Group 5. The metals of this group (barium, strontium and calcium) are isolated, in the general scheme, in the form of carbonates. To separate them from one another, the mixed carbonates are dissolved in dilute acetic acid, and a portion of the solution is tested for barium by the addition of potassium chromate, K,CrO. (not the bichromate, IC.CriOs). If barium is present, a straw-yellow precipitate of barium chromate, BaCr04, is thrown down. If barium is not present, we may proceed at once to the tests for calcium and strontium; but if it is pres ent, it must first be removed from the solution by cautiously adding potassium chromate until no further precipitate is formed, and then filter ing. In this case it is necessary to remove the excess of chromate of potassium before testing for calcium and strontium, because these two metals cannot be separated in the presence of that reagent. For this purpose the filtrate con taining the strontium and calcium (which must be perfectly free from barium chromate, even though several successive filtrations may be necessary in order to make it so,) is made alka line by ammonia, and ammonium carbonate is then added until the carbonates of strontium and calcium are all precipitated, the potassium chro mate remaining in solution. After filtration and thorough washing, the mixed carbonates of strontium and calcium are dissolved in hot acetic acid, and the solution is boiled. Dilute sulphuric acid is then added; and if the acid is sufficiently dilute, a white precipitate of sul phate of strontium is thrown down, while the sulphate of calcium (which is formed at the same time) remains in solution. After stand ing for 15 minutes or more, in order that the precipitation of strontium sulphate may be complete, the solution is filtered, and the clear filtrate is tested for calcium by first adding ammonia until an alkaline reaction is obtained, and then adding a solution of oxalate of am monia. If calcium is present, a white precipi tate of calcium oxalate is obtained.
Group 6. This °group') contains magnesium only. Hence the presence of magnesium will be made apparent at once in the course of the preliminary separation into groups.
Group 7. Potassium, sodium, lithium and ammonium belong to this class. These metals are characterized by the high solubility all their important salts. Ammonia, being used in the separation of the groups as a reagent, is sure to be present in the final solution that has been designated as containing the metals of °Group 7.° Hence if it is desired to test for that substance, the test should be made upon a specimen of the original solution, before any ammoniacal reagent has been added. The test is very simple, and consists merely in heating some of the proposed solution with milk of lime (calcium hydrate in suspension). All salts of ammonia are decomposed in this way, with the liberation of ammonia gas, which may be recog nized by its smell or by its turning moist red litmus paper blue. The general nature of the re action involved in the liberation of ammonia gas may be illustrated by the case of ammonium sulphate. We have (NH4)1SO4-1-Ca(OH)2=
CaSO4-1-2NH,+2H20. Sodium, potassium and lithium may be sought for in the final fil trate obtained in the separation of the funda mental groups, since no compounds of those metals have been used as agroup reagents.° They are best identified by means of the colors that they communicate to the flame of a Bunsen burner. For performing a test of this sort, the filtrate containing these metals should be evapo rated to dryness in a porcelain dish, and the residue ignited until any ammoniacal salts present are eliminated by volatilization. The dish is then allowed to cool, and its contents are moistened with a few drops of distilled water. A piece of platinum wire is next thoroughly cleaned, moistened with hydrochloric acid and held in the Bunsen flame until it ceases to com municate any color. The wire is then dipped into the solution in the dish, and again held in the flame. Potassium salts give a fine blue color, lithium salts a red and sodium salts an orange yellow. If sodium is present in any con siderable amount, its strong, brilliant flame color is almost certain to obscure the colors due to any other elements present. Chemists there fore make use of colored glasses that are prac tically opaque to sodium light, using them as screens through which to view the Bunsen flame. The commonest glass of this kind is the "cobalt-blue" glass, which is used for the de tection of potassium, since it is quite trans parent to the flame-color of that metal.
For methods of examination adapted to the detection of gold, platinum and others of the less common metals, reference must be made to books on chemical analysis.
The metals that exist in a proposed substance being known, it remains to discover in what chemical combinations they are present. It is usually impossible to learn, by mere qualitative analysis, which of the acid radicals that may be present is combined with any given one of the metals; but it is possible to ascertain that certain kinds of compounds (such as sulphates, chlorides and the like), are present and a chem ist of experience, who is familiar with the kind of work in hand, can often infer, with con siderable probability, how the bases and acids are associated with one another. In the present article it is not possible to discuss this difficult phase of analysis, but a few of the more com mon tests for the presence of acid radicals may be given.
There is no general scheme for the detection of these radicals, by which they are separated into groups like the metals, and eventually iso lated singly. In examining a substance for them, all that can be done is to apply certain tests, largely independent of one another, and best carried out by dividing the original solu tion into a number of parts, each of which is to be examined for a single class of acid radi cals, and then thrown away.