ing tape ages and loses its sticking abilities. Any remaining glue from old masking tape can be removed with acetone.

3.3.2 Preparation for Use

Any dust, fibers, or particulate material between the inner and outer members of a joint or stopcock can cause jamming and/or excessive wear on glass or scratch Teflon.

If you examine the cleaned plug from an old burette's stopcock (especially one from a freshman chemistry lab), you will see a lot of scratching and wear on either side of the hole. This wear is caused by particulate matter collected in the hole (see Fig. 3.32). In time, this wear circumscribes the entire plug, causing a leak that nothing can stop save replacement of the entire stopcock. Frequent cleaning (often not provided to freshmen chemistry burettes) can prevent unnecessary wear and subsequent problems.

All glass stopcocks and joints should have both inner and outer members wiped with a clean Kimwipe® before use to remove any particulate matter. In addition, to remove any fingerprints or other simple greases, a simple glass cleaner can be used to clean the glass. Rinse with water followed by a distilled water rinse. Ground sections of high-vacuum pieces should not be handled after cleaning because perspiration can degas slowly within the system.

Teflon items (Teflon stopcock plugs, rotary valve plugs, and Teflon sleeves) should be wiped off with some acetone (using a Kimwipe) before being assembled for use.

3.3.3 Types of Greases

When selecting a grease, keep in mind that a grease can only lubricate and/or seal. Stopcock grease cannot repair leaks or imperfections of damaged or poorly made stopcocks or joints. A grease's lubrication abilities are necessary for stopcocks to rotate, as well as to facilitate the easy separation of joint members. A grease's sealing abilities are necessary to help the joint or stopcock maintain the separation between a vacuum and atmospheric pressure, or between one (or more) compound^).

A

Fig. 3.32 Scratches on glass (or Teflon) plugs caused by particulate matter collected on grease (and never cleaned) causes, scratches, or grooves on a plug.

A sealant, such as Apiezon W, is a good example of a material that was designed for an extremely limited application. Apiezon W is a hard black wax that needs to be heated to 80°-90°C before it is soft enough to apply to the members you wish to join. At room temperatures it is hard and has no lubrication abilities whatsoever and is therefore not usable for a stopcock. It has a relatively high vapor pressure (103 torr), so it cannot be used for most vacuum work. However, at temperatures of about 100° to 150°C, it becomes very thin like hot honey and can easily be applied to joint members. Therefore, if you have a standard taper joint that will not be in a high-heat or ultrahigh-vacuum environment, there is no need to separate often, and you don't want it to accidentally separate, Apiezon W is the type of sealant you need.

Table 3.4 Hydrocarbon-Based Stopcock Greases

Table 3.4 Hydrocarbon-Based Stopcock Greases (continued)

There are three types of stopcock greases: hydrocarbon-based, silicon-based, and fluorinated hydrocarbon-based. The general characteristics and attributes of the various hydrocarbonand silicon-based greases are listed in Table 3.4 and Table 3.5, respectively (the third category, the fluorinated greases, requires no table because there are only two fluorinated greases, both made by the same manufacturer, and no cross distinctions are necessary). Table 3.6 lists the available technical information on all of the greases. These tables are by no means inclusive of all the greases available, nor is it complete. Regrettably, much technical information is not available. Occasionally, the technical information that was available was contradictory, and when this was observed, only the less grandiose of the claims was listed.

Hydrocarbon-based stopcock greases are made (usually) from a refined hydrocarbon base. They offer efficient sealing and good to excellent vapor pressures. Some are refined specifically for stopcock use, whereas most others are made for the simpler requirements of joints. Most stopcock greases can be easily cleaned with hydrocarbon solvents, which is also part of their problem: Most solvents can easily strip the grease from a stopcock or joint. Working with solvents, solvent vapors, high temperatures, or UV wavelengths can break down organic stopcock greases (such as the Apiezon brand of stopcock grease). One option is to use a sil- icon-based stopcock grease.

Silicon-based stopcock grease maintains constant viscosity over a wide range of temperatures (-40°C to 300°C). These greases are water insoluble and inert to most chemicals and gases. Silicone grease is inexpensive compared to many other stopcock greases.

There are, however, several drawbacks to using silicone grease: Silicone grease is not impervious to all solvents and ethers. Silicone grease ages rapidly and it breaks down under UV irradiation. Some brands of silicone grease begin to age once they leave the factory and are only good for about 18 months once you

Table 3.5 Stopcock Greases Containing Silicone

receive the tube. Old silicone grease becomes very hard, which makes rotation of a stopcock very difficult. When hard, silicone grease will not flow as a joint is being rotated. This lack of flow causes the grease to "crack," leaving a network of air lines (called a spider web). These air lines will leak to the environment or to the other sections of the stopcock. Despite its low vapor pressure, silicone grease "creeps" within a vacuum system, which can lead to poor visibility, make glassblowing repair difficult to impossible, and increase cleaning complications.

Both hydrocarbon and silicon-based greases show signs of aging, which are typically increases in viscosity. When hydrocarbon-based greases age, lighter factions of the grease evaporate leaving heavier factions of the same material remaining. Softening this material can be done by heat in the same manner that heat is used to soften cold honey.

When silicon-based greases age, light factions evaporate, leaving behind primarily nonvolatile components. Heat cannot soften this material. Thus, silicone grease requires more maintenance than hydrocarbon-based stopcock grease. For best results, old silicone grease can be (sufficiently) removed with a chlorinated hydrocarbon and new grease applied every two to three months. Reapplication of new silicone grease should be done regardless of whether a joint was used or not.

