781 



CHEMICAL ANALYSIS. 



CHEMICAL ANALYSIS. 



782 



examination of a mineral, we may be satisfied with the detection of 

 iron, or with the estimation of its amount ; or, on the contrary, it 

 may be required to be known in what stage or stages of oxidation the 

 iron exists, or how much of the different oxides are contained in the 

 mineral. 



It is, however, clear that, whatever be the kind of information sought 

 regarding the nature of a substance, the processes to be adopted will 

 have greater or less similarity according as the substances examined 

 are more or less alike ; and it follows hence that as substances may be 

 grouped together in classes, whose members resemble one another 

 chemically ; so the processes of analysis applied to the members of the 

 same class bear also a likeness to one another. 



Thus those substances which were (historically) first derived from 

 the animal and vegetable kingdoms, and the artificially prepared bodies 

 resembling them in composition the so-called organic bodies contain 

 invariably carbon, generally hydrogen, frequently oxygen and metals, 

 sometimes nitrogen, and occasionally sulphur, chlorine, phosphorus, &c. 

 In many instances, such bodies may have to be divided into their 

 proximate constituents : in such cases special knowledge must be had 

 of the nature and behaviour of these constituents. But a question 

 more frequently to be solved with regard to organic bodies, is the 

 determination, qualitatively or quantitatively, of their ultimate con- 

 stituents or elements. The importance of this species of analysis, and 

 the similarity of the methods employed upon the different organic 

 substances, renders a special article necessary for their consideration. 

 [ORGANIC ANALYSIS.] 



Again, the physical state in which a body is presented for analysis 

 wkether as a solid, a liquid, or a gas, renders corresponding variations 

 of method and apparatus necessary. This is especially the case with 

 gases and vapours, and hence a separate article will be devoted to this 

 branch of analysis. [GASOMETBIC ANALYSIS.] 



Inasmuch as most inorganic chemical compounds consist of one or 

 more bases, acids or salts [Acm ; BASE ; SALT], qualitative and quan- 

 titative inorganic analysis consists mainly of the detection, separation, 

 and estimation, of the various bases and acids (for the detection and 

 separation of these of course includes those of the salts resulting from 

 their union). Again, as most bases are metal, or contain a metal, the 

 detection, separation, and estimation of the metals is of the first im- 

 portance hi qualitative and quantitative analysis. 



It ia proposed, in the limits of the present article, to give a general 

 outline of the chemical analysis of inorganic liquids and solids. To do 

 this it will be necessary to describe, in qualitative analysis 1st, how 

 the bases are separated into groups ; 2nd, how the inembers of the 

 groups are separated from one another ; 3rd, the detection of the 

 meet important acids. A general sketch will then be given of the 

 principles and practice of quantitative analysis. 



Qualitative analyrit. The chief means used in separating the 

 metallic bases from one another is founded upon the difference of 

 solubility of their analogous compounds in the same medium. 



Thus if the metals be made to combine with oxygen, the so-formed 

 oxides are found to be soluble in water in the following different 

 degree! : 



Difficultly 



Very Soluble. Less Soluble. Soluble. Insoluble. 



Oxirlcs of . K,Na, NH,* Ba, 8r, Ca Mg Al, Cr, Fe, Mn, Zn, Ni, 



Co, 8n, Ft, Au, Hg, 

 Pb, Ag, Bi, Cd, Cu. 



Again, on examining the chlorides of the metals, we find that 



Difficultly 



Very Soluble. Soluble. Insoluble. 



Chlorides of K, Na, NH t , Ba, Sr, Ca, Mg, Al, Cr, Fe, Pb Hg, (Hg,Cl), Ag. 

 Mn, Zn, Ni, Co, Pt, Au, Hg, (HgCl), 

 Cd, Cu 



The compounds formed by the union of other elements, or groups of 

 element* (acids), with the metals, show again a different order of solu- 

 bility ; and it is by this difference in solubility in different media, of 

 the analogous compounds of the metals, that their separation from 

 each .other is chiefly effected. 



Thus, if a solution contains silver and copper, and by any means both 

 the metals are converted into chlorides, it is clear that, the chloride 

 of silver being an insoluble substance in water, may be separated from 

 the soluble chloride of copper; that is, the silver may be separated 

 from the copper. When one metal has been thus converted into an 

 insoluble compound in a solution, it is said to be precipitated or 

 thrown out of solution in a solid form ; and it may generally be sepa- 

 rated from the soluble compounds of other metals present by filtration 

 through unsized paper. 



Very frequently analogous compounds of different metals, which 

 are equally soluble or insoluble in water, show different degrees of 

 solubility in other media. Often an acid or alkaline medium affects 

 differently in this respect analogous compounds of bodies which are 

 equally soluble or insoluble in water : and this is in most cases due to 

 the difference which such compounds possess in their liability to be 



* The nalts of the quasi-metal ammonium (NIT, ) are so analogous to those of 

 the metals proper, and ammonium is of so frequent occurrence in nature, that 

 It to included among the metals. 



decomposed by such media : sometimes, however, to a true difference 

 in the solvent power of the medium. An example of the difference of 

 solubility in another medium of analogous compounds (which are 

 equally insoluble in water), is furnished by the sulphides of zinc and 

 of cadmium. The former is dissolved in (and decomposed by) hydro- 

 chloric acid, the latter is unchanged. Again, the sulphides of tin and 

 copper are insoluble both in water and hydrochloric acid. But the 

 former is soluble in caustic potash (and combines with it) ; the latter 

 is insoluble. Further, the chlorides of barium and of strontium are 

 both soluble in water ; but the latter alone is dissolved by alcohol. 



