Atomic absorption spectroscopy in the v.u.v.-range: determination of bromine

The conditions for the determination of elements by atomic absorption spectroscopy (AAS) in the vacuum-ultraviolet range are discussed. The requirements for the absorption vessel are considered and the equilibrium concentrations of possible dissociation products of different chemical compounds are calculated. In this way it is possible to estimate the detection limit attained with such an AAS arrangement. When losses by diffusion of sample vapour are disregarded a theoretical value of 5 × 10^?10 g Br results. AAS measurements are described using a special graphite oven in conjunction with a vacuum-ultraviolet monochromator. For KBr as a representative substance we obtained the experimental detection limit of 1.5 × 10^?9 g Br with Br I 148.86 nm.     

Emission characteristics of phenyl isocyanate and parachlorophenyl isocyanate in polar media

The fluorescence and phosphorescence quantum yields (Φ_Γ and Φ_Π) and observed triplet decay time (τ_P) of phenyl isocyanate and its chloro derivative, parachlorophenyl isocyanate have been measured in the polar solvent ethanol. Kinetic parameters for the energy depletion process have been generated K_P the triplet-singlet transition probability is much more sensitive to Cl substitution than K_ISC the intersystem crossing rate.     

The effect of processing conditions on the u.v. stability of polyolefins

It is shown that the lifetime of polyolefins in the presence of u.v. light is markedly dependent on the degree of oxidation occurring during the prior processing operation. This is influenced strongly by the presence of pro-oxidants and anti-oxidants, and their effect on subsequent u.v. life can be rationalized on the basis of their functions during processing.     

Diffusion mechanism of the synergism of u.v.-absorbers and antioxidants

The mechanism of synergism in mixtures of u.v.-absorbers with antioxidants is considered. The synergism is attributed to diffusion of the antioxidant from the deeper-lying layers of polymer film, protected from the action of light by a u.v.-absorber, towards surface layers where the photoreaction takes place. The effect of the antioxidant diffusion process on the duration of the induction period of the unbranched chain oxidation reaction of the polymer is analysed. The experimental investigation of the polybutadiene photo-oxidation process has verified the diffusion mechanism so allowing finding of the optimum relationship between the concentrations of u.v.-absorber and antioxidant and explanation of the observed enhancement of the effect of synergism in thicker polymeric films, for higher antioxidant diffusion coefficients and lower light intensities.     

Spectral (u.v.-vis,i.r.MS), electrochemical and metallochromic properties of 2-hydroxy-benzoylhydrazones of acetylpyridines

The synthesis, electronic, infra-red and mass spectra of salicyloylhydrazones of 2-, 3-, and 4-acetylpyridine are reported. Ultraviolet absorption spectra have been applied to determine the dissociation constants which are ranged between 4.0–4.5 (pyridine nitrogen) and 6.2–6.3 (OH group). Infrared spectra show that strong intramolecular H-bonding exists in the solid compounds. Fragmentation of these compounds were found to undergo skeletal rearrangement in addition to either CON or NN simple bond cleavage; pyridine nitrogen position contributes in the paths leading to ions. These aroylhydrazones exhibit polarograms that show several reduction waves, the positions of which change with the pH value. Of the three compounds, 3-pyridyl derivative was the most difficult to reduce. The chelating properties of the compounds towards metal ions were investigated. The 2-acetylpyridine derivative results to be the most adequate ligand, as well as a good preconcentrating agent for metal ions.     

Emission phenomena in electrochemical systems: Investigation of solvated and delocalized electrons in polar media

The principles of using the results of studies of emission phenomena in electrochemical systems for determination of the energy levels of excess (solvated and delocalized) electrons in electrolyte solutions are formulated. The energy characteristics of excess electrons in hexamethylphosphoric triamide have been estimated on the basis of data obtained by the author and literature data (results of photo- and thermoelectron emission measurements and equilibria studies in solutions of solvated electrons). The energy levels of solvated electron and bi-electron, their photoionization energies and solvent reorganization energies have been found. The nature of the absorption spectra is discussed.     

Determination of the ionization quantum yield and thermalization distance of photoelectrons created by v.u.v. photoionization in pure liquid alkanes and tetramethylsilane

The photocurrents, in liquid n-hexane, n-pentane, cyclohexane, cyclopentane, 2-2-4 trimethylpentane, 2-2 dimethylbutane, neopentane and tetramethylsilane, produced by v.u.v. irradiation, are measured at electric fields from 2 to 35 kV cm^-1. Assuming the one- and three-dimensional Onsager probabilities are respected, the values of the attenuation parameter of the photon flow B₁, the quantum yield for the production of an ion-pair θ_i, and the attenuation parameter of the photoelectron flow B₃ can be deduced. These values are compared with those provided by other injection methods. Both the validity of these values and the theoretical models are discussed. The discussion is extended to other parameters: V₀, μ₀ and K_J, all correlated to emission and propagation mechanisms in non-polar liquids.     

