Colloidal Metal Colors

The red color produced in many glasses containing gold, known as gold-ruby glasses, is due to the presence of very fine colloidal gold 

A number of other colloidal species, including, but not limited to Pb, As, Sb, Bi, Sn, and Ge, can be formed in glasses. The properties of the 

This reaction, which is called striking, occurs spontaneously upon reheating an originally colorless glass to the correct temperature. A similar process can be used to produce glasses colored by silver or copper. The color is distributed uniformly throughout the glass. 

Silver colloids will form spontaneously if silver films on float glass are heated in air to temperatures above 300 °C. The process involves ion 

---

Because colloidal metal (color) is produced by irradiation of small band gap azides, they may be useful as photographic materials. In fact, silver azide emulsions have been made and their properties compared with emulsions of silver halides [208,209]. No attempt is made here to review in detail the photographic properties of emulsions. Instead a few characteristics of crystalline material are discussed briefly. 

Colloidal metal coloration generally requires careful control of the overall oxidation state of the glass throughout the process. The melt must be kept sufficiently oxidized at the melting temperatures to keep the metal dissolved in its oxidized state until the melt is cooled, then it must be made sufficiently reduced to allow precipitation of the metal particles. [The metal must transform from the +1 oxidation state to the neutral (metallic) oxidation state as... 

When stannous chloride is added to solutions of platinum salts a red coloration is produced, which is due to the formation of so-called red colloidal platinum, which is kept in a fine state of division by the colloidal products of hydrolysis of the stannous chloride.7 In the absence of a protective colloid the red platinum changes to brown colloidal platinum. An interesting analogy may be traced between this red. colloidal metal and the better-known colloidal gold, termed purple of Cassius. ... 

The hydrosilylation reaction can also be conventionally conducted by reaction of an olefin and an SiH-f mctional polydimethylsiloxane in the presence of a standard transition metal catalyst, and after the reaction the catalyst can be extracted with an ionic liquid. In some cases, the use of an ionic liquid in the hydrosilylation process even improved the quality of the polyethersiloxanes with respect to color compared to the standard process. An explanation might be the avoidance of catalyst reduction leading to the formation of colloidal metal particles, which tend to color the product slightly brownish. In other words, the ionic liquid seems to have a stabilizing effect on the catalyst. 

Colloidal metal is produced in the alkali halides both by additive coloration and by irradiation [109]. In either case, the F center, an electron trapped at a negative ion vacancy, is the stable defect at room temperature. Clustering of F centers takes place during heat treatment or in some cases during irradiation. When a region consists almost exclusively of F centers, a coUapse of the lattice takes place and colloidal metal is formed. It is unlikely that colloids are formed in the small band gap azides in this manner. F centers have not been detected in these azides and are thus not dominant defects. In addition, to allow clustering, the mechanism requires F centers to be mobile at 12°K, which is unlikely. Colloids are not formed at low temperatures in the alkali halides, presumably because the F centers are not sufficiently mobile. 

The arsenic(lll) chloride will be reduced by hypophosphorous acid to amorph colloidal metallic arsenic, which appears more as a coloration than as a precipitate. 

One other model which was proposed primarily to explain the color of the solutions was the colloidal model by Kruger. According to this model, the metal is considered to be present as a colloid with negative charge, the colloidal metal particles exhibiting the observed blue color. However, the theory has met with little success in explaining the optical, magnetic, and other properties and will not be discussed any further. 

The formation of (II) provides a quite selective spot test for palladium. Gold must be removed prior to the test because it will cause the development of a deep ruby red in the spot plate test and a diffused violet spot on the paper, apparently due to the reduction of the gold ions to the colloidal metal. Interference may also arise from 0s04 , Os+, Ru+, and RuCle ions because they have distinct self-colors. Mercurous ion causes partial interference by the reduction of part of the palladium to the elementary state, but a positive response can still be seen. It is possible to detect I part of palladium in the presence of 200 parts of platinum or 100 parts of rhodium. Less favorable ratios should be avoided because of the color of these salts. No interference is caused by mercuric and iridic chloride, but free ammonia, ammonium ions, stannous, cyanide, thiocyanate, fluoride, oxalate, and tetraborate ions do interfere. Lead, silver, ferrous, ferric, stannic, cobaltous, nickel, cupric, nitrite, sulfate, chloride, and bromide ions do not interfere. 

Allied to the purple of Cassius are those substances composed of colloidal metals and colloidal stannic acid. A good instance of this is silver purple. Silver nitrate and stannous nitrate form a blood-red solution that gradually turns brown and from which a reddish brown precipitate falls out. Ditte mistook this for silver stannate, but L. Wohler f has demonstrated that the substance is analogous to gold purple. Lottermoser J has succeeded in preparing another silver purple synthetically by mixing colloidal silver and colloidal stannic acid. Acids throw out a dark violet precipitate that dissolves in alkali with a deep brown color. The same product may be obtained by mixing solutions of silver nitrate, stannic chloride and anunonium citrate. 

Many reagents have been suggested for the detection of gold, often involving its reduction to the colloidal metal, which imparts various colors to the system, depending on the particle size. 

Similarly, simple exact solutions can be obtained for small elhpsoidal particles of any arbitrary axial ratio. Such solutions have been found extremely useful in explaining the colors produced by colloidal metal particles suspended in glass, the coloration and polarizing effects found in photochrvmic glasses, and the color and performance possibilities of... 

---