WCO Explanatory Notes reproduced for reference. © World Customs Organization. Verify against official WCO publication.
28.43 - Colloidal precious metals; inorganic or organic compounds of precious metals, whether or not chemically defined; amalgams of precious metals. 2843.10 - Colloidal precious metals - Silver compounds : 2843.21 2843.29 2843.30 2843.90 - - Silver nitrate - - Other - Gold compounds - Other compounds; amalgams (A) COLLOIDAL PRECIOUS METALS This headin covers recious metals as listed in Chapter 71 (i.e., silver, old, platinum, iridium, osmium, pafiadium, &odium and ruthenium), provided they are in collol a1 suspension. These precious metals are obtained in this state either by dispersion or cathodic pulverisation, or by reducing one of their inorganic salts. % Colloidal silver occurs in small grains or flakes, bluish, brownish or greenish-grey, with a metallic glint. It is used in medicine as an antiseptic. Colloidal gold may be red, violet, blue or green, and is used for the same purposes as colloidal silver. Colloidal platinum is in small grey particles and has remarkable catalysing properties. I These colloidal metals (e.g., gold remain classified in this heading when put up in colloidal solutions containing protective col oids (such as gelatin, casein, fish glue). (El) INORGANIC OR ORGANIC COMPOUNDS OF PRECIOUS METALS, WHETHER OR NOT CHEMICALLY DEFINED These are : (I) Oxides, peroxides and hydroxides of precious metals, analogous to the compounds of sub-chapter IV. (11) Inorganic salts of precious metals, analogous to the compounds of sub-chapter V. (IU)Phosphides, carbides, h drides, nitrides, silicides and borides, analogous to the compounds of headings 2H48 to 28.50 (such as platinum phosphide, palladun hydride, silver nitride, platinum silicide). (IV)Organic compounds of precious metals, analogous to the compounds of Chapter 29. Compounds containing both recious metals and other metals (e.g., double salts of a base metal and a precious metal, comp ex esters containing precious metals) are also covered by this heading. P The most common compounds of each of the precious metals are listed below : (1) Silver compounds. (a) Silver oxides. Disilver oxide (Ag20) is a brownish-black powder slightly soluble in water, turning black on exposure to light. Silver oxide (Ago) is a greyish-black powder. Silver oxides are used, inter alia, in the manufacture of batteries. (b) Silver halides. Silver chloride (AgCl) is a white mass or dense powder, insoluble in water, darkenin when exposed to light; it is packed in dark-coloured opa ue containers. Use in photography, in the manufacture of ceramics, in medicine an in silvering. h Cerargyrites (or horn silver), natural silver chlorides and iodides, are excluded (heading 26.16). Silver bromide (yellowish), silver iodide (yellow), and silver fluoride are used for similar purposes to the chloride. (c) Silver sulphide. Artificial silver sulphide (A&S) is a heavy grey-black powder, insoluble in water, used for glass-making. rite Natural silver sulphide (argentite), natural sulphide of silver and antimony (pyrar 01 basite) and natural sulphide of silver and arsenic (proustite) are exc uded $?."dtFi& (d) Silver nitrate (AgN03) for silvering lass or manufacture o indelible ink; " lunar caustic ", though this uantity of sodium or potassium ?om a cauteriser of Chapter 30. B (e) Other salts and inorganic compounds. Silver sulphate (Ag2S04)crystals. Silver phos hate (Ag3P04), yellowish crystals not very soluble in water; used in medicine, p! l otography and optics. Silver cyanide (AgC.N}, a white powder darkenin on ex osure to li water; used in med~c~ne and for silver-plating. Eilver &ocyanate similar appearance and is used as an intensifier in photography. Complex cyanide salts of silver and potassium (KAg(CN)S or silver and sodium (NaAg(CN)2)are soluble white salts, used in electroplatmg. Silver fulminate, white crystals, exploding at the slightest shock, dangerous to handle; used for the manufacture of detonattng caps. Silver dichromate used for painting ruby-red powder, slightly soluble in water; red, purple red). Silver permaganate, a crystalline deep violet powder, soluble in water; used in gas masks. Silver azide, an explosive. (f) Organic compounds. These include : (i) Silver lactate (white powder) and silver citrate (yellowish powder); used in photography and as anhseptics. (ii) Silver oxalate, which decomposes and explodes when heated. (iii) Silver acetate, benzoate, butyrate, cinnamate, picrate, salicylate, tartrate and valerate. (iv) Proteinates, nucleates, nucleinates, albuminates, peptonates, vitellinates and tannates of silver. Gold compounds. An insoluble, dark violet powder. Auric oxide (hZO3) the corresponding acid is auric hydroxide or acid decomposed on exposure to light, from which alkali (b) Chlorides. Aurous chloride AuCl , a yellowish or reddish crystalline powder. Gold trichloride (AuC13) (auric c lori e, brown chloride), a reddish-brown powder or crystalline mass, very hygrosco ic, often resented in sealed flasks or tubes. Tetrachloroauric (111) acid (Au 13.HC1.4H2 ) (yellow chloride) yellow crystds, hydrated, and alkali chloroaurates, reddish-yellow