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CALIBRATION OF Powered By Docstoc

    In any experiment, you must ascertain the limits and range of your measuring
    instruments. The experiments in this course are at a sufficient level of
    sophistication to warrant the calibration of the instruments used in order to
    correct for instrumental errors and to explore the limits of their measurement
    capability. In many of our experiments, we will need to measure accurately
    temperature and liquid volumes. The main objective of this experiment is to
    determine accurately the actual volume of your transfer pipet and selected
    volumes of your buret, and to calibrate your thermometer.

Before coming to the laboratory you should read and understand Appendix B. Also read the
sections on volume measurements and re-read the reliability of laboratory measurements in the
introductory section of this manual (pp. 19-13 and 17-27, 29-41).

                             EXPERIMENTAL PROCEDURE

The de-ionized water (about 500 ml) used in this experiment should be put into a large flask at
the very beginning of the lab period and allowed to stand until it comes to room temperature.
Cover it with a small watch glass. Before starting the experiment you should make certain that
your 50-ml buret, and 25-ml pipet are scrupulously clean; they do not need to be dry. The
criterion for clean walls is that on draining water from them they retain a continuous, invisible,
unbroken film on the walls; no droplets. If cleaning is required, see pp. 9, 21-22 of the
introductory section of this manual.


Leave your thermometer immersed for some five minutes or longer in the large beaker of
continuously stirred water provided with a thermometric standard. At some convenient time
record the temperature reading (TT) of the standard and the temperature reading (To) of your
thermometer, both to the nearest 0.1°C. The "correction" that should be applied to your
thermometer reading when it is used at approximately room temperature is TT - To.

Calibration of Volumetric Equipment                                                       1

1. Calibrating the Buret. Use de-ionized water for all operations. Check your buret for
leakage and drainage time as described on p. 22-23. In order to calibrate your buret, fill it and
lower the meniscus to just below the "0" mark. Follow the recommended technique with
respect to the buret tip; then read the buret to the nearest 0.01 ml. Withdraw approximately 25
ml of water into a weighed (but not necessarily dry) 50-ml conical flask. This flask should be
covered with a small watch glass while it is being weighed and at all times except when having
water added to it; the watch glass minimizes evaporation. The flask should be dry on the
outside because water will be removed as you set the flask down changing the mass. Remember
to touch the tip of the buret to the inside of the flask wall before covering it with the watch
glass again. Read the buret to the nearest 0.01 ml, and reweigh the flask and water on the
analytical balance.

Now withdraw an additional quantity of water into the flask so that the meniscus stands at
about the 35-ml mark. Again read the buret to the nearest 0.01 ml, and again reweigh the flask
on the analytical balance. Repeat the process with the meniscus at the 45-ml mark.

Take the temperature of the water used in each run. This can be done at the end of each series
by putting the thermometer in the water in the flask after the final weighing.

2. Repeating the Calibration. Empty the 50-ml conical flask and again weigh it empty (but
not necessarily dry on the inside). Repeat the entire series of three measurements at the 25-, 35-
and 45-ml marks, starting at approximately "0" again.

In order to check rapidly whether the corrections at a given volume (35 ml, for example) are in
close enough agreement between the two series of measurements merely calculate the
difference between the total observed volume and the total mass of water delivered at that
volume. When for any volume the differences between the first two series disagree by more
than 0.03, a third measurement at that volume should be made. If it is value at the 45-ml mark
that needs be repeated, say, it is not necessary to also repeat those at the 25-, and 35-ml marks.
Simply draw water from the "O" mark to the 45-ml mark, taking the same precautions as
before. These differences serve no purpose other than as a quick indicator of whether your
experimental work is satisfactory.

Calibration of Volumetric Equipment                                                       2
3. Calculating the Buret Correction. For each of the marks on the buret (25, 35 and 45 ml) to
which water was withdrawn, calculate the observed volume (VO) withdrawn determined from
the initial and final buret volume readings. Also for each of the marks to which water was
withdrawn, calculate the mass of water withdrawn. Correct these masses for buoyancy. (The
density of the weights used for calibrating the analytical balances is 8.4 g/ml.) Knowing the
temperature of the water and its density (Appendix G), calculate from these corrected masses
the true volume (VT) of water withdrawn. With the "correction" defined as VT - VO, calculate
the correction that must be added at each of the three marks where calibration was carried out;
for each of these marks, average the two or more values you obtained. The following table
shows a sample calibration.

