scholarly journals THE KINETICS OF DARK ADAPTATION

1927 ◽  
Vol 10 (5) ◽  
pp. 781-809 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Dark Adaptation ◽  
Photochemical Reaction ◽  
Bimolecular Reaction ◽  
Chemical Mechanism ◽  
Constant Intensity ◽  
Photosensitive Material ◽  
Chemical Combination ◽  
Other Substances ◽  
Primary Photochemical Reaction ◽  
Kinetics Of

1. Data are presented for the dark adaptation of four species of animals. They show that during dark adaptation the reaction time of an animal to light of constant intensity decreases at first rapidly, then slowly, until it reaches a constant minimum. 2. On the assumption that at all stages of adaptation a given response to light involves a constant photochemical effect, it is possible to describe the progress of dark adaptation by the equation of a bimolecular reaction. This supposes, therefore, that dark adaptation represents the accumulation within the sense cells of a photosensitive material formed by the chemical combination of two other substances. 3. The chemical nature of the process is further borne out by the fact that the speed of dark adaptation is affected by the temperature. The velocity constant of the bimolecular process describing dark adaptation bears in Mya a relation to the temperature such that the Arrhenius equation expresses it with considerable exactness when µ = 17,400. 4. A chemical mechanism is suggested which can account not only for the data of dark adaptation here presented, but for many other properties of the photosensory process which have already been investigated in these animals. This assumes the existence of a coupled photochemical reaction of which the secondary, "dark" reaction is catalyzed by the products of the primary photochemical reaction proper. This primary photochemical reaction itself is reversible in that its main products combine to form again the photosensitive material, whose concentration controls the behavior of the system during dark adaptation.

scholarly journals SENSORY EQUILIBRIUM AND DARK ADAPTATION IN MYA ARENARIA

1919 ◽  
Vol 1 (5) ◽  
pp. 545-558 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Reaction Time ◽  
Latent Period ◽  
Dark Adaptation ◽  
Photochemical Reaction ◽  
Short Interval ◽  
Bimolecular Reaction ◽  
Mya Arenaria ◽  
Rational Explanation ◽  
Order Of Magnitude ◽  
Photosensitive Substance

1. The reaction time of Mya to light is composed of two parts. The first, a sensitization period, is an exceedingly short interval of the order of magnitude associated with photographic processes. The second is a latent period of about 1.3 seconds, during which Mya need not remain exposed to the stimulating light. 2. The process of dark adaptation in Mya is orderly. Its progress may be represented by the formation of a photosensitive substance according to the dynamics of a bimolecular reaction. See PDF for Structure 3. Photosensory equilibrium as represented by the light- and dark-adapted conditions finds a rational explanation in terms of the "stationary state" of a reversible photochemical reaction involving a photosensitive substance and its two precursors. 4. There are two corollaries to this hypothesis. The first requires that the reaction time at sensory equilibrium for a given intensity should vary inversely with the temperature; the second, that the rate of dark adaptation should vary directly with the temperature. Experiments verified both of these requirements.

scholarly journals DARK ADAPTATION AND THE LIGHT-GROWTH RESPONSE OF PHYCOMYCES

1929 ◽  
Vol 12 (3) ◽  
pp. 391-400 ◽  
Author(s):  
E. S. Castle
Keyword(s):  
Latent Period ◽  
Dark Adaptation ◽  
Growth Response ◽  
Light Adaptation ◽  
Exposure Period ◽  
Bimolecular Reaction ◽  
Dark Reaction ◽  
Action Time ◽  
Kinetics Of ◽  
Photochemical Action

1. A single-celled, elongating sporangiophore of Phycomyces responds to a sufficient increase in intensity of illumination by a brief increase in growth rate. This is the "light-growth response" of Blaauw. 2. The reaction time is compound, consisting of an exposure period and a latent period (this comprising both the true latent period resulting from photochemical action and any "action time" necessary for the response). During the latter period the plant may be in darkness, responding nevertheless at the end of the latent period. 3. Both light adaptation and dark adaptation occur in the sporangiophore. The kinetics of dark adaptation can be accounted for on the basis of a bimolecular reaction, perhaps modified by autocatalysis. Attention is called to the bimolecular nature of the "dark" reaction in all other photosensory systems that have been studied, in spite of the diversity of the photosensitive substances themselves and of the different forms of the responses to light.

Study on Adsorption of Crosslinked Starch Microspheres towards Heavy Metal Ions

2013 ◽  
Vol 834-836 ◽  
pp. 531-535
Author(s):  
Li Yan Yang ◽  
Yi Hui Guo ◽  
Li Li Yu ◽  
Jing You
Keyword(s):  
Metal Ions ◽  
Reversed Phase ◽  
Adsorption Rate ◽  
X Ray Diffraction ◽  
Adsorption Selectivity ◽  
X Ray ◽  
Starch Microsphere ◽  
Other Substances ◽  
Effects Of Media ◽  
Kinetics Of

A type of cross-linking starch microsphere (CSMs) has been synthesized via reversed phase suspension method. Crosslinked starch microsphere has good adsorption performance to metal ions in water. The adsorption kinetics of Co (II) on the CSMs, selectivity of adsorption CSMs towards Co (II),Cu (II),Pb (II),Cd (II) and adsorption effects of media towards Co (II) were investigated. The CSMs and its adsorption product were comparatively characterized by X-ray diffraction (XRD). The results showed that The adsorption rate is mainly controlled by liquid film diffusion, and the constant of adsorption rate is 0.0686min-1 at 308K. The crystal structure of the CSMs decreased greatly after the incorporation of Co (II). Co (II) has better adsorption selectivity on CSMs. Ions coexist and other substances in the solution have certain impact on adsorption. Those data are helpful for treatment of the wastewater containing heavy ions.

