398 ANALYTICAL CHEMISTRY. 



Special remarks. Often a solution is presented for analysis instead of a 

 solid substance, in which case some of it is evaporated to dryness. If a dry 

 residue is left, this is tested for acids, as already described. But no residue may 

 remain, and if the solution has a strongly acid reaction, the presence of the 

 volatile acids is indicated, and the student, guided by the odor and change of 

 color upon evaporation, should make tests for the following acids: hydro- 

 chloric, hydrobromic, hydriodic, nitric, sulphurous (hydrocyanic, acetic, 

 formic). 



If a strongly acid, fuming, oily residue is left, sulphuric acid is indicated. 



A strongly acid, pasty, non-fuming residue indicates phosphoric acid. 



If the solution is strongly acid and leaves a solid residue, the substance may 

 be either an acid salt, or a salt held in solution by an acid, such as hydrochloric, 

 nitric, sulphuric, etc., in which case several acids would have to be looked for. 

 The presence of the volatile acids would be indicated by holding wet blue 

 litmus-paper in the vapor as the liquid approached high concentration. If the 

 residue upon evaporation is decidedly alkaline, this maybe due to a salt having 

 an alkaline reaction, or to a hydroxide, or both. The presence of a hydroxide 

 is shown by adding some solution of silver nitrate to the diluted solution, when 

 a dark precipitate of silver oxide is formed at once. In the absence of carbon- 

 ate, the presence of hydroxide is also shown by adding some dry ammonium 

 chloride to the solution and warming, when ammonia is liberated. A solution 

 containing a hydroxide must, of course, be neutralized before applying the tests 

 for acids. The acid usually employed for this is dilute nitric, but if tests are 

 also to be made for the latter acid, another portion of the solution is neutralized 

 with hydrochloric or acetic acid. 



A solution may be colorless, odorless, practically neutral, leave no residue 

 upon evaporation, and still not be plain water. In such a case, the student may 

 suspect hydrogen dioxide. He would have reason to suspect this compound if 

 he proceeded to search for metals before evaporating and found none, but got a 

 precipitate of sulphur when using hydrogen sulphide, showing an oxidizing 

 action. 



The presence of some metals interferes with certain tests for acids, and these 

 should be removed. After determining the kind of metal or metals in a sub- 

 stance or a mixture, and it is seen that there will be interference with the tests 

 for acids, boil some of the substance with a slight excess of sodium or potassium 

 carbonate for some time and filter. Non-alkali metals, except arsenic and 

 antimony, remain behind, while the acids pass into the filtrate as alkali salts 

 (with few exceptions). The filtrate is then exactly neutralized with nitric acid 

 and boiled to expel all carbonic acid, and used for the various tests for acids. 

 Arsenic and antimony may be removed by passing hydrogen sulphide into the 

 warm acidified solution and filtering. 



Substances insoluble in water. When a single substance, or that part of a 

 mixture which is insoluble in water, is treated with hydrochloric acid in order 

 to prepare a solution for the analysis of metals, something can be learned as to 

 the nature of the acids in combination. Carbonates, sulphites, phosphates, 

 arsenates, and arsenites behave the same as when treated with concentrated 

 sulphuric acid in Table IX. Sulphides give the odor of hydrogen sulphide. If 

 chlorine gas is given oif, the presence of a higher oxide, like MnO 2 , PbO 2 , BaO 2 , 

 etc., or a chromate is indicated. If effervescence takes place and an inflamma- 



