calibrations may wash off with thedirt. The tolerances of disposable pipettes are shown inTable 2.19.

The biggest problem with disposable pipettes (or any disposable labware) is that they are wasteful and not environmentally sound. The concept of a disposable lab is not practical inthe long run foreither the individual orthe earth.

An alternate approach to measurement is touse a syringe rather than a pipette. Although tolerances of syringes are not very good, syringes are particularly useful when adding materials tolabware through a stopcock (see Fig. 2.21), and they are essential when delivering material past a septum.

Syringes aremade of plastic orglass. Like other plastic ware, you must be onguard that solvents do not dissolve theitem. Glass, on the other hand, is more likely tobreak, have the piston stick, orhave the fluid leak past the piston plunger. The latter problem canbe a greater danger than just a nuisance if the material is toxic and/or is likely todissolve any protective gloves you should bewearing.

2.3.13 Burettes

Burettes are a specialized form of measuring pipette. To control the rate and amount of material flow from the tip, a pinch clamp, stopcock, orvalve is attached to the bottom of a burette, thus providing control onthe liquid outflow and allowing accurate dispensing. Burettes are made with Class A, Class B, and Student Grade tolerances and are calibrated at 20°C. Class A are marked as such, and other grades arenotrequired to identify their volumetric tolerances. As opposed to pipettes, burettes are made out of standard tolerances laboratory borosilicate glass. This means that they can easily be added to other apparatus by a glassblower. The tolerances and general characteristics of burettes are listed in Table 2.20.

Like pipettes, Class Aburettes have slower outflows than Class Bburettes. They allow the user to better control the dispensing fluid. The slower speed also allows

Fig. 2.21 If the plug is opened, a syringe may beinserted into the stopcock to dispense liquid.

Table 2.20 Requirements for Burettes"

a From theASTM document E 287, Table 1. Omitted were columns titled "Class A Scale Length, mm, 0 to Capacity," "Class B Scale Length, mm, 0 to Capacity," and "Distance, Top of Burette to"0" Graduation, mm."Reprinted with permission.

the liquid in the burette ample time to flow from the walls at the rate of thedispensing liquid. Thus the user does not need to wait before making a reading.

A burette should be mounted securely in a vertical position with a burette clamp or several three-fingered clamps. A single three-fingered clamp is likely to wobble and swing off vertically. If the accuracy of your work is critical, a plumb is recommended to achieve a true vertical orientation.

A burette can be filled from a side tube or three-way stopcock on the bottom. If there is no bottom-filling capability, the liquid can be poured into the top of the burette with a funnel. If any liquid spills on the outside of a burette, wipe the burette dry with a laboratory tissue.

Unless a burette is automatic and one wishes to fill to the 0.00 mL mark, overfill the burette about 10 mm past the zero line. Let the liquid settle a minute, then release some of the liquid into a beaker or some other receptacle by slightly opening the stopcock. Let the fluid lower to the zero line. Wait another minute to allow the fluid to settle to the new level, and re-check the level of the meniscus at the zero line. Release or add more liquid as necessary.

To make a burette reading, first read and record the liquid volume in the burette, dispense the required amount, and reread the liquid volume in the burette. Then subtract the first reading from the second reading to calculate the amount of fluid delivered. Self-zeroing burettes do not require a first and second reading, because all readings start from 0.00 mL.

2.3.14 Types of Burettes

There are four types of burette designs. Not every type of burette design is made in all tolerances, and some burettes have limited use.

Some burettes have a tube for easier filling attached between the calibration lines and above the tip. Flexible tubing is attached to the tube used for filling the burette. There are two methods to stop liquid flow. One technique uses a pinch clamp on the flexible tubing, and the other has a stopcock on the side tube. The pinch clamp can lead to inherent errors as the pressure from the weight of the liquid in the burette causes a small expansion of the flexible tube. As the burette is emptied, this pressure decreases, and the amount of error decreases as well. If your work requires limited accuracy, these changes are well within tolerance. Alternatively, a stopcock can be used instead of a pinch clamp. There are no hydrostatic complications with the stopcock.

The least accurate burettes are Mohr burettes (see Fig. 2.22). Mohr burettes do not have stopcocks at their tips and therefore require flexible tubes with pinch clamps to control dispensing liquid.

The standard lab burette is the Geissler, which can be identified by the stopcock at the bottom. It can be refilled by pouring liquid in at the top or by a filling tube from the side. Some Geissler burettes provide three-way stopcocks for easier fill-

Fig. 2.22 Different types of burettes.

"The three-way stopcock receives its name because it has three arms leading to the barrel. A twoway stopcock has two arms leading to the barrel.

the plug (see Fig. 3.17). If the hole is offset, or not in sight, try a different plug until a good match is found.

If you suspect that a stopcock is leaking, remove all grease (if present) from the stopcock, and replace the plug (which should be wet with water) ungreased into the barrel in the closed position. Push the plug in firmly, but do not rotate the plug (if the plug is glass, rotation without grease may jam the plug in the barrel). Next, fill the burette with water and see the rate of water loss over a period of about a half-hour. If the water drops more than 3 mm in length, the stopcock can be considered poor. If there is no significant loss, pull the stopcock out, rotate 180°, replace, and retest. Any loss of water less than 3 mm will easily be assisted by the application of grease. However, any attempts to repair a leaky stopcock by extra grease will be doomed to failure. Adding excess grease to repair imperfections in a stopcock is like adding extra lacquer to fill up the rough surface of unsanded wood: It just does not work. Regardless, the excess grease is likely to fill the plug hole, resolving the leaking problem in an unacceptable manner.

How often you need to clean and regrease a stopcock depends on the quality and type of grease you are using, how often the burette is being used, and the nature of the fluids within the burette. It is safe to say that an inexpensive grease will require cleaning and replacement more often than a grease of higher quality.

Do not use silicon-based stopcock grease on burettes unless the nature of your chemicals absolutely requires it. The reason is that silicon-based greases require constant cleaning and replacement to maintain their slippery nature. Also, the most effective way to clean silicon grease is with a base bath. However, the base bath is considered one of the two worst cleaning methods for use on volumetric ware. Silicon grease may be inexpensive in the short run, but it can prove to have many hidden costs. If the chemicals in your work require you to use silicon grease, it would be better to use a Teflon stopcock or rotary valve as a substitute. They are a much better choice because they do not require any grease and require very little maintenance.

The plugs from Teflon stopcocks and rotary valves must be protected from scratches. So, when a stopcock is disassembled for cleaning, lay the plug down where it is not likely to be scratched or pick up paniculate matter. Wipe the plug with some methanol (or acetone) on a Kimwipe before reinserting it into the stopcock or valve barrel. Be sure to follow the correct sequence of end pieces when reassembling the stopcock: The sequence is (white) washer, (black) O-ring lock washer, and (colored) plastic nut (see Fig. 3.21). If the washer and lock washer are reversed on a Teflon stopcock, the plug will tighten as you rotate it clockwise (CW), and loosen (and probably leak) as you rotate it counterclockwise (CCW).

When storing burettes with stopcocks, leave the locking nut on a Teflon stopcock loose. Then, if the stopcock gets warm, the swelling Teflon plug is less likely to swell and stick, or split the walls of the barrel.

Do not store solutions in a burette, and never store alkaline solutions in a burette. Alkaline solutions will react with the glass and cause a glass stopcock to