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a metal belonging to the first subdivision. And this fact brings us to the consideration of a point of great consequence, viz.THE ABSENCE OF ALL BASIC RADICALS EXCEPT POTASSIUM AND SODIUM.
It has been already stated that the presence of acid-radicals often interferes most materially with the detection of the basic constituent of a salt; the converse of this statement is true in even a more extended sense. The presence of metallic radicals other than potassium or sodium is found extremely inconvenient, chiefly on account of the insoluble salts which they form by union with the acid-radicals of the salts added as reagents or tests. Ammonium indeed should, as a general rule, be excluded, on account of the interference which its salts (e.g. NH, CI, NH, NO,, and (NH), So) frequently exert by their solvent action on other saline combinations. Thus the borates and phosphates, the citrates and tartrates of barium and calcium (salts characteristic of their respective acid-radicals) are held in solution when reagents appropriate for their formation are added, if a salt of ammonium also be present in the liquid. It must, however, be stated here, that stable ammonium salts (NH, HO and (NH),CO, being thus excepted) do not inferfere with the action of two frequently employed reagents, viz. AgNO, and Fe, Ciz.
The separation of the original basic constituent of a salt is sometimes very difficult, and in other cases very easy of accomplishment.
1. If the salt is soluble in water, and the basic constituent belongs to Subdivision I., no such separation is needed, except in the special cases just mentioned, where the radicals of borates, phosphates, and organic salts are present, and we intend to employ BaCl or CaCl as a test; then, if ammonium be the base, it must be removed. This is accomplished by adding to the solution some KHO, and boiling the liquid until the addition of a few drops more of the alkali no longer produces an odour of NH,
2. If the salt is soluble in water, and contains for its basic constituent a member of Subdivision II., it may be immediately
converted to the requisite condition of a suitable alkaline salt by boiling its solution with a solution of Na, Cog:
2BaBr+Na, C0,=2NaBr+Ba, COZ. 3. If the salt contains a member of Subdivision II. as its basic radical, but is insoluble in water though soluble in acids, the treatment with Na, co, may be employed. Care must, however, be taken to add a quantity of Na, Co, sufficient not only to neutralize the acid employed as solvent, but also to effect the decomposition of the original salt. If this precaution be not observed, the reagent will simply neutralize the solvent acid, and reprecipitate the original salt, thus
Ba, R+2HNO3+Na, CO, =Ba, R+2NaNO3+H,CO,; salt sol. in acid
reprecipi. acid only. solvent.
tated salt. solution. the search for the desired salt (Na, R) in the solution will therefore in this case fail. But if enough Na, CO, has been added, the reaction is as follows:
Ba, R+2HNO3+2Na, C0,=Na, R+2NaNO3+H, CO,+Ba, CO2, where the desired salt (Na, R) is obtained. In these experiments the smallest possible quantity of acid solvent should be used.
Moreover, in certain instances where Na, CO, does not effect a rapid decomposition of the salt (e.g. BaCrO2, Ba, HPO ), it is well to filter off the precipitate first formed, and to boil it with a fresh portion of Na, Coz.
4. These observations and methods of treatment apply also generally to all salts, whatever basic radicals they contain, provided that their acid constituent is not a complex organic radical containing C,H, and 0, or C,H, N, and 0.
5. But when we try to decompose salts containing the metallic radicals of the third and fourth subdivisions united with a complex organic constituent, we find that complete decomposition is not effected, that a perfect precipitation of the basic radical as carbonate is not made, but that in the filtrate the acid-radical to be detected exists in combination partly with Na, and partly with the original basic radical. Nor can this latter be removed by successive repetitions of the process; here the
analyst must use his judgment and the knowledge previously acquired. These organic radicals do not allow of the perfect precipitation of many bases belonging to the third and fourth subdivisions by any reagents except certain combinations of sulphur (H, S or NH, HS). Some of the sulphides thus formed are, it will be remembered, soluble in alkaline solutions, some in acids, while others are soluble in none of these reagents. The method of separation must therefore be adapted to the nature of the basic constituent present, which the student will have already recognized. If the sulphide of the radical be insoluble in acids, it is better to make a solution of the salt in HNO, as dilute as possible, and to pass H, S through the acid solution till no more precipitate is formed. The sulphide should then be removed by filtration, and the residual H, S expelled by gently warming the filtrate. The solution thus treated may then be immediately tested for acid-radicals. Should the sulphide of the radical in the salt be soluble in acids, but insoluble in alkaline solutions, H, S is to be passed into the solution obtained by boiling the original salt with Na, Co, (this plan is better, though longer, than adding NH, HS); thus the removal of the small quantity of metallic radical retained in the alkaline solution is effected. Having filtered off the precipitate thus formed, the student must acidify the filtrate with dilute HNO,, to decompose the NaHS present, and then gently warm the solution, to expel H, S.
