Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1846
    or the whole may be mixed together and concentrated by evaporation in a porcelain capsule; but either the concentration should not be carried so far as to produce a deposit from the solution, or the deposited matter should be separated by filtration, and separately examined. When the solution, as is often the case, contains but little organic matter, the testing for soluble saline substances may be immediately proceeded with; for although organic matter does interfere with the action of some of the reagents to be employed, when it is present only in small quantity, this interference will not be of great consequence. When, however, the dark colour of the solution indicates the presence of much organic matter, it is necessary to destroy it before proceeding with the testing. For this purpose, the solution is evaporated in a porcelain capsule, till it becomes of syrupy consistence; after which, it is transferred to a platinum crucible and evaporated to perfect dryness. The crucible is then heated to in- cipient redness, and continued in that state till all organic matter is burnt off, and the contents lose their blackness. It is to be kept in mind, how- ever, that if the solution contained volatile substances, such as nitric acid or ammoniacal salts, they are entirely dissipated in this process, and must be tested for in a portion of the solution reserved for the purpose, and not evaporated to dryness and ignited. It is also to be ob- served, that the heat is kept as low as possible during the burning off of the organic matter, to avoid, as far as it can be done, the loss of chlorides, which are almost invariably present, and are liable to be volatilised at high temperatures. After the organic matter is destroyed, the residuum is treated with dis- tilled water, to which a little nitric acid has been added; and if a small portion of insoluble matter still remains, it is silica, and may be separated by filtration. The slightly acid solution is now rendered exactly neutral by ammonia, or as nearly so as can be effected without causing precipita- tion; and thus prepared, the saline solution, which contained much organic matter, is ready for being tested. The operation of testing is carried on in wine glasses, or small conical test glasses, (Fig. 29,) un- less otherwise specified. For each trial, a small quantity of the solution (about 60 grains) is taken, and the reagent is added in quantity corresponding to its strength, and to the concentration of the solution. After the addition of the test, the mix- ture is shaken, or stirred with a glass rod. Trial of Acidity or Alkalinity.—This is to be done by dipping a grey litmus-paper into that portion of the solution first obtained, for the more diluted portions cannot be expected to give a decided reaction. The solution obtained, in cases where it has been found necessary to destroy the organic matter, must, of course, be acid, from the nitric acid used in the process. The most useful litmus paper is prepared by staining the surface of perfectly pure drawing paper, (Harding’s), with an infusion of litmus, to which as much nitric acid has been added, as to change its blue to neutral-tint. If the colour of the neutral or grey tint is changed to red, the indication is acid, if to blue, it is alkaline. The saline solu- tion is generally neutral, but occasionally otherwise. Detection of Nitric Acid.—It is very seldom, indeed, that soils in this
    country contain nitrates in any appreciable quantity. Even in cases where nitrate of soda has been used as a manure, so soluble is the salt, and so small a dose is required, that unless the specimen of soil were taken soon after its application, nitric acid could not readily be detected in it. If, on evaporating a portion of the solution to dryness, and heating nearly to redness, as directed for the destruction of organic matter, that peculiar mode of combustion, called deflagration, takes place, it arises from the presence of nitrates. This proof of the presence of nitrates, however, cannot be implicitly relied on, for the presence of common salt often renders the deflagration so slow and inconspicuous that it can readily escape observation. The presence of nitrates may also be detected by evaporating a portion of the solution to dryness, and adding sulphuric acid and solution of sul- phate of iron, when the solution assumes a brown tint, more or less deep, according to the quantity of nitric oxide evolved. Detection of Ammonia.—A portion of the first washings, concentrated by evaporation if necessary, is transferred to a small test tube, and a few drops of a solution of caustic potash are introduced by a small funnel, to avoid soiling the mouth of the tube. The tube is then heated, while a grey litmus-paper is held within its mouth. If ammonia is present, the test-paper will become blue, and after being exposed to the air for some time, or gently heated, the blue tint will disappear. The smell of amon- nia is so peculiar, that it forms an excellent indication of its presence; but ammonia may be detected by the means above mentioned, when pre- sent in too small quantity to be recognised by the smell. Detection of Sulphuric Acid.