Polyethyleneoxide-capped phthalocyanines: limiting phthalocyanine aggregation to dimer formation

The synthesis and characterization of a soluble metal-free polyethyleneoxide-capped phthalocyanine and the corresponding lead compound are described. This phthalocyanine was designed to allow the formation of dimers but to inhibit formation of higher aggregates. The monomer/dimer equilibrium constant in chloroform solutions is 750 ± 20 l mol -1. No evidence for higher aggregates was found. The molecular extinction coefficient of the metal-free polyethyleneoxide-capped phthalocyanine in chloroform is one of the lowest known (2.5 × 104 l mol -1 cm -1). The lead-substituted material was demonstrated to be a reverse saturable absorber from 532 nm to about 610 nm. It possesses a large nonlinear absorption coefficient in the visible and is a promising optical limiter material.

Disubstituted hydroxamic acids and the determination of vanadium

A number of disubstituted hydroxamic acids were investigated as possible replacements for benzoylphenylhydroxylamine in the determination of vanadium; none is superior. At 378 mμ., p-phenylazobenzoylphenylhydroxylamine shows a marked sensitivity increase but selectivity is poor. In the visible region, there is an increase in molecular extinction coefficient as the reagent π system is extended; how ever, complete coplanarity of the aromatic and metal chelate rings is not possible and the increase is not large enough to be of much practical value.

THE STABILITY OF ASCORBIC ACID IN SOLUTION

The stability of ascorbic acid in aqueous solution was increased under certain conditions by oxalic acid, metaphosphoric acid, glutathione, thiourea, and sodium diethyldithiocarbamate. A slight protective effect was exerted by creatinine; but formic, phthalic, and orthophosphoric acids, creatine, and caffeine had no demonstrable effect. In all these instances the pH, concentrations of reagents, etc., must be considered. In oxalate and thiourea maximum stability occurred at pH 2.5 to 3.0 and in glutathione at pH 3.6 to 4.2. The latter substance itself was also most stable at pH 3 to 4. At the optimum pH a concentration of 40 mM of oxalate gave maximal protection, this being independent of the initial concentration of ascorbic acid over the range 2 to 20 mM. Thus a stoichiometric relationship between the concentrations of the ascorbic acid and the oxalate required for protection was not found. A region of minimum solubility of oxalate in water occurred at pH 2.4 to 3.0, which coincides with the pH at which the maximum protective effect occurs and with the highest relative concentration of sodium hydrogen oxalate (or sodium hydrogen oxalate monohydrate). The absorption of ultraviolet light by ascorbic acid was altered by pH, the maximum shifting from 244 to 268 mμ from pH 2.8 to 4.5. The molecular extinction coefficient of ascorbic acid also changed with pH and was minimal at pH 4.0. This effect occurred in oxalate, which has a specific protective effect, and also in formate and orthophosphate, which have no specific protective effect. The possible mechanisms for the protection of ascorbic acid by oxalate are discussed.

A new method for the determination of nitrogen peroxide

The ordinary spectrograpbic method of estimating a substance from its absorption spectrum when photographed under standard conditions has the merit of providing permanent records, but suffers from necessitating the use of an expensive instrument as well as being laborious and as a rule very slow. A method will now be described by means of which nitrogen peroxide, a substance often determined spectrographically, may be estimated in concentrations of 1 : 100,000 and upwards with high accuracy and in a few seconds when once a simple calibration has been made. The method utilises the fact that the spectral region in the visible whereover nitrogen peroxide absorbs most strongly, is close to that at which a potassium photoelectric cell is most sensitive and where it can be used to record, with high accuracy, the light transmitted by the gas under consideration (fig. 1). The optical basis of the method may first briefly be discussed. If a beam of monochromatic light be passed through a column of absorbent medium, the Beer-Lambert law gives log 10 (I 0 /I) = ε . c . d , (1) where I 0 = intensity of light transmitted at zero absorption, I = intensity of light transmitted at measured absorption, ε = the molecular extinction coefficient, which is a constant for the absorbent medium at any given wave-length of the light concerned, c = concentration of absorbent medium, d = length of absorbent column in the direction of the path of the light. Since for a vacuum photocell with an applied voltage in excess of the saturation value ( i.e ., ca . 40 volts), the photoelectric current varies directly, over a wide range, as the intensity of the light incident upon the cell, we may re-write equation (1) as log i = log i 0 — ε . c . d , (2) where i 0 = photoelectric current corresponding to I 0 , and i = photoelectric current corresponding to I.

XXXVI.—The Absorption of Light by Inorganic Salts. No. I.: Aqueous Solutions of Cobalt Salts in the Infra-Red

The present article is intended to be the first of a series on the absorption of light by solutions of inorganic salts of different elements. With a few scattered exceptions all the work hitherto done on the absorption spectra of inorganic salts has been merely qualitative and has been confined to the visible spectrum. Kayser in his Spectroscopie, vol. iii. p. 45, states that in this field there is work for years and for numerous observers. E. C. C. Baly states in his Spectroscopy, p. 407, that not much is known about the absorption of light in inorganic salts. Merely for its own sake, then, an accurate determination of the molecular extinction coefficient for as many salts under as many different conditions of temperature and concentration, and for as many wave-lengths as possible, would be very valuable.