                             CALIBRATION OF A BURET AT 25°C
                            (only one series of measurements is shown)

  Nominal            Vo, Observed         Actual        VT, Actual             VT - VO
Vol. Interval        Vol. Delivered     Mass Delivered Vol. Delivered         Correction
  (ml)                    (ml)              (g)           (ml)                  (ml)

  0-25                    25.04              24.97           25.07              +0.03

  0-35                    34.76              34.61           34.75              -0.01

  0-45                    45.06              44.90           45.08              +0.02

4. Graphing the Corrections. Construct a simple graph in your notebook, plotting
"correction" as ordinate and "buret reading" as abscissa; this is the calibration curve for your
buret. In this graph the "buret readings" plotted should be the nominal values of 25, 35, and 45
ml. Unlike all the other graphs to be constructed from experimental data in this course, this one
should have the points connected consecutively by straight line segments, not by a smooth
curve. The resulting graph should not be interpreted as meaning that your buret suddenly
changes diameter at certain points along the tube. Instead, you should realize that you lack
specific information about the exact manner in which the buret diameter varies along the tube,
that you will assume a gradual linear change from one calibration point to the next, and that in
any case you will round off any correction taken from the graph to the nearest 0.01 ml.

Calibration of Volumetric Equipment                                                      3

1. Calibrating the Pipet. Just as in the case of the buret, check your pipet for proper drainage
time. Fill it to the mark with de-ionized water and observe the time for it to empty while it is
held vertically. Consult your instructor if the time of efflux differs greatly from that
recommended on page 22.
Using the necessary care for the proper use of the pipet (pp. 20-21), pipet 25 ml of water into a
weighed 50-ml flask. Don't forget to wait for proper drainage or to touch the tip to the inside
wall of the flask. Cover the flask and reweigh it on the analytical balance. Note that you should
measure the masses to 0.1 mg because you may be able to measure to a precision of 0.001 ml.
Repeat this operation so as to give five successive additions of water from the same pipet. After
the final weighing take the temperature of the water.

2. Calculations. Calculate the mass of water obtained from each addition to the flask. Correct
these masses for buoyancy. Knowing the temperature of the water and its density (Appendix G),
calculate the true volume (VT) of water delivered in each of the five additions to the flask from
these corrected masses. Calculate the average value of VT. Defining "correction" as VT -
25.000, calculate the correction which must be applied to your pipet.

For your five values of VT, calculate the standard deviation and the 95% confidence interval for
a single measurement.

Calibration of Volumetric Equipment                                                     4
Name__________________________________ Lab Section________________

Date Report Submitted___________________


1. Thermometer

          Thermometric Standard, true reading (TT)                  ___________°C

          Desk Thermometer, observed reading (To)                   ___________°C

          Thermometer Correction (TT - To)                          ___________°C

2. Buret
Series A:          Observed            Mass of       Corrected       True        Vol. Correction
T(H2O), °C         Volume, Vo           Water          Mass        Volume, VT        VT – Vo

__________        ___________         ___________    ___________   ___________     ___________

                  ___________         ___________    ___________   ___________     ___________

                  ___________         ___________    ___________   ___________     ___________
Series B:
T(H2O), °C

__________        ___________         ___________    ___________   ___________     ___________

                  ___________         ___________    ___________   ___________     ___________

                  ___________         ___________    ___________   ___________     ___________
 Series C:
(if needed)
T(H2O), °C

__________        ___________         ___________    ___________   ___________     ___________

                  ___________         ___________    ___________   ___________     ___________

                  ___________         ___________    ___________   ___________     ___________

Calibration of Volumetric Equipment                                                5
Calibration of Volumetric Equipment   6

     Nominal buret reading                 0 - 25              0 - 35          0 - 45

    Average volume correction           __________          __________      __________

    3. Pipet

        T(H2O), °C _____________

         Mass of Water                 Corrected Mass             True Volume, VT

        ______________                ______________             ________________

        ______________                ______________             ________________

        ______________                ______________             ________________

        ______________                ______________             ________________

        ______________                ______________             ________________

                                                     Average      ________________

                                            Standard deviation    ________________

              95% Confidence interval of a single measurement ________________

                                 Volume correction, VT - 25.0000 ________________

4. Show sample calculation for pipet calibration on the reverse side.

5. Solve the following problