342 METHODS OF ANALYSIS [Chap. 



8 DETERMINATION. 



In order to find the allowable rapidity of the air current employed during the 

 determination, proceed as follows: Charge the apparatus exactly as for an analysis 

 leaving out the carbonate. Begin to aspirate at the rate of about 50 cc. per minute. 

 After 2 liters have been aspirated weigh the tubes, M and N. If they have lost in 

 weight, repeat the experiment with 40 cc. per minute, and so on until the weight of 

 the tubes remains constant. If the work has been done with due precaution, the 

 first tube should have lost just as much as the second has gained. Do not exceed the 

 safe speed thus found. 



Weigh the tubes M and N at the air temperature of the balance room. Shortly 

 before weighing open the tubes for a moment to allow equalization of air. Note 

 the thermometer and barometer readings. Connect the tubes with the apparatus 

 and test the tightness of the joints by closing A at the bottom, opening all the cocks, 

 starting the aspirator, and observing P, in which the liquid should soon come to a 

 standstill. Then disconnect the aspirator, close B, remove F, put in the substance, 

 using about 1 gram of sodium carbonate or calcium carbonate or about 2 grams of 

 baking powder, connect F, and start the condenser {G). Introduce 50 cc. of 10% 

 hydrochloric acid through D, lifting E slightly and allowing only small quantities 

 of the dilute acid to enter at a time. Light the burner under F, heat to boiling 

 and reduce the flame to keep the liquid just at the boiling point. If no more air 

 passes P, start the aspiration. When the water stops running from S, open B care- 

 fully and adjust the outflow of the aspirator by raising or lowering the syphon to 

 one half the safe speed. 



After M has become cool increase the current to the full safe speed and aspirate 

 altogether 3 liters, continuing boiling to the end of the aspiration. After the tubes 

 have assumed the temperature of the balance room, open for a moment and weigh. 

 When extreme accuracy is desired, note again the thermometer and barometer 

 readings and apply correction according to the following formula: 



- (A2 - Ai) X T and + (B^ - BO X B in which 



A^ = the temperature at first weighing in degrees C. ; 

 A* = the temperature at second weighing in degrees C; 

 Bi = the air pressure at first weighing in mm.; 

 B'' = the air pressure at second weighing in mm.; 



T and B are constants found from the following formulas: 



T = V X 0.0000039 gram; 



B = V X 0.0000015 gram in which 



0.0000039 = change in weight of 1 cc. of air for TC; 

 0.0000015 = change in weight of 1 cc. of air for 1 mm. pressure; 



and the value of V is obtained from 



2.7 2.0 8.5 ' 



representing the differential volume affected by temperature and pressure and being 

 a constant for the tubes and in which 



