Fusible white precipitate

The diammine [Hg(NH3)2Cl2], descriptively known as fusible white precipitate , can be isolated by maintaining a high concentration of NH4+, since reactions (2) and (3) are thereby inhibited, or better still by using non-polar solvents. It is made up of a cubic lattice of Cl ions with linear H3N-Hg-NH3 groups inserted so as to give the common, distorted octahedral coordination about Hg (Hg N = 203 pm, Hg-Cl = 287 pm) (Fig. 29.4a). 

Several min after addition of ethanol to a mixture of the amide chloride ( fusible white precipitate ) and iodine, an explosion occurs. Addition of the compound to chlorine gas or bromine vapour leads to a delayed violent or explosive reaction. Amminemetal salts behave similarly, and formation of A-halogen compounds is involved in all cases. 

The product decomposes on sublimation forming mercury(ll) chloride, ammonia, and nitrogen. However, in the presence of ammonium chloride, the same reaction with ammonia with HgCb in aqueous solution yields fusible white precipitate, (NH3)2HgCl2. Similar product also is obtained by reaction of gaseous ammonia with solid mercury(II) chloride. However, the solid mer-cury(II) chloride is more stable than the above infusible product and can be melted without decomposition. 

EXERCISE 72. FUSIBLE WHITE PRECIPITATE, HgCl2-2NHa... 

Figure 19 The elementary cell of the fusible white precipitate Hg(NH3)2Br2...

Mercury forms direct covalent as well as co-ordinate links with nitrogen. Mercury(II) chloride reacts with gaseous ammonia to give fusible white precipitate ... 

Fig. 281. Random arrangement of Hg(NH,ions in fusible white precipitate, Hg(NH,)jCl,.

If PgCy is slowly added to a hot mixture of NH3 and the fusible white precipitate , Pg(NH3)2]Cl2, is formed, an example of (1) above. However, if NH3 is added to PgCy the infusible white precipitate , HgNH2Cl, is obtained, an example of (2) above. The addition of NH3 to Pgy or, more readily, the reaction between NH3 and Nessler s reagent , pgU] and OH, precipitates the reddish-brown iodide of Millon s base [an example of (3) above], i.e., Hg2Nl. This is a test for NH3 and is sometimes inconveniently over-sensitive. Rather similar bromides are Pg(NH3)2]Br2, HgNH2Br and Hg2NBr. 

To obtain it, bismuth is dissolved in nitric acid, and the solution thrown into water, when a copious white precipitate of subnitrate of bismuth is formed. This is washed, dried, and ignited, and protoxide of bismuth is left. It is a yellow powder, fusible at a red-heat. With acids it fonns colourless salts. Most of these salts are decomposed when thrown into water and this character, coupled xvith the black caused by sulphuretted hydrogen, enables us easily to recognise the presence of bismuth, and to separate it from other metMs. 

Is formed when hydrochloric acid or a soluble chloride is added to any solution of a salt of protoxide of lead H Cl + Pb 0 = PbCl + HO. It is deposited in strong solutions as a white precipitate, sparingly soluble in cold water. It dissolves in hot water, and forms white needles on cooling. It is fusible below a red-heat, and forms on cooling a horny mass. 

The potassium tantalum fluoride first precipitated is a fusible substance. Its aqueous solutions on being boiled predpitate a very insoluble potassium tantalum oxyfluoride, 4KF.Ta2Os.2TaFs or K4Ta4OfiFu, as a white powder. This reaction is stated to constitute a sensitive test for tantalum.1... 

Silver pyrophosphate, Ag4P207.—Precipitation of sodium pyrophosphate with silver nitrate yields the pyrophosphate in the form of a white powder, fusible at red heat. Secondary sodium orthophosphate converts it into silver orthophosphate. A double salt of the formula NaAg3P207 is also known. On heating, its colour changes from white to yellow, a phenomenon which has been attributed to decomposition into sodium metaphosphate and silver orthophosphate, but which is considered by Balareff 4 to have probably a physical origin. 

Morphine or Morphi This is the chief active principle of opium. The morphia of commerce is a white crystalline powder but when crystallized in alcohol, mnns brilliant, prismatic, transparent, and colorless crystals, which turn nitnc acid red. In powder, unlike strychnine, it is fusible without decomposition, and strongly decomposes iodic acid. It is insoluble iu ether, scarcely soluble iu water, and freely soluble in alcohol. Potassa and ammonia precipitate morphia from the solntions of its salts. 

Zinc Oxid—Zinci oxidiun (XT. S. Br.)—ZnO—80.9—is prepared either by calcining the precipitated carbonate, or by burning Zn in a current of air. An impure oxid, known as tutty, is deposited in the flues of zinc furnaces, and in those in which brass is fused. When obtained by calcination of the carbonate, it forms a soft, white, tasteless, and odorless powder. When produced by burning the metal, it occurs in light, voluminous, white masses. It is neither fusible, volatile, nor decomposable by heat, and is completely insoluble in neutral solvents. It dissolves in dilute acids, with formation of the corresponding salts. 

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