Wavelength controlled production of SO4- radicals in the ultraviolet photolysis of ceric sulfate solutions

1984 ◽  
Vol 37 (3) ◽  
pp. 475 ◽  
Author(s):  
RW Matthews
Keyword(s):  
Rate Constants ◽  
Photochemical Reaction ◽  
Transfer Reaction ◽  
Quantum Yields ◽  
Oh Radical ◽  
Transfer Energy ◽  
Fluorescence Measurements ◽  
Sulfate Solutions ◽  
Ultraviolet Photolysis ◽  
Primary Photochemical Reaction

Solutions of cerium(III)/(IV) and formic acid in 0.4 M sulfuric acid have been photolysed under 254 nm and 365 nm light. Marked differences in the reaction kinetics and quantum yields are observed at the two different wavelengths. At 365 nm, the reactions leading to cerium(IV) reduction are caused almost exclusively by the SO4- radical. The ratio of rate constants, k(SO4- + CeIII)/ k(SO4- + HCOOH), is 116 � 11 and the quantum yield of sulfate radicals, ф(SO4-), is 0.023 � 0.002. At 254 nm, the reactions leading to cerium(IV) reduction are caused mainly by the OH radical, but approximately 35% of the oxidizing radicals formed in the primary photochemical reaction are SO4-. Cerium(III) species, excited at 254 nm, transfer energy to cerium(IV) and this results in an additional yield of OH and SO4- radicals. Fluorescence measurements confirmed the efficiency of the energy transfer reaction. The ratio of rate constants, k(OH+CeIII)/k(OH+HCOOH), is 2.22 � 0.18 and ф(CeIV*) and ф(CelIII*) giving oxidizing radicals are 0.116 � 0.010 and 0.0083 � 0.0008 respectively. Thus about 5 times more total oxidizing radicals are produced from excited cerium(IV) species at 254 nm than at 365 nm.

scholarly journals The thermal decomposition of acetaldehyde and the existence of different activated states

1933 ◽  
Vol 141 (843) ◽  
pp. 41-55 ◽  
Keyword(s):  
High Pressures ◽  
Initial Pressure ◽  
Bimolecular Reaction ◽  
Bimolecular Reactions ◽  
First Order ◽  
Straight Line ◽  
Gas Reactions ◽  
Pass Through ◽  
Low Pressures ◽  
Kinetics Of

Homogeneous thermal gas reactions were at one time tacitly assumed to possess a definite order, unimolecular and bimolecular reactions, for example, being sharply distinguished. The kinetics of the decomposition of acetalde­ hyde, CH 3 CHO = CH 4 + CO, over the pressure range of 100 to 400 mm. were found to satisfy the criterion of a bimolecular reaction, namely, that the reciprocal of the time for half change (1/ t 1/2 ) )plotted against the initial pressure ( p 0 ) gave a straight line inclined to the axes. The line, however, did not pass through the origin, as may be seen in fig. 1 of the present paper. This indicated the presence of some first order reaction, the nature of which was not determined. Subsequently, in accordance with the collision theory of activation and deactivation, it was shown that certain reactions, sometimes called quasiummolecular, change their order from the second at low pressures to the first at high pressures. This apparently was the reverse of the behaviour shown by acetaldehyde.

scholarly journals The catalytic decomposition of nitrous oxide on the surface of gold: a comparison with the homogeneous reaction

1925 ◽  
Vol 108 (745) ◽  
pp. 211-215 ◽  
Keyword(s):  
Nitrous Oxide ◽  
Catalytic Decomposition ◽  
Bimolecular Reaction ◽  
Homogeneous Reaction ◽  
Gaseous Reaction ◽  
Measurable Velocity ◽  
Rare Class ◽  
The Moment ◽  
Kinetics Of ◽  
Decomposition Of Nitrous Oxide

The bimolecular reaction 2N 2 O = 2N 2 + 2N 2 was recently shown to belong to the rather rare class of homogeneous reactions. Decomposition of two molecules of nitrous oxide takes place when a collision of a certain critical degree of violence occurs in the gas. At the moment of collision the two molecules must possess a combined energy of at least 58,000 calories (per 2 gram molecules), and it is probable that most of the collision in which this condition is fulfilled are fruitful. A comparison between the kinetics of this homogeneous decomposition and the corresponding reaction proceeding catalytically at the surface of a solid might be expected to throw light on the mechanism of heterogeneous catalysis. Previous efforts ( loc. cit .) to accelerate the reaction catalytically by the introduction of metals into the bulb in which the homogeneous reaction was going on were fruitless, since the reaction, if any, which took place at the surface of the metal was slow in camparison with the gaseous reaction. This difficulty was overcome by using as a catalyst a fine metal wire heated electrically. This could be raised to a sufficiently high temperature to cause the surface reaction to proceed with measurable velocity while the bulk of the gas was kept cold, thus eliminating the homogeneous reaction. Experiments made with platinum wires in this way were successful. The kinetics of the decomposition of nitrous oxide on the surface of platinum are summarized in the equation – d [N 2 O]/ dt = k [N 2 O]/1 + b [O 2 ]. The reaction is unimolecular, but is complicated by the strong retarding action of the oxygen formed.

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