Nafion-PAN optical chemical sensor: optimization by FIA

A copper-sensitive optical chemical sensor (optrode) is described. The optrode is based on a Nafion membrane and an immobilized organic ligand coupled with a flow injection (FI) system. The FI system includes a flow-through removable measuring cell and a simple spectrophotometer. Owing to the miniature size of the system and the efficient use of optical fibers, this optrode is well suited for monitoring environmental water samples. The success of the described optrode system depends on the effectiveness of the FI reagent delivery system. Optimum contact time with the membrane (as determined by the reagent flow rates) and the injected sample volume are critical. Environmental water samples were analyzed for copper content using the optimized optrode system. To validate the optrode's performance, the same water samples were analyzed using the atomic absorption spectrophotometer.     

The use of a polymer inclusion membrane in a paper-based sensor for the selective determination of Cu(II)

A disposable paper-based sensor (PBS) is described for the determination of Cu(II) in natural and waste waters at approximately 2 cents per measurement. The device makes use of a polymer inclusion membrane (PIM) to provide the selectivity for Cu(II). The PIM consists of 40 wt% di(2-ethlyhexyl) phosphoric acid (D2EHPA) as the carrier, 10 wt% dioctyl phthalate (DOP) as a plasticizer, 49.5 wt% poly(vinyl chloride) (PVC) as the base polymer and 0.5 wt% (m m⁻¹) 1-(2′-pyridylazo)-2-naphthol (PAN) as the colourimetric reagent. High selectivity under mildly acidic conditions (HCl, pH 2.0) is achieved for Cu(II) in the presence of frequently encountered metal ions in natural and waste waters such as Fe(III), Al(III), Zn(II), Cd(II), Pb(II), Ca(II), Mg(II), and Ni(II).     

Kinetics and structural studies of the atmospheric corrosion of carbon steels in Panama

The corrosion of a carbon steel was studied in different atmospheres at sites in the Republic of Panama. The weight loss (corrosion penetration) suffered by the carbon steel is related to time by a bilogarithmic law. Mössbauer spectroscopy indicated the rust was composed of non-stoichiometric magnetite (Fe_3-xO₄), maghemite (γ-Fe₂O₃), goethite (α-FeOOH) of intermediate particle size, lepidocrocite (γ-FeOOH) and superparamagnetic particles. Magnetite formation is related to the alternating dry--wet cycles. Goethite is related to corrosion penetration by a saturation type of behavior, following a Langmuir type of relationship. Goethite in rust protects steel against further atmospheric corrosion.     

The lengths of Hermitian self-dual extended duadic codes

Duadic codes are a class of cyclic codes that generalize quadratic residue codes from prime to composite lengths. For every prime power q, we characterize integers n such that there is a duadic code of length n over F_q² with a Hermitian self-dual parity-check extension. We derive asymptotic estimates for the number of such n as well as for the number of lengths for which duadic codes exist.     

Atomic Proximity Due to Molecular Congestion: Rational Design of Bis(phosphite) Ligands by Restriction of Molecular Motion(1)

The synthesis and conformational analysis of the sterically congested bis(phosphite) ligand {2-{1-{3,5-(t)Bu(2)-2-[2,2'-CHCH(3)(4,6-(t)Bu(2)C(6)H(2)O)(2)PO]C(6)H(2)}Et}-4,6-(t)Bu(2)C(6)H(2)O}(PhO)(2)P (5) are reported. X-ray crystallographic, dynamic (31)P{(1)H} NMR, NOE, DNOE, CP-MAS (31)P NMR, and calculational studies of 5 as well as the structurally related bis(phosphites) 1 and 6 suggest that the conformational freedom of the molecule is severely restricted because of geometric restraints due to steric congestion. A through-space mechanism of coupling is suggested to explain the observed eight-bond P-P J coupling of 27.5 Hz in the (31)P{(1)H} NMR spectrum of 5, which is a result of the proximity of the two phosphorus atoms in 5. The results of this study support the contention that the restriction of molecular motion by steric congestion can be used to rationally design a ligand favoring a particular disposition of phosphorus atoms.     

Nafion-based optical sensor for the determination of selenium in water samples

An optical chemical sensor responsive to selenium (SeO₃²⁻) in water samples was developed. Its matrix was nafion membrane suffused with an organic ligand p-amino-p’-methoxydiphenylamine or variamine blue (VB). The method of analysis was flow injection (FI) where in the membrane was fixed in a flow-through demountable measuring cell and connected to a computer-controlled simple spectrophotometer.

Variamine blue was previously established to determine amounts of selenium in water and other media by means of a spectrophotometer. The method involved reacting selenite with potassium iodide to generate iodine gas, which reacts with variamine blue to form a colored species.

