Determination of citric acid by means of competitive complex formation in a flow injection system

A photometric method for the determination of citrate and other organic acids based on their ability to complex Fe³⁺-ions is presented. The red colored complex of [Fe(SCN)₂]⁺, used as reagent, is destroyed upon contact with the sample because the organic acid complexes the Fe³⁺-ion. The decrease in absorption is monitored at 460 nm. The reaction is carried out in a simple flow injection system either in single or preferably double channel configuration.The influence of pH was investigated. Best results were obtained by adjusting the carrier stream to pH 2.0–2.5 with a KCl/HCl-buffer. With an increasing concentration of reagent the linear range is shifted to higher citrate concentrations. The slope of the calibration graph and the linear range are influenced by the sample volume. Other variations of parameters include flow rate, reactor volume and diameter of tubing. Generally speaking, optimum conditions for the flow system are not specified because they vary with the application.The typical conditions for a calibration graph from 1 to 8 mmol/l citrate were a reagent concentration of 2.6 mmol/l [Fe(SCN)₂]⁺, a flow rate of 2.4 ml/ min, a reactor length of 50 cm with tubing of 0.97 mm inner diameter and a sample volume of 100 l. At these system settings the coefficients of variation were 2.5% and 1.6% for eight replicate measurements of samples containing 4 mmol/l and 8 mmol/l citrate, respectively. Up to 180 samples can be analyzed per hour.Naturally the method is disturbed by all other ions that form complexes or precipitates with Fe³⁺-ions. Therefore its application is limited to samples with a known matrix, which was given in the analysis of citrate in lemon flavored soft drinks, where the citric acid usually accounts for 95 to 99% of the total acidity and other interfering ions are absent.     

Metabolites of microorganisms. 162nd Communication. The crystal and molecular structure of lysolipin I

The triacetate of the antibiotic lysolipin (1) crystallizes in the space group P2₁, a = 11.059, b = 21.453, c = 8.423 Å, β = 109.76°. X-ray analysis was used to determine the structure of this compound.     

Ozonolyse d'olefines perfluorees

We have obtained unusually stable secondaxy ozonides ($\text{2a-c}$) by heating neat perfluoroalkenes ($\text{1a-c}$) with ozone.     

Über Pterinchemie. 62. Mitteilung. Protonenkatalysierte [1,2]-H-Verschiebung bei der Umlagerung von 6,7-Diphenyl-5,6-dihydropterin zu 6,7-Diphenyl-7,8-dihydropterin

Proton catalysed [1,2]-H-shift in the rearrangement of 6,7-diphenyl-5,6-dihydropterine (I) to 6,7-diphenyl-7,8-dihydropterine (III) The arrangement from I to the thermodynamically more stable III undergoes through a acid catalysed [1,2]-H-shift (intramolecular 6,7-hydride rearrangement) (see Scheme 1).     

Pyrrolothienopyrazines: Synthèse de la pyrrolo[1,2-a]thieno[3,2-e]pyrazine et de la pyrrolo[1,2-a]thieno[2,3-e]pyrazine

The synthesis of pyrrolo[1,2-α]thieno[3,2-e]pyrazine and pyrrolo[1,2-α]-thieno[2,3-e]pyrazine is described. These syntheses could be achieved by intramolecular cyclization of 2- (and 3-) (1-pyrrolyl)-3- (and -2)-thienyl-amines obtained by hydrolysis of carbamates or by cleavage of the corresponding ureas. An original way giving better results was also studied via a Curtius rearrangement by reaction between the azide and aldehyde groupings. The synthesis of 2- (and -3)-2-formyl-1-pyrrolyl)-2- (and -3)-thenoylazide is described.     