Stability of dispersions in polar media

The stability of dispersions in polar organic media was reviewed comparing with that of in aqueous and non-polar media. Structure and properties of the double layer of AgI in water-ethylene glycol were studied and the stability of the dispersions was related to the study of the double layer.Coagulation concentrations of Ag,AgI and α-FeOOH dispersions in methanol, ethanol 1,2-propanol and acetone by the addition of electrolytes were determined.Coagulation by monovalent counterions is due to double layer compression. With bi- and multivalent counterions or in low dielectric constant-solvents coagulation is primarily caused by charge neutralization.The effect of an ionic surfactant, AOT on the stability of TiO₂ or α-Fe₂O₃ in water-p-dioxane mixtures was studied. AOT behaved as a coagulating agent in polar media ( ϵ = 10–50) , though it worked as a dispersing agent in non-polar (dioxane) and water rich media. AOT accordingly coagulated the dispersion a carbon black in 2-butanone, though a nonionic surfactant did not so. Desolvation from the particle surface is considered to be an important origin of coagulation by AOT in polar media, because AOT is not adsorbed on particles and comsiderably dissociates into ions in 2-butanone.     

Mechanisms of antioxidant action: Metal complexes as u.v. stabilisers in polyethylene

In order to evaluate the importance of u.v. screening, peroxide decomposition and excited state quenching in the mechanism of action of a variety of metal complex stabilisers, screening and additive studies were carried out in high density polyethylene and in model hydrocarbons. All the metal complexes studied were found to be u.v. screening agents when contained in a separate phase between the u.v. source and the polymer and all were rather more effective than a typical commercial u.v. absorber (2-hydroxy-4-octyl-oxybenzophenone, HOBP). However, when used as additives, all the stabilisers had an additional function. In the case of the transition metal dibutyl dithiocarbamates (NiDBC and FeDBC), the additional stabilisation mechanism is consistent with their known peroxide decomposing behaviour. In the case of the 2-hydroxy-acetophenone oxime complexes (NiOx and CuOx), the initial function is a stoichiometric reaction with hydroperoxides to give non-radical products. An amine complex of nickel bis-octylphenol sulphide (NiBOPS) appeared to behave primarily as a screen. Only one of the metal complexes (FeOx) appeared to be capable of functioning as an excited state quencher and this complex is effectively sensitised by triplet carbonyl to give a photo-pro-oxidant effect in cumene. It is concluded that the concept of excited state quenching, although it may occur with some metal complexes, is not a sufficient criterion of u.v. stabilising activity.     

Solvent effects in u.v. absorption spectra. IV. Substituted phenols,anisole and phenetole

The u.v. absorption spectra of some substituted phenols, mostly with the substituents in the ortho positions, have been measured in mixtures of cyclohexane and ethanol and, in some cases, in mixtures of cyclohexane and alternative hydrogen bonding solvents. The loss of the vibrational fine structure has been used as a measure of the ability of the solute molecules to form hydrogen bonds. The behaviour of o-isopropyl phenol does not differ significantly from that of phenol; hence the isopropyl group does not produce hindering to hydrogen bonding. A detectable degree of hindering occurs with 2,6-xylenol and 2,6-dichlorophenol. The vibrational structure of the latter does not disappear completely in ethanol and this is the result of intramolecular bonding between the chlorine atoms and the hydroxyl group. Although 3,5-dichlorophenol is comparable in bonding behaviour to phenol some residual band structure is present even in very strongly associative solvents; possible explanations are discussed. Anisole and phenetole associate rather weakly with ethanol and this is confirmed by the measurement of the shift in the hydroxyl stretching frequency in the i.r. spectrum. This result is interpreted in terms of a marked delocalisation of the ether oxygen lone-pair electrons.     

Solvent effects in u.v. absorption spectra. I. Phenol in cyclohexane ethanol mixtures

The u.v. and i.r. absorption spectra of phenol in a series of solvent systems have been studied and the changes in the spectra, particularly the loss of fine structure in the u.v. spectra, have been qualitatively interpreted in terms of hydrogen bonding effects. The systems studied were chosen to favour either self bonding of the phenol or bonding between the phenol and the minor component of the solvent system. In the latter case the solvent systems were such that the phenol could act either as a proton donor (with an aliphatic ether) or as a donor and acceptor (with ethanol).     

