Silver halides physical development

In a modified and more complicated form (pre-fixation physical development) the exposed sensitive layer is placed directly in the developing solution without first dissolving out the silver halide. The developing solution contains a silver halide solvent in addition to the soluble silver salt, and part of the developed silver in this process comes from the original silver halide. Some direct development also occurs. 

Sheppard s mechanism was formulated primarily in reference to direct development (Sheppard, 3 Sheppard and Meyer, 3a) and he assumed that the complex was formed in the act of or as a result of adsorption of the developing agent by the silver halide. Since he did not specifically suggest application of the basic mechanism to physical development, consideration of his mechanism will be deferred until direct development is treated in detail. 

Direct development by sulfite ion does not take place, probably because the rate of reduction of silver ions by the sulfite is much smaller than the rate of solution of the silver halide (see Section YI of this chapter). The writer has obtained physical development, however, with a solution of silver nitrate and sodium sulfite. A specially hardened gelatin film which was able to withstand the action of the solution at 70° for 40 minutes was used. The developing solution contained 1.7 g. silver nitrate and 13 g. sodium sulfite per liter. Fog formation was relatively high, as might be expected. 

The gold treatment undoubtedly results in a replacement of at least some latent image silver by gold. It is possible, as already mentioned, that the nuclei have been increased in size by a physical development effect, but this seems unlikely for the short times involved. The maximum effect is obtained within 5 minutes under the experimental conditions used. Accordingly, it appears that the increased rate of development is the result of the changed properties of the metal/silver halide interface, which now involves metallic gold instead of silver. 

Under the usual conditions of commercial practice, the development reaction does not occur entirely at the silver/silver halide interface. Some reduction of silver ions from solution takes place. Such reduction presumably can occur at any point on the silver/solution interface, and the mechanism should be the same as that for post-fixation physical development. The relative extent of the physical development in comparison with that at the silver/silver halide interface will depend upon the silver halide solvent action of the developing solution and upon the rate of the direct development. 

Physical development. The developer used in this process contains a soluble silver salt, and the developed image is obtained by reduction of this salt. The process is chemical, but the term is retained for historical reasons. Development by the same mechanism can occur when silver ions from silver halide grains pass into solution and are subsequently reduced at latent image centers or developed silver. This process is termed solution-physical development. 

The primary photochemical process that occurs when a photon is absorbed by the silver halide is the transfer of an electron to the conduction band. The quantum efficiency of this process is one. The efficiency of formation of a developable silver nucleus, however, depends on various chemical and physical secondary processes and may be much smaller. 

Silver halide grains are used as the light-sensitive component in many solution processes involving physical development. The latent images or partially developed latent images comprise the nuclei upon which the physical development reactions are catalyzed. There are many other nuclei-forming compounds, however, which find use in systems where the speed of silver halide is not required, its expense is unsuitable, or a more efficient catalyst for the physical development reaction is desired. 

Notes Potassium thiocyanate can produce actual reduction in the size of individual silver halide grains by virtue of its dissolving action. As a result of physical development, it produces a more homogeneous deposition of silver. It is used in concentrations of 1.0 to 1.5g/liters in certain developers, such as Kodak DK20 (DK-20 is not included in the formulas found in this edition). 

In another version of the system described above the physical development nuclei can be coated in the same sheet below the silver halide layer, which is designed to wash off during processing, leaving the positive image behind. 

Superadditivity has been observed in both chemical and physical development [52] thus the presence of silver halide is not a necessary condition for its occurrence but is likely to modify its detailed course. This would appear to rule out the charge-barrier theory in its original form, although charge effects might also occur at silver as well as at silver halide surfaces. 

This indicates that superadditivity arises by the removal of an inhibiting species in Phenidone oxidation, probably the Phenidone radical as in case 1 above. Lee and Miller [53] showed that development of silver halide by the Phenidone radical generated in a flow system was much slower than that by Phenidone itself by a factor of about 24, which agreed very closely with the results of Levenson and Twist [52c], and Shiao and Dedio [54] also found that superadditivity of ascorbic acid and a Phenidone derivative (MHP) was explained by scavenging the MHP radical, thus eliminating any inhibition. A similar inhibition by oxidation products of development was proposed to explain the behavior of ascorbic acid physical developers [55]. 

The latent image catalyzes the reduction of silver ion either from the solid silver halide phase, as in chemical development, or from a soluble source of silver ion, as in physical development (Figure 21). One view of chemical development is that interstitial silver ions move through the silver halide crystal and are reduced on the underside of the latent image speck. In purely physical development complexed silver ion moves through the solution and is reduced on the nucleus. In this sense physical development and the early stages of chemical development are similar. 

Figure 21. Chemical and physical development. In chemical development the silver ion source is solid silver halide. In physical development the silver ion source is a soluble silver ion complex,...

It should be said that development of the latent image can occur by virtue of the reduction of the original light sensitive silver halide either by virtue of the cata-lytically enhanced reduction rate described above (known as chemical development in the trade ) or by the use of the catalytic action on a soluble silver salt introduced simultaneously with the reducing agent or by virtue of solubilising the primary halide and effecting reduction from it in its solubilised state. This is known as physical development. Physical development can be carried out either before fixation (see later) or post fixation the latter is clearly not an option for chemical development. Despite the name, it is, of course, still a chemical development process. 

Alternatively, it is possible to install fibre optic probes directly in the main stream in-line while the IR spectrophotometer remains remotely in a low vibration laboratory environment. In-line analysers, which do not remove any sample from the line, have the minimum possible lag time and do not change the sample physically or chemically from its nature in the process. Recently, bundles of 500 /xm optic fibres have been developed for the 5000-900 cm (2000-11,000 nm region), which permit transmission of IR energy over distances of several metres. Lowry et al. [76] have evaluated fibre-optic cables that might prove useful in FTIR remote sampling applications. The various optical fibres (chalcogenide, silver halide, heavy metal fluoride or sapphire) differ in their spectral window [77]. Due to the thermal stability and the spectral window, sapphire fibres are considered suitable for in-line characterisation of polymer melts in a production line (e.g. in an extruder head) as an alternative to discontinu-ously operating conventional off-line transmission IR spectroscopy of polymer films [78]. 

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