crystals, are also classified xn this heading. These products are used in hotography (preparation of toning baths), in the ceramic or glass industries and in me icine. \ d a The heading excludes purple of Cassius, a mixture of stannic hydroxide and colloidal gold (Chapter 32); this is used in the manufacture of paints or varnishes, and especially for colouring porcelain. (c) Other compounds. Gold sul hide (Au2S3)a blackish substance which, in combination with alkali sulphides, forms goaurates. Double sul hites of gold and sodium (NaAu(S03)) and of gold and ammonium ( N & ~ ( S & ) )marketed as colourless solutions, are used in electroplating. Sodium aurothiosulphate is used in medicine. Gold cyanide (AuCN), a crystalline yellow powder decomposable if exposed to heat, is used for electrolytic gilding and in medicine. Reacts with alkali cyanides to give cyanoaurates, such as potassium tetracyanoaurate (KAU(CN)~),which is a soluble white salt used in electroplating. Sodium aurothiocyanate, crystallising in orange-coloured needles; used in medicine and in photography (toning baths). Ruthenium dioxide (Ru02) is a blue product, while ruthenium Ruthenium trichloride (RuC13) apd ruthenium chlorides with alkali chlorides and ammno or nitroso of ruthenium-oralkali metals. onds to rhodium oxide with alkali (4) Rhodium compounds. The (*03) a black powder. chlorides, and there are, a sulphate complex nitrites; also cyanorhodites (5) Palladium compounds. The most stable oxide is palladous oxide (PdO), the only basic one. It is a black powder decomposed by heat. Palladous chloride (PdC12), a brown deliquescent powder, soluble in water and crystallising with 2 H20, is used in the ceramic industry, in photography and in electroplating. Potassium chloropalladite (K2PdC14),a brown salt, fairly soluble, used as a detector of carbon monoxide, is also classified here. There also exist chloropalladates, amrnino compounds (palladium diammines), thiopalladates, palladonitrites, cyanopalladites, pallado-oxalates and palladous sulphate. (6) Osmium corn ounds. Osmium dioxide (OsOz) is a dark brown powder. Osmium tetraoxide ( 0 s h is a volatile solid, crystallising in white needles; it attacks the eyes and lungs; used in histology and micrography. This tetraoxide ives osmates such as pofassium osmate (red crystals), and, by treatment with ammonia an alkali hydroxides, osmiamates such as osmiarnates of potassium or sodium, yellow crystals. % Osmium tetrachloride (OsC14) and trichloride (OsC13) give alkali chloro-osmates and chloro-osmites. (7) Iridium compounds. In addition to iridium oxide there are an iridium tetrahydroxide blue solid, a chloride, chloroiridates and chloroiridites, double sulphates anld ammino compounds. (8) Platinum compounds. (a) Oxides. Platinous oxide (PtO) is a violet or blackish owder. Platinic oxide (PtO?) forms several hydrates of which one, the tetrahydrate &P~(OH)~)is a complex ac~d (hexahydroxyplatinic acid) to which correspond salts such as alkali hexahydroxyplatinates. There are also corresponding ammino complexes. (b) Other compounds. Platinic chloride (PtC14) occurs in the form of a brown powder or a yellow solution; it is used as a reagent. Commercial platinum chloride (chloroplatinic deliquescent prisms, brownish-red in colour, soluble in water; used in toning), in platinum- latin , for ceramic glazing or for sponge. There are correspon!c ng p atinum ammino complexes. K There are also ammino complexes corresponding to chloroplatinous acid (H2PtC14), itself a red solid. Cyanoplatinites of potassium or barium are used for obtaining fluorescent screens for radiography. (C)AMALGAMS OF PRECIOUS RWTALS These are alloys of precious metals with merc . Amalgams of gold or silver, which are the most common, are used as intermediate products or obtaining these precious metals. The heading includes amalgams containing both precious metals and base metals (e.g., certain amalgams used in dentistry); but it excludes arna1ga.mwholly of base metal (heading 28.53). Mercury corn ounds, whether or not chemically defined, other than amalgams are excluded (heading 28.56.
1.- Except where the context otherwise requires, the headings of this Chapter apply only to : (a) Separate chemical elements and separate chemically defined compounds, whether or not containing impurities; (b) The products mentioned in (a) above dissolved in water; (c) The products mentioned in (a) above dissolved in other solvents provided that the solution constitutes a normal and necessary method of putting up these products adopted solely for reasons of safety or for transport and that the solvent does not render the product particularly suitable for specific use rather than for general use; (d) The products mentioned in (a), (b) or (c) above with an added stabiliser (including an anti-caking agent) necessary for their preservation or transport; (e) The products mentioned in (a), (b), (c) or (d) above with an added anti-dusting agent or a colouring substance added to facilitate their identification or for safety reasons, provided that the additions do not render the product particularly suitable for specific use rather than for general use.