CHEMICAL ANALYSIS. 



CHKMICAl, ANALYSIS. 



mure its completeness. Hence, as a general rule, when, 

 , a precipitation bu been effected and the innnluble com- 



in order to mure its 



for instance, 



pound separated by filtration, the filtrate should be treated with some 



more of the same reagent which ha* been employed in the precipita- 



tion, and if the latter b*i been perfect, no further precipitation will be 



thereby produced. 



It is also clear that when a precipitate has been formed in a solution 

 and separated from the non-precipitated constituents by filtration, the 

 precipitate is still moistened with the solution of the non-precipitated 

 portions, and must be therefore cleansed from them by washing with 

 water before further examination. 



Fig*. I, 2, S, show the successive foldings by which a filter is formed 



from a round piece of unsized paper ; 4 shows how the folds are 

 opened so as to form a conical sack, having three folds of paper on one 

 side and one on the other, and how it is placed in a glass funnel. 

 Fig. 5, the washing bottle, is a narrow-necked glass flask, through 



whose air-tight cork two tubes, open at both ends, pass ; the one, A c, 

 having its internal end beneath the water, the other, B, just passing 

 through the cork. Air forced into B from the mouth presses the water 

 into c, and projects it from A, where the tube is narrowed to a fine 

 orifice. 



The various substances added in order to separate the above groups 

 of metals from one another, and each of which consequently behaves 

 in the same manner to the members of the group which it separates, 

 are called the genrral nagtntt of those groups. Thus carbonate of 

 ammonia is the general reagent for the alkaline earths, barium, stron- 

 tium, and calcium. 



Space will not admit of a more detailed description of the separation 

 of the rariout groups from one another, but from the following sum- 

 mary an idea may be formed of the process in each group. 



A. Of the chlorides chloride of lead (PbCl), chloride of silver 

 (AgCl), and subchloride of mercury (Hg,Cl), the chloride of lead 

 alone is soluble in boiling water : of the two chloride of silver and 

 subchloride of mercury, chloride of silver alone is soluble in ammonia, 

 subchloride of mercury being blackened but not dissolved by it 



B. Of the sulphide of mercury (HgS), bismuth (IMS), cadmium (Cd 

 8), and copper (CuS), the sulphide of mercury alone is insoluble in 

 strong nitric acid. From the nitrates of bismuth, cadmium, and cop- 

 per, the bismuth alone remains permanently precipitated (as an oxide) 

 on the addition of an excess of ammonia. From the ammoniacal solu- 

 tion* of cadmium and copper, the cadmium alone is precipitated (as a 

 carbonate) by carbonate of ammonia, 



C. The sulphides of arsenic, antimony, tin, gold, and platinum, (Un- 

 solved in sulphide of ammonium are all precipitated as insoluble sul- 

 phide* on the addition of hydrochloric acid ; of these sulphides that of 

 arsenic alone is soluble in carbonate of ammonia. Of the remaining 

 sulphides, those of antimony and tin are converted into oxides (SbO,, 

 SnO,) on ignition with nitrate of ammonia, while the sulphides of gold 

 and platinum are reduced to the metallic state. Oxide of antimony 

 is soluble in tartaric acid, while neither binoxide nf tin nor metallic 

 gold or platinum are dissolved by it. But gold and platinnm are 

 both dissolved by nitro-hydrochloric acid, in which binoxide of tin is 



D. The sesquioxides of iron, chromium, and aluminium, are soluble 

 in hydrochloric acid. All these three metals are precipitated by cold 



caustic potash (as oxide*) from the so-formed chlorides, the oxide of 

 iron alone is insoluble in excess. Boiling caustic potash precipitates 

 the oxide of chromium, but not the alumina. 



E. The sulphides of zinc, nickel, cobalt, and manganese, are dissolved 

 as chlorides by nitro-hyilrochloric acid. From this solution carbonate 

 of ammonia precipitates them all ; but when added in excess, dissolves 

 them all excepting manganese. Caustic potash precipitates oxides of 

 nickel, cobalt, and zinc, and in excess only dissolves the oxide of /.inc. 

 Hydrochloric acid dissolves the oxides of nickel and cobalt ; cyanic!.- . ; 

 potassium precipitates and dissolves in excess both nickel and cobalt 

 from the acid solution of their chlorides. An acid added in excess 

 to the cyanide of potassium solution, only precipitates the nickel as 

 cyanide. 



