METHOD OF DE KONINCK AND NIHOUL 



451 



to the branch T-tube h. Between the T-tube * and the wash- 

 bottle for the carbon dioxid is placed a short piece of glass tub- 

 ing, s, containing a strip of filter paper, slightly moistened with 

 iodid of starch solution. This tube j is really hardly necessary, 

 as no chlorin escapes backwards if a moderate current of carbon 

 dioxid is kept passing, but it serves as a check. A glance at the 

 joints o, p, and q, which are of narrow india-rubber tubing, is 

 sufficient to show that, by using this arrangement, practically no 

 rubber is exposed to the action of the chlorin. The tiny piece 



Figure 35. 



McGowan's Apparatus for the lodometric 

 Estimation of Nitric Acid. 



of rubber tubing at the joint o may be done away with, the nar- 

 rower tube there being accurately ground into the wider one ; this 

 makes the condensing apparatus practically perfect. 



The actual operation is performed in the following manner: 

 The evolution flask is washed and thoroughly dried, and the 

 nitrate (say, about 0.25 gram of potassium nitrate) is introduced 

 from the weighing tube. Two cubic centimeters of water are 

 added, and the bulb is gently warmed, so as to bring the nitrate 

 into solution, after which the stopper of the flask is firmly inserted. 

 About 15 cubic centimeters of a solution of potassium iodid (one 

 in four) are run into the first condensing tube, any iodid adher- 

 ing to the upper portion of the tube being washed down with a 

 little water, and five cubic centimeters of the same solution, mixed 



