Cleaning and Storing Procedures

The use of clean glassware is of utmost importance when doing a chemical analysis. In addition to the obvious need of keeping the solution free of contaminants, the walls of the vessels, particularly the transfer vessels (burets and pipets), must be cleaned so that the solution will flow freely and not bead up on the wall as the transfer is performed. If the solution beads up, it is obvious that the pipet or buret is not delivering the volume of solution intended. It also means that there is a greasy him on the wall that could introduce contaminants. The analyst should examine, clean, and reexamine his or her glassware in advance so that the free how of solution down the inside of the glassware can be observed. For the volumetric flask, at least the neck must be cleaned in this manner so as to ensure a well-formed meniscus. 

Both hand washing and machine washing and drying procedures are in use in industrial analytical laboratories. The variety of available soaps include alkaline phosphate-based and phosphate-free soaps. While phosphate-based soaps are quite satisfactory for cleaning purposes because phosphate helps to 

Hand washing typically involves the appropriate soap and a brush. For burets, a cylindrical brush with a long handle (buret brush) is used to scrub the inner wall. With flasks, a bottle or test tube brush is used to clean the neck. Also, there are special bent brushes available to contact and scrub the inside of the base of the flask 

Pipets pose a special problem. Brushes cannot be used because of the shape of some pipets and the narrowness of the openings. If soap is to be used, one must resort to soaking with a warm soapy water solution for a period of time proportional to the severity of the particular cleaning problem. Commercial soaking and washing units are available for this latter technique. Soap tablets are manufactured for such units and are easy to use. 

In the past, chromic add solutions have been used for cleaning. These solutions consist of concentrated sulfuric add in which solid potassium dichromate has been dissolved. Because of safety concerns and concerns with chromium contamination in the environment, chromic add has been essentially eliminated from use. 

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