By the means just described, or by a combination of them, acid-radicals may be separated from their original saline combinations, and obtained in the form of an alkaline salt, the base of which will not interfere with the effects produced by the reagents employed.
We now proceed to the method of testing the solution for acid-radicals—the actual analysis. But first it may be useful to give a list of the acids whose salts we may expect to be present, distinguishing by small type those which are of rarer occurrence. These, when they occur, are often reduced to simpler forms, or else are separated, in the preparation of the solution. They generally require for their recognition the employment of the special tests detailed in Chapter VII., although evidence of their existence is occasionally obtained in the preliminary examination for the acid constituent (which see):Subdivision I.-HCI, HBr, HI, HF; HCIO, HC103, HC104,
HBrO2, HIO3, HIO,, HPtC1,, &c. Subdivision II.-H,O, H, S, H, Se, H, Te; H, SO2, H, SO,
H, SeO3, H, SeO4, H, TeO3, H, Teod; HCrO2, HWO,, HM002,
HVO, Subdivision III.—H, Si, F,; H, CO,, H,C,0,, HB00,, HSi0, ;
HCy, HCYO, HCyS, H, Cfy, H, Cfdy, H, Cocy; HĀ, HĒZ,
H, L, H, J, H, T, H, Õi, H, G, H, Qt, H, Ū. Subdivision IV.-HNO, HNO3, H, PO,, H, PO3, H, PO,
H, As, O., H, Asoc. Of these acid-radicals, some, as has been previously observed, can be detected in the preliminary examination, or by tests based upon the results of that examination. These are the radicals the acids of which are represented by the formulæ HCIO, HC10,; H, S, H, Se, H, SO; HCyO and HNO,. Some radicals, again, though found in analysing by the moist way, are best detected in the previous examination : the acids of these radicals are HCI, HBr, HI, HF, HCy, and HĀ.
From the solution prepared as above directed, the original basic radical has been separated; CO, has also been expelled by a gentle heat applied to the liquid after it has been slightly acidified with dilute HNO,* : this expulsion of the CO, is necessary, for its presence in the liquid would falsify the results. The next step should be to exclude another class of metallic acids: they may all be removed by the passage of H, S through the liquid ; but if this has been already done, it need not be here repeated. The H, S gas is passed through the solution, which has been very slightly acidified with HNO . By this means we remove the radicals +
* Unless the solution be very dilute, and the nitric acid very weak, HBz and H, S may be here precipitated, while H, Ū and H, Qt will certainly separate.
+ Chromic acid, and the other metallic acids in general, are detected and separated among the basic radicals by their behaviour with H, S.
PtCl,, Cro,, Sn,03, WO,, M00,, VO,, As, Os, AsOy; we also convert Bro,, 10,, and 10, into the types MBr and MI; and if cyanogen compounds be present, they will generally be found to produce a sulphocyanide, while ferricyanogen is changed to ferrocyanogen : these facts must be borne in mind when testing. The H, S remaining in the liquid must now be removed by gently warming the solution placed in an evaporating dish.
The list of radicals now existing as acids will be as follows:-
not all expelled by warming the acid solution), HCsy;
and the other organic acids. Subdivision IV.-H, PO,
No really good plan of detecting and separating the various acids has been yet devised; and the Tables we are about to give must not be deemed in any way faultless. More reliance must be placed on the preliminary examination and special testings, than upon the actual analysis.
To detect, then, with more or less certainty, in a single salt, the radical of any one of the above-mentioned acids, we may employ the following plan :