Volumetric ware

Select calibration hierarchy. By selecting a working calibrator, its calibration hierarchy is determined by the available documentation. Attention should also be paid to the calibration and metrological traceability of measurement results for input quantities to a measurement function measured by accessory equipment such as balances, thermometers, and volumetric ware. 

The original idea of the metric system was that either approach would provide the same unit of metric volume. Unfortunately, it did not work because of the subtle differences in density caused by subtle differences in temperature. Thus, the kilogram-based milliliter equaled 1.000,027 cubic centimeters. Because of the discrepancy, the International System for Weights and Measures had to make a choice between which approach would be accepted to obtain volume measurements, and the nod was eventually given to the cubic length technique. The use of liters and milliliters in volumetric ware is therefore misleading because the unit of volume measurement should be cubic meters (cubic centimeters are used as a convenience for smaller containers). The International System of Units (SI) and the ASTM accept the use of liters and milliliters in their reports, provided that the precision of the material does not warrant cubic centimeters. Because the actual difference in one cubic centimeter is less than 3 parts in 100,000, for most work it is safe to assume that 1 cm3 is equal to 1 mL. 

Table 2.6 Cross Comparison of Class A Volumetric Ware (25 mL)fl...

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. 

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. 

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. 

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 ... 

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... 

Cleaning. Volumetric ware must be cleaned both to prevent contamination of other materials and to ensure accurate measurements. Because volumetric ware is often used as a temporary carrier for chemicals, it comes in contact with a variety of different materials which may be interreactive. Also, any particulate or greasy material left behind can alter a subsequent measurement. 

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. 

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. 

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. 

Resistance to Chemicals. Exposure to all alkalines should be kept to a minimum (minutes). Exposure to hydrofluoric and perchloric acid should be limited to seconds. Volumetric ware used once for these materials should be downgraded (glassware that was Class A should now be considered Class B) or not used at all. If measurements of these acids or alkalines are required, use plastic ware because it is resistant to these chemicals. 

Contrary to common belief, it is safe to place borosilicate volumetric ware in a drying oven. While there has not been a study indicating the effects of heating on Type I, Class B or Type II glass, there was a study on Type I, Class A volumetric ware. The study was done by Burfield and Hefter,6 and the results (which are shown in Table 2.8) clearly indicate that any variations from the original volume are within tolerances even for Class A glassware. 

Because of contamination concerns, it is not recommended that you place volumetric ware on a standard drying rack because the pegs may introduce foreign... 

The parallax problems of linear measurement are compounded with volumetric ware because there are two distinct lines to read. One line is where the liquid makes contact with the walls of the volumetric container, and the other is in the center of the volumetric tube (see Fig. 2.15). 

It may take a bit of practice to properly see the correct part of the meniscus line for accurate measurement. Fortunately, there are tricks and devices to facilitate the reading. For instance, if the graduation lines on the volumetric ware mostly encircle the tube, it is easy to line up your vision so that you can avoid parallax problems (see Fig. 2.16). 

After using volumetric ware, wash if necessary and always rinse thoroughly, first with water and then with distilled water. Although it is not necessary to dry to deliver volumetric ware between measurements (if the same chemical is being... 

The volumetric marks on volumetric ware are called calibrations. How they were located on the volumetric ware is called calibration. All volumetric ware is calibrated to provide its stated volume at 20°C. The International Standards Organization has recommended that the standard volumetric temperature should be changed to 27 °C. However, so far there has not been any significant movement toward this goal. The ASTM recommends that those labs in temperate climates that are unable to maintain an environment at 20°C should maintain one at 27°C. 

Another concern for accuracy is based on how accurately the user can read calibrations on the volumetric ware. The reproducibility of an individual user will be more consistent than the reading made by a variety of users. Therefore, if there will be a variety of users on any given apparatus, all who are likely to use it should make a series of measurements. This way, the individual errors can be calculated. The ASTM has analyzed the range of errors made by trained personnel, and the reproducibility of these results are shown in Table 2.9. 

Volumetric readings can be made two ways. The easiest and most common is simply reading the volume directly from a piece of volumetric ware. Alternatively, you can weigh a sample and, if you know the molecular weight of the material, you can calculate the volume. Each approach can be affected by barometric pressure, humidity, and temperature. The calculations and tables needed to obtain true volume from observed volume or calculated weight are not difficult to use but should only be used when necessary—that is, when accuracy or precision demand their use. 

There are two different approaches for properly correcting volumetric readings caused by environmental variations because there are two approaches to making volumetric readings those done by reading volume directly from volumetric ware, and those made indirectly by weight. 

The simplest corrections are made when reading directly from volumetric ware. As the volumetric container (and the liquid contained) expands and contracts by temperature variations from 20°C, volumetric corrections are required. These corrections can be found on Table 2.10. 

You may occasionally see a soda-lime glass equivalent for Table 2.10, but I have not included it in this book. Although soda-lime glass volumetric ware was common many years ago, it is not used for any accurate volumetric purposes... 

Therefore, glassware should not soak in a base bath for an extended period of time, and the base bath should never be used for volumetric ware. 

Use a small amount of heated sulfuric acid in a fume hood. Roll the item around so the acid comes in contact will all areas of the piece. Unless the item needs to soak (for example, 30 minutes of soaking time is recommended for the removal of silicone grease), immediately empty the glassware and rinse. Although more dangerous to use than a base bath, this technique can be used to remove silicone grease from volumetric ware without concerns about altering the volume of volumetric ware. 

Drying ovens are typically set at 120°-130°C, which is well below the recommended temperature of about 400°C needed to remove all adsorbed water. Contrary to misconception, it is completely safe to place any borosilicate volumetric ware in a drying oven. The temperature is way below what is needed to cause any distortion and resultant volume changes (see Sec. 2.3.5). Objects placed in drying ovens should remain three to four hours, so that the drying of the glass (the adsorbed water) can reach an equilibrium. Otherwise, if items look dry, they probably are dry. 

Most of the glassware companies listed in Chapter 3 sell, or can provide standard or custom volumetric ware as described in Chapter 2. 

---