i. GENERAL APPARATUS AND 

 MANIPULATION 



A. VESSELS, STOPPERS, HOLDERS, ETC. 



Vessels. Most of the reactions employed in the various chemical 

 techniques are carried out in simple glass vessels. The tube shown 

 in Figure 35 is especially useful; it is nothing more than a small test 

 tube having a total capacity of 0.25 ml. (available from A. H. 

 Thomas Co. and E. Petersen, Carlsberg Laboratory, Copenhagen 

 Denmark). Norberg (1937) employed the tubes (Fig. 36) for use 

 in centrifugation and the apparatus shown in Figure 37 for removal 

 of supernatant fluid from centrifuged precipitates. By applying 

 suction at A the fluid is drawn into the reservoir; the low-power 

 microscope is used to enable careful control of the operation. As 

 indicated in Figure 37, the tube may be surrounded by a larger 

 vessel filled with a clear liquid, such as alcohol, to permit better 

 observation, particularly when the vessel B (Fig. 36) is used, since 

 the bottom part of the tube has a dark zone due to its form. 



The tubes may be cleaned conveniently by immersing in the clean- 

 ing liquid, heating to drive the air out of the tubes, cooling to let 

 them fill up with the liquid, and shaking out the liquid from each 

 one. Usually the process is repeated two or three times. To place 

 films of liquid across the upper portion of a reaction tube, as in 

 iodometric titrations, Holter and Doyle ( 1938) employed the vessel 

 shown in Figure 38, which has a total volume of 0.20 ml. These 

 vessels must be given an inner hydrophobic coating to prevent the 

 liquid from spreading on the glass surface. The method of doing 

 this is described on page 169. 



Levy (1936) used the 2.5 ml. tube illustrated in Figure 39 for the 

 Kjeldahl digestions and Linderstr0m-Lang, Weil, and Holter (1935) 

 employed the two-piece unit shown in Figure 40 for ammonia dis- 



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