2.3Volume

" Based on ASTM guidelines.

are updates to any document, the number will be followed by the acceptance year of the update. For example, the preceding example ASTM document cited shows that it was accepted by the ASTM for publication in 1989. If there is a conflict in ASTM guidelines, the document with the later publication date takes precedence.

All manufacturers abide by the standards set by these organizations. Therefore with the exception of quality and control, one manufacturer's volumetric ware (of comparable type) should not be more accurate than another manufacturer's. It is important to keep these guidelines in mind so that ASTM standards are not allowed to be used as marketing hype.

Manufacturers refer to these standards in their catalogs, both to let you know what you are buying and to enhance importance that may not otherwise be there. For instance, only glassware made to specific established tolerances can have the symbol of "A" or "Class A" on their sides, signifying highest production quality. For example, for a standard tolerance graduated cylinder, you may see statements in catalogs like "... conforms to ASTM Type I, Style 1 specs for volumetric ware." This description translates to mean "the graduated cylinder is made out of borosilicate glass (Type I) and has a beaded lip with a pour spout (Style 1)." In reality, this enhanced description is harmless and is much safer than statements saying "the most accurate graduated cylinder in town."

Do not let the designation Class A mean more than it was meant to. Class A can only mean that it is the best tolerance readily available for that specific type of volumetric ware. Class A volumetric ware is not consistent across volumetric ware type. For example, a Class A volumetric pipette does not have the same degree of tolerance as a Class A measuring pipette. Equally, a Class A graduated cylinder does not have the same degree of tolerance as a Class A volumetric flask. See Table 2.6 for a representative cross comparison of Class A tolerances.

In addition, do not be misled by ASTM designations. The words "Class," "Style," and "Type" are constantly used to describe different attributes to different types of variables in ASTM literature. They seldom refer to the same attribute. Thus it is important to know what the identifying word is attributed to before assuming that you know what it is signifying. An example of these differences are shown in Table 2.7. The ASTM always refers to itself in its own specifications for equipment when describing one of its own procedures for a given test. That is, when they are performing a test, they must ensure that their guidelines are used when their equipment is selected. However, when you see manufacturer state-

88 Measurement

The stated volume of a flask will approximately fill to where the neck attaches to the flask.

Fig. 2.13 The volume of flasks

abbreviations IN for to contain and EX for to deliver. These abbreviations come from the Latin root found in intra (within) and extra (out of). This classification has been accepted for use in the United States by the ASTM and is slowly being introduced by U.S. manufacturers. Because there have been no formal deadlines set, manufacturers are waiting for their current silkscreen printing setups to wear out by attrition and then replacing them using the new classification. Although some volumetric ware may be labeled with both English and metric calibrations, glassware is seldom labeled with both TD and TC calibrations. If you are ever using a volumetric container that is double-labeled, be careful that you are reading the scale relevant to what you are doing.

Other common laboratory containers such as beakers, round bottom flasks, and Erlenmeyer flasks often have a limited graduated volume designated on their sides. These markings provide an approximate volume and cannot be used for quantitative work. The required accuracy of these containers is only 5% of volume. When there are no calibration lines on a flask, it still is possible to obtain an approximate volume measurement based on the stated volume: In general, the stated volume will approximately fill any given nonvolumetric container to the junction of the neck and container (see Fig. 2.13). Thus, if you need about 500mL of water, it is safe to fill a 500-mL flask up to the neck and you will have approximately the needed volume.

The quality of any given volumetric ware is based on how accurate any given calibration line will deliver the amount it claims. For example, say that a one-liter flask is accurate to ±5%, meaning that the flask is likely to contain anywhere from 950 mL to 1050 mL of liquid. For comparison, a one-liter Class B volumetric flask is accurate to ±0.60 mL, or +0.06% accuracy, and a one-liter Class A is accurate to ±0.30 mL, or ±0.03% accuracy. Needless to say, it costs more for greater accuracy.

Precision volumetric glassware must be made following the standards set by ASTM Standards E542 and E694. These standards establish not only the degree of tolerance, but how the container is to be made, how the lines are engraved, the width and length of calibration lines, the type of glass used, the design and type of base (if any), the flow rate of liquids through tips, and many other limitations. In