58 



SELECTION OF TECHNIQUES 



Construction of Parts and Properties of Materials: Once it has been 

 decided to build equipment or component parts for larger assemblies, 

 the choice of material becomes important. It is always worthwhile to 

 study any properties of a possible material which will be important in 

 the operations of construction and in the finished product. Many metals 

 and alloys are available. Brass, even though it is expensive, has been 

 found to be one of the best metals for general use in constructing small 

 parts. It is easy to cut and machine, it can be soldered easily, and it is 

 resistant to corrosion. If organisms are to be kept in a brass container, 

 however, toxic levels of Cu or Zn mav leach out of the metal. Aluminum 

 is less dense, usually cheaper, but somewhat harder to work and more 

 likely to corrode by electrolysis. One of the favorite materials is acrylic 

 plastic (Plexiglas or Lucite), available in rods, tubes, or sheets of many 

 sizes. It comes as the clear, transparent plastic, in a variety of colors or in 

 opaque black. It machines beautifully on the lathe or milling machine, 

 and pieces can be cemented together with a chlorinated hydrocarbon like 

 chloroethane or with a cement containing the monomer from which the 

 plastic is polymerized. For many uses this plastic is unexcelled. 



Glass Apparatus: Glass has a number of properties which make it a 

 desirable laboratory material. It is chemically inert, transparent, avail- 

 able in many forms, cheap, not too 

 difficult to work with, etc. Glass com- 

 ponents can be assembled with con- 

 necting pieces of flexible tubing, but 

 for many applications ground joints 

 are superior. The glassware manu- 

 facturers provide tapered joints or ball 

 joints in standard, interchangeable 

 sizes. Both are illustrated in Fig. 5-1. 

 Table 5-1 lists some of the features 

 of Standard Taper joints, along with 

 some representative sizes. The joints 

 themselves can be fused to glass tub- 

 ing of almost any size. Flasks, con- 

 densers, extractors, and many other 

 glass items are available with ground joints, so that a great variety of 

 combinations can be assembled. Stopcocks of many designs and sizes 

 complete the assembly. A few minutes spent examining the pictures in 

 a glassware catalogue can be quite instructive. 



Fig. 5-1. Left, standard ball joint; 

 right, standard taper joint. (Courtesy 

 Corning Glass Works.) 



