Volumetric Glassware

Section: II-B

Most figures did not transfer properly; table may also be misplaced 8/17/01

The specific directions are quite detailed, often trying to explain why following the procedure minimizes a specific error. Use these notes to check your technique and to remind you of the proper steps. Hopefully, you already know this material.

At the risk of nagging, it important that glassware be very clean and that water flows evenly across the surface without beading up into droplets. These droplets will remain in position and will alter the volume of liquid, undermining the precision and accuracy of the glassware. For aqueous solutions it is usually unnecessary (and often unwise) to dry the inside of the glassware. In particular, do not use unfiltered bench top compressed air since it usually contains traces of oil. Volumetric flasks can be used wet, since you will be adding solvent anyway; pipets and burets are easier to rinse with the intended solution than to dry.

To prepare a solution from a solid:

method A (sample requiring heating, reaction)

• 1. Weigh out the sample
• 2. Transfer to a beaker or regular flask.
• 3. Add solvent and dissolve; heat if necessary. A stirring bar and magnetic stirrer may be useful.
• 3. Transfer to the flask using a funnel. Completely rinse the beaker into the funnel, including any liquid at the pouring lip. (Don't let the magnetic stirrer fall into the flask.)
• 4. Add solvent to fill the flask 1-2 cm below the mark on the neck of the flask.cap and mix by inverting several times.
  • It is important to dilute in a two step process because the combined volume of solvent and solution can be slightly different from the final volume and we'd like this possible shrinkage to occur before we make the final dilution.
  • now slowly add solvent until liquid is at the calibration mark (use a dropper.)
  • mix the solution by inverting and swirling the flask; do this ten five. (Inverting is the only effective means of mixing a filled volumetric flask.)

• 1. Weigh out the sample
• 2. Transfer the solid to the flask.
  • use a wide neck funnel to avoid loss
  • rinse out the weighing boat (if used)
  • rinse down the funnel using the wash bottle; you must not leave any solid in the funnel.
  • slowly lift and rinse the outside stem of the funnel and the upper neck of the flask.

• 3. Add enough solvent to fill the flask about 1/3 and swirl the flask to dissolve the solid.
  • Add more solvent if necessary. (If the flask is more than half filled, swirling is ineffective and you must mix by repeated inversion.)
  • volumetric flasks should not be heated. If necessary, you can assist dissolving by placing the flask in a large beaker of heated water. Of course you must cool the contents before the final dilution.
• 4. Finish diluting and mixing as above.

• 1. use a pipet to transfer a measured volume into the flask. (Be sure to "touch out" the tip of the pipet.)
• 2. Fill the flask within 1-2 cm of the mark; cap and invert several times.
• 3. Now carefully fill the flask to the mark; cap and mix by inverting ten times.
  • always pipet using a bulb or other mechanical device to draw up the liquid. Never pipet by mouth

If the pipet is wet

• Use a tissue; dry the outside especially the tip.
• Pour a small amount of the liquid into a dry beaker. Don't pipet directly from the sample bottle; you may contaminate (dilute) the entire sample.
• Use a bulb and draw up a small amount of the sample. Tilt the syringe and draw the liquid into the body of the pipet; rotate to rinse the complete inner surface; drain this liquid into a waste container.
• repeat the process (a total of three rinses.)
• verify that the pipet drains cleanly; if not clean it or use another pipet.

• Pour solution into a convenient container such as a small beaker.
• Using the bulb, draw liquid until the level is about 1 cm above the mark on the stem.
• Raise the pipet, tilt slightly and touch the tip to the wall of the container.
• Place your eye at the level of the mark on the pipet stem (to avoid parallax errors.)
• Slowly allow the liquid to drain until the bottom of the meniscus is level with the mark. You may find this is easier if you slightly tilt the pipet, up to 45^o.
• withdraw the tip from the container wall and tilt the pipet back to about 30-45^o from horizontal. (This will cause the liquid to withdraw lightly from the tip and prevent liquid from accidentally escaping.)
• use a tissue to wipe any liquid from the outside of the tip; be careful not to absorb liquid from within the pipet tip.
• move the pipet to the intended vessel, hold it nearly uniform and allow the liquid to drain freely. Allow about 5 seconds after the liquid stop flowing.
• gently touch the tip of the pipet to the surface of the liquid or the wall of the container. This "touching off" technique allows the correct amount of liquid to remain inside the tip.

