402 ANALYTICAL CHEMISTRY. 



38. METHODS FOR QUANTITATIVE DETERMINATIONS. 



General remarks. Quantitative determination of the different 

 elements or groups of elements may be accomplished by various 

 methods, which differ generally with the nature of the substance to 

 be examined. But even one and the same substance may often be 

 analyzed quantitatively by entirely different methods, of which the 

 two principal ones are the gravimetric and volumetric methods. 



In the gravimetric method, the quantities of the constituents of a 

 substance are determined by separating and weighing them either as 

 such, or in the form of some compound the exact composition of 

 which is known. For instance : From cupric sulphate, the copper 

 may be precipitated as such by electrolysis and weighed as metallic 

 copper, or it may be precipitated by sodium hydroxide as cupric oxide, 

 CuO, and weighed as such. Knowing that every 79.6 parts by weight 

 of cupric oxide contain of oxygen 16 parts and of copper 63.6 parts, 

 the weight of copper contained in the cupric oxide found may be 

 readily calculated. 



In the volumetric method, the determination is accomplished by add- 

 ing to a weighed quantity of the substance to be examined, a solution 

 of a reagent of a known strength until the reaction is just completed, 

 no excess being allowed. For instance : We know that every 80.12 

 parts by weight of sodium hydroxide precipitate 79.6 parts by weight 

 of cupric oxide, containing 63.6 parts by weight of copper. There- 

 fore, if we add a solution of sodium hydroxide of known strength to a 

 weighed portion of cupric sulphate until all the copper is precipitated, 



QUESTIONS. Why is sulphuric acid added to a solid substance when it is to 

 be examined for acids ? Mention some acids which cause the liberation of 

 colorless, and some which cause the liberation of colored gases when the salts 

 of these acids are heated with sulphuric acid. Mention an acid which is pre- 

 cipitated by barium chloride in acid solution, and some acids which are pre- 

 cipitated by the same reagent in neutral solution. Which acids may be pre- 

 cipitated by silver nitrate from neutral solutions, and which from either neutral 

 or acid solutions? Mention some acids which form soluble salts only. Mention 

 three soluble, and three insoluble carbonates, phosphates, arsenates, sulphates, 

 and sulphides respectively. Which oxides or hydroxides are soluble, and 

 which are insoluble in water? Mention some metals the solutions of which 

 are precipitated by soluble chlorides, iodides, and sulphides. State a general 

 rule according to which most insoluble salts may be formed from two other 

 compounds. Why is it sometimes impossible to render a substance soluble in 

 order to test for the acid in the solution obtained ? 