F. The carbonates of barium, strontium, and calcium are soluble in 

 hydrochloric acid, the corresponding chlorides being formed. In a 

 neutral or ammoniacal solution of these chlorides, eliminate of potash 

 precipitates the barium alone. Of the strontium and calcium, sulphate 

 of potash precipitates the strontium alone, from dilute solution-. 



O. Requires no separation. 



H. It will be well to describe more fully, by way of example, tin- 

 separation of the members of one group. For thia purpose we will 

 take that of the alkaline metals, potassium, sodium, and ammonium. 



Ammonium is first tested for ; this is done by adding caustic potash, 

 soda, or lime to a portion of the solution, and heating. With \vh.it- 

 ever acid the ammonium U combined, the latter is liberated by the 

 free base added, and is detected in the form of ammoniacal gas by its 

 peculiar smell, by ite alkaline reaction in turning reddened litmus 

 blue, and by its fuming when a rod moistened with hydrochloric acid, 

 of sufficient strength to diffuse sensibly in the air, in held in its pre- 

 sence. If no ammonia is found in this manner, the original solution 

 is tested for potash. To a portion of it tartaric acid is added, and the 

 sides of the vessel vigorously rubbed with a glass rod. A white crys- 

 talline precipitate of the tartrate of potash, whose formation is much 

 accelerated by the agitation, shows the presence of potash. To a fresh 

 portion bichloride of platinum (PtCl,) is added, the solution is gently 

 evaporated to dryness, and a mixture of equal parts of alcohol and 

 ether is added ; a heavy yellow crystalline insoluble residue shows the 

 presence of potash. A perfectly clean platinum wire is dipped in a 

 portion of the original solution (previously concentrated by evapora- 

 tion), and held in the outer flame of the blowpipe. If on looking at 

 it with the naked eye the flame appears yellow, this shows the presence 

 of soda; if purple, it is evidence of potash. If the flame be yellow, 

 potash may be present, because the yellow colouring power of the soda 

 is much greater than the purple colouring power of the potash. If 

 both are present, the purple colour due to the potash may be seen by 

 viewing the flame through a piece of blue glass (cobalt glass), wlik-li 

 intercepts the yellow light produced by the soda, and allows the purple 

 of the potash to be recognised. 



If ammonia was found in the first instance, it must be got rid of 

 before testing for potash ; because, like jratash, it forms insoluble com- 

 pounds with tartaric acid and with bichloride of platinum. This i 

 done by evaporating the original solution to dryness and igniting it, 

 The non-volatile residue is then dissolved in water, and examined as 

 above for potash and soda. 



After the metals have been separated into groups by the general 

 reagents, and the members of each group separated from one another 

 according to the preceding outline, the presence of the different metals 

 found should be confirmed by the application of as many charm 

 reactions as possible, that is, by applying special reagents to the solution 

 which has been submitted to analysis ; for when a substance shows a 

 characteristic behaviour towards any reagent, the like of which is n..t 

 shared by any other substance, the reagent is called special with regard 

 to the substance. It follows that when only one or few substances are 

 present or have to be tested for, the application of certain appropriate 

 special reagents is often sufficient to indicate their presence or absence, 

 and saves the time and trouble of the systematic separation above 

 sketched, in which the whole of the metallic bases are supposed to be 

 praent. 



The following are some characteristic reactions of the metallic bases, 

 which reactions are not included in the above scheme : 



Copper is precipitated from an acid solution in the metallic state 

 upon metallic iron. It is precipitated as a green carbonate by the 

 carbonate of a fixed alkali ; as a greenish blue hydrated oxide by a 

 caustic fixed alkali in the cold ; as a black oxide when heat is employed. 

 It forms a mahogany-coloured precipitate with ferrocyanide of po- 

 tassium. 



Cadmium is a volatile easily oxidisablc metal, whose sulphide is 

 yellow. 



ffitmuth is precipitated from its acid solutions by water, basic in- 

 soluble salts being formed. 



Ma-eury is a volatile metal ; all its compounds are reduced to the 

 metallic state cither bv heat alone, or in the presence of alkalies or 

 alkaline carbonates. The salts of ita peroxide form a red chromate, 

 a red iodide, and a yellow oxide. It. is precipitated in the metallic 

 state by metallic copper. 



Lead is precipitated as a white hydrated oxide by a fixed alkali. It 

 forms white insoluble carbonates and sulphates. Its chromate and 

 iodide are yellow and insoluble ; most of ita compounds are reduced to 



