Co-crystallization of solvents with polymers: The x-ray diffraction behavior of solvent-containing and solvent-free polyacrylonitrile

Differences in the x-ray diffraction pattern of polyacrylonitrile (PAN) have been observed, both here, as well as in the work reported in the literature. Various workers have cited grossly differing unit cells for PAN and this has never been explained. In this work, it is shown that in dry samples, the diffraction pattern contains only two equatorial peaks, d₁ ≈ 5.3 Å and d₂ ≈ 3.0 Å. This is consistent with a hexagonal packing of rod-like chains of PAN with no chain-axis order. However, in solvent-plasticized films, the diffraction pattern is altered. It is proposed that certain solvents cause a hexagonal to orthohombic transition. However, the orthorhombic polymorph appears to be a solvated form, where the solvent has co-crystallized with the polymer. After removal of solvent, the structure reverts to hexagonal packing. Hence, it is suggested that the discrepancy in cell parameters cited in the literature, particularly when large cell dimensions are quoted, may be due to the inadvertent presence of solvent which could have been introduced during sample preparation. Similarities with solvated crystls in other polymeric systems are also briefly discussed. © 1994 John Wiley & Sons, Inc.     

Titrimetric determination of phenol resorcinol and phloroglucinol

A new and sensitive titrimetric method with an amplification procedure has been worked out for the determination of 10-1000 mug of phenol, resorcinol or phloroglucinol. The method is based on reaction of the phenols with an excess of bromine to form bromosubstituted aryl hypobromites. After removal of excess of bromine with formic acid, the hypobromites are treated with iodide to liberate an equivalent amount of iodine, which is extracted into chloroform, then reduced to iodide and determined by the Leipert procedure with 6-fold amplification. The coefficient of variation does not exceed 0.4% for amounts of determinand > 100 mug, but increases to 1.5% at the 10-mug level.     

NMR and x-ray studies on the morphology of thermoreversible polyacrylonitrile gels

The morphology of thermoreversible polyacrylonitrile–propylene carbonate (PAN-PC) gels was examined using solid-state carbon-13 nuclear magnetic resonance (NMR) spectroscopy and x-ray diffraction. Following complete dissolution of the polymer at elevated temperature and cooling of the concentrated PAN-PC solutions, a gel was formed. The PAN-PC gels consisted of regions of mobile polymer chains, rich in PC, “cross-linked” by regions of rigid polymer. The mobile regions of the gels showed solution-type NMR spectra with resolution of tacticity effects. The rigid component detected by NMR would correspond to the crysttallites detected previously by x-ray diffraction. Wide-angle x-ray diffractograms of the gels showed different peaks when compared with the dry polymer powder. After solvent extraction and drying of the gel, the diffractogram reverted to that of the original dry powder. This new result is the strongest evidence to support the view advanced earlier that the new peaks found in the diffraction pattern of the wet gels arises from solvated polymer crystallites rather than from ordinary polymer crystallites. © 1993 John Wiley & Sons, Inc.     

Titrimetric determination of chloramine-T and some aldoses by amplification reactions

A titrimetric method with amplification has been worked out for the determination of chloramine-T and certain aldoses. It is based on Leipert determination of iodide produced in the case of chloramine-T by reduction of the determinand with excess of iodide, and extraction and subsequent reduction of the iodine liberated. In the other case, the aldoses are oxidized with iodine, the surplus oxidant is extracted, and the residual iodide is determined. The method is applicable to chloramine-T in the range 0.01-3 mg, with a relative error between -3.0 and -0.4% and a relative standard deviation of 0.6-0.8%, depending on the amount present, and to 0.05-1 mg of glucose, galactose or arabinose, or 0.1-2 mg of lactose or maltose, with a relative standard deviation of 1.4% for > 0.5 mg of aldose.     