When silicone grease becomes hard, the material remaining is not the same material that has volatilized off. Thus, using a hot air gun (as one would use on hydrocarbon grease) for softening the remaining material cannot be used on silicone grease. You must replace the old grease immediately. Blowing a hot air gun on silicone grease with the intention to soften it up will in fact hasten its aging by removing the remaining volatile materials.

When all volatile compounds are removed from silicone grease, all that remains is silica. This material can be very difficult to remove from glass and can severely

200 Joints, Stopcocks, and Glass Tubing

hinder any repair of glass items by a glassblower. Therefore, silicone grease must be completely removed before any intense heat (>400°C) is applied. Silicone grease can be removed from small systems by letting them soak in a base bath for a half hour (see Sec. 4.1.7).

The last grease type is fluorinated greases. These greases go under the brand name of Krytox® (registered and manufactured by Du Pont). They are all derived from the base oil Krytox 143. To this oil are added different amounts of a thickening agent and other materials that impart a variety of characteristics. The thickening agent for Krytox greases is a component of a compound related to Teflon, called Krytox® DR* Krytox DF is the lubricating agent used on all stainless steel razor blades. Krytox stopcock grease comes in two forms for common lab use: one for general use, Krytox® GPL, and one with low vapor pressure, Krytox® LPV(l(r12torrat20oC).

Krytox seems like a product from heaven, as many of its attributes are spectacular. It is a fluorinated grease and is therefore impervious to hydrocarbonand water-based solvents. It cannot burn, so it has no flash point and can be exposed directly to oxygen, even at high pressures. It can be used over the widest temperature range of all the greases and works equally well with stopcocks as with joints. It has remarkable stability characteristics and can withstand long-term abuse. Best of all, Krytox greases are nontoxic.

There are, however, two problems with the Krytox greases. Although they do not burn, they break down in high heat environments (>260°C) into lethal fluorine compounds. Thus, a habit such as smoking while you work can truly become a real killer: If any Krytox grease remains on your hands when you handle a cigarette, burned Krytox fumes from the grease transferred to the cigarette can kill you. Because of the dangerous fumes generated by highly heated Krytox grease, it is obvious that any glassware contaminated with Krytox must be completely cleaned before any repairs can be made.

The cleaning of Krytox grease presents the second problem. The removal of Krytox grease is problematic at best. Originally, the only way to remove Krytox from glass apparatus (or your hands), was to scrub it off (simple wiping is not sufficient) with a chlorofluorocarbon solvent such as Freon® 113 or Freon® TF. Unfortunately, both of these compounds have been directly implicated in the destruction of the world's protective ozone layer and are no longer commercially available. But that wasn't all. If you simply tried to wipe off the grease with a small amount of Freon 113 or Freon TF, a joint or stopcock still wasn't safe. As mentioned before, the thickening agent for Krytox grease is called Krytox DF whose burning fumes are also toxic. Krytox DF can smear into the rough surfaces of a ground joint or stopcock, adding to the lubrication capabilities. To remove

'Krytox DF stands for Krytox Dry Film. It was previously known as Vydax.

Krytox from these surfaces requires rough scrubbing with a toothbrush or fingernail brush and also requires sufficient solvent to remove the material.

To resolve these problems, the researchers at Du Pont tried a variety of cleaning solutions and eventually discovered that an industrial solvent, BH-38,* did a very effective job in removing the Krytox grease. The author did some basic tests and observed that the detergent did a good job of removing the vast majority of the grease. However, it was observed that an apparent film did remain on the ground section of the stopcock and joint, even with considerable scrubbing. The composition of this film is unknown at the time of this writing. The author plans a future experiment of heating the remaining material and determining the composition of any products. While there is no question that BH-38 is removing the Krytox, at this point in time the author is unaware of any independent studies verifying the safety of heating glassware, from which Krytox grease has been sufficiently removed, so that further glassblowing can be safely performed.

*It should be pointed out that there is nothing particularly special about this specific industrial solvent, and others may work just as well.

"oftens at 250°C, but does not melt to a free flowing liquid.

*Apiezon greases are made in England and are sold by many suppliers.

cDesigned for ultrahigh vacuum.

dGood for general use.

eDesigned for stopcocks. •^Designed for high temperature uses.

s Ultra high-vacuum use.

* Not leached by aggressive solvents, yet are removable with aqueous detergents.

'Moderate vacuum use.

'Fisher Scientific product.

* Heavier and darker than Cello-Seal, high-vacuum use.

' Gas tight seals between glass and rubber tubing; can be coated outside of tubing to prevent diffusion.

mDow Corning product.

"Constant viscosity in that temperature range.

0 General Electric product (Silicone Products, Waterford, NY).

pCorrosion protection; heat sink.

qNonlubrication; dielectric grease.

rMade by 3M and M.W. Kellog Co.

sMade by E.I. Du Pont de Nemours & Co., Inc.

'Fluoronated grease.

" More refined than Lubriseal.

vContains silicone and graphite.

wMade by Podbielniak.

xInsoluble with most organic and hydrocarbon materials, but soluble in water.

Krytox, Teflon, and other similar products can be removed by incineration, and the amount of fluorine released from several stopcocks or joints is a relatively small amount. However, if the oven used is not properly vented in the confined