tice can easily exacerbate the problem and cause other problems as well. However, before you complain, be sure that pieces are assembled correctly and that matching parts are assembled with their correct pairs.

3.Seals with folds and/or large quantities of internal bubbles. When glass is first attached to other glass, it will show folds, or ripples, that need to be worked smooth. Sometimes in production, speed overtakes quality and these flaws are not adequately worked out. These folds can be weak spots requiring little stress for fracture. One approach a glassblower may use to speed the "working out" of these folds is to use a hotter-than-necessary flame. If the glass is inadequately cleaned (another victim of production speed), the intense heat will cause the glass in the region of the seal to develop many bubbles. These bubbles contribute to (and are an indication of) weak glass. Although many seals may have bubbles, it is the overabundance of bubbles that should elicit concern.

4."Burnt" glass. The "burning" of glass is caused by severe overheating of glass during glassblowing operations. This process removes some of the component materials. Because this new glass has a different chemical composition than the glass it was made from, it cannot flow or mix properly with the surrounding glass. It can be identified by a spot of glass that does not look as though it has properly fused with the surrounding glass and may be confused with a fold.

1.2Flexible Tubing

1.2.1Introduction

There is an enormous variety of flexible tubing available for use in the laboratory because no single tubing type or size is right for all purposes. Within most laboratories, flexible tubing is used for such purposes as connecting vacuum systems to mechanical pumps and manometers, transferring nitrogen or argon gas around the lab, and connecting water lines to condensers, coolers, and constant temperature baths. In addition, you may connect a source for natural gas (or propane) and oxygen to a torch and even temporarily connect a mechanical pump to drain used pump oil.

Comparing the various brands and types of flexible tubing is not unlike comparing a variety of stereos at competing stores. Because of the many different brands and models of tubing available, and because not every manufacturer uses the same

analysis parameters, a cross comparison of features (or prices) from one type of flexible tubing to another is very difficult.

Purchasing flexible tubing has a few other complications. First, few laboratory supply house catalogs identify the manufacturers of the tubings listed. Furthermore, just as stereo stores do not carry all brands of stereos, no single laboratory supply company carries all manufacturers' tubing. Therefore, if a particular type of tubing has features that you require, you may need to ask your laboratory supply house if it carries (or can obtain) a particular manufacturer's product. This section will detail the differences and similarities between the types of tubing that may be used in the chemical laboratory.

Flexible tubing is identified by its inside diameter and wall thickness (see Fig. 1.10). This identification is unlike glass tubing, which is identified by its outside diameter and wall thickness. When ordering flexible tubing, specify the inside diameter and the wall thickness.

When ordering tubing, size is only one of several variables to consider. Two others are the tubing's physical characteristics (see Table 1.4) and its chemical resistance properties (see Table 1.5). Table 1.6 compares some of the advantages and disadvantages of the various types of flexible tubing.

IMPORTANT NOTICE

Unless otherwise stated, all data in this section (test conditions) are based on tubing that has 1/4"I.D. and 1/16" wall thickness.

1.2.2 Physical Properties of Flexible Tubing

The following physical characteristics may be important in your selection of flexible tubing (see Table 1.4).

1. Color and/or Transparency. If it is important to see a solution flowing through tubing, then transparency or translucency of the tubing is important. Color cannot be used as an indicator of physical properties or chemical resistance. Color can be used by manufacturers to distinguish between various tubing types however. For example, the manufacturer of Nalgene® tubing uses blue imprinting on its clear tubing to

Wall thickness

Fig. 1.10 The measurements of flexible tubing.

designate its 8000 line of tubing, red imprinting to designate its 8000vacuum line, and black imprinting to designate its 8007 line of tubing. Color can also aid in laboratory setup. For example, labs that use color to identify operations may use orange tubing for natural gas, green tubing for oxygen, blue tubing for water going into a condenser, and red tubing for water leaving a condenser.

2.Durometer Range. The durometer range is a measure of a tubing's physical hardness, which is indirectly related to the tubing's flexibility and resilience. The harder a tubing's composition, the less flexible and resilient it is likely to be. The hardness of tubing material is calibrated with an instrument called a durometer. The durometer is a device that measures the amount of reflected bounce by a special hammer off the material being tested. Most tubing is tested using what is called the Shore A technique. When very firm tubing is tested, the Shore D technique is used. Shore D measurements can be interpolated to provide approximate Shore A measurements for comparison. For simplicity's sake, I have chosen to interpolate all Shore D measurements to Shore A. Except where advised to the contrary, please assume that all durometer readings preceded with a "=" symbol are Shore D measurements that have been interpolated to Shore A.