• Most glassware can be cleaned effectively by rinsing with hot soapy water. If water tends to bead, soaking in a stronger detergent may be helpful. Laboratory detergents are generally more effective than dishwashing liquids.
• Severe problems may yield to a brief soaking in 6M NaOH or KOH in ethanol.
• The ultimate laboratory cleaning solution is a commercial oxidizing bath (NoChromix) which is prepared in concentrated sulfuric acid.
  • Obviously this is a very hazardous material and it must be used carefully.
  • Spills must be avoided and cleaned up immediately.
  • Containers must be carefully labeled and must be kept in an area where you and others are unlikely to have accidental contact. (We continually find unlabeled and even uncovered pots of cleaning solution near the sinks in research labs.)
  • Most glassware can be cleaned by a 10-30 minute soaking in the cleaning solution, followed by careful rinsing.
  • (The brand name reflects the prior use of a chromic acid cleaning bath made from potassium dichromate and sulfuric acid. Since the exposure to and the disposal of Cr(VII) is highly regulated, chromium free baths are much preferable.)

Most volumetric Flasks are Class A and are calibrated to contain at 20^oC.. The calibration is for water and dilute aqueous solutions; it is not affected by viscosity. The calibration is slightly affected by surface tension because the liquid meniscus changes shape, so there may be a minor error with hydrocarbons and other solvents.

Temperature can also affect the accuracy since the flask expands with increased temperature. Since the flask and water expand at different rates, you cannot assume that the volume will come back to the correct value when the solution and flask cool to 20^oC. You can expect that a 5 degree change in temperature will change the volume of a flask by about 0.01%; the volume of water changes by about 0.1%.

Note that small volumetric flasks and pipets are less accurate than larger units. Modern instruments can use very small samples so it is tempting to use small flasks. This is certainly more economical in the use of reagents and in disposal of solutions. However, you should determine that the accuracy of your results is not downgraded by the use of the smaller devices.

vol flask size, ml	tolerance, ml	relative tolerance
5	0.02	0.4%
10	0.02	0.2%
25	0.03	0.12%
50	0.05	0.1%
100	0.08	0.08%
250	0.1	0.04%
500	0.2	0.04%
1,000	0.3	0.03%

Most volumetric pipets we use will be class A, but some may be labeled as Class B. These are calibrated to deliver at 20^oC. Pipets will retain a small amount of liquid in the tip; since this is taken into account on calibration, this liquid should not be blown out. Again, a 5 degree change in temperature can affect the volume by about 0.01-0.1 %.

The calibration is for water and should be valid for most dilute aqueous solutions. With other liquids there can be a significant error due to viscosity and surface tension. This can change to completeness of draining and the amount of liquid remaining in the tip. For precise work the pipet should be recalibrated for the liquid being used. A sample of the pipetted liquid should be weighed and the volume calculated from the density.

Table II-- Volumetric Transfer Pipets

Measuring pipets are convenient, especially for irregular volumes. Be aware, however, that they are much less accurate than Volumetric Transfer Pipets or Burets. The calibration is generally valid for any fluid (since you measure the difference in position of the fluid.) The accuracy is such that a 5-10 degree temperature change does not make a significant change in volume. The accuracy is much less when the pipet is used to deliver less than full volume: a 10 ml pipet used to deliver 1.0 ml has an accuracy of 0.1 ml or 10%.)

A 50 ml buret can typically be read to +0.02 ml (estimating between the 0.10 ml calibrations.) Since a buret volume is determined by two readings, an overall precision of + 0.04 ml. is typical. This is + 0.08% when the sample is nearly 50 ml; it is 0.4% for a 10 ml sample.

For smaller volumes a 10 ml buret is preferable. These are calibrated in 0.05 ml increments and a sample can be measured to + 0.02 ml ( +0.2%.)

These devices use a piston to displace air and the air is used to draw up and then deliver a sample of liquid. The sample is drawn into a disposable tip. This makes them fast and convenient to use. They are available as fixed volume and as variable volume devices (usually with a digital display.) They are generally used for small samples (1-500 microliters.)

The calibration is for water at 20^oC. Viscosity and surface tension can affect the volume delivered, so other fluids may require calibration. You might also switch to a Positive Displacement (piston) Pipet for such liquids.

Note that the accuracy of adjustable units is much less than the precision (reproducibility once the volume is set.) The accuracy also appears to become worse with use, so a test calibration run on the device is advisable.

Note also that the relative accuracy/precision is much poorer when delivering small samples. For example a 100 ml device is accurate to 0.8% when delivering 80-100 ml, but the figure falls to 2.5% when delivering 10-25 ml.

Air Displacement Pipets (Eppindorf Series 2000, adjustable)	accuracy, %	precision, %
size microliters
0.5-10	5-1.0 %	2.0-0.3 %
10-100	2.5 -0.8 %	0.7 - 0.15%
50-200	1 -0.6 %	0.3 - 0.2%
100-1000	1.6-0.6%	0.3 - 0.2%