CALIBRATION OF VOLUMETRIC GLASSWARE

 

Glass apparatus used to measure the volume of a liquid or gas is called volumetric glassware.  Some examples are pipets, volumetric flasks and burets.  Despite their simplicity these items are capable of giving very accurate volume measurements provided they are calibrated and properly used.  There are some pitfalls and subtle sources of error that you must be aware of in order to make full use of the available accuracy.  Read the appropriate sections of your textbook (or the accompanying CD) that deal with the correct use of pipets, volumetric flasks and burets.

 

Accuracy Desired

Analytical techniques that make use of the balance, pipets, burets, volumetric flasks and similar apparatus are often called classical or “wet” methods to distinguish them from more modern techniques that make rather extensive use of electronic instruments.  This is not to say that the classical methods have become old fashioned or out of date.  They still have certain advantages, among them speed and the absence of expensive and sometimes temperamental instruments.  Another important advantage is that classical wet methods usually give higher accuracy.  Analytical chemists have come to think of a relative uncertainty of about one part per thousand as an ideal goal to be sought in such measurements. (Note 1) Although few measurements actually attain this high degree of accuracy many come very close.

 

The reasons for choosing one part per thousand as a goal are that pipets, burets, volumetric flasks and the analytical balance are capable of giving this degree of accuracy with relative ease, whereas the difficulties and time required to attain higher accuracy are so great that it is seldom worth the effort.

 

Nominal and True Values

The value marked on a piece of apparatus is called the nominal value.  However, there is no guarantee that this number is actually correct and, in fact, the true value is frequently slightly different from the nominal value.  For example, a pipet that is nominally marked “25-mL” may really deliver a true volume of 25.04 mL.  (Note 2)

 

Whether the nominal value is considered to be “correct” or not depends on the accuracy required by the user.  Suppose, for example, that an analyst wishes to deliver 25.0 mL of a solution with a volumetric pipet.  Note that this is only three significant digits, meaning that any volume between 24.9 and 25.1 mL is close enough.  Analysts know from experience that volumetric pipets are at least this accurate.  Because higher accuracy is not required it is safe to use the nominal value.  On the other hand, if one wishes to obtain the highest possible accuracy it is necessary to calibrate.

 

“To Contain” vs. “To Deliver”

Volumetric flasks are designed to contain the indicated volume of liquid.  The burets and pipets used in this laboratory are designed to deliver the stated volume of water or dilute aqueous solution.  To indicate this difference, volumetric glassware is usually marked “TD” meaning to deliver or “TC” meaning to contain.  Certain types of pipets, especially micropipets, are designed to contain a certain volume of liquid and these are marked “TC”.  (Note that for apparatus that is calibrated "to deliver", there may be a substantial error in volume if the solvent is something other than water.)

 

Blow-out

After using a pipet to make a delivery there is always a small amount of liquid remaining in the tip.  Certain types of pipets, especially serological pipets, are designed for the operator to blow out this last bit of liquid.  This is not the case for the “volumetric” pipets used in this laboratory.  The proper technique is to allow 20 seconds for drainage, touch the tip of the pipet to the inside wall of the container or the surface of the liquid, and leave the remaining liquid undelivered.  By convention, a pipet that is calibrated for blowout is marked with a white ring around the top end.

 

Basis for the Calibration

The basic measuring device in the laboratory is the analytical balance.  The accuracy of the counterweights inside the balance is much better than one part per thousand and the balances are serviced and calibrated at regular intervals to ensure their accuracy.

 

The volume of a pipet is therefore determined by weighing the water delivered into a clean dry container.  From the weight and the density of water one can calculate the true volume delivered.  (Note 3)  Similarly, the volumetric flask is first weighed empty, clean and dry.  It is then filled to the mark with water and again weighed.  As before, the volume is calculated from the weight of water and its density.

 

In the most accurate work two corrections are required.  One is to correct for the difference between an object weighed in air and the same object weighed in vacuum.  According Archimedes' principle an object is buoyed up by a force equal to the weight of air it displaces.  Second, is the fact that glass expands with increasing temperature, so the volume of a container also increases.  By convention, volumetric glassware is always calibrated at 20 C. Since the temperature at which you do the calibration may be somewhat different there is a small correction for the cubic coefficient of expansion of glass.  Fortunately the correction is very small within a few degrees of 20 C and can be neglected in ordinary work.

 

The data in Table 1-1 incorporate these corrections into the density.  In order to find the true volume simply multiply the weight of water by the correct factor in this table.

