QUALITATIVE ANALYSIS ] 



CHEMISTRY. 



399 



(h.) But the solution effected in the last process by 

 hydrochloric acid, 'after the silica is removed, may con- 

 tain both alumina and oxide of iron. It may be testec 

 in exactly the same manner as indicated under d and e, 

 when these substances, which had been held in chemica 

 union with the silica, will also be discovered. 



By such means the presence of silica, alumina, lime, 

 and iron may be detected in clay ; and also the combi- 

 nation of the three last-named bodies in union with the 

 silica as sand. In this analysis, we still further discover 

 the necessity of a successive process in analytical investi- 

 gations, if we would command reliable results. We may 

 recapitulate our analysis as follows : 



1. We removed matter soluble in water. 



2. We effected the solution of a portion of the clay in 

 hydrochloric acid, leaving the insoluble part for further 

 experiments (6.) 



3. We next got rid of the carbonate of lime in the 

 form of a sulphate (c. ) 



4. The iron was removed by the next process (d.) 



5. The alumina was precipitated as a hydrate in e. 



6. We afterwards effected the decomposition of the 

 sand, and obtained its silica as a white powder (J and g.) 



1. And lastly, the other constituents of the sand, in 

 chemical union with the silica, were also discovered (A.) 



The foregoing examples of qualitative analysis may 

 serve the student as types of the methods generally to 

 ursued in inorganic qualitative analysis, when earths 

 al .:e are sought for ; and to the agriculturist such may 

 prove a kind of general guide to a rough analysis of an 

 average soil. We shall next give some instances, in 

 which the detection of metals proper are more ex; 

 involved, and which may prove useful to those desirous 

 of investigating the nature of veins of metallic ores, 

 which may occur in their neighbourhood. Before pro- 

 ceeding to the following experiments, the reader is ad- 

 vised to peruse the tables of tests for the metals, given 

 at a previous page ; * by which he will l>e the better pre- 

 pared to understand the operation of each reagent in 

 any special case. 



i-iment 84. Dissolve together, in a test-tube, a 

 few crystals of sulphate of copper, and proto-sulphate of 

 iron. Yon will thus obtain a mixed solution of the two 

 mrtals ; the object being to detect the presence of each 

 separately. Now, if the ordinary ineihod of precipi- 

 tating the oxide, carbonate or prussiate, were adopted, 

 by means of those salts of the alkalies, the salts of the 

 metals would be thrown down together. Other means 

 must therefore be tried : and either of the two following 

 methods may be adopted : 



1. Divide the solution into two parts, in separate 

 test-tubes. Into one immerse a piece of clean iron 

 plate, having first added a few drops of sulphuric acid. 

 Chemical action will at once ensue, and the copper will 

 be reduced to a metallic state, completely from the 



liquid. 



2. Immerse two pieces of platina foil, attached to 

 each wire proceeding from a single cell of a voltaic 

 battery, in the other solution, and allow the current to 

 pass for some time. Eventually the copper will be re- 

 duced at the negative pole in the metallic state. 



3. The copper may be removed from an acid solution 

 of the two metals, by means of sulphuretted hy<i 



For this purpose, some sulphuric acid is first to 

 be added to the solution of the salts of the two metals, 

 anil then a current of sulphuretted hydrogen is to be 

 passed through. The black sulphide of copper will be 

 at once produced ; whilst the iron still remains in solu- 

 tion. It can afterwards be precipitated by means of any 

 of the alkalies ; but the solution must first be boiled, to 

 remove any hydro-sulphuric acid left after the precipi- 

 tation of the copper. This experiment illustrates methods 

 by which voltaic action may often be employed for the 

 removal of platina, silver, mercury, and copper, from 

 mixed metallic solutions. 



