Rapid and accurate titrations with a convenient and compact automatic derivative spectrophotometric titrator

A convenient and compact automatic derivative spectrophotometric titrator is described which is suitable for automatically performing a wide variety of the well known color indicator titrations without any procedure modifications and usually with few, if any, pretitration considerations and instrument adjustments The results for many acid-base and oxidation-reduction titrations are presented to illustrate the general applicability and important characteristics of the instrument     

Reaction-rate method for the determination of phosphorus in agricultural products

A new automated reaction-rate method for the quantitative determination of phosphorus in grains and feeds is presented. The chemical methodology has been investigated, resulting in a high analytical throughput with precise, accurate results. The phosphomolybdenum blue reaction is used for the reaction-rate determination. After digestion of the samples by the official AOAC block digestion procedure, automated instrumentation is used for precise and rapid combination of the reactants and transfer of the mixed solution to an automated spectrophotometer. The rate of formation of the product during 5 sec is automatically determined and compared with rates obtained for phosphorus standards to determine the phosphorus content of the sample. Relative standard deviations of about 0.3% are obtained and results for determinations of phosphorus in grains and feeds are accurate as indicated by comparison with the average obtained by all laboratories participating in the AAFCO check sample programme.     

Automated catalytic ultramicrodetermination of manganese in natural waters with a miniature centrifugal analyzer

The application of a miniature centrifugal analyzer to trace analysis by kinetic methods has been studied. A spectrophotometric reaction-rate method based on the potassium periodate—diethylaniline reaction which is catalyzed by manganese has been developed. Ultramicro amounts of manganese in the range 0.13–3.4 ng in 45 μl of sample were determined with relative errors and relative standard deviation of about 2%. The method was applied to the determination of manganese in natural waters.     

Precision null-point potentiometry direct microdetermination of iodide in solutions with high chloride concentrations

The determination of micro amounts of iodide in the presence of large concentrations of chloride, such as found in brines or iodized salt, can be performed directly and rapidly by the precision null-point potentiometric method. The complete; analysis time is about 3 min. Iodized salt samples containing 5–100 p.p.m. iodide were determined with an average deviation less than 0.3 p.p.m. and with relative errors of about 0.5% at 100 p.p.m. and about 5% at 10 p.p.m. iodide. The measured samples contained 1–20 μg of iodide in 20 ml of solution, and the sodium chloride to iodide weight ratio was as high as 200,000 to 1. Similar precision and accuracy were obtained for prepared brines containing 10–200 p.p.m. iodide and a wide range of chloride to iodide ratios.     

Automated determination of nitrate in waters with a reduction column in a microcomputer-based stopped-flow sample processing system

An automated method for the determination of nitrate in waters with a microcomputer-based stopped-flow mixing system is described. Nitrate is reduced to nitrite with a copperized cadmium—silver alloy or cadmium tube column fitted to the stopped-flow system. Nitrite is determined using fast kinetic, multi-point or single-point procedures with N-(1-naphthyl)ethylenediamine dihydrochloride as the color reagent. Reduction time parameters are evaluated and optimized. Water samples in the range of 0.025–3 ppm NO^-₃—N can be processed with a throughput of up to 100 samples per hour and a detection limit of 0.013 ppm. Interference studies show that cyanide; dichromate, iodide, sulfide, copper and tin ions cause negative results.     

Lipid bilayers covalently anchored to carbon nanotubes

The unique physical and electrical properties of carbon nanotubes make them an exciting material for applications in various fields such as bioelectronics and biosensing. Due to the poor water solubility of carbon nanotubes, functionalization for such applications has been a challenge. Of particular need are functionalization methods for integrating carbon nanotubes with biomolecules and constructing novel hybrid nanostructures for bionanoelectronic applications. We present a novel method for the fabrication of dispersible, biocompatible carbon nanotube-based materials. Multiwalled carbon nanotubes (MWCNTs) are covalently modified with primary amine-bearing phospholipids in a carbodiimide-activated reaction. These modified carbon nanotubes have good dispersibility in nonpolar solvents. Fourier transform infrared (FTIR) spectroscopy shows peaks attributable to the formation of amide bonds between lipids and the nanotube surface. Simple sonication of lipid-modified nanotubes with other lipid molecules leads to the formation of a uniform lipid bilayer coating the nanotubes. These bilayer-coated nanotubes are highly dispersible and stable in aqueous solution. Confocal fluorescence microscopy shows labeled lipids on the surface of bilayer-modified nanotubes. Transmission electron microscopy (TEM) shows the morphology of dispersed bilayer-coated MWCNTs. Fluorescence quenching of lipid-coated MWCNTs confirms the bilayer configuration of the lipids on the nanotube surface, and fluorescence anisotropy measurements show that the bilayer is fluid above the gel-to-liquid transition temperature. The membrane protein α-hemolysin spontaneously inserts into the MWCNT-supported bilayer, confirming the biomimetic membrane structure. These biomimetic nanostructures are a promising platform for the integration of carbon nanotube-based materials with biomolecules.     

Precision null-point potentiometry: A simple,rapid and accurate method for low concentration chloride determinations

A new, rapid and accurate potentiometric technique is described for the determination of chloride in the concentration range 10^-6 to 10^-1M. For many samples results can be obtained in less than a minute with 0.1% accuracy. The method consists simply of changing the chloride concentration of an unknown solution, while maintaining the ionic strength constant, until it is of identical concentration to a known reference solution. This can be accomplished experimentally in such a way that temperature control and calculation corrections for chloride from reagents or electrodes are not necessary. The procedure can be set up so that the calculation for the result is merely an addition or subtraction of two numbers. The general principles and simple experimental techniques are described and results for chloride presented.     

Hydrogel-encapsulated lipid membranes

We have encapsulated free-standing planar lipid membranes in a polymer hydrogel via in situ photopolymerization. These encapsulated membranes were measured to last at least 5 days and provided a stable environment for single-molecule measurements of incorporated alpha-hemolysin channel proteins. The translocation of single-stranded DNA through alpha-hemolysin incorporated into these encapsulated membranes is slowed over 100 times relative to that of the gel-free state. These stabilized membranes, therefore, have application to the single-molecule analysis of DNA as well as broader application to biomolecular sensors, membrane protein biophysical studies, and drug discovery.     

Pseudo-steady-state solutions for solidification in a wedge

Polubarinova-Kochina's analytical differential equation methodis used to determine the pseudo-steady-state solution to problemsinvolving the freezing (solidification) of wedges of liquidwhich are initially at their fusion temperature. In particular,we consider four distinct problems for wedges which are: freezingwith the same constant boundary temperature, freezing with thesame constant boundary heat fluxes, freezing with distinct constantboundary temperatures and freezing with distinct constant fluxesat the boundaries. For the last two problems, a Heun's differentialequation with an unknown singularity is derived, which in bothcases admits a particularly elegant simple solution for thespecial case when the wedge angle is . The moving boundariesobtained are shown pictorially.