228 ANALYTICAL CHEMISTRY. 



acid very rarely) contain arsenic, and should, therefore, be 

 examined by Marsh's test 



Sulphuric acid may contain sulphurous or nitric acid ; the 

 former may be recognized by potassium permanganate which is 

 decolorized by sulphurous acid; the latter is detected by care- 

 full} 7 pouring solution of ferrous sulphate upon the acid, when, 

 if nitric acid be present, a brownish zone appears at the line of 

 contact. 



Sulphurous add frequently contains sulphuric acid ; it should 

 not give more than a very slight turbidity with barium chloride 

 (limit of sulphuric acid). 



Nitric acid diluted with 5 parts of water should afford no pre- 

 cipitate with either barium chloride or silver nitrate (absence of 

 sulphuric and hydrochloric acids). 



Phosphoric acid should be tested for phosphorous, nitric, sul- 

 phuric, hydrochloric, pyrophosphoric, and metaphosphoric acids. 

 Phosphorous acid is indicated by the formation of a dark color 

 when silver nitrate is added to the diluted acid ; sulphuric and 

 hydrochloric acids by the formation of white precipitates on the 

 addition of barium chloride or silver nitrate, respectively, to the 

 diluted acid; pyrophosphoric and metaphosphoric acids, by the 

 formation of a precipitate upon adding to the acid an equal 

 volume of tincture of chloride of iron. 



Hydrochloric add, after being diluted, should not give a pre- 

 cipitate with barium chloride (absence of sulphuric acid), and 

 should not liberate iodine from potassium iodide (absence of 

 free chlorine). 



The officinal acids should have the strength required by the 

 U. S. P. The amount of actual acid can either be determined 

 by the specific gravity of the liquid acid, or by the quantity of 

 an alkali required to neutralize a certain quantity of the acid. 



Examination of potassium, sodium, and ammonium compounds. 

 The acidulated solution of the hydrates, carbonates, bicarbonates, 

 sulphates, nitrates, and phosphates of potassium, sodium, and 

 ammonium should afford no precipitate with hydrosulphuric 

 acid, or with the hydrate, sulphide, carbonate, or phosphate of 

 ammonium (absence of heavy metals, alkaline earths, and mag- 

 nesium). 



The nitrates, hydrates, carbonates, and bicarbonates (the three 



