Determination of ferrous and ferric iron in aqueous biological solutions

A solvent extraction method was employed to determine ferrous and ferric iron in aqueous samples. Fe³⁺ is selectively extracted into the organic phase (n-heptane) using HDEHP (bis(2-ethylhexyl) hydrogen phosphate) and is then stripped using a strong acid. After separation, both oxidation states and the total iron content were determined directly by ICP-MS analysis. This extraction method was refined to allow determination of both iron oxidation states in the presence of strong complexing ligands, such as citrate, NTA and EDTA. The accuracy of the method was verified by crosschecking using a refinement of the ferrozine assay. Presented results demonstrate the ability of the extraction method to work in a microbiological system in the presence of strong chelating agents following the bioreduction of Fe³⁺ by the Shewanella alga BrY. Based on the results we report, a robust approach was defined to separately analyze Fe³⁺ and Fe²⁺ under a wide range of potential scenarios in subsurface environments where radionuclide/metal contamination may coexist with strongly complexing organic contaminants.     

Oscillating plate viscometer in the Hz to kHz range

A low and audio frequency viscometer with a frequency range 2 Hz to 1 kHz and possibly up to 10 kHz is described, which shears a liquid sample (shear volume < 0.3 ml) between an oscillating glass plate and the wall of a cuvette. Alternating velocity and force exerted on the oscillating plate are measured by a mechanical impedance head, designed for the frequency range 1 Hz to 10 kHz. This device permits the determination of frequency dependent viscous liquid properties. Real and imaginary part of the complex viscosity can be obtained from the complex mechanical impedance. Inertial forces from the oscillating plate are compensated in an adjustable electronic circuit.Significant shear parameters, as the frequency, the exciting wave form and the amplitude, can be set independently. In this way viscous spectral data and also nonlinear properties can be studied, e.g., in aqueous DNA solutions. In addition, processes with time constants >0.1 s in (relaxing) viscoelastic systems, which do not obey the Boltzmann superposition principle, can be monitored by means of the (nonlinear) modulation of an LF sampling signal. Some examples of measurements will be shown. Model calculations for water and for glycerol give typical values for the complex impedances to be obtained with this oscillatory rheometric device.