 

Procedure for Calibration of a Volumetric Flask

The flask must be completely dry.  If not, rinse it with a small portion of acetone and evaporate the acetone by drawing air through the flask with an aspirator.  The aspirator must have a narrow rubber hose that is long enough to reach down into the bottom of the flask.  (Note 4)

 

Avoid breathing acetone vapor.  Discard wet acetone in the sink in the fume hood.

 

Using the top loading balance, weigh the dry empty flask with its stopper to the nearest + 0.1 g.  Fill the flask exactly to the mark with distilled water that has been allowed to reach room temperature.  Any drops of water clinging to the inside of the neck of the flask must be removed with a rolled-up piece of filter paper.  Weigh the flask, stopper, and water.  Calculate the true volume of the flask using the data in Table 1-1.  Repeat the determination until you are confident that you have the correct volume with an uncertainty of less than one part per thousand.

 

Volumetric flasks must never be heated, either with an open flame or on a hot plate.  The high temperature causes irreversible changes in the shape of the glass and the flask must be recalibrated.

 

Procedure for Calibration of a Pipet

Test the pipet to make sure it runs clean by drawing water up above the mark and allowing it to run back out.  Look carefully at the inside wall of the pipet to see if it is completely clean.  The inside should be completely smooth, if there are any droplets of water on the inside the pipet is dirty and it must be cleaned.

 

Pipeting by mouth is not allowed.  Use a pipeting bulb or the water aspirator for suction.

 

Two cleaning solutions are available.  One is a strong acid and the other is a strong base.  Both are dangerous to use.  You may use these solutions only under the direct supervision of the lab instructor.  You must wear goggles and rubber gloves.  Return used solution to its container.  DO NOT DISCARD.

 

Check out four 25-mL Erlenmeyer flasks with corks.  Number the corks and flasks.  If they are not dry, rinse with acetone and draw air through the flasks with an aspirator to evaporate the acetone.  Weigh the dry empty flasks, with their corks, on the analytical balance.  With the pipet to be calibrated, carefully deliver the measured volume of distilled water into each flask.  Allow 20 seconds for the pipet to drain.  Be careful to keep the neck of the Erlenmeyer flasks dry where the cork fits.  After draining, touch the tip of the pipet to the inside wall of the flask, or to the surface of the water.  Watch to make sure the water inside the tip drops when contact is made.  Stopper the flasks and weigh.  Calculate the volume delivered using the data in Table 1-1.

 

After obtaining four values calculate the average volume delivered and the relative standard deviation in parts per thousand (ppth).  It this value comes out greater than 1 ppth you need more practice using the pipet.

 

Table 1-1

 

Volume of 1.0000 gram of water weighed in air with brass weights at various temperatures.  These data take into account the change of density of water with temperature, the buoyancy correction, and the correction for the expansion of glass with increasing temperature.

 

               Temp C          Vol., mL                                         Temp C               Vol., mL

 

               16                               1.0022                                    23                               1.0034

               17                               1.0023                                    24                               1.0036

               18                               1.0025                                    25                               1.0038

               19                               1.0026                                    26                               1.0041

               20                               1.0028                                    27                               1.0043

               21                               1.0030                                    28                               1.0046

               22                               1.0032                                    29                               1.0048

 

 

REQUIRED MEASUREMENTS

            You must present the calibration of one of each type of volumetric flask and pipet to the TA.  You should present your value (the mean of at least three measurements) and the uncertainty (the corresponding relative standard deviation.)

 

NOTES

 

1)         In analytical chemistry the relative uncertainty of measured numbers is often expressed in parts per thousand (ppth).  For example, suppose you measure out 1000 mL of water with an uncertainty of + 1 mL.  The volume is actually somewhere between 999 and 1001 mL and the relative uncertainty is said to be 1 part per thousand.  On the other hand, if you measure out 5000 mL of water to within + 1 mL the relative uncertainty would be one part in 5000 or 0.2 part per 1000.  Similarly, an uncertainty of + 0.02 mL in a 25-mL pipet is the same as 0.8 parts per 1000.

 

2)         Past experience in this laboratory indicates that the true volumes of pipets and volumetric flasks are often in error by 2-3 parts per thousand.  Burets, on the other hand, are generally within one part per thousand of the nominal value.  Calibration of the buret is therefore omitted.

 

3)         Note that for accurate calculations such as required in this calibration, the density of water is not exactly 1.00 g/mL.  Furthermore, it depends on the temperature.  For example, at 25 C it is 0.99705 g/mL.  (See Table 1-1.)

 

4)         A common error is not getting the flask completely dry.