CHEMICAL ANALYSIS. 



( HKMICAL ANALYSIS. 



mutually iii>cial rragenU. And jut as moat of the chemical character* 

 of a Jtase are exhibited in its soluble salta, *o most of thuno uf the acids 

 appear in their soluble compounds (for the so-called free acids are 

 hydrogen alU). But in regard to their physical properties, thin U not 

 the case ; for many acids when in uch combination with hydrogen 

 lone, that is, as free acids, are very much more volatile than when in 

 union with metallic bam. 



It U almo*t self-evident that all those acids which in the free state 

 (under the ordinary conditions of pruesure and temperature) are 

 gasenus, are also volatile. When such an acid is bvr any means ex- 

 pelled from combination (from its base) more or less of it will be 

 volatilised according as the free acid is less or more soluble in the. liquid 

 in which it is liberated; and this solubility, an with other gases, 

 depends upon the nature of the acid, upon the nature of the liquid in 

 which it U freed (prmci(nlly in regard to the amount of solid matter 

 it holds in solution), and upon the temperature to which the freed acid 

 is subjected (leaving out of account the pressure upon it). According 

 to their solubility in most liquids, and therefore, crtrri* paribut, their 

 degree of volatility, volatile acids may be divided into five tolerably 

 well defined groups. 



I. Most volatile, and therefore moat easily driven off from solution 

 when in the free state: Carbonic acid (CO,), hydrosulphuric acid 

 (H8), sulphurous acid (SO,). 



II. More soluble, therefore leas volatile ; requiring a higher tem- 

 perature, or more concentrated solution for expulsion : Hydrocyanic 

 acid (HCy), hydriodic acid (HI),hydrobromic acid (HBr), hydrochloric 

 acid (HC1), nitric acid (HONO,), hydrofluoric acid (HF1). 



III. Much more difficultly volatile : Sulphuric acid (HOSOA 



IV. Further, there are the non- volatile : Boracic acid (BoO,), phos- 

 phoric acid (PO.), silicic acid (SiO,). 



V. Those which are decomposed by heat, Chloric acid (CIO,), 

 Oxalic acid (HOC.O,). 



A* a general rule, the less soluble and more volatile an acid is, the 

 more readily is it driven from its base by a more soluble or lens 

 volatile acid. Hence sulphides, sulphites, and carbonates, are generally 

 decomposed, even at low temperatures, by almost any other acid such 

 as one of the second group. These again are expelled from their salts 

 by sulphuric acid, when in concentrated solution, or when heat is 

 applied. The latter is expelled and volatilised at a strong heat, by one 

 of the third group. 



Hence the gases which are given off on adding hydrochloric u<-i<l t.i 

 a dilute solution of a substance are to be examined for group I. 'I'll' > 

 which are expelled from a strong solution, or on heating (between 100- 

 305 Fahr.) with sulphuric acid (for, as before stated, concentration 

 has here the same effect as heat), are to be examined for II. For the 

 lean volatile HI. the non-volatile IV. and the decomposed V. other 

 methods are employed. 



The detection of the individual acidg in based upon their following 

 characters: 



Free Carbonic acid precipitates lime from alkaline solutions, and 

 dissolves it in excess ; it is non-inflammable, and extinguishes 

 dame. 



Free Jiydntulphir acid gives the characteristically coloured sulphides 

 with the metals (described under the bases) ; its smell is peculiar ; it 

 U inflammable, being converted, on combustion, into sulphurous acid 

 aod water. 



The luljJiurou* acid U uninflammable ; of peculiar smell ; forma an 

 insoluble sulphite with lime ; and decomposes hydrosulphuric acid, 

 solid sulphur being liberated. 



Free hydrofluoric acid has the power of attacking silicic acid and the 

 silicate*, forming gaseous fluosilicic acid. If therefore a piece of ghvss 

 be partly coated with wax, and exposed to the gas given off when a 

 substance is warmed with strong sulphuric acid, it will be etched in 

 those irts which are unprotected, if the original substance contain a 



Nitric add. A* all nitrates are soluble, nitric acid cannot be preci- 

 pitated by any base. Free nitric acid oxidises metallic copper, I icing 

 iUelf deoxidised, and the formation of red fumes attend -H. h dooxida- 

 tion. Free nitric acid is also turned brown by the protosulphate of 

 iron. On adding, therefore, strong sulphuric acid to a nitr 

 heating with metallic copper, red fumes are given off. If a solution ,.f 

 protosulphate of iron be poured upon another cold portion to which 

 sulphuric acid has been added, a brown ring is formed at the junction 

 of the two liquids. 



Hydrocyanic acid. When a cyanide is treated with strong sulphuric 

 acid, hydrocyanic acid is given off. Free hydrocyanic acid combine* 

 with sulphide of ammonium, containing an excess of sulphur (over the 

 monoiulphide), and forms with it sulphocyanide of ammonium, a 

 substance which, after the excess of sulphide of ammonium is expelled 

 by heat, give* a blood-red colour with the persalts of iron. Hydro. 

 cyanic acid forms a white cyanide of silver, which is insoluble in 

 water aod acids, but soluble in ammonia. It is decomposed by heat 



UydrocMnric and fiyiirooromic aridi both form salt* with silver which 

 are insoluble in water and acids, but soluble in ammonia. The 

 bromide and chloride of silver are decomposed on being fused with 

 carbonate of soda, the inrtal being reduced and the bromide and 

 chloride of sod* formed. Bromide* and chloride*, li.-n heated with 



sulphuric acid and binoxide of manganese give off bromine and chlorine 

 respectively. 



