graxos brutos e refinados são apresentados. O método proposto apresenta vantagens em relação a método potenciométrico em termos de rapidez e simplicidade.

A titrimetric method for determination of lead(II), based on the reaction between plumbite and silver ions, is described. Sodium hydroxide solution is added to the sample until the precipitate of lead hydroxide has redissolved. The solution is then made 0.025M in sodium chloride and titrated with silver nitrate. The titration is monitored with a silver electrode. An error smaller than 0.5% has been obtained for 0.05M lead (II).

The acid–base behaviour of arginine, lysozyme and ovalbumin has been studied by potentiometric and catalytic thermometric titrimetry in a mixture of dimethyl sulphoxide–water, with acrylonitrile as the end-point indicator in the latter technique. It was observed that, with the exception of the SH groups, all the protonated groups, including the guanidine groups of lysozyme and ovalbumin, were titrated by catalytic thermometric titrimetry. By using potentiometric titrimetry, all the titratable groups of ovalbumin were determined, whereas the guanidine groups of lysozyme were not determined by this technique.

A flow-injection (FI) method was developed for the determination of oxalate in urine. It was based on the use of oxalate oxidase (E.C. 1.2.3.4) immobilized on ground seeds of the BR-303 Sorghum vulgare variety. A reactor was filled with this activated material, and the samples (200 µL) containing oxalate were passed through it, carried by a deionized water flow. The carbon dioxide produced by the enzyme reaction permeated through a microporous PTFE membrane, and was received in a water acceptor stream, promoting conductivity changes proportional to the oxalate concentration in the sample. The results obtained showed a useful linear range from 0.05 to 0.50 mmol dm-3. The proposed method, when compared with the Sigma enzymatic procedure, showed good correlation (Y = 0.006( + 0.016) + 0.98( + 0.019)X; r = 0.9995, Y = conductivity in µS, and X = concentration in mmol dm-3), selectivity, and sensitivity. The new immobilization approach promotes greater stability, allowing oxalate determination for 6 months. About 13 determinations can be performed per hour. The precision of the proposed method is about + 3.2 % (r.s.d). Desenvolveu-se um método por injeção em fluxo para determinação de oxalato em urina, baseado na utilização da oxalato oxidase (E.C. 1.2.3.4) imobilizada em sementes trituradas de Sorghum vulgare, variedade BR-303. Amostras de 200 µL contendo oxalato são introduzidas num fluxo de água deionizada que passa por um reator, em forma de coluna, preenchido com o material enzimático ativado. O dióxido de carbono produzido pela reação enzimática é conduzido, pelo fluxo, até uma cela de permeação, contendo uma membrana de PTFE, onde permeia para um outro fluxo de água deionizada. Este fluxo passa por uma cela de condutividade. A presença de dióxido de carbono provoca uma diferença na condutividade, proporcional à concentração de oxalato originalmente presente na amostra. Os resultados obtidos mostram uma faixa de linearidade entre 0,05 e 0,50 mmol dm-3. O método proposto, quando comparado com o procedimento enzimático da Sigma, mostra uma boa correlação (Y = 0,006( +0,016) + 0,98( + 0,019) X, r = 0,9995, Y = condutividade em µS e X = concentração em mmol dm-3), seletividade e sensibilidade. O novo procedimento de imobilização promove grande aumento de estabilidade da enzima permitindo a determinação de oxalato por cerca de seis meses. Cerca de 13 determinações podem ser realizadas por hora. A precisão do método proposto é bastante satisfatória (d.p.r. = + 3,2 %).

The use of paraformaldehyde (PF) as an end-point indicator in hydrolytic titrations involving precipitation, complexation and redox reactions was investigated. The decrease in temperature caused by depolymerization of PF, catalysed by hydroxyl ions produced by hydrolysis of excess of titrant, was employed to locate the end-point of the titration. The results of titrations of alkaline earth metals with carbonate, several metal ions with EDTA and iodine with arsenite are presented.

A flow injection system for the determination of acetylcholine, based on the use of immobilized cholinesterase in glass beads, a diffusion membrane and a conductimetric cell is described. In this approach acetic acid formed by the action of acetylcholinesterase on acetyl choline permeates through a PTFE membrane and is received by a stream of deionized water, which carries the solution to the conductivity cell. Linearity for acetylcholine concentration and conductimetric response with a useful range of 10–5–10–3 mol l–1 was observed.

The use of formalin solution as an end-point indicator in the titration of both acids and bases in aqueous solutions was investigated. In both instances, a lowering in temperature at the end-point, due to depolymerization of poly(oxymethylene glycols), was observed. Endothermic end-points were also obtained by using paraformaldehyde as an end-point indicator in titrations of acids in aqueous solutions. The use of paraformaldehyde in the titration of very weak acids and of some sparingly soluble acids solubilized in micellar suspensions of cationic detergents was also investigated.

The influence of the concentration of reactants on the results of titrations of salicylic acid in acetone with potassium and tetramethylammonium hydroxides in propan-2-ol were investigated. The titrations were performed by catalytic thermometric titrimetry using acetone as the end-point indicator and by potentiometric titrimetry. These and other results are discussed in terms of the equilibrium and kinetic aspects involved in the titrations of polyfunctional acids capable of forming intramolecular hydrogen bonds.

The use of the decomposition reaction of cumene hydroperoxide for indication of the end-point of the titration of organic bases with perchloric acid was investigated. The rise in temperature at the end-point is of the order of 25 °C. Only strong bases can be determined when the titrand solvent is acetic acid-acetic anhydride (92 + 8 v/v), but even weak bases such as caffeine can be determined in 1,2-dichloroethane. Some titration curves resulting from titration of bases with boron trifluoride (a Lewis acid) are presented.

Solution of silver ion, which is stable in the presence of appropriate concentrations of ferrous and ferric salts, is reduced by ferrous salts if fluoride is added. This fact has been utilized to develop a spot test for fluoride. 2μg of fluoride ion can be detected. A method for the removal of fluoride from interfering ions is also described.

Application of a linear algebraic method to the potentiometric titration of a mixture of tartaric and malic acids makes it possible to determine the individual concentrations of both acids in the same sample. These 2 acids have also been determined in grape juice free of citric acid after their separation from the juice matrix by precipitation as barium salts, followed by selective solubilization. It is also possible to determine tartaric acid and the sum of malic acid and citric acid in grape juice when the latter is present.

The application of a linear algebraic method to the potentiometric titration of a mixture of tartaric and citric acids and sodium carbonate and sodium hydrogen carbonate (previously treated with strong acid), permits the determination of the concentration of each individual acid and the total equivalent of carbonate in antacids. It is also possible to determine individual concentrations of sodium carbonate and sodium hydrogen carbonate by combining the results of the described titration with the acidimetric titration of a sample previously treated with an excess of strong base.

A procedure for the determination of total sulphur in ethanol by reduction of the sulphur compounds to sulphide using Raney nickel, conversion to H2S and detection by differential-pulse cathodic stripping voltammetry (DPCSV) is presented. The supporting electrolyte that gave the best results was a sulphide antioxidant buffer (SAOB II, 10%). The calibration graph covered the concentration range 1–40 p.p.b., and the method could be used for ethanol samples with sulphur contents ranging from 15 ng g–1 to 20 µg g–1. The precision and accuracy of the method compare well with values in the literature.