reality, if the manufacturers are properly following the ASTM guidelines, there is little room for any volumetric ware to be poorly made.

ASTM guidelines also establish that volumetric ware is to be calibrated at 20°C. This (or any) constant is required because gases, liquids, and solids vary in size as temperature changes. Thus, any individual item is accurate only at one temperature. If manufacturers did not adhere to one temperature calibration, constant mathematical adjustments would have to be made as one switched from one brand of glassware to another.

There are four basic grades of volumetric ware:

1.Special Tolerance, also called Calibration Tolerance and Certified Ware. This glassware is calibrated not only to Class A tolerances, but to each mark on the glass. This calibration ensures high accuracy for every measurement. It needs to be specially ordered and calibrated.

2.Class A. This code is for the highest-class glassware that is production made. Some manufacturers refer to this grade as precision grade. High-quality work can be performed using glassware that has this designation. The tolerance is the same as Special Tolerance, but in Class A it is based on a container's total capacity, not for each measurement division. Containers of Class A tolerance are always designated as such in writing on the side.

3.Class B. This classification is average grade glassware and can be used for general quality work. Some manufacturers refer to this quality as standard grade or standard purpose. By ASTM designation, Class B tolerances are twice those of Class A. The tolerance is based on a container's total capacity, not for each measurement division. Containers of Class B tolerance are seldom, if ever, designated as such in writing on their side.

4.General Purpose. This glassware is not accurate for any level of quality work. It is often referred to as student grade or economy grade. Because the calibration is never verified, it is the most inexpensive volumetric glass available. The tolerance is limited to plus or minus the smallest subdivision of the container. Containers of general purpose tolerance are never specifically identified as such and therefore may be confused as Class B volumetric glassware. Such glassware made by Kimble is identified as "Tekk." Corning does not have a generic name for its student ware. General purpose glassware is often made out of soda-lime glass rather than borosilicate glass.

2.3.4 Materials of Volumetric Construction #1 Plastic

There are four types of plastic that are commonly used in volumetric ware: polypropylene (PP), polymethylpentene (PMP or TPX), polycarbonate (PC), and polystyrene (PS). Plastic can be less expensive than glass, can be more difficult to

7. Organic solvents can be used cautiously only with polypropylene. However, test each solvent before use for possible negative effects on the outside of the volumetric ware, away from a calibrated area.

Autoclaving. Polypropylene can be autoclaved, and polycarbonate can be autoclaved for a limited time (<20 minutes) but can be weakened from repeated autoclaving. The quality of volumetric measurements is likely to be eroded with repeated autoclaving. Polystyrene should never be autoclaved.

2.3.5 Materials of Volumetric Construction #2 Glass

Although glass is more likely to chip, crack, or break than plastic ware, it is safe to use with almost all chemicals. Type I glass can safely be put into a drying oven, and Type I and II glass can be autoclaved (glass types are explained below). No volumetric ware should ever be placed over a direct flame. Occasionally, tips and/ or ends of burettes or pipettes are tempered to provide additional strength. The tempering process typically used is heating followed by rapid cooling.

General Characteristics of Each Glass Type. Volumetric ware made of glass provides different properties depending on the type of glass used. The ASTM refers to the different glasses used in volumetric ware as Types. The use of the terms "Class A" and "Class B" in reference to types of glass have no relationship to volumetric quality.

1.Type I, Class A glass is usually Pyrex, Kimex, or Schott glass. It can be used for all classifications of volumetric ware. This glass is chemically inert, is repairable (in nonvolumetric regions away from calibration), can safely be placed in a drying oven, and can be fused onto other similar borosilicate laboratory glassware.

2.Type /, Class B glass is an aluminosilicate glass. It can be used for all classifications of volumetric ware. The trade names used are Corning's "Corex" glass or Kimble's "Kimex-51." They are more chemically resistant than standard borosilicate. However, they have larger coefficients of expansion and therefore cannot directly be fused onto

other lab glass items made out of Pyrex or Kimex. An intermediary glass with a coefficient of expansion of about 40 x 10"6 - 42 x 10"6 is

required for fusing Type I, Class B glass to Type I, Class A glass.

3.Type II glass is often identified as soda-lime, flint, or soft glass. Sodalime glass is not as chemically resistant as Type I glass, but short duration containment is acceptable (it is generally a good idea to not leave chemicals in soda-lime volumetric ware, particularly an alkaline chemical).

Type II volumetric ware cannot be repaired if cracked or chipped, nor can it be fused onto other laboratory ware. On the other hand, it is inexpensive and therefore commonly used for disposable ware and lower standard calibration ware.