io Operations used in Quantitative Analysis [n 



Exercise IV.— Weigh a clean watch glass, place upon it 

 about half a gram of a mixture of sand and chalk, and weigh 

 again. Enter the results in your note book. Place a funnel in 

 the mouth of a conical flask, as shown in fig. n, and wash the 

 powder off your watch glass into the funnel with water from a 

 wash-bottle jet. When all the powder has thus been transferred 

 to the flask, pour dilute hydrochloric acid, a little at a time, 

 through the funnel into the flask until effervescence ceases. Heat 

 the liquid to boiling in order that all the carbonic acid may be 

 expelled. Remove the funnel, and wash it, both inside and out, 

 with the spray of a wash bottle, allowing the drops to fall into 

 the flask. Cover the glass with a watch glass, and keep for 

 Exercise VI. 



PRECIPITATION 



ii. The student will be perfectly familiar with this opera 

 tion after having studied qualitative analysis. Certain special 

 precautions must, however, be taken when the contents of the 

 filter have to be weighed. 



(a) The precipitation must be complete. This is best 

 explained by taking an example. Suppose that it were neces- 

 sary to determine how much iron is contained in a certain 

 solution of ferric chloride. The addition of ammonia will 

 give a precipitate of ferric hydrate ; but before this precipitate 

 is weighed it is necessary to ascertain whether sufficient am- 

 monia has been added to precipitate the whole of the iron. 

 This may be done by allowing the precipitate to subside and 

 adding a little more ammonia. If no further precipitate be 

 formed, then the precipitation is complete. 



{b) Large excess of the precipitant is to be avoided. 



(c) The precipitate must be obtained in a condition which 

 will not readily pass through the filter paper. In some cases 

 this takes place naturally, as in the precipitation of Fe 2 (OH) G ; 

 but in others it causes considerable difficulty — e.g., barium sul- 

 phate. In many cases a good granular precipitate may be 



