Licence: Public Domain Mark
Credit: Qualitative chemical analysis / by C. Remigius Fresenius. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![-^^'^ OXIDE OF MERCURY. (-^ ^hT\ u^^ saj^^svery poisonous and most of them are colourless Ihe soluble neutral salts redden litmus-paper. The nitrate and sulphate are decomposed by a large quantity of water into soluble acid and insoluble basic salts. 3. The addition of a very small quantity of sulphuretted hydrogen •or sulphide of ammonium produces, after shaking, a perfectly white precipitate ; a somewhat larger quantity of the reagent causes the pre- cipitate to acquire a yellow, orange, or brownLsh-red colour; whilst an excess produces a black precipitate of mercuric sulphide, HgS [HgS] Ihis progressive variation of colour from white to black, which depends on the proportion of the sulphuretted hydrogen or sulphide of ammonium added,_ distinguishes mercuric oxide from all other bases. The white precipitate which forms at first consists of a double compound of mer- curic sulphide with the still undecomposed portion of the salt of of mercuric chloride, for instance, 2HgS-l- HgCl [2HgS,HgCIJ; the graduaUy increasing admixture of black sulphide causes the precipitate to pass through the several gra- dations of colour above mentioned. Sulphide of ammonium dissolves the merest trace of mercuric sulphide; least of all when the precipitate is digested with hot yeHow sulphide of ammonium. Potassa and cyanide of potassium do not dissolve mercui^c sulphide, and it is -entirely insoluble in nitric acid, even on boiling. By the very pro- tracted action of hot concentrated nitric acid the precipitate is converted into a white compound, consisting of 2HgS + H:gO,N'05 L2HgS,Hg(N0,)J Hot concentrated hydrochloric acid dissolves it comnaratively easily but It is less soluble in the cold acid; if dilute, the acid dissolves it to some extent when boiling but scarcely at all in the cold. Sulphide of potassium and sulphide of sodium in the presence of potash or soda dissolve the precipitate completely, but it is insoluble in hydrosulphide ■of potassium, and in hydrosulphide of sodium. Aqua regia decom- poses the precipitate and dissolves it with ease. In solutions of mer- curic oxide containing a large excess of concentrated mineral acid, sul- phuretted hydrogen produces a precipitate only after the solution has been diluted with water. _ 4. Potassa or soda added in small quantity to neutral or slightly acid solutions produces a reddish-brown precipitate, which becomes yellow when the reagent is in excess; the reddish-brown precipitate is a basic salt; the yellow precipitate consists of mercuric oxide. It is not soluble in an excess of the precipitant. In very acid solutions this reaction does not take place at all, or at least the precipitation is very incomplete. In presence of salts of ammonia, potassa produces white precipitates. The precipitate thrown down by potassa from a solution of mercuric chloride containing an excess of chloride of ammonium is an- alogous in composition to the precipitate produced by ammonia (see 5). 5. Ammonia produces white precipitates quite analogous to those produced by potassa in presence of chloride of ammonium; thus, for instance, ammonia precipitates from solutions of mercuric chloride NHgHg^Cl [NHjHgCI], the so-called infusible white precipitate, which may be regarded as a chloride of dimercur-ammonium, or as mercurammonic chloride, or as mercury amidochloride. If the solution ■contains much free acid, ammonia produces no precipitate. The white precipitate is insoluble in ammonia, but easily soluble in hydro- chloric acid.](https://iiif.wellcomecollection.org/image/b21966953_0168.jp2/full/800%2C/0/default.jpg)
