GENERAL APPARATUS AND MANIPULATION 169 



bottom of the outer or larger vral. When the vial is inverted and 

 set on the plate this solution forms a hanging drop over the liquid 

 which is liberating ammonia. 



A small open porcelain dish {Micro chemical Specialties Co.) (Fig. 

 44 heknv) was used by Kirk and associates as a titration vessel. 



P^ig. 44. Titration dish, actual size. 

 Frotn Kirk and Bentley (1936) 



Coating Vessels with a Hydrophobic Layer. At times it is 

 desirable to coat reaction vessels with a hydrophobic layer to pre- 

 vent aqueous liquids from spreading on the glass surface, as in 

 iodometric titrations where liciuid films are placed across the lumen 

 of the neck of the titration tube. Linderstr0m-Lang and Holter 

 ( 1933a) used paraffin and Holter and Doyle ( 1938) employed 

 ceresine. The procedure followed by the latter was to boil about 50 

 vessels for 5-10 min. in 75 ml. of water to which 0.1 g. ceresine was 

 added. After the water had cooled, the vessels were emptied and 

 dried for at least 3 hr. at 100-110°. 



The procedure finally employed at the Carlsberg Laboratory for 

 paraffin coating was described by Brliel et al. (1946). The clean, 

 dry glass tubes are immersed in melted paraffin at 150-200° (the 

 synthetic paraffin used had a melting point of 82°), picked out one 

 at a time with forceps, quickly emptied and rotated in a clean towel 

 between the fingers until the paraffin solidifies. A heavy layer of 

 paraffin on the bottom of the tube and a thinner one on the upper 

 part is desirable. The outside of each of the tubes is wiped free of 

 paraffin and they are stored protected from dust and fumes. After 

 the vessels have been used, they are cleaned by rinsing first with 

 water, then with acetone, hot toluene, acetone, and water in the order 

 given. 



Stoppers. For most work it is sufficient to stopper reaction tubes 

 with a short piece of rubber tubing one end of which is plugged with 

 a glass bead or short piece of glass rod. A stopper consisting of a 

 cap with a small hole (Fig. 45) is useful in some cases as in the 

 addition of alkali in the method of Linderstr0m-Lang and Holter 

 (1933b I for ammonia. In this same method a stopper was used 



