Silver halide

The subfluoride Ag2F can be prepared by reaction of silver with aqueous AgF or by electrolysis of AgF in HF  

It has the anti-CdI2 structure, with alternating double layers of silver and intercalated fluorides. It is a metallic conductor. 

Anhydrous AgF is best made by fluorination of finely divided silver at room temperature alternatively it can be made by dissolving silver(II) oxide in hydrofluoric acid and crystallizing  

As expected from the similarity of ionic radii between Ag+ (1.15 A) and Na+ (1.01 A), one form has the NaCl structure (it is trimorphic) with other forms having the CsCl and inverse NiAs structures. Unlike the other silver(I) halides, it is very soluble in water (up to 14 M) and forms di- and tetra-hydrates it is decomposed by UV rather than visible light and melts unchanged at 435°C. 

The other silver(I) halides are traditionally prepared by ionic precipitation, on account of their insolubility  

MX3 Bright red Gold-yellow, dec. SOOX Red, dec. 254X Dark brown, dec. 9TC  

The analog photography community is well-versed with the fact that silver halides are tunable band gap semiconductors. Thus the photoactivity of AgCl can be tuned from the UV into the visible light region by a process known as self sensitization, which is due to the formation of Ag clusters during the photoreaction. The formation of these clusters introduces new levels within the forbidden gap that can now be populated by visible light (Fig. 7). 

The photocatalytic oxidation of water to O2 on thin nanostructured AgCl layers has been reported.568 In subsequent work by the same group, AgCl photoanodes have been combined with either p GaMV69,570 or an amorphous silicon solar cell571 in the cathodic part of an electrochemical cell to split water. Modification of AgCl with gold colloids was found to enhance H2 and O2 production.571 572 

AgBr nanoclusters have been also synthesized by y-radiolysis under various conditions of irradiation dose and dose rate. The final cluster size is governed by the competition between two mechanisms of particle growth, one by the coalescence (controlled by second order kinetics and dose rate sensitive)  

At the end of irradiation with a given dose, the cluster nuclearity decreases ( max blue-shifted) with increasing the dose rate. 

Absorption spectra of CdS clusters in solutions of CdS04 andHO(CHj)2SH. 

Inset Evolution of the optical density at 282 nm and 320 nm as a function of the dose rate.  

Pulse radiolysis provides also means to determine the size of chemically prepared CdS clusters in measuring their reaction rate with OH radicals. The mechanisms of such an electron transfer process from the surface of CdS, CdTe and ZnTe, PbS semiconductor particles to OH radicals, and from solvated electrons or alcohol radicals to the particle were studied. The absorption spectrum of CdS with an excess electron is blue-shifted. Colloidal nanoparticles (2 nm) of chalcopyrite CuFeSj are oxidized in their first monolayer by Fe ions induced by a pulse. 

---

CH rCHCH NHCSNH. Colourless crystalline solid with a faint garlic-like odour m.p. 74 C. Manufactured by treating propenyl isothiocyanate with a solution of ammonia in alcohol. It has been given by injection in the treatment of conditions associated with the formation of excessive fibrous tissue. Toxic side reactions may occur. Propenyl thiourea is a chemical sensitizer for photographic silver halide emulsions. 

Na COj and Na2S with SO2 or from Na2S03 plus sulphur. Forms many hydrates. Used-in photography ( hypo ) because it dissolves silver halides. Also used in tanning, preparation of mordants, as a fermentation preventative in dyeing and in chemical manufacture. 

While much early work with dispersed electrochemical systems focused on silver halide sols [16], more recent studies by Rusling and co-workers and others exploited... 

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

Many ionic halides dissolve in water to give hydrated ions. The solubility of a given halide depends on several factors, and generalisations are difficult. Ionic fluorides, however, often differ from other halides in solubility. For example, calcium fluoride is insoluble but the other halides of calcium are highly soluble silver fluoride. AgF, is very soluble but the other silver halides are insoluble. 

All the silver halides are sensitive to light, decomposing eventually to silver. In sunlight, silver chloride turns first violet and finally black. The use of these compounds in photography depends on this (see below). (All silver salts are, in fact, photosensitive—the neck of a silver nitrate bottle is black owing to a deposit of silver.)... 

REACTIONS AND IDENTIFICATION 325 drop. JNote that the white precipitate of silver salts turns black (showing the presence of silver cyanide). Now add an excess of mercurous nitrate the black precipitate redissolves, but the silver halide remains undissolved. 

To determine which halogen is present, take 1-2 ml. of the filtrate from the sodium fusion, and add dilute sulphuric acid until just acid to litmus. Add about 1 ml. of benzene and then about 1 ml. of chlorine water and shake. A yellowish-brown colour in the benzene indicates bromine, and a violet colour iodine. If neither colour appears, the halogen is chlorine. The result may be confirmed by testing the solubility of the silver halide (free from cyanide) in dilute ammonia solution silver chloride is readily soluble, whereas the bromide dissolves with difficulty, and the iodide not at all. 

Principle. A known weight of the substance is heated with fuming nitric acid and silver nitrate in a sealed tube. The organic material is thus oxidised to carbon dioxide and water, whilst the halogen is converted quantitatively into the corresponding silver halide. The latter js subsequently washed out of the tube, filtered and weighed. 

Place in the tube sufficient organic compound to give subsequently about 0-3 g. of the silver halide, and weigh again. Now allow the small tube to slide carefully down the inclined Carius tube until it finally adopts the position shown in D (Fig. 72). If the compound readily loses halogen in the presence of nitric fumes, the Carius tube should first be rotated in an oblique position to wet the tube for about 10 cm. from the bottom the small tube, if cautiously inserted into the Carius tube, will now come to rest when it first reaches the wet portion of the tube and will thus be held above the main bulk of the acid until the tube is sealed. 

Principle. This is essentially a small-scale modification of the macro piethod described on p. 416, the substance being completely oxidised in a sealed tube with fuming nitric acid in the presence of silver nitrate, the halogen being thus converted into silver halide. The collection and weighing of the silver halide require special techniques on the semi-micro scale. 

The apparatus for collecting the silver halide is described later. 

Collecting the silver halide. Meanwhile assemble the apparatus shown in Fig. 91. G is a filter-funnel having a sintered glass filter-plate... 

Remove the tube K and the bung L. Examine the underside of the bung for any particles of silver halide that might be adhering wash any such particles down into the funnel G with a small quantity of ethanol. Then wash the inside walls of the funnel, under gentle suction, with 5 ml. of ethanol from a wash-bottle, and finally the walls and precipitate with two 5 ml. portions of acetone, and drain well. 

Sulphur, as sulphide ion, is detected by precipitation as black lead sulphide with lead acetate solution and acetic acid or with sodium plumbite solution (an alkaLine solution of lead acetate). Halogens are detected as the characteristic silver halides by the addition of silver nitrate solution and dilute nitric acid the interfering influence of sulphide and cyanide ions in the latter tests are discussed under the individual elements. 

Many patents and studies are still published in the field of thiazolo dyes because the photographic industrx is always looking for new sensitizing dyes with improved efficiency and eager to know more about the mechanisms of their action on silver halide. 

The use of sensitizing dyes in photography has been the subject of many studies and constitutes. still now. one of the most studied areas in specialized periodic publications (125, 126) or in textbooks (88. 127). It can be ascertained that one hundred years after Vogel s discovery of spectral sensitization, the basic mechanisms of action of dyes on their silver halide support still remain not fully understood. However, the theoretical reasons explaining why among many other dye families practically only cyanine methine dyes appear to be spectral sensitizers (128) are better known. 

---