789 



CHEMICAL ANALYSIS. 



CHEMICAL ANALYSIS. 



790 



The acids formed by the union of metals with oxygen, such as 

 chromic, manganic, arsenious, &c., are found when the examination 

 for the bases is performed. Moreover, when certain acids are present 

 along with certain bases, and the salts so formed are soluble in hydro- 

 chloric acid and other acids, but insoluble in ammonia, it is clear that 

 in the examination for the bases, when ammonia is added to precipitate 

 the oxides of chromium, aluminium, and iron, salts of such a nature 

 will be precipitated along with, or in place of these three oxides. The 

 salts having this property are formed by the union of any of the 

 three acids, 



Phosphoric J 

 Boracic 

 Oxalic ) 



with any of 

 the base, 



Baryta 



Strontia 



Lime 



Alumina 



Oxide of chromium 



Sesquioxide of iron, 



as also phosphoric acid when in combination with magnesia. 



Hence the precipitate obtained by ammonia must be examined for 

 these acids and bases, as well as for the simple oxides of chromium, 

 iron, and aluminium. For this purpose it is dissolved in nitric acid, 

 and treated with an excess of cold caustic potash. If a precipitate is 

 formed the nitrate is boiled, whereby chromium, if present, is pre- 

 cipitated; in the nitrate from the chromium, acetic acid in excess 

 precipitates any phosphate of alumina ; and from the filtrate from this, 

 phosphate of soda precipitates the original alumina as a phosphate. 



The precipitate got by adding cold caustic potash to the nitric acid 

 solution of the original ammonia precipitate is redissolved in nitric 

 acid, treated with tartaric acid, and then supersaturated with ammonia. 

 If a precipitate (A) is formed, it is owing to the presence of the phos- 

 phates or oxalates of the alkaline earths. The precipitate is dissolved 

 in nitric acid, and treated with protonitrate of mercury. A precipitate 

 shows the presence of oxalic acid, which is separated from the mercury 

 by digesting the oxalate of mercury with sulphide of ammonium, and 

 filtering. The nitrate from the oxalate of mercury is saturated with 

 ammonia, which precipitates the original phosphoric acid in combina- 

 tion with mercury ; the two are separated by digestion with sulphide 

 of ammonium, and filtration. The filtrate from the phosphate of 

 mercury is examined for baryta, strontia, lime, and magnesia (which, 

 if present, exist now as nitrates), according to the method described 

 among the bases. The nitrate from precipitate A contains the phos- 

 phate of iron, and also the nitrate of the iron which was originally 

 precipitated as an oxide. To a portion of this filtrate, acetic acid and 

 then ferrocyanide of potassium are added ; the formation of Prussian 

 blue shows the presence of iron. To another portion chloride of 

 ammonium and then sulphate of magnesia are added : the latter pre- 

 cipitates the phosphoric acid as a phosphate of magnesia and ammonia. 



In order to test for the non-metallic elements when they are not 

 present in the form of acids, they are in some cases separated by simple 

 solvents, in others made to combine with oxygen, and the so-formed 

 acids examined. Sometimes both methods are united. Free carbon 

 and carbides are converted respectively into carbonic acid and carbon- 

 ates on heating with oxide of copper. Free sulphur and phosphorus 

 are soluble in bisulphide of carbon : the former is converted into sul- 

 phuric, the latter into phosphoric acid on boiling with nitric acid. 

 Phosphides are decomposed by water or alkalies, phosphide of hydrogen 

 being evolved. Compounds of nitrogen, as the nitrides, are decom- 

 posed by caustic alkalies, ammonia being evolved. The method of 

 testing for the free gaseous elements is too obvious to need description. 



Some salts resist the action of all solvents in the cold. Such are 

 binoxide of tin, sulphate of lead, sulphate of baryta. On fusing such 

 bodies with carbonate of soda, their acids unite with the soda, and 

 since all soda salts are soluble, they may be thus obtained in solution. 

 Thus Btannate and sulphate of soda would be formed from the above. 

 The base, when 'such is present, combines with the carbonic acid, and 

 the so-formed carbonate, after separation from the soluble soda salt of 

 the acid, is dissolved in an appropriate acid. 



Quantitative Analysis. The methods used in determining the quan- 

 tity of a constituent in a compound substance, are mainly based upon 

 the experience that in all chemical compounds the constituents are 

 united with one another in definite proportions. For it is a conse- 

 quence of this fact, that if we can unite a constituent A of a compound 

 with any other body B, so as to produce a substance, the definiteness 

 and invariableness of whose composition is established ; then, from 

 the amount of the product AB, we may deduce the amount of the sub- 

 stance A contained in the original compound. Thus, if we wish to 

 ascertain the quantity of hydrogen which a substance contains, our 

 object is effected if we can succeed in combining the whole of the 

 hydrogen with oxygen in such a manner as to form water, and in 

 determining the amount of water so produced. Because, from the 

 fact that water consists of eight parts by weight of oxygen to one of 

 hydrogen, one-ninth of the weight of the water found, will be the 

 weight of the hydrogen which the original substance contained. In 

 other words, from the weight of a compound whose constituents are 

 in definite proportions we know the weight of each of its constituents. 

 Hence, in determining quantitatively the amount of a substance pre- 

 sent, we have to seek to combine it in such a manner with some other 

 substance, that the resulting compound may so differ in physical 

 property (solubility or insolubility, volatility) from all other bodies 

 .t as to admit of perfect separation from them. 



