174 CHEMICAL TECHNIQUES 



pipette. If employed in the assembly shown in Figure 50, a quick 

 squeezing of the rubber tubing over K will be sufficient to initiate 

 the emptying process. Greater accuracy is obtained by adjusting the 

 dimensions so that the pipette delivers automatically without apply- 

 ing excess pressure w^hen the tip touches the vessel or the liquid. 

 The tip and the constriction should be constructed as in A (Fig. 53) , 

 and not as in B. Holter and Doyle ( 1938) employed a constriction 

 pipette surrounded by a water jacket to control the temperature of 

 the liquid being pipetted (Fig. 54). 



In the procedure for the determination of total nitrogen (pages 

 234 and 283) the pipettes used must meet certain dimensional re- 

 quirements as defined by Briicl et al. (1946). Thus, the pipette used 

 to transfer the digested sample must have a tip, the opening of 

 which is not so narrow as to become blocked by small crystals or 

 other particles; but neither must it be so wide as to make it difficult 

 to empty the pipette without blowing air through the tip, which 

 might cause spattering of the liquid delivered. Furthermore, the 

 pipette stem must be thin enough for use in a narrow tube without 

 causing the liquid to be drawn up between the tube and pipette, and 

 yet it must be thick enough to have mechanical strength. A suitable 

 pipette is illustrated in Figure 55, and the allowable variation in the 

 dimensions is shown in Figure 56, which gives the dimensions of a 

 rather thin and a rather thick pipette, either of which may be used. 



The dimensions of a suitable constriction pipette for pipetting the 

 acid used to absorb ammonia are given in Figure 57. 



Placing a water seal of known volume across the lumen of a reac- 

 tion tube is best performed with the type of constriction pipette 

 shown in Figure 58. Water is drawn up to the point X; the entire 

 amount is blown out to form the seal, and then the excess water is 

 sucked back into the pipette to Y. The amount left in the seal is 

 then the volume between X and Y in the pipette. Acid-selenium mix- 

 ture is pipetted with the horizontal pipette illustrated in Figure 59. 

 Each division corresponds to 1 //.I. These pipettes are available from 

 E. Petersen, Carlsberg Laboratory. 



Automatic Pipettes. An automatic pipette was designed by 

 Linderstr0m-Lang and Holter (1931) and it was used by them for 

 the accurate delivery of 20—40 /xl. alcoholic acid to stop enzyme ac- 

 tion (Fig. 60) {A. H. Thomas Co. and E. Petersen, Carlsberg Labo- 

 ratory). The pipette consists of a narrow glass tube drawn out to a 