The difference of solubility of the analogous compounds of the 

 different metals in water, acids, and alkalies is most marked in the 

 case of the sulphides. On this account, and because a metal is often 

 characterised by the colour of its sulphide, one of the earliest processes 

 in the detection and separation of the metals is their conversion into 

 sulphides. 



If we look at the following series of facts, we at once see the possi- 

 bility of separating the metals into families or groups : 



I. Of all the metals, the following chlorides alone are insoluble, or 

 nearly so, in cold water, and in dilute hydrochloric and other acids, 

 AgCl, PbCl, Hg a CL A. 



II. Of all the metals not in A, the following sulphides alone are 

 insoluble in hydrochloric acid, HgS, BiS, CdS, CuS, AsS,(S.), SbS 

 (S B ), SnS(S,)PtS,AuS s . . . . . . . . . B! 



III. Of all the sulphides in B, the following alone are soluble in 

 alkalies and alkaline sulphides, AsS 3 (S,), Sb S 3 (S 5 ), SnS (S, ), PtS., 

 AuS, c. 



IV. Of all the metals not in A and B, the following alone are pre- 

 cipitated by excess of ammonia (as oxides), even when soluble salts of 

 ammonia are present, Fe,O 3 , Cr a 3 , AL.O,. D. 



V. Of all the metals not in A, B, and D, the following alone form 

 sulphides which are insoluble in water, MnS, ZnS, NiS, CoS. . E. 



VI. Of all the metals not in A, B, D, and E, the following alone 

 form carbonates which are insoluble in water, even in the presence of 

 soluble salts of ammonia, BaO CO,, SrO CO,,, CaO CO, . . F. 



VII. Of all the metals not in A, B, D, E, F, the following alone 

 forms an insoluble arseniate and phosphate, Mg . . . G. 



VIII. The remaining metals, whose chlorides, sulphides, oxides, 

 carbonates, phosphates, and arseniates are soluble, are the alkaline 

 metals, K, Na, NH, ......... H. 



According to a part of Berthollet's doctrine of the conditioning of 

 chemical change, the entire union of one of two bodies in a solution 

 with another is dependent upon the insolubility of the resulting com- 

 pound. So that by bringing two compounds together which by 

 reciprocal interchange of their constituents may give rise to an in- 

 soluble new body, that body will be formed. 



A. If, therefore, we have a solution containing all the metals, and 

 add to it an excess of hydrochloric acid, those metals whose chlorides 

 are insoluble in water and hydrochloric and other acids, will be pre- 

 cipitated as chlorides, and, as such, may be separated by nitration 

 from all the other metals. 



B. and C. On passing sulphydric acid through the acid, filtrate from 

 the precipitated chlorides A, until it is perfectly saturated with the gas ; 

 all the metals of the group B, and none others, will be precipitated as 

 insoluble sulphides, and may be separated from all the remaining 

 non-precipitated metals by filtration. 



D. The filtrate from B and C contains all the remaining metals, and 

 is saturated with sulphydric acid, which, though it has not precipitated 

 the iron, has reduced it to the state of protoxide. On boiling this 

 filtrate to expel the sulphydric acid, then boiling with nitric acid to 

 peroxidise the iron, and adding chloride of ammonium and ammonia 

 in excess, the metals D are precipitated as oxides, and may be separated 

 from the remaining metals by filtration. If the hydrosulphuric acid 

 be not expelled, an alkaline sulphide (sulphide of ammonium), would 

 be formed on the addition of ammonia, which would precipitate the 

 metals E. If the iron be not peroxidised, a portion of the protoxide 

 would be dissolved by the ammoniacal salts formed on the addition of 

 ammonia, and would appear in the filtrate. If a large quantity of 

 chloride of ammonium be not added, in addition to the salts of ammo- 

 nia formed, the manganese will not remain entirely non-precipitated. 



E. The oxides D having been separated by nitration, sulphide of 

 ammonium added to the filtrate from D precipitates the metals E as 

 sulphides. 



F. On boiling the filtrate from E with an excess of carbonate of 

 ammonia, the metals F are precipitated as carbonates. Magnesia ia 

 not precipitated on account of the soluble ammoniacal salts present. 



G. The filtrate from the alkaline carbonates F, on being treated 

 with arseniate of ammonia, forms a precipitate of arseniate of magnesia 

 and ammonia. G. 



G. The nitrate from G on evaporation to dryness and ignition until 

 all the arsenical and ammoniacal salts introduced are volatilised or 

 decomposed, contains the alkalies H. 



Since ammonia and its salts have been employed in the separation 

 of the groups, ammonia itself must be tested for in the original solu- 

 tion, or in the filtrate from A. 



It is self-evident that in every case where a reagent is used to sepa- 

 rate a group, that reagent must be used in excess, that is, in more than 

 sufficient quantity to effect the separation (precipitation or solution), 



