Silver halide sols

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... 

For the determination of chlorine ions add to the solution 5-10 drops of a 0101 per cent alcoholic solution of dichlorofluorescein and titrate with neutral 0-02-0-025 N silver nitrate solution, using a micro-burette with 0-02 c.c. scale divisions. At the beginning of the titration the solution is only slightly opalescent deep turbidity develops as the end-point is approached. At this stage continue to titrate cautiously with vigorous shaking until the silver halide sol suddenly coagulates to reddish-pink flocks. 

Particle electrophoresis studies have proved to be useful in the investigation of model systems (e.g. silver halide sols and polystyrene latex dispersions) and practical situations (e.g. clay suspensions, water purification, paper-making and detergency) where colloid stability is involved. In estimating the double-layer repulsive forces between particles, it is usually assumed that /rd is the operative potential and that tf/d and (calculated from electrophoretic mobilities) are identical. 

Numerous studies have been made of the interaction of polyvalent metal ions with the silica surface. The development of Zr-O-Si bridges has been examined by Nikolskii et al. (432), who concluded that as the basic Zr ions polymerized they were less reactive. Healy, cooper, and James (433) found that Fe and Cr are adsorbed on silica particles, giving them a positive charge. The chemistry of hydrolysis of cations Al, Zr, Th, Cr has been summarized by Baes and Mesmer (434). Matijevic, Janauer, and Kerker (435) have shown that it is the polymeric hydrolyzed species in aluminum nitrate solution and not the single AF ion that can cause charge reversal in silver halide sols. The same is true in charge reversal of silica sols. 

This equivalent flocculation ii also encountered in silver halide sols, pepti2 ed by halide ions which are flocculated by equivalent amounts of silver ions or in Fe(OH)3 sols, peptized with HCl which are flocculated by NaOH. In these last two cases a more than equivalent addition of the flocculating electrolyte leads to a second region of stability, where the charge of the sol proves to be inverted in sign ... 

When an irregular series is produced by addition of potential-determining ions (negative silver halide sol -f Ag NO3, positive Fe(OH) sol -j- NaOH) the explanation is simply that the surface passes its zero point of charge. 

The dispersion (or sols) are only slightly viscous. Examples sols of metals, silver halides, metallic sulphides, etc. 

The viscosity of the sols is similar to that of the medium. Examples sols of the metals, silver halides, metallic hydroxides, and barium sulphate. 

Pentahydrate, odorless crystals or granules, mp 48° when rapidly heated. Effloresces in warm dry air slightly deliquesces in moist air. d 1.69. Loses all its water at 100° dec at higher temp. Sol in water insol in alcohol. Slowly dec in aq soln at ordinary temp, more rapidly when heated. The aq soln is practically neutral. pH 6.S-S.0. It dissolves silver halides and many other salts of silver. Incompat Iodine, acids lead, mercury, and silver salts. LD j.v. in rats >2.5 g/kg (Voegtlin). 

Until comparatively recently, the only reported stable inorganic hydrosols were primarily sols of elements such as gold, sulphur, selenium, etc. and compoimds such as silica, lead iodate, silver halides, etc. A considerable amount of attention is now being paid, however, to the preparation of mono-dispersed hydrous metal oxides, which are chemically considerably more complex than other crystalline or stoichiometrically well-defined materials and are of interest as potential catalysts. Examples include the hydrous oxides of chromium and aluminium (spheres) and copper and iron (polyhedra) with particle sizes < 1 pm. One manufacturing procedure consists of ageing aqueous... 

Double decomposition is the usual way of forming sols of insoluble salts Silver halides easily give colloidal solutions by mixing of silver nitrate and alkali halide solu-> tions. A sol of arsenic trisulphidc is obtained by introducing hydrogen sulphide into a saturated solution of arsenic trioxidc ... 

Apart from the conJIuo. of low total electrolyte content it is often found that the presence of small traces of specific electrolytes is necessary to obtain a stable soL Taking again the case of silver halides a slight excess of either Ag or halide ion is necessary to stabilize the sol formed. 

Somewhat more complicated were the experiments by Kruyt and Kunst on various hydrophobic sols (silver halides, arsenic sulphide). They found an increase of the D.C of the order expected from eq (40) with k a 0.5 or 1.0. The dispersion frequency was also in accord with eq. (41). So in these experiments the double layer probably played the most important role. Nevertheless the influence of added electrolytes was not clear in all respects. More experiments and a more refined theory are desired here. 

The low-frequency modes associated with the halides were seen also in colloidal systems. Wetzel and Gerischer found on a silver sol Raman features at 235, 163, and 112 cm for chloride, bromide, and iodide. Garrel et monitored the replacement of chloride by bromide on a silver colloid. 

The possibility of detecting metal-molecule vibrations in SERS has induced several studies in the absence of halides. Lombardi et have investigated a silver sol and found for adsorbed pyridine a wide band at about 230 cm This band was not seen when acetate or formate ions replaced the pyridine, but, instead, an unresolved feature at slightly higher frequencies (26Ocm 0 appeared. They... 

Sol 1. (i) Reaction of allyl halide and silver trifluoroacetate generates the methylallyl cation, which then reacts with cyclohexadiene to give the seven-membered ring cation. Like other tertiary cations, it loses a neighboring proton to give the alkene, i.e., 3-methylbicyclo[3.2.2]nona-2,6-diene. 

The second method is the opposite of the first that is to say, the potassium bromide solution is in the buret and is added to the solution of silver nitrate. The phenomena are quite analogous, but the hydrosols differ in one important particular. In the first case as long as the halide ion is in excess of the silver, the ultramicrons are charged negatively, while in the second case where the silver ion is in excess the ultramicrons are charged positively. The two halide hydro-sols mutually precipitate each other, as is to be expected from a mixture of oppositely charged colloids. 

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