Experimental results showed the optrode to be an effective tool in analyzing the selenium content of water samples particularly for remote or in situ applications. Interference studies proved that the method is free of intervention from tested ions.     

Sterically congested tripodal phosphites: conformational analysis,solid-state polymorphism,metal complexation,and application to the asymmetric hydrosilation of ketones

The synthesis as well as isolation and crystallographic analysis of two solid-state polymorphs of the tripodal ligand tri[2,2',2' '-tris[(2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl)oxy]ethyl]amine (3) is described. Form I crystallized from ethyl acetate in the space group P2(1)/n with the unit-cell parameters a = 20.070(10) A, b = 17.477(2) A, c = 27.620(3) A, and beta = 93.050(10) degrees, V = 9674.5(14) A(3), and Z = 4. Form II crystallized from a mixture of acetone and toluene in the space group P1 with the unit-cell parameters a = 12.493(1) A, b = 19.701(2) A, c = 21.027(2) A, alpha = 116.23(1) degrees, beta = 100.15(1) degrees, and gamma = 91.07(1) degrees, V = 4542 A(3), and Z = 2. Differences in the relative absolute stereochemistry of the stereoaxes in the seven-membered dibenzo[d,f][1,3,2]dioxaphosphepin ring are discussed. The synthesis and X-ray characterization of enantiomerically pure (S,S,S)-tri[2,2',2' '-tris[(2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl)oxy]propyl]amine [(S,S,S)-7] are reported. Two crystallographically independent molecules exist in the unit cell that cannot be superimposed with each other by either a translation or a symmetry operation. The two solid-state conformers in the unit cell differed predominately by the absolute stereochemistry of the stereoaxes in the seven-membered dibenzo[d,f][1,3,2]dioxaphosphepin ring. The Rh(I)-catalyzed hydrosilation of acetophenone with the chiral ligands (R,R,S)-7 and (S,S,S)-7 showed significant differences in chiral induction. Chiral cooperativity between the stereoaxes and stereocenters in (S,S,S)-7 is observed. The mechanism of the communication between the stereocenters and stereoaxes leading to chiral cooperativity in the stereoselective transition state is suggested to be primarily steric in nature.     

On-line preconcentration and speciation of arsenic by flow injection hydride generation atomic absorption spectrophotometry

A flow injection-column preconcentration-hydride generation atomic absorption spectrophotometric (FI-column-HGAAS) method was developed for determining μg/l levels of As(III) and As(V) in water samples, with simultaneous preconcentration and speciation. The speciation scheme involved determining As(V) at neutral pH and As(III + V) at pH 12, with As(III) obtained by difference. The enrichment factor (EF) increased with increase in sample loading volume from 2.5 to 10 ml, and for preconcentration using the chloride-form anion exchange column, EFs ranged from 5 to 48 for As(V) and 4 to 24 for As(III + V), with corresponding detection limits of 0.03-0.3 and 0.07-0.3 μg/l. Linear concentration range (LCR) also varied with sample loading volume, and for a 5-ml sample was 0.3-5 and 0.2-8 μg/l for As(V) and As(III + V), respectively. Sample throughput, which decreased with increase in sample volume, was 8-17 samples/h. For the hydroxide-form column, the EFS for 2.5-10 ml samples were 3-23 for As(V) and 2-15 for As(III + V), with corresponding detection limits of 0.07-0.4 and 0.1-0.5 μg/l. The LCR for a 5-ml sample was 0.3-10 μg/l for As(V) and 0.2-20 μg/l for As(III + V). Sample throughput was 10-20 samples/h. The developed method has been effectively applied to tap water and mineral water samples, with recoveries ranging from 90 to 102% for 5-ml samples passed through the two columns.     

Spectrophotometric study of the solubility and the protolytic properties of 1-(2-pyridylazo)-2-naphthol in different ethanol–water solutions

The solubility and the protolytic constants of 1-(2-pyridylazo)-2-naphthol (PAN) have been accurately determined at temperatures between 20.0 and 30.0°C in ethanol–water solutions with ethanol concentrations ranging from 10.0% to 95.0% (v/v). The measurement of the protolytic constants is based on the spectrophotometric determination of the concentrations of the corresponding conjugate acid–base pairs in ethanol–water solutions containing PAN and hydrochloric acid or sodium hydroxide. The traditionally used pH measurements in such studies in non-aqueous solutions are thus avoided. The solubility and the protolytic constants of PAN in pure water have been calculated by extrapolating the ethanol–water results to pure water where PAN is practically insoluble. The determination of the thermodynamic data mentioned above is necessary for elucidating the interactions of PAN with various metal ions in solutions and in cation-exchange membranes (e.g., Nafion^®) used as PAN-based optodes.