Table 2.14 Corrections for Water Weight Determinations for Borosilicate Glass Using a One Pan Balance (Nominal capacity 100 mL)

also be used for Type I, Class B (aluminosilicate glass) by adding 0.0006 for every degree below 20°C. Likewise, subtract 0.0006 for every degree above 20°C.

2.3.10 Volumetric Flasks

Volumetric flasks are used to measure very precise volumes of liquids for making standard solutions or weighing for density calibrations. Each flask has one measurement line* for the specific volume of the flask (see Fig. 2.18). Volumetric flasks are of either Class A or Class B quality; the tolerances for Class B are twice those for Class A. There are no general purpose volumetric flasks. The tolerances

'There are specialized volumetric flasks with calibration lines up the neck.

102 Measurement

for the standard sizes of volumetric flasks are listed in Table 2.15. Volume calibrations were made at 20°C.

All volumetric flasks can be capped, thus preventing material in the flask from evaporating as well as maintaining the material's purity. There are three ways to close the top of a volumetric flask. Each type of cap can only be used with the proper corresponding volumetric flask. For example, a volumetric flask designed for a screw cap cannot use a snap-on cap or standard taper stopper. Each following type of cap is available for Class A and Class B flasks:

1.Plastic caps that snap on

2.Plastic caps that screw on

3.Standard taper stoppers made out of plastic or glass

The advantage of snap-on caps is that they are inexpensively replaced. They can also provide a small amount of protection against leakage if a flask is knocked over or is being agitated to mix the contents. The major disadvantage of a snap-on cap is that you need both hands to remove the cap. Screw-on plastic caps provide the best protection against spillage, but are more expensive. They also require both hands for removal.

Standard taper stoppers will prevent spillage only so long as the stopper is firmly in place, but they can be removed with only one hand. Plastic standard taper stoppers do not require grease, but may be chemically attacked by materials within the flask. Glass plugs placed over alkaline solutions without grease are likely to be frozen in place. On the other hand, the contents of the flask may dissolve any grease used.

The proper procedure for filling a volumetric flask is as follows:

1. Carefully place the liquid to be measured into the flask using a pipette, burette, or any other device that can supply a steady stream against a wall, about a centimeter above the calibration line. Be careful not to splash. The tip may touch the wall.

Volumetric

mark

Fig. 2.18 The volumetric flask.

fluid. The surface tension in the tip of the pipette should be sufficient to draw out the fluid. Once completed, cap the container.

7.Weigh the filled flask.

8.If Step 3 was performed, repeat it to verify the readings.

9.Subtract the weighed flask by the empty flask weight.

10.If Steps 3 and 8 were performed, use the appropriate table and make any necessary corrections.

The procedure for using a volumetric flask for making a solution of a definite strength is:

1.Clean and dry (if necessary).

2.Carefully place the precalculated concentrated liquid into the flask using a pipette, burette, or any other device that can supply a steady stream against a wall about a centimeter above the calibration line. Be careful not to splash. The tip may touch the wall.

3.Follow this by adding to the flask distilled water up to the calibration line using a pipette, burette, or any other device that can supply a steady stream against a wall about a centimeter above the calibration line trying to rinse any of the concentrated liquid remaining on the flask's neck. Be careful not to splash. The tip may touch the wall. When the level is just below the calibration line, cap the container and stop for a few minutes to let the liquid drain from the walls of the container.

4.Complete the filling process using Step 3 again. Try not to overshoot because any excess liquid will dilute the intended concentration.

For emptying a volumetric flask, the following procedure should be followed:

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

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

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

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

2.3.11 Graduated Cylinders

Graduated cylinders are generally not used for high-quality volumetric work. Although they are available in Class A, Class B, and Student Grade, the acceptable tolerance of graduated cylinders is considerably greater than volumetric

flasks (compare Table 2.15 to Table 0.1). Following standard practice, Class B tolerances are twice those of Class A. For specific capacities, graduations, and tolerances, see Table 0.1. Earlier federal specifications required more accurate tolerances on to contain than to deliver containers, but this case no longer applies and now both have the same tolerances.

There are three styles of graduated cylinders: Style 1 has a beaded lip and a pour spout, Style 2 has a ground standard taper joint on the top,* and Style 3 is just like Style 1 but has a heavy glass reinforcing bead just below the top for strength. Graduated cylinders of Style 1 are manufactured in both to contain and to deliver designs. Graduated cylinders of Styles 2 and 3 are manufactured in to contain designs only. Often, graduated cylinders come with plastic or foam rubber cylinder guards to protect their tops if they are accidentally tipped. Use of these guards is highly encouraged.

Because liquids cannot wet plastic walls, plastic cylinders can provide constant volume for both to deliver and to contain. However, they are only accurate to Class B tolerances and susceptible to chemical attack from organic solvents. Regardless of the quality, or construction design, graduated cylinders have no calibration lines at the section closest to the base because when the body is fused onto the base, the overall shape of the tube is distorted and accurate calibration on a production basis is not possible. This feature is equally applicable to graduated cylinders that use a plastic press-on foot.

Pharmaceutical graduated cylinders, which can be identified by their non-verti- cal walls,* have poor accuracy. Their accuracy varies at any given inside wall diameter, even on the same flask. For instance, at regions where the inside diameter is between 21 and 25 mm, the accuracy is +0.4mL. However, at regions where the inside diameter is between 46 and 50 mm, the accuracy is ±1.8 mL.

To fill a to contain cylinder (Style 1, 2, or 3), the following procedure should be followed:

1.Clean and dry (if necessary).

2.Carefully place the liquid to be measured into the flask using a pipette, burette, or any other device that can supply a steady stream against a wall about a centimeter above the calibration line, being careful not to splash. The tip may touch the wall.

face tension in the tip of the pipette should be sufficient to draw out the fluid. If you are using a Style 2 cylinder, be sure not to get the

*To receive standard taper stoppers.

trThose smaller than 15 mL have vertical walls.

Table 0.1 Dimensions and Tolerances of Graduated Cylinders"

" From the ASTM document E 1272, Table 1. Omitted were columns titled "Distance From Scale to Top, mm (Min and Max)," "Minimum Wall Thickness (mm)," and "Maximum Inner Diameter (mm)." Reprinted with permission.

bMain graduations may be each 5 or 10 mL. If main graduations are 5 mL each 5 mL, intermediate graduations do not apply.

cLines below the first numbered line may be omitted.

''Applies to Style 2 only.

ground joint wet with fluid. If the cylinder is a Style 2, cap the container to prevent evaporation or contamination.

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

1. Clean (if necessary) for the first filling. If there is a water film, you may wish to rinse the cylinder with the liquid about to be measured to stabilize the molarity. Subsequent fillings do not require the rinse nor will they need a dry container.

2.Carefully place the liquid to be measured into the flask using a pipette,

burette, or any other device that can supply a steady stream against a