PREPARATION OF THE CITRIC ACID EXTRACT 2O? 



of 10 per cent, citric acid solution, which is preserved by the ad- 

 dition of half a gram of salicylic acid per liter. From this store 

 the other solution of citric acid can be prepared. It is important 

 that the citric acid solution which is used, should be as nearly as 

 possible at a mean temperature of 17.5. Any departure from this 

 temperature is apt to produce errors. For this reason, the shaking 

 machine should be in a room approximately of the same tem- 

 perature. It is advisable to have the apparatus protected with 

 felt. 



In pouring 500 cubic centimeters of citric acid solution on five 

 grams of the sample it is sometimes noticed that small lumps 

 of the slag are produced, which resist for a long while the en- 

 trance of the solvent. For this reason it is advisable to use, pre- 

 viously to the introduction of the citric acid solution, five cubic 

 centimeters of alcohol into which the sample of basic slag is 

 poured. 



It is inadvisable to use a shaking apparatus in place of the ro- 

 tating apparatus described. Wagner uses a rotating apparatus 

 made in Darmstadt, which is constructed of metal and driven by 

 a gas motor. The flasks which are to be used in the rotating ap- 

 paratus are made especially for this purpose according to the 

 specifications of Wagner, and have a neck diameter of at least 

 20 millimeters and the mark is at least eight centimeters 

 below the mouth. Some care must be exercised in this matter, 

 for if the diameter of the neck is too narrow or the mark too high, 

 the movement of the fluid during rotation is restricted, and there- 

 by the results may be influenced. The rotating apparatus should 

 have a velocity of from 30 to 40 rotations per minute, but any 

 variation of the rate of rotation between these two figures is 

 without any marked influence upon the results. 



The filtration ought to take place immediately after the end of 

 the 30 minutes rotation, and it is advisable to use a folded filter 

 of sufficient size so that the whole contents of the flask may be 

 brought at once upon the filter. Small and badly working filters 

 by reason of delay in the filtrations, can easily produce errors in the 

 result. If the filtrate should be at first turbid, it is thrown back 

 upon the filter. 