Determination of α-amylase activity using Fourier transform infrared spectroscopy

A new method for the determination of -amylase activity in aqueous solutions and human serum with FTIR-spectroscopy is proposed. The chemical reaction catalyzed by the enzyme under study can be followed directly when applying FTIR-spectroscopic detection also in the case, where no colored or electrochemical active species are generated or consumed during the course of the reaction of -amylase with simple starch. Therefore the determination of the -amylase activity could successfully be performed by recording two FTIR-spectra, one immediately after mixing the sample and a substrate (starch-) solution and the other after a 20 min reaction time. From these two FTIR-spectra a difference spectrum was calculated hereby eliminating an unspecific absorption of the matrix. The intensities of the resulting difference spectra corresponded to the extent of the reaction which took place during the investigated time interval and hence could be related to the activity of the enzyme in the sample. The developed method is linear from 80 to 1400 U/l (r.s.d.=5% for 700 U/l) in aqueous solutions and was also successfully applied to the determination of -amylase activity in human serum where a linear working range from 100 to 800 U/l (r.s.d.=11% for 150 U/l) was achieved.     

IR-spectroscopic investigations on solute-solvent interactions Part 1: Solvation of triethylphosphine oxide in aprotic and CH-acidic solvents

A systematic study has been carried out on the solvent and concentration dependence of the P=O stretching vibrationv(PO) of the model compoundEt ₃PO in various aprotic and several CH-acidic solvents. Evidence has been found for the existence of dipole-dipole complexes in concentrated solutions ofEt ₃PO in solvents of low acceptor numbers. In dilute solutions, however, the P=O stretching band generally occurs as a single symmetrical peak which shows that only one kind of solvate species is present in solution. In CH-acidic solventsEt ₃PO is present in the form of hydrogen bonded complexes. Informations about the structure of these complexes have been obtained by means of spectrophotometric titrations. It has been shown that the wavenumbersv°(PO) obtained by extrapolation ofv(PO) values to zero concentration are linearly related to the acceptor numbers of the solvents, previously derived from³¹P-chemical shift measurements. The existence of this linear relationships proves that both parameters are linearly related to the strength of the intermolecular interactions and are virtually free from unspecific contributions such as magnetic anisotropy or vibrational coupling effects. The results of the present study show that the P=O stretching vibration ofEt ₃PO represents an ideal probe for the investigation of solvent effects and solvation mechanisms.

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Infrarotspektroskopische Untersuchungen über Substrat-Lösungsmittel-Wechselwirkungen. Teil 1: Solvatation von Triethylphosphinoxid in aprotischen und CH-aciden Lösungsmitteln

Zusammenfassung Es wurde die Konzentrations- und Lösungsmittelabhängigkeit der P=O-Valenzschwingungv(PO) der ModellverbindungEt ₃PO in verschiedenen aprotischen und CH-aciden Lösungsmitteln systematisch untersucht. In hinreichend verdünnten Lösungen tritt die P=O-Valenzschwingung als einzelne symmetrische Bande auf, was beweist, daß nur eine einzige Solvatform vorliegt. In Lösungsmitteln niedriger Akzeptorzahl konnte bei höheren Konzentrationen die Existenz von Dipol-Dipol-Komplexen nachgewiesen werden. In CH-aciden Lösungsmitteln liegtEt ₃PO in Form von Wasserstoffbrückenkomplexen vor. Aussagen über die Struktur dieser Komplexe konnten mit Hilfe spektrophotometrischer Titrationen erhalten werden. Die durch Extrapolation auf Konzentration null erhaltenen Wellenzahlenv° (PO) hängen linear von den aus den³¹P-chemischen Verschiebungen vonEt ₃PO bereits früher bestimmten Akzeptorzahlen der Lösungsmittel ab. Die Existenz dieser linearen Beziehung beweist, daß beide Parameter frei von störenden Einflußfaktoren (magnetische Anisotropie- und Schwingungskopp-lungseffekte) sind und tatsächlich linear mit der Stärke der intermolekularen Wechselwirkung variieren. Die Ergebnisse der vorliegenden Studie zeigen, daß die P=O-Valenzschwingung vonEt ₃PO eine hervorragende Sonde zur Untersuchung von Lösungsmitteleffekten und Solvatationsmechanismen darstellt.