Joints, Stopcocks, and

Glass Tubing

3.1 Joints and Connections

The ability to assemble and disassemble apparatus components has been a longstanding requirement in laboratories. Theuse of ground joints for this purpose is not new, butstandardizing thetaper sothat onepiece from onelabcanbe attached to another section of apparatus in another lab hasonly been in common use since just after World WarII. Currently, there are many standardized ways to join and separate laboratory glass apparatus.

3.1.1 Standard Taper Joints

Standard Taper Ground Joints. These joints have thesame taper throughout the world. This taper, a 1:10 ratio, provides for the ground section to decrease in size

These male joints show thetwoareas that are measured and which thereby identify any Standard Taper " s " joint. These pairs of numbers arealways given in X,Y coordinates. Thefirst number (X) is the widest part of theground area. Thesecond number (Y) is thevertical length of the ground section.

nid

Mid-length joints differ from full-length joints only in their "Y"dimension. It is asif the short endhadbeen cut off. The example to the left shows a regular and mid-length joint.

Fig. 3.1 Full- andmid-length standard taper joints.

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Fig. 3.2 Various types of inner joints.

one unit in width for every 10 units of length. These units are always in "mm" (see Fig. 3.1).

Standard taper joints are used in matching inner and outer (also called male and female) members. They are made in a variety of standardized sizes and shapes. Figures 3.2 and 3.3 illustrate the shape variations available. In general, how a joint will be used determines which joint design should be selected.

Inner Joints. Common varieties of inner joints are shown in Fig. 3.2. Figure 3.2(a) is a standard full-length joint, the one most commonly seen in the lab. Figure 3.2(b) has a small "drip tip" extended from the small end of the joint. This joint is commonly placed on condensers to prevent distillate from coming in contact with, and/or dissolving, stopcock grease. Figure 3.2(c) has an extended drip tip that a glassblower can bend to aim the liquid flow, or seal to other pieces of glass. The manufacturer adds these extensions during initial fabrication, before the joint is ground to the proper taper. If a glassblower were to try to seal such an extension onto a Type (a) joint, the distortion caused by the sealing action would alter the taper of the joint, necessitating regrinding. Manufacturers also provide joints that already have hooks for springs on them, as in (d).

Outer Joints. These joint also have a variety of forms (see Fig. 3.3). Both (a) examples have heavy walls and can receive a moderate amount of physical abuse. Example (b) has thinner walls on the ground joint section. Ribs are placed around the joint for extra strength. Thin walls are important for experiments with large

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(a) (b) (c) (d) Fig. 3.3 Various types of outer joints.

The "old" style female joint is very good to use with two-finger clamps. It is not neccessary to squeeze the wing nut tightly to securely hold the joint. Rather, the bulb of the joint rests easily on the clamp.

Fig. 3.4 Old style outer joint in a two-finger clamp.

temperature fluctuations that, in turn, might cause glass expansion or contraction. Changes in glass size could cause separation of the inner and outer joints, thus causing leaks. Example (c) is often referred to as an "old style" joint. It can easily be identified by the small bulb just below the ground section. This bulb provides a resting place for the joint on a two-finger clamp without requiring extra pressure (see Fig. 3.4). The squeezing of a column to maintain position (when there is no bulb to support the weight) is the exclusive cause of cracks on the sides of columns. If the joint is not held vertically, the bulb can also cause a minor hold-up of fluids. Example (d) shows outer joints with preattached hooks.

Standard taper joints are formed by a process called "tooling," which establishes the physical form from a simple glass tube. After the joint is made, it is ground using various grades of carborundum grinding compounds to achieve the proper tapered smooth finish. In addition to the standard ground surface joint, Wheaton Scientific Co. also makes a joint with a smooth polished glass surface. These joints are known as Clear-Seal® joints. Instead of being ground, the joint is heated until soft and then suctioned over a mandrel. The suctioning causes the glass to collapse onto and conform to the mandrel shape.

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mercury

Fig. 3.5 Examples of mercury joint seals.

Fig. 3.6 Sample of a water-cooled condenser using a mercury seal joint.

condenser where the joint also needs to be water cooled, as in Fig. 3.6). Without the extension of glass, a seal of this type would distort the walls of the joint, making it useless. This outer joint, therefore, provides the same advantage afforded by extended drip tip inner joints (Fig. 3.2(c)).

European Joint Sizes. In Europe a different set of joint lengths are used than what are common in North and South America. The European joints are just a bit longer than the ASTM standardized midlength joints found in North America. See Table 3.1 to see how these joints match up with standard and midlength joints.

Normally, laboratories in the America are not likely to encounter European joints. However, it is possible to encounter a Standard Taper Joint that is neither full nor midlength if one is doing joint research with European labs, or when obtaining laboratory apparatus from European companies.

Because the taper (1:10) and the width are the same, it is easy to insert and use European joints in any ASTM Standard Taper Joint. However, the European joint will extend just beyond the midlength joint, and the full-length joint will extend just beyond the European joint. Because of this mismatch, there is a greater chance for stopcock grease to be exposed to materials within the containers. This can be resolved by either being more judicial when applying the stopcock grease (don't overdo it), using a Teflon sleeve, or changing one or the other joints to

Stoppers (solid or hollow) are all measured on the width of the ground section (X). This measurement (in mm) is the size (number) of the stopper. The length is consistent for any given width.

Fig. 3.7 Stopper plugs.