,iment 85. We shall now offer a more complex 

 experiment, in which two metals and an earth are found 

 in solution ; the object being to detect and remove each 



See ante, p. 393. 



separately. Dissolve a few grains each of the nitrates of 

 silver, lead, and baryta together in a test-tube ; and 

 from this solution precipitate each substance as follows : 

 (a.) A.dd common salt in solution to the above solu- 

 tion. The chlorides of silver and lead will be precipi- 

 tated together. Filter these off by means of the funnel, 

 <tc., preserving the clear liquid, and washing the preci- 

 pitate with distilled water, which must then be added 

 to the original solution left after filtration. Put the 

 precipitates into a test-tube, and add liquid ammonia. 

 This will dissolve the chloride of silver, and leave the 

 lead untouched, by which that metal will be removed 

 entirely. 



(6.) To the solution of silver by ammonia, produced 

 in a, add an excess of dilute hydrochloric acid, when the 

 chloride will be precipitated, and may be known by 

 changing to a dark colour by the action of light. 



(c.) The original solution will now only contain the 

 salt of baryta, which is readily precipitated by adding 

 a sulphate, or dilute sulphuric acid ; and thus the baryta 

 will also be removed. 



Now, had we used sulphuric acid in the first instance 

 in o, we should have precipitated together the sulphates 

 of each of the bases, and would have lost almost all chance 

 of dividing them. This, again, is another illustration 

 of the necessity of rule in using chemical reagents. 

 It is of the utmost importance, in adding a test, that we 

 take care not to introduce any substance which may in 

 any way invalidate or modify the different steps of an 

 analysis. Thus the chloride of barium, or nitrate of 

 baryta, may sometimes be used to precipitate sulphuric 

 acid from solutions. At others, the introduction of 

 chlorine would cause insoluble precipitates, and so render 

 the result fallacious in that or other ways, as the follow- 

 ing experiment will show : 



Experiment 86. Add a few drops each of sulphuric 

 and hydrochloric acid together. To the solution add 

 some of nitrate of baryta, when an insoluble sulphate of 

 baryta will be precipitated. Then add, after the re- 

 moval of the sulphuric acid, a little nitrate of silver to 

 the original solution : a precipitate of chloride of silver 

 will be produced ; and the hydrochloric acid will then 

 be also removed. Now, hail the chloride of barium been 

 used instead of the nitrate, then it would of itself have 

 produced a precipitate of the cliloride of silver, indepen- 

 dently of the hydrochloric acid already present ; and 

 hence if a quantitative analysis had been desired, the 

 amount of chlorine would have been indicated far in 

 excess of the truth. We may here mention another 

 source of error, which occasionally arises in testing with 

 either of the salts we have named. Both the chloride of 

 barium and the nitrate of baryta are apt to be precipi- 

 tated as very minute crystals, when used as tests in con- 

 centrated solutions. They, however, immediately dis- 

 appear when water is added, being readily soluble in 

 that fluid. They are, of course, thus at once removed 

 from the insoluble sulphate. 



Wo may make a practical application of some of 

 these facts in undertaking a quantitative analysis of 

 mineral waters. But as some of our readers may not 

 have access to any, we shall suggest a way of imitating 

 the composition of one. For this purpose dissolve a few 

 grains of each of the following salts in a pint of distilled 

 water; viz. Sulphate of magnesia(Epsom salts); chloride 

 of sodium (common salt) ; sulphate of liuie (plaster of 

 Paris) ; sulphate of iron (copperas). 



Now, these substances are generally found in most 

 mineral waters ; and very often in common river-water. 

 The salts are all to be well shaken up ; and will dissolve, 

 with the exception of the sulphate of lime, of which only 

 a small portion will enter into solution. It is required 

 ;hat we should detect the bases, i.e., the magnesia, lime, 

 and iron, and the common salt. The solution must be 

 iltered so as to be perfectly clear ; and the following 

 steps are to be taken : 



(a.) Pour a portion of the solution into an evaporating 

 dish, capable of holding about two fluid ounces ; ami 

 evaporate by the heat of a spirit-lamp ; adding more of 

 the fluid as it is lost in vapour. By this means the 