But it is also an immediate result of the definite nature of chemical 



change, in respect to the quantities of the bodies concerned, that from 

 the amount of substance B required to be added to a substance A, in 

 order, only just, but perfectly to complete the chemical change, the 

 quantity of A present may be deduced. As, in most cases of this 

 species of determination, the reagents are vised in solution of known 

 strength, and the amounts of solution employed are determined by 

 measure, this kind of quantitative analysis has received the name of 

 Volumetric Analysis, or Analysis by Standard Solutions. (See VOLU- 

 METRIC ANALYSIS.) 



The separation of the constituents of a substance by precipitation is 

 the method most frequently employed for their determination, and 

 hence the collection and weighing of precipitates is an operation of very 

 frequent occurrence, and of the greatest importance, in quantitative 

 analysis. 



It is evident that all the operations of quantitative analysis m\>st be 

 performed with the utmost nicety of manipulation with respect to 

 cleanliness, avoidance of loss, and thoroughness. 



The determination of the weight of a substance is performed on a 

 balance of the most sensible kind, a balance which will bear fifty 

 grammes on each pan, and which will turn, when so loaded, when an 

 additional milligramme is placed on one of the pans. The weights 

 most usuually employed are grammes and their decimal parts. Milli- 

 grammes are usually indicated, not by weights having this absolute 

 value, but by placing a centigramme fashioned of wire at various 

 distances from the fulcrum on one of the arms of the balance, which, 

 for this purpose, is divided into tenths. 



In general, when a substance which has been precipitated from a 

 solution and collected on a filter is weighed, it is previously strongly 

 ignited, in order to free it from all traces of adhering moisture ; and 

 since the solid substance cannot be separated from the filter without 

 loss, the filter must be burned with the substance. The ash which the 

 filter would leave on being burned alone is thus weighed with the 

 substance estimated ; it is necessary, therefore, to know the weight of 

 such ash, and to deduct it from the weight found, in order to get the 

 weight of the substance. On accoxmt of the smallness and constancy 

 of its ash, Swedish filtering-paper is ordinarily employed : if a sheet of 

 such paper, of known area, be burned and its ash weighed, the 

 weight of the ash of a circular piece of known diameter can be easily 

 reckoned. 



The hypothetical analysis of a substance will give the best idea of 

 the manipulatory processes generally employed. Let us suppose that 

 we have to determine quantitatively the composition of crystallised 

 sulphate of copper (CuOS0 3 + 5HO) ; that is, to determine quanti- 

 tatively (1) the water, (2) the sulphuric acid, and (3) the copper. A 

 clean platinum crucible is heated to redness over a smokeless lamp ; it 

 is placed, by means of pliers tipped with platinum, while still hot, in a 

 jar closed at the top with a well-fitting plate, and containing some 

 strong sulphuric acid. The crucible is supported over the acid, on an 

 iron triangular tripod, whose legs are cased in glass tubes closed at the 

 bottom, and whose sides, on which the crucible rests, are coated with 

 platinum foil. When the crucible is quite cold, it is weighed. Let its 

 weight be C. About 1'5 grammes of sulphate of copper, which has been 

 recrystaUised, powdered very finely, and dried, to remove mechanically 

 adhering water, by being pressed for some hours between folds of 

 blotting-paper, is placed in the crucible, and the whole is weighed. If 

 the new weight be C + S, then S is the weight of the sulphate of 

 copper. The crucible, with the sulphate, is now kept for an hour at a 

 temperature a little below redness. After cooling as before in the 

 dry-air chamber, its weight is found to be C + S W ; then W is the 

 weight of the water which has been driven off, and this is the whole of 

 the water which was contained in the original sulphate. 



The sulphate which has thus been deprived of water, but which has lost 

 neither sulphuric acid nor copper, is now transferred without loss into 

 a beaker glass containing distilled water, and the adhering particles of 

 sulphate are washed out of the crucible into the beaker, whose con- 

 tents are thereon heated to boiling, whereon the whole of the sulphate 

 is dissolved. A clear solution of chloride of barium is gradually added, 

 until fresh portions cause no further precipitation. When the sulphate 

 of baryta so formed has subsided, the supernatant liquid is poured off 

 upon a moistened filter of Swedish paper, and the whole of the filtrate 

 is collected in another beaker. The sulphate of baryta is then washed 

 out of the first beaker on to the filter. When as much liquid as pos- 

 sible has drained through, a stream of boiling distilled water is directed 

 upon the sulphate of baryta from the wash-bottle previously de- 

 scribed, until the filter-paper is filled to within about a quarter of an 

 inch of the top ; when this has run through, a fresh portion of water 

 is added, and the same process is repeated six or eight times, the whole 

 of the wash-water being collected in the second beaker. The washing 

 is complete when a drop of the wash-water leaves no residue when 

 evaporated on platinum foil. When thoroughly washed, the funnel 

 containing the sulphate of baryta is covered with paper, and set apart 

 in a warm place to dry. The filtrate contains chloride of copper along 

 with the excess of chloride of barium used. The latter is removed by 

 boiling, adding dilute sulphuric acid until further precipitation ceases, 

 filtering and washing as before : the filtrate alone is preserved. The 

 filtrate from the second precipitate of sulphate of baryta is collected in 

 a silver basin, and evaporated at a temperature below its boiling point 

 as it is being collected. When its bulk is reduced to two or three 



