Silver bromide sol

Mix equal amounts of 0.020 mol din sodium bromide solution and 0.018 mol dm silver nitrate solution. A colloidal dispersion of silver bromide is formed immediately. A silver iodide sol maybe prepared in a similar manner. 

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Figure 9 shows plots of layer thickness as a function of spin-fluid ionic strength obtained for four silver bromide sols at pH 6. 

Observe that both the gold sol and the ferric hydroxide sol appear quite dear when viewed in ordinary light. Now pass a beam of light through each of them and observe a Tyndall cone. Suitable light sources are a beam from a slide projector or, in a darkened room, a beam from a hand torch. Similar observations can be made with diluted samples of the sulphur and silver bromide sols and with microemulsions. 

Stumm, Huper, and Champtin (195) reviewed the interaction of polysilicic acid as coagulants with other colloids. Polysilicic acids coagulate positive colloids at low concentration, but in excess can reverse the charge and restabilize the system. Specific interactions can outweigh electrostatic repulsion thus negative polysilicate ions can flocculate negative silver bromide sols. 

Figure 16. Electrophoretic mobilities of a silver bromide sol in statu nascendi (AgBr,...

Figure 17. Plot of the critical coagulation concentrations (ccc, solid lines) and of the critical stabilization concentrations (esc, dashed lines) of a silver bromide sol in statu nascendi as a function of the pH for ethlenediamine (en. A), 2,2"-dipyridyl (dipy, o, ) and 1,10-phenanthroline (phen, , ). AgBr, I.O IO" excess Br-, 1.9-10 mol dm [28].

Figure 18. Experimentally determined critical coagulation concentrations (c.c.c., o) and critical stabilization concentrations (c.s.c.) due to charge reversal ( ) of 2,2"-dipyridyl (dipy) as a function of the pH for a negatively charged silver bromide sol. The molar concentrations...

Basinski carried out a more complete investigation on concentrated silver bromide sols. I he 2 ero point of charge was situated slightly asymmetrically at a Br ion concentration of 2-4- iV. and a Ag ion concentration of 1-2 10 N. The adsorption isotherms resemble those of the Agl sols. The capacity of the double layer is of the order of 10 This value is rather uncertain owing to the difficulty of the estimation of the total interface of the sol particles., ... 

A typical graph demonstrating this region of oriented flocculation is given by Fig. 29 representing the extinction of a silver bromide sol as a function of the pAg 20 hours after the mixing of the solution of silver nitrate and potassium bromide. 

Colloids are classified according to the phases of the substances involved (Table 15.10). A colloid that is a suspension of solids in a liquid is called a sol, and a suspension of one liquid in another is called an emulsion. For example, muddy water is a sol in which tiny flakes of clay are dispersed in water mayonnaise is an emulsion in which small droplets of water are suspended in vegetable oil. Photographic emulsions also contain solid colloidal particles of light-sensitive materials such as silver bromide. Foams are suspensions of a gas in a liquid or solid. Foam rubber, Styrofoam, soapsuds, and aerogels are foams. Zeolites (Box 13.4) are a type of solid foam in which the openings in the solid are comparable in size to molecules. 

When aqueous solutions of silver nitrate and sodium bromide are mixed rapidly, the silver bromide may form a hydrophobic colloidal suspension rather than precipitating. The tiny particles are kept from settling out by Brownian motion, the motion of small particles resulting from constant bombardment by solvent molecules. The sol is further stabilized by the adsorption of ions on the surfaces of the particles. The ions attract a layer of water molecules that prevents the particles from adhering to one another. 

Ottewill, R.H. and Woodbridge, R.F. 1961. The preparation of monodisperse silver bromide and silver iodide sols. J. Colloid Sci., 16, 581-594. 

The numerous tables in the article of Teague and Buxton go to show that the maximum precipitation occurs when the two dyes are in equivalent amounts. This Avould indicate that the acid and basic dyes in the solution unite to form a salt which falls out because of its insolubility, just as silver iodide is precipitated from solutions of silver nitrate and potassium iodide mixed together. That the precipitation is not complete when one or other of the substances is in excess has been explained in the discussion on sol formation of silver bromide, page 180. When the cathion and anion have united to form a salt the tendency of the neutral particles is to join together in complexes which finally precipitate out. If, however, one or other of the dyes is present in excess the salt particles adsorb some one ion more than another, and thus attain a positive or negative charge. The chaise on the particles increases the stability to such an extent that little or no precipitation may result. If the acid dye is present in excess the particles will... 

The abnormally low flocculation values of complicated organic ions have already been cited in sect. 3. Closer inspection shows that many of these ions give rise to reversal of charge and an irregular series, e.g, in the case of the silver iodide sol, laurylpyridi-nium bromide or strychnine nijrate... 

K bromide solns v si sol in w. It occurs in nature as the mineral born silver and is formed when a soln of a sol chloride is added to a soln of a Ag salt. Silver chloride in contact with metallic Na or K is exploded by percussion (Ref 3). Its ammoniacal soln absorbs acetylene, forming Silver Acetylide Chloride complexes which are expl (See Vol 1, p A80 l. Silver chloride is used in medicine as an antiseptic nerve sedative, in the manuf of pure Ag Ag salts, and in photography photometry(Refs... 

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. 

Light yellow, odorless powder slowly darkened by light. Crystals are hexagonal Or cubic, d 5.67 mp 552. Practically insol in water (0.03 mg/l) in acid (except coned HI in which it dissolves readily on heating) in ammonium carbonate. Freely sol in solns of alkali cyanides or iedides 35 mg dissolve in a liter of 10% ammouia appreciably sol in coned solns of alkali bromides, chlorides, thiocyanates, thiosulfates, mercuric and silver nitrates. It is slowly attacked by boiling coned acids, but not affected by hot solns of alkali hydroxides. 

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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