Influence of additives on some physical properties of high density polyethylene-II. commercial u.v. stabilizers

The changes in mechanical properties of processed high density polyethylene, when “u.v. stabilizers” are added, are examined using tensile and melt flow measurements. The results show that, at low additive concentrations, antioxidant effects predominate; at high concentrations, a reinforcement effect is observed. Degradation measurements show that the u.v. stabilizers behave as excellent oxidation inhibitors in the dark; this finding has implications regarding their mechanistic action.     

Electronic structure of xanthine and its biological methyl derivatives by u.v. photoelectron spectroscopy

He(I) excited photoelectron spectra of xanthine, theophylline, theobromine and caffeine are presented and discussed. The spectra are unequivocally assigned using CNDO calculations and comparison arguments to some related molecules. Methyl substitution effects proved to be a powerful tool for reliable assignments of the spectra.     

Mechanisms of antioxidant action: Antioxidant behaviour of nickel complex u.v. stabilisers

The antioxidant mechanisms involved in the u.v. stabilising activity of two nickel complexes, nickel dibutyl dithiocarbamate (NiDBC) and nickel acetophenone oxime (NiOx), have been investigated both by oxygen absorption measurements and in peroxide decomposition studies. NiDBC gives rise to a powerful catalyst for non-radical hydroperoxide decomposition; the rate of the reaction is faster in the presence than in the absence of light although the overall mechanisms appear to be very similar. NiOx is decomposed by hydroperoxides in a series of stoichiometric reactions which involve the consumption of at least six molecules of hydroperoxide per molecule of NiOx. Again the reaction is catalysed by light but the mechanism, which does not involve free-radical formation, is the same in the presence and absence of light. NiOx also appears to have weak radical trapping properties. Attempts to show that NiDBC and NiOx might interact sacrificially with photo-excited states of molecules (triplet carbonyl and singlet oxygen) were unsuccessful; the rates of photo-destruction of both complexes were unaffected by the presence of ketones both in the presence and absence of oxygen. This work confirms earlier conclusions that the nickel complex u.v. stabilisers function by auto-synergistic mechanisms involving u.v. screening and conventional chain-breaking and preventive antioxidant processes which operate during thermal processing operations as well as during environmental exposure. Although excited state quenching processes may occur, they appear to be less important in the overall scheme than the well established antioxidant mechanism.     

Mechanisms of antioxidant action: Synergism between tin stabilizers and polymer-bound antioxidants in the u.v. stabilization of PVC

It is shown that a conventional tin stabilizer (dibutyltin maleate) (DBTM) forms an effective synergistic u.v. stabilizing combination when used in conjunction with a bound phenolic antioxidant (BHBM) and a bound u.v. stabilizer (EBHPT). These additives become bound to the polymer through sulphur during processing in the presence of DBTM and it is suggested that the sulphide of the adduct makes its own contribution to u.v. stabilization by acting as a peroxide decomposing (PD-C) antioxidant during photo-oxidation. However, the 2-hydroxybenzophenone function appears to be an essential part of the system and is probably involved in quenching of excited states of the intermediate sulphoxides which precede the formation of the PD-C antioxidant.     

Determination of nitrate in suspended particulate matter by high-performance liquid chromatography with u.v. detection

For the determination of nitrate in suspended particulate matter (s.p.m.), high-performance liquid chromatography (h.p.l.c.) with a u.v. detector at 210 mm gives precise and accurate results. Chloride, bromate, iodide, nitrite, thiocyanate and various cations do not interfere. Calibration graphs are linear over the range 0–20 ppm of nitrat-enitrogen, and the limit of detection is 0.25 ng of nitrate-nitrogen. The coefficients of variation at 5.0- and 10.0-ppm levels are 3.4 and 2.9%, respectively. Results obtained by the h.p.l.c. method and by two 2,4-xylenol spectrophotometric methods for aqueous extracts of s.p.m. are compared. Agreement is generally good, particularly when choride is removed in the 2,4-xylenol method, but the spectrophotometric methods are much more prone to interference.     

Determination of bromide in suspended particulate matter by high-performance liquid chromatography with u.v. detection

High-performance liquid chromatography on a Zipax SAX column with u.v. detection at 200 nm gives precise results for bromide. Chloride, bromate, nitrate, nitrate, iodide, thiocyanate and various cations do not interfere; prior treatment with activated charcoal removes interfering organic compounds. Calibration graphs are linear over the range 0–50 mg 1 ^?1 bromide in aqueous extracts of the particulates; the limit of detection is 10 ng of bromide. The relative standard deviations at the 0.5 and 1.0 mg 1^?1 levels are 2.3 and 1.0%, respectively.