Photometric determination of nitrite

A new sensitive colour reaction for nitrite determination is presented. The method is based on the reaction of nitrite with p-aminobenzoic acid to form a diazonium ion, which is coupled with 8-hydroxyquinoline, in alkaline medium, to form a reddish-orange water-soluble azo dye. Diazotization and coupling are very fast and control of temperature is unnecessary. Moreover, the colour is stable and its intensity is in direct proportion to nitrite concentration over a wide range.     

Study of the complexation chemistry and speciation of thallium for on-line preconcentration with immobilised quinolin-8-ol

The effects of various buffering reagents and pH conditions were investigated by flame atomic absorption spectrometry to optimise complexation of T1³⁺ with quinolin-8-ol (8-Q), immobilised on controlled-pore glass beads in a 5-cm column. As Tl³⁺ is a softer acid than the other trivalent cations of the Group III elements, the effects of the buffers are different from those observed previously for Al³⁺, Ga³⁺ and In³⁺. A mixed buffer of 0.1 mol l^–1 acetate and 0.1 mol l^–1 ammonium chloride at pH 10 proved most successful, although 0.1 mol l^–1 maleate was also satisfactory over a pH range of 4–10. As thallium normally exists as Tl⁺ in solution, an oxidation method was developed to convert the ions to Tl³⁺, which is more efficiently complexed by 8-Q. Addition of 1–10 l of bromine per 100 ml of sample was sufficient to oxidise Tl⁺ without heating. Excess bromine was removed by addition of phenol. With a flow-rate of 6 ml min^–1, the detection limit of Tl³⁺ is 3 ng ml^–1, for a 3-miri preconcentration time. The enrichment factor under these conditions is 55 and the characteristic concentration is 2 ng ml^–1. The major ions in sea water did not interfere with Tl³⁺ preconcentration and the tolerable limits of Fe³⁺, Cu²⁺ and Al³⁺ are high enough to permit analysis of river and sea waters. The method was applied successfully to the determination of thallium in potassium-enriched table salt. It was also shown that the concentrations of Tl⁺ and Tl³⁺ in a solution can be derived using the described procedure, allowing speciation of inorganic thallium.     

Spectrophotometric determination of aniline in aqueous solution by azo-dye formation with diazotized p-nitroaniline

A simple, sensitive, and rapid Spectrophotometric method for the trace determination of aniline in aqueous solution is described. The method is based on coupling aniline with diazotizedp-nitroaniline in the presence of ethanol and sodium hydroxide to form an intense red water-soluble azo-dye which shows maximum absorption at 505 nm and greater stability and sensitivity in the presence of cetyltrimethylammonium bromide. Beer's law is obeyed over the aniline concentration range 0.2–2.4g/ml in the solution measured; the molar absorptivity is 3.46×10⁴l· mole^–1·cm^–1, and the relative standard deviation 1–2%, depending on the aniline concentration.     

Microdetermination of acetone in aqueous solution

Diazotized anthranilic acid has been found to be a possible reagent for the spectrophotometric microdetermination of acetone in aqueous solution. The determination range is 10–130 μg/ml. The coefficient of variation does not usually exceed 12% for 60–130 μg of acetone but increases to 2.8% at the 10-μg level. The average relative error for five determinations ranges from ?1.5 to 2%. The molar absorptivity is 9.28 × 10³ liter · mol^?1 · cm^?1 in the presence of starch as a surfactant. Possible reaction path mechanism has been suggested for the colored body formation.     

Indirect spectrophotometric determination of cerium(IV)

A simple and very fast photometric method for the determination of trace levels of cerium(IV) has been developed. Use has been made of the reaction in which iron (II), on oxidation with cerium(IV), gives iron(III) which is complexed with tiron, in acidic medium, to form a stable blue color, the intensity of which is in direct proportion to the amount of cerium(IV) originally present in the sample solution. In addition, the method has a wide range of determination and reasonable sensitivity, and it avoids both temperature control and the tedious extraction step.