wall about a centimeter above the calibration line, being careful not to splash. The tip may touch the wall.

3.When the level is just below the calibration line, stop for a few minutes to let the liquid drain from the walls of the container.

4.Finish the fill using Steps 2 and 3 again. If you overshoot the calibration line, a pasture pipette can be used to draw out excess fluid. The surface tension in the tip of the pipette should be sufficient to draw out the fluid.

For emptying a to deliver cylinder (Style 1 only), the following procedure should be followed:

1.Slowly incline the cylinder to provide a steady stream of liquid from the spout. Be careful not to splash.

2.Continue inclining the cylinder until it is vertical and hold for about half a minute.

3.Touch the drop at the tip of the spout to the wall of the receiving container.

4.Remove the cylinder horizontally from the wall of the receiving container with no vertical motion.

2.3.12Pipettes

There are three types of volumetric pipettes: volumetric transfer, measuring, and serological. The differences are based on whether the volume within the pipette is subdivided and if the volume in the tip is included in the calibration (see Fig. 2.19).

The serological pipette is used to dispense varying volumes of liquid similar to the burette. Like a burette, the serological pipette is calibrated volume does not include the tip region. Volumetric pipettes are designed to dispense one volume of liquid, whereas the measuring pipette is calibrated to dispense varying volumes. Both of these pipettes are designed to include the tip region in their entire dispensed volumes.

There are two different methods for completely draining a volumetric transfer or measuring pipette, and each method requires a different pipette design. One method is to leave the tip against the side of the receiving vessel and let the material drain into the vessel. This method leaves a small amount of solution remaining at the tip. The difference between this remaining portion and what has been transferred has been accounted for in the pipette's calibration. The other method of pipette draining requires the user to blow out the remaining liquid in the pipette; after all the liquid has drained out naturally, an extra burst of air is applied to the end of the pipette to expel any remaining fluid out of the tip. All blow-out

Serological

pipette

Measuring

pipette

Volumetric transfer pipette

Fig. 2.19 The three types of pipettes.

pipettes are identified by either (a) an opaque band one-quarter of an inch wide or

(b) two bands one-quarter inch apart at the end of the pipette.

There are pros and cons to both types of pipettes. The decision to use one or the other depends on the reliability, and repeatability, of your laboratory technique as well as the nature of the liquid you are pipetting. All non-blow-out pipettes are calibrated with water. Thus if your liquid has different surface tension or viscosity characteristics than water, your measurements will not be accurate. On the other hand, not everyone will exert an equal amount of blow-out force. Thus different people may deliver different volumes using the same equipment.

Pipettes often have color-coding bands on their ends to help identify them. Although the colors are designated by the ASTM, they do not necessarily specify a specific volume. Their volumes do not necessarily correspond to any other pipette design. In fact, a given color may identify two volumes for any given type of pipette: however, pipettes with the same color band are always distinctively different in volume.

Traditionally, pipettes were filled by sucking fluid into the tube with the mouth. This procedure, however, carries the risk of getting chemicals in the mouth. To protect the user, some pipette ends are designed to receive a cotton plug (see Fig. 2.20).

The cotton plug allows gases past, but inhibits the flow of liquids. Thus, if a user sucked too hard, the liquid would clog the cotton plug and prevent the liquid from

Fig. 2.20 Regular and cotton plug ends for pipettes.

pipettes include the contents of the tip (see Fig. 2.19). Measuring pipettes should never be drained or blown out when delivering solution because the extra volume in the tip is not part of the pipette's calculated volume. Some serological pipettes are blow-outs, and others are not. You need to examine the end for the one-quar- ter-inch opaque mark

Measuring pipettes come in both Class A and Class B and are available in two styles: Style 1 is a standard taper tip, and Style 2 is a long taper tip (Class B only). Serological pipettes are only made to Class B tolerances and have no special styles. Volumes, tolerances, and other data of measuring pipettes are provided in Table 2.17. Information for serological pipettes is provided in Table 2.18. All calibrations were made at 20°C.

Class A pipettes have slower outflow times than Class B pipettes. This slowness provides the user with more reaction time to control liquid flow and thereby

Table 2.18 Requirements for Serological Pipettes"

a From the ASTM document E 1044, Table 1. Omitted was the column titled "Outside Diameter of Graduated Portion, min., mm." Reprinted with permission.