Uydrimlif acid. Iodide of silver U yellow, insoluble in acids, and 

 very difficultly soluble in ammonia. Iodides are decomposed, K' V '"~ 

 :!'ii.-t- iodine when heated with binoxide of manganese and strong 

 sulphuric in ill. 



Suljihuric acid in solution precipitates baryta from solution as a 

 sulphate, which is insoluble in hydrochloric acid. Sulphates are con- 

 verted into sulphides by fusion with carbon. 



The members of group IV. form baryta and lime, salts whii -li 

 are insoluble in water and ammonia, but soluble in free hydro 

 old. 



SUlelf acid in the free state, after drying, is insoluble in water and 

 acids, except hydrofluoric acid. It forms a transparent colourless bead 

 when fused with carbonate of soda. Silicates which are not decom- 

 posed^by acids give up their bases to carbonic acid when fused with an 

 excess of carbonate of soda ; the silicic acid itself combining with the 

 soda to form a basic silicate of soda from which an acid separates the 

 base, leaving the silicic acid, after drying, as an insoluble po d.-r. 



Photphoric acid. Phosphoric acid forms a yellow phosphate of 

 silver, which is soluble in nitric acid and in ammonia. The phosphates 

 of the alkaline earths, and of iron, chromium, and alumina, are soluble 

 in acids, but not in ammonia. Compare magnesia. 



Boracic acid. The borates of the alkaline earths, anil of iron, 

 chromium, and alumina, are soluble in acids, but not in ammonia. If 

 a burate be treated with strong sulphuric acid, the boracic acid is 

 liberated. Free boracic acid has on alkaline reaction, and give" 

 racteristic green colour to the flame of alcohol. The borate of 

 is soluble in nitric acid. 



Chlvric acid, like nitric acid, forms an insoluble salt; a hi" 

 being ignited gives off its oxygen, and is converted into a chloride. 



Ojcalic acid forms with lime an oxalatc which is insoluble in water, 

 but soluble in acids (excepting acetic acid). Many oxalates on 

 heated give off carbonic oxide, and are converted into carbonates. 



Space will not allow a detailed description of the separation of the 

 acids ; moreover they do not admit of a systematic separation, similar 

 to that described with the bases. The following outline may serve t" 

 give some idea of the process employed. We will at first suppose that 

 the alkaline earths, alumina, and the oxides of chromium and iron are 

 absent. 



I. The substance is treated with dilute hydrochloric acid ; if a gag 

 is evolved on warming, it is passed through lime water. A precipit.de 

 is formed, both with carbonic and sulphurous acids. The latter is 

 detected by its bleaching power, its peculiar smell, and by it*, il.i 

 sulphur when mixed with hydrosulphic acid. A piece of paper 

 moistened with a solution of acetate of lead is held in the evolved gas ; 

 if it is blackened, hydrosulphic acid is present. 



II. Another portion is evaporated to dryness with a fixed alkali, and 

 treated with strong sulphuric acid ; the gas, if any is evolved, is tested 

 as before described for hydrocyanic and hydrofluoric acids. 



III. Another portion is treated with strong sulphuric ociil. ami 'li.- 

 tilled (this is at first done with a small quantity, lest a elil. : 

 present, in which case the action is attended with explosion), 

 being placed in the receiver. A portion of the distillate is tested for 

 nitric acid with sulphate of iron and metallic copper as already 

 described. To another portion nitric acid and nitrate of silver are 

 added. If any precipitate is formed, it is collected and treated with 

 dilute ammonia. The insoluble portion is treated with (troogsulpharlo 

 acid and binoxide of manganese. Iodine is evolvitl if hyilriiul 



was present (iodine would also bo evolved on distilling the MI) 

 with sulphuric acid). Nitric acid added to the anunonUcal i 

 precipitates the bromide and chloride of silver, in \\hii-li the h 

 are either at once detected in the same manner as was the iodine, or 

 else after combining them with sodium by fusing the silver salts with 

 carbonate of soda. 



IV. To another portion, hydrochloric acid must be added (if a pre- 

 cipitate bo fonned it must be removed by filtration), and then solution 

 of chloride of barium ; if a precipitate is formed, sulphuric acid was 

 present. 



V. A portion from which the carbonic acid has been cxpclliil is 

 neutralised with ammonia, made acid with acetic acid, and treated with 

 chloride of calcium. If a precipitate is formed, it is oxalatc of lime. 

 In this cose it is collected, dried, and heated in a closed tub. 

 lioiiie oxide is thereby driven off, and may be inflamed; a white 

 carbonate of lime remains, which gives off its carKmic acid when 

 treated with hydrochloric acid. The filtrate from the oxalate of lime 

 U evaporated to dryness, at lil 1 .!". with hydi-.K-lil..i ic aci.l. If a residue 

 remains which is insoluble in water, silicic acid i* present ; this gives a 

 clear glass when fused with carbonate of soda, A portion of the dry 

 residue is moistened with alcohol , treated with sulphuric acid, and 

 inflamed; a green flame shows the presence of boracic acid. T<> 

 another |rtion of the dry lime salt, chloride of iron in add. -d. and then 

 acetate of soda in excess. On I Milling to ux|>el the liberated acetic acid, 

 l,h,..|.li.iie of iron (and subacetate) is precipitated. This is filtered 

 t'lointliccli! le.iiiin formed, and after washing, decomposed 

 by sulphide of ammonium. The iron remains insoluble as a sulphide ; 

 the pluwphoric acid in in the filtrate as the phosphate of ammonia, from 

 which a soluble salt of magnesia precipitates the phosphoric acid. 



