The solutions of silver are also preci pitated by sulphuretted hydrogen, and by hydro-sulphurets ; but the precipitate is then reddish, or brown, or black ; or it may be at first white, and afterwards be come speedily brown or black. It is so luble, in great part, in dilute nitrous acid, which is not the case if occasioned by muriatic or sulphuric acid.
The solutions of silver are precipitated by extractive matter ; but in this case also the precipitate has a dark colour, and is soluble in nitrous acid.
.Rcetate of Lead —Acetate of lead is a test of sulphurettecl hydrogen and of by dro-sulphurets of alkalies, which occasion a black precipitate ; and if a paper, on which characters are traced with a solu tion of acetate of lead, be held over a portion' of water containing sulphuretted hydrogen gas, they are soon rendered vi . Bible, especially when the water is a little warmed.
alitriate, Nitrate, and 4cetate of Barytes. —These solutions are all most delicate tests of sulphuric acid and of its combina tions, with which they give a white pre cipitate, insoluble in dilute muriatic acid. They are decomposed, however, by car bonated alkalies ; but the precipitates, occasioned by carbonates, are soluble in dilute muriatic or nitric acid, with effer vescence, and may even be prevented by adding previously a few drops of the same acid as that contained in the barytic salt, which is employed.
One hundred grains of dry sulphate of barytes contain (according to Klaproth, vol. i. p. 168.) about 45i of sulphuric acid, the specific gravity 1850: accord ing to Clayfield, (Nicholson's Journal, 4to. iii. 38.) 33 ofacid, of specific gravity 2240; according to Thevnard, after calcination, about 25 ; and according to Mr. Kirwan, after ignition 23.5 of real acid. The same chemist states, that 170 grains of ignited sulphate of barytes denote 100 of dried sulphate of soda ; while 136.36 of the same substance indicate 100 of dry sulphate of potash ; and 100 parts result from the precipitation of 52.11 of sul phate of magnesia.
From Klaproth's experiments, it ap pears that 1000 grains of sulphate of ba rytes indicate 595 of desicated sulphate of soda, or 1416 of the crystallized salt. The same chemist has shown, that 100 graMs of sulphate of barytes are produced by the precipitation of 71 grains of sul phate of lime.
Prussiates of Potash and of Lime.—Of these two, the prussiate of potash is the most eligible. When pure, it does not speedily assume a blue colour, on the ad dition of an acid, nor does it immediately precipitate muriate of barytes.
l'russiate of potash is a very sensible test of iron, with the solutions of which in acids it produces a Prussian blue pre cipitate, in consequence of a double elec tive affinity. To render its effects more certain, however, it may be proper to add, previously to any water suspected to contain iron, a little muriatic acid, with a view to the saturation of uncombined al kalies or earths, which, if present, pre vent the detection of very minute quan tities of iron.
1. If a water, after boiling and filtra tion, does not afford a blue precipitate, on the addition of prussiate of potash, the solvent of ithe iron may be inferred to be a volatile one, and probably the carbonic acid.
2. Should the precipitation ensue in the boiled water, the solvent is a fixed acid, the nature of which must be ascer tained by other tests.
In using the prussiate of potash for the discovery of iron, considerable caution is necessary, in order to attain accurate re sults. The prussiate should, on all occa sions, be previously crystallized ;,and the quantity of oxide of iron essential to its constitution, or at least an invariable ac companiment, should be previously ascer tained in the following manner : Expose a known weight of the crystallized salt to a low red heat in a silver crucible. After fusing and boiling up, it will become dry, and will then blacken. Let it cool ; wash off the soluble part ; collect the rest on a filter ; dry it, and again calcine it with a little wax. Let it be again weighed, and the result will show the proportion of o side of iron present in the salt which has been examined. This varies from 22 to 30 and upwards pet cent. When the test is em ployed for discovering iron, let a known weight of the salt be dissolved in a given quantity of water : add the solution gra dually; and observe how much is ex pended in effecting the precipitation. Before collecting the precipitate, warm the liquid, which generally throws down a further portion of Prussian blue. Let the whole be washed and dried, and then ignited with wax. From the weight of the oxide obtained, deduct that quantity, which, by the former experiment, is known to be present in the prussiate that has been added ; and the will denote the quantity ofoxide of iron pre sent in the liquor which is under exami. nation.