Precipitate

a. (from the verb.) Steeply falling (Raleigh), 2. Tead long; hasty; rashly hasty (Clarend.). 3. Hasty; violent (Arbuthnot). (PRECIPITATE, in chemistry, (heederschlag, Germ.) In a more limited and appropriate sense a substance thrown down from its solution in a ménstruum by the application ofa third or additional material, which detaches it, either by a greater affinity to itself or to the menstruum with which it has thus far been combined. (See the article PRECIPITATION.) It is nece ary, however, for the name of a precipitate in this restricted sense, that the substance thus subsiding should fall in a flocculent or pulverulent form, since if the different particles unite in an angular polyhedral figure it will be a erystallization. The term, however, is often used in@ more extended sense to in i part any visible substance, simple or compound, separated from its menstruum whether it sink orswim, or whether it be a crystal or a powder; and in this view precipitation forms one of the great operations in chemistry, and instead of being opposed to the phenomenon of crystallization, is directly opposed to that of solution. It is chiefly from the proce of piecipitation, as Berthollet has shown in his valuable e ays on cheinical statics, a that philosophers have been led to deduce the supposed order of affinities between different substances. The precipitates of most importance to attend to are those which occar in the case of acids, alkalies, or earths, and those in the case.of metals and metallic compounds. We shall subjoin from Berthollet. a few observations upon both these, connected with remarks offered by the Me rs. Aikin. The second kind of precipitate is that which has been usually supposed to be simple, and which takes place when to a compound of an -acid and a hase, insoluble or nearly so in water, an alkali or soluble earth is added, which by. elective affinity unites with the acid and displaces the former base: and this latter now supposed to be uncombined, and insoluble in the quantity of Hquid present, falls to the hot. ‘om. ‘This kind of precipitation is constantly resorted to when ‘we wish to obtain the simple earths, as in adding an alkali to a solution of alum in order to precipitate the alumine, to Epsom salts to precipitate magnesia, . But the precipitate in all such cases is not (as has been usually supposed) the simple earth, since it always retains after the first operation a small and variable portion of the acid with which it was before combined. ‘Thus, if the alumine, however well edolcorated, be re-di olved in muriatic acid, and a salt of barytes be added, there will be a sensible precipitate of sulph at of barytes. This retention of acid is variable, and may be reduced to an insensible quantity, if not absolutely taken away by subsequent digestion in very concentrated alkali. ‘Such precipitations therefore are not effected merely by simple affinity, but by a division of the acid into two portions, one of which, and by much the largest, goes to the alkali, and the other remains with the earth, and accompanies it in its precipitation. Even when no precipitation eccurs, the acid may be proved to distribute itself in this manner; for as Bergman has remarked, if a calcareous salt be di olved in fifty times its weight of water and an exce of potash or soda be added, no precipitation takes place, so that the acid continues in part to act upon the lime, and renders it soluble in a much le quantity of water than the lime alone would require for solution. t may be a umed, therefore, as a general rinciple, and one that is of great importance in the explanation of chemical decomposition, that when a salt is composed of an acid and a base, insoluble, or little soluble by itself, the decomposition effected by any soluble base is the result not of a simple but a compound action, in which, on the one hand, the decomposing base unites with more or le of the acid and remains in solution, while the other base resumes its insolubility by the separation of part of its acid, and forms a precipitate with the remainder. This observatian will be found to apply still more strikingly in the formation of mo precipitates, and of those salts which (like the sub sulph at of mercury or turbith) were formerly taken for simple oxyds, but are now found to be salts with exce of oxyd. ‘The metal of any metallic solution in any acid is always in the state of an oxyd, so that the solution must contain at least three substances, the acid, the metal, and oxygen united with the metal. But the proportion of the latter is subject to a great number of variations, and is different both with the several metals and with the same metal in different circumstances, A metallic solution may be decomposed and its metallic part precipitated either in the reguline state or as an oxyd; and these two methods which have been adopted in the analysis of metallic bodies require to be considered separately, When a carbonated alkali is added to a meéailie solution the precipitate is a carbonated oxyd of the metal, and like the other carbs nats, effervesces on the addition of a stronger acid. But when a pure acid is added only to saturation the precipitate is in some instances! nearly a pure oxyd, but in the greater number is an oxyd retaining a small proportion of the acid’ with which it was united. This acid, however, is not sufficient to render it soluble, and it may be generally entirely taken away by subsequent digestion in a concentrated fixed alkali, provided” the oxyd is not soluble therein. Hence the first action of alkalies on most metallic solu-_ tions is to cause an unequal division of the acid, the greater part of which unites with the alkali, and the remainder falls down along with the — oxyd now rendered insoluble. If ammonia be the alkali employed there is also in general a partition of the alkali, and the precipitate is a quadruple compound of the metal, the oxygen united with it, and a small portion of the acid, — and of the.ammonia; and the solution contains the greater part of the acid of the alkali, and ofien a sinall part of the metallic oxyd sufficient — to be very's ensible to the taste and to chemical — tests. According to Proust, lime also,when used — as a precipitant, enters in a small proportion into the precipitate, Metallic solutions sometimes yield precipi-'_ tates on the mere addition of water, by the simple distribution of the constituents into two portions, a soluble and an insoluble one, the former of which has an exce of acid, and the latter of oxyd. Thus when the clear nitrat of. bismuth is diluted with water a white precipitate falls down, which is chiefly oxyd of bismuth, and the supernatant liquor contains an exce of acid and a small portion of metal, which may be separated by evaporation, or by _ an alkali. Jn like manner, if sulphuric acid and mercury be treated together, sulphureous acid is given out, and the white ma that remains is an uniform acid sulph at which deliquesces into a dense solution. But if hot water be added, the turbeth or yellow sub sulph at of mercury is precipitated, and the solution contains an acid sophie The second species of metallic precipitates: are those in which the metal’ appears in its reguline form. This takes place when one metal is precipitated by another, as when a piece of iron is immersed ina solution of copper, and also when phosphorus is kept for some time in certain metallic solutions, ake The order of the precipitation of one metal by another is the full owing among those that have been most accurately examined,—zine, tron, lead, tin, copper, silver, mercury, cold: that is to say, zine precipitates all the other metals; jron precipitates all but zinc; lead all but zine and iron, . But. sometimes the precipitation fails with solutions in one acid though it succeeds in others: thus zinc separates iron in the metallic state from muriat of iron, but only as an oxyd from the nitric. A smali.exce of acid is nece ary to begin the proce in all cases. During this operation the following affinities _must act, namely, that of the precipitant (or metal by which the separation is effected) for the oxygen of the metal already di olved; that of the oxyd thus produced for the acid of the solution; and to this Berthollet adds, that of the two metals for each other when in the reguline state. The sum of these affinities must overcome that which exists between the constituent parts of the metallic salt intended to be _decom posed. - The metals which phosphorus separates from their solutions in the reguline state are principally gold, silver, mercary,and copper. Thus ifa stick of clean newly-melted phosphorus be immersed in an acidulated solution of sulph at of ‘copper, in a day or two it becomes beautifully spangled with little knobs of bright metallic copper which gradually increase to a thick crust, after which if the stick be put into boiling water the phosphorus will melt out and leave a delicate hollow cylinder of copper. -PRECYPITATELY. ad. (from precepit ate.) 1. Headlong; steeply down, 2. Hastily; in blind hurry (Pope). _* PRECIPITA’TION. s. (from precipitate.) 1. The act of throwing headlong (Shakspeare). 2. Violent motion downward (Woodward). Tumultuous hurry; blind haste (Woodward). _PrecieiTaTioNn. (from preecipilo, to cast down.) In chemistry. When two bodies are united, for instance, an acid and an oxyd, or metallic ealx, and a third body is added, such as an alkali, which has a greater affinity with the acid than the metallic oxyd, the conse- quence is, that the alkali combines with the acid, and.the oxyd, thus deserted, appears ina separate state at. the bottom of the ve el in which the operation was performed. This decomposition is commonly known by the name of precipitation,.and the substance that sinks is ndmed a precipitate. ‘he substance, by the addition of which the phenomenon is pro- duced, is denominated the precipitant. By this operation, bodies are recovered from _ their solution, by means of the addition of some other substanee, with which either the men- strnum or the body di olved have a greater af- finity than they have with each other. 658 659