—To a portion of the solution, a few drops, of nitric acid are added, and then solution of nitrate of baryta. If a white precipitate is formed, the presence of sulphuric acid is indicated. Detection of Chlorine.—To a portion of the solution, a few drops of nitric acid are added, and afterwards solution of nitrate of silver. When chlorine is present, a white curdy precipitate falls. This precipitate be- comes dark on exposure to light. Chlorides are very rarely absent, but it should be observed, that after the addition of nitrate of silver, a solution in which a white haze was barely perceptible at the time, will become dark on exposure to light if organic matter is present. Detection of Carbonic Acid.—If the solution has given an alkaline re- action, the presence of carbonic acid may be inferred. If the quantity of carbonates is considerable—and this is very rarely the case—effervescence will take place, when a portion of the concentrated solution is poured into a glass containing muriatic acid. When the quantity of carbonates is small, no effervescence is perceptible, the carbonic acid being retained in solution. It is to avoid the interference of carbonic acid, that it is necessary to acidulate with nitric acid before testing for sulphuric acid or chlorine. Remarks on the Subsequent Testing.—Several of the substances next to be tested for might be easily detected by simple operations on separate portions of the saline solution; but as some of the number cannot be de- tected till after the removal of others, it becomes necessary to follow a particular order; taking those first, whose removal is required for the detec- tion of those that follow. As some of the substances do not always occur, and when they do, only in small quantity, it is better to operate on a con- siderable portion of the solution; and as several of them could readily escape observation in the presence of much organic matter, it is safest to
    use a portion of solution, freed from organic matter in the manner before described, (page 139.) If the previous examination has shewn the presence of more than a trace of sulphuric acid, as much chloride of barium is added as is suffi- cient to precipitate all the sulphuric acid and no more, and the sulphate of baryta is separated by filtration. Ammonia is then added to the filtered solution in excess, and from the appearance of the precipitate thereby produced, some information will be obtained regarding the kind of bases present. If the precipitate is dark brown, peroxide of iron is present; and if, on exposure to the air, the colour becomes darker, it is owing to the presence of manganese. If the precipitate is white and gelatinous, it is due to alumina or magnesia, or both ; but it may have a colour more or less deep when all the four are present. Detection of Peroxide of Iron.—To the solution containing the above more or less brown precipitate, muriatic acid is added till the precipitate is com- pletely redissolved, and excess of ammonia is again added. If magnesia and oxide of manganese were present, they now remain in solution, and the precipitate that falls consists of peroxide of iron and alumina—sup- posing both to be present. This precipitate is separated from the solution by filtration, which should be conducted as quickly as possible, and after being washed it is treated on the filter-paper with muriatic acid, while still moist, whereby it is dissolved; the solution is re- ceived in another glass, and the filter-paper is washed clean. These small filtrations are most conveniently per- formed by means of small and thin filter-papers, set in Clark’s filtering rings, Fig. 30, X X X the ring sup- ported on the edges of the test-glass, o the filter-paper. It is a still better arrangement when the ring is sup- ported by a single arm fixed in a separate stand, at such a height, that the apex of the filter is in contact with the inner edge of the test glass. To the acid solution of peroxide of iron and alumina, caustic potash is added, which at first precipitates both bases, but on being added in excess, redissolves the alumina, leaving the peroxide of iron. Water is now added lest the caustic alkali should corrode the filter-paper, and the peroxide of iron is separated by filtration. Detection of Alumina.—To the alkaline solution filtered from the per- oxide of iron, muriatic acid is added till it is neutral; on the addition of ammonia, alumina is precipitated as a white gelatinous hydrate. Should phosphates exist in the saline solution,—and nothing beyond traces are likely to be met with—both the peroxide of iron and alumina will contain phosphoric acid. This, however, is of no importance in a qualitative analysis, where the object is merely to ascertain what bodies are present, as a separate process for detecting phosphoric acid must be had recourse to. Detection of Oxide of Manganese.—To the alkaline solution out of which peroxide of iron and alumina were precipitated, muriatic acid is added till neutral, after which, on the addition of hydro-sulphate of ammonia, sulphuret of manganese falls as a flesh-coloured precipitate. Should the quantity of sulphuret of manganese be small, the characteristic colour will not become apparent till the precipitate has completely subsided, and can be viewed without the intervention of the liquid coloured by excess of the test. For the sake of beginners, it may also be mentioned, that the pre- Fig. 30.
    cipitate is not contaminated, although, on exposure to air, its colour changes to brown ; for this arises from the oxidation of the manganese. Detection of Lime.—The solution, filtered from the sulphuret of mangan- ese, is acidulated with muriatic acid, and heated till it ceases to smell of sulphuretted hydrogen. If, after this, the solution appears muddy, it arises from precipitated sulphur, and it must be freed from this by filtra- tion. The filtered solution is now rendered in the feeblest degree alkaline by ammonia, and oxalate of ammonia is added in excess, whereby a white precipitate of oxalate of lime is produced, either immediately, or after stand- ing for some hours, according to the quantity of lime present. This precipi- tate, as already mentioned, is very liable to run through the filter paper; hence, any attempt to filter, before it has stood for some hours in a warm place, will be unsuccessful. Detection of Magnesia.—The solution filtered from the oxalate of lime, is concentrated by evaporation, in a porcelain basin, till it is reduced to a small bulk ; after which, it is completely transferred to a platinum crucible or capsule, and cautiously evaporated to dryness. It is next heated to dull redness, to drive off volatile substances, and the heat must be so re- gulated as to occasion no loss. The saline residuum is dissolved in a small quantity of water ; and if any thing insoluble remains, it may be taken up by a few drops of muriatic acid, and added to the aqueous solu- tion. Pure red oxide of mercury (red precipitate free from nitric acid, and all basic impurities) is added to the solution, and the whole is evaporated to dryness. On treating the dry mass with water, magnesia and the excess of oxide of mercury remain undissolved : they are separated by filtration, washed, and ignited, to drive off the oxide of mercury. The white powder which remains is magnesia.. Detection of Potash and Soda.—The solution, filtered from magnesia and oxide of mercury, is evaporated to dryness and ignited: chlorides of potassi- um and sodium alone remain. A small portion of the saline mass is taken up on a scrupulously clean platinum wire, curved at its extremity, so as to retain the assay, and heated in the apex of the reducing flame of the blowpipe. If soda is present, the outer flame is coloured of a fine yellow tint, remarkably apparent in the portion of the oxidating flame, which is colourless, or faintly blue, when no assay is held in the point of greatest heat. The saline mass is now dissolved in a small quantity of water, and a strong aqueous solution of tartaric acid is added If a crystalline precipi- tate falls immediately, or on standing, and after frequent stirring, potash is also present. [I have examined no specimen of soil from which potash was entirely absent.] In place of tartaric acid, chloride of platinum may be employed, which gives a yellow precipitate when salts of potash are present. A method of precipitating soda has recently been proposed by M. Fremy, which is capable of detecting the jj-^th of a salt of soda in a mixed solution of soda and potash salts. Although the blowpipe test above mentioned is quite conclusive, it may be satisfactory after the pre- cipitation of potash by tartaric acid, to obtain evidence of the presence of soda by precipitation. For this purpose, the solution filtered from bitar- trate of potash, is rendered exactly neutral by solution of pure caustic potash, and again filtered from bitartrate, after which, solution of anti- moniate of potash is added, which, if soda is present, will, either imme- diately, or after stirring, throw down a crystalline precipitate very similar in appearance to bitartrate of potash; it is antimoniate of soda.
    Detection of Phosphoric Acid.—The soluble saline matter of the soil rarely contains more than traces of phosphoric acid, and in many cases no phos- phoric acid can be detected. Such, however, is the importance of this substance as a constituent of plants, and consequently of soils and manures, that careful search should be made for it even when present only in traces. Sprengel states that when peroxide of iron and alumina are present, only traces of phosphoric acid can be met with. For the detection of phosphoric acid, a separate portion of the aqueous saline solution is necessary; and the organic matter should be destroyed. If peroxide of iron or alumina was found in the previous examination, no addition is necessary; but if they were absent, or occurred merely as traces, a few drops of permuriate of iron are added to the solution. Ammonia in excess is next added, and the precipitate is separated by filtration, washed, and treated on the filter-paper with concen- Fig. 31. trated acetic acid. The soluble matters are taken up by the ^ acetic acid, and pass through the filter ; while the phosphate of \ [ iron and phosphate of alumina remain upon it undissolved. As, however, traces of these substances might easily remain on the filter-paper unobserved and scarcely appreciable by the most accurate weighing, it is better to throw down the pre- cipitate by ammonia, in a tall narrow test-glass, (Fig. 31,) and to wash it by affusion and subsidence ; that is, as soon as the precipitate has subsided, to pour off or draw off by a syphon, sd— \ or a sucker, (Fig. 32,) as much of the supernatant liquid as can be done without loss of the precipitate. After this, the vessel is filled up with distilled water, and the precipitate is well stirred up. After sub-Fig. 32. sidence, the clear water is again drawn off, and this process is yr repeated till the precipitate is washed clean. When this point has been attained, and the wash-water as completely removed as possible, the precipitate is digested in strong acetic acid : if any J portion of the precipitate remains undissolved, phosphoric acid is present. In the foregoing qualitative examination, no notice has been taken of substances that very rarely occur, such as oxide of cop- per, baryta, &c. The methods by which they may be distinguished and separated from other bodies may be readily ascertained by reference to works on chemical analysis. Every conclusion arrived at in the qualitative analysis, should S/ he carefully noted down at the time ; and when the investigation is at an end, the operator will be in possession of data, from which, it is easy to draw up a scheme of quantitative analysis, with no more complication than is absolutely necessary for the estimation of the bodies actually present. QUANTITATIVE ANALYSIS OF THE SALINE MATTER OF SOILS SOLUBLE IN WATER. For the quantitative analysis, a solution is to be prepared from a known weight of air-dried soil; and this weight will be regulated by the quantity of saline matter present, a pretty accurate idea of which will have been obtained in performing the qualitative analysis. To obtain weighable quantities of some substances that may he present in very small propor- tions, Sprengel recommends as much as two pounds of soil to be operated upon; but a half, or even a fourth of this quantity, will in most cases suffice; but whether a large or small quantity has been employed, it is