3.Flame Resistance. Some tubing materials are naturally resistant to flames, while others are flammable to various degrees. This quality must be considered if the tubing may be exposed to a high, or direct, heat source. Unfortunately, although we know that a given tubing may be flammable, flame-resistant, or nonflammable, there is no information currently available by which one can quantitatively compare the flammability of tubing.

4.Flexibility. Is the tubing flexible or stiff? Is the tubing prone to kinking? Transport of gases or fluids may be impaired by nonflexible tubing.

5.Gas Impermeability. The permeability of tubing depends on the gas being used: Tubing that is fine for nitrogen may be totally unacceptable for helium. Unfortunately, comprehensive data on the permeability of various gases through tubing are not readily available. In most cases, it is necessary to contact the manufacturer for information.

6.Resilience (Memory). Over time, some tubings mold themselves to new shapes, whereas others will always return to their original shape. For example, rubber has the best resiliency of any tubing, and thus can maintain a constant grip on a hose connection. On the other hand, Tygon® tubing, which is not very resilient, will "learn" the new shape of a hose connection and will eventually lose its grip on the nipple. Internal pressure (i.e., water pressure) on a nonresilient hose can cause it to slip off its connection. Low-resilience tubing should be

Fig. 1.11 A typical screw clamp.

attached with screw clips (see Fig. 1.11), for a more secure attachment.

7.Temperature Range. As temperature increases, a tubing's ability to withstand internal pressure decreases. Conversely, if the temperature drops below the recommended temperature range, a normally flexible tubing may crack when flexed.

8.Vacuum. Under vacuum conditions, thin wall tubing will collapse. In general, it is safe to use tubing for vacuum work if the tubing satisfies Eq. (1.6). However, heat may cause this equation not to hold up:

In many labs it is standard to use red heavy-wall rubber tubing to connect mechanical pumps to vacuum systems. However, there is no technical reason for using red (colored) tubing. Any tubing that meets the qualifications for vacuum work (or your specific vacuum work) should suffice, regardless of color.

Pressure. There are many variables that interact to affect the maximum potential pressure tolerance of any given tubing. These include:

1.Inside Diameter: The smaller the I.D., the greater the potential pressure tolerance.

2.Wall Thickness: The greater the wall thickness, the greater the tolerable pressure.

3.Temperature: The higher the temperature, the less the tolerable pressure.*

4.Time: If you are working over the recommended pressure, it is only a matter of time before the tube will burst.

5.Material Transmitted: Most types of tubing can handle most materials for at least a short time. However, if tubing is undergoing chemical attack, the tubing will eventually fail.

6.Braiding: Internal or external braiding provides extra strength for pressure systems.

7.External Surface. If you wear gloves and/or work in a glove box, you

*This statement does not necessarily mean that the lower the temperature, the greater the tolerable pressure because at very low temperatures, tubing becomes brittle and prone to failure.

may want a tubing with friction rather than a smooth and/or slippery surface. To improve its handling capabilities, natural rubber tubing is sometimes cloth-wrapped during the curing process. Once the curing process is completed and the cloth is removed, the cloth impression remains on the tubing, providing a better grip.

1.2.3 Chemical Resistance Properties of Flexible

Tubing

A tubing's resistance to chemical attack depends on the nature, quantities, and length of time it is exposed to particular liquids or gases. Some tubing manufacturers have tested their tubings against a variety of chemicals and gases. Some manufacturers have even made these studies at various temperatures. The Nalgene Corporation has listed the effects of many chemicals on a variety of its polymers, and this list is included in Appendix B. If you have a question about the resistance of a particular type of tubing to a given material, contact the manufacturer. However, remember that there are many possible combinations and permutations of chemicals. Unless you are sure that a given chemical is not likely to affect your tubing, it is best to test the tubing with the chemical in an environment that duplicates your conditions. Then you can properly determine if your system, setup, and/or chemicals will affect your tubing (or vice versa).

If in doubt, test it!

In general, the following materials are those you should consider to be potential

Table 1.4 Physical Characteristics of Flexible Tubing0

" Material for this table came from:

Gates® Specialized Products, circular 63009-C, © 1976, Printed in U.S.A. Nalgene Labware 1988, from Nalge Company, #188, © 1988, Printed in U.S.A. Norton Performance Plastics, circular #10M0035 1184, © 1983, Printed in U.S.A.

Norton Performance Plastics, circular #5M-321023-485R, © 1978, Printed inU.S A.

fcThe manufacturers (address) list is in theAppendix, Sec. C.

eThe smaller the I.D.,the greater the potential pressure.

Table 1.5 Chemical Resistance Characteristics of Flexible Tubing

Code for Resistance Characteristics

Table 1.6 Comparison of Flexible Tubing Characteristics