C.Mongay Fernández

Formation and instability of o-phthalaldehyde derivatives of amino acids

o-Phthalaldehyde reacts with amino acids in the presence of a thiol to give highly fluorescent 1-alkyl-thio-2-alkyl-substituted isoindoles. However, the instability of the derivatives limits the general utility of the reaction. Mechanistic descriptions of isoindole formation and degradation proposed in the last years, which permit a better understanding of the factors affecting isoindole stability, are presented. The use of alternative thiols and o-phthalaldehyde-like reagents is also reviewed.

Studies on the formation and stability of isoindoles derived from amino acids, o-phthalaldehyde and N-acetyl-L-cysteine

Abstract A kinetic-spectrophotometric study is performed on the formation and degradation of the isoindole derivatives of amino acids with o -phthalaldehyde and N -ace-tyl- l -cysteine. The experimental and structural factors which affect the formation and stability of the compounds are considered, and the results are compared with those obtained for mercaptoethanol. N -Acetyl- l -cysteine derivatives are highly stable, not requiring a strict control of the time of reaction as in the case of mercaptoethanol.

Preparation d'un tampon universel de force ionique 0,3 M.

The paper describes preparation of a universal buffer with ionic strength 0.3M. The calculated pH values agree well with those experimentally observed.

Spectrophotometric determination of N-acetylcysteine in drug formulations with o-phthalaldehyde and isoleucine

A spectrophotometric procedure is proposed for the assay of N-acetylcysteine (NAC) in the range 0.5–49 µg ml–1, based on the reaction of the thiol group with o-phthalaldehyde and isoleucine at pH 9.5 (λmax.= 335 nm). The procedure was applied successfully to the determination of NAC in several pharmaceutical formulations. The recoveries ranged from 98 to 106%, with relative standard deviations of less than 1.5%. The method is free from interferences and is simple, rapid and easy to automate.

Comparative study of several programs used in the potentiometric evaluation of equilibrium constants including an error sensitivity analysis

Abstract The programs MINIQUAD, MINIPOT, SUPERQUAD and PHCONST are applied to the evaluation of the protonation constants of several hypothetical polyprotic substances using simulated titrations, and the precision and accuracy of the results are discussed and compared. When statistical weights are used the results are very similar, the accuracy being better with PHCONST. Error sensitivity analysis was used as a means of establishing the influence of a systematic error of an experimental variable on the accuracy of the determination. An algorithm for the accurate calculation of error sensitivities is proposed and checked. Error sensitivities can be used to make a choice between the various minimization functions or to decide if a variable must be included in the optimization process.

Spectrophotometric determination of mercury(II) and silver(I) with copper(II) and diethyldithiocarbamate in the presence of triton X-100

Procedures for the determination of mercury and silver by displacement of diethyldithiocarbamate (DDTC) from its copper complex in the presence of 1% Triton X-100, and measurement of the decrease in the Cu(DDTC)(2) absorbance, are described. The use of the surfactant avoids the need for an extraction step. Reproducibility within 1% and detection limits of 0.25 ppm Hg(II) and 0.45 ppm Ag(I) have been obtained, and linear calibration ranges up to 13 ppm Hg(II) and 15 ppm Ag(I). In the presence of 0.1M EDTA very good selectivity is achieved.

Information content of data and variables and types of weighting in least-squares regression methods

Abstract Algorithms are given for evaluating the relative amount of useful information related to a particular parameter which is carried by individual data points and intervals of the variables. The algorithms provide an efficient means of using the information contained in a set of data. Applications to the optimization of weighting in regression methods are described. Several informational and combined informational-statistical types of weighting are studied as a means of improving the accuracy and precision of the parameters obtained by non-linear regression.

Graphical representation of polynuclear acid-base and complex equilibria

A calculation procedure for diagrams of log C(M) = f(pL) at various values of the ratio of analytical concentrations, C(L)/C(M), in polynuclear binary systems is described. The diagrams are useful in the resolution of analytical problems of practical interest involving acid-base equilibria (such as the preparation of buffer solutions) and complex equilibria. The presence of a solid phase is also considered.

Thermometric determination of reaction enthalpies using a polypropylene cell in an isothermic air bath

Abstract An isoperibolic system consisting in a polypropylene cell of thin walls surrounded by an isothermic air bath is proposed. Heat leaks are evaluated and corrected for each experiment. The system is calibrated making use of the NaOH-HCl reaction and tested by carrying out the determination of the enthalpies of neutralization of acetic, oxalic and citric acids.

On the preparation of simple and universal buffers including polynuclear species

A simple generalized procedure for the calculation of electrolyte concentrations in pH-buffers is proposed. Mixtures of acid-base systems and formation of polynuclear species at high ionic strengths are considered, and a diagram useful for the study and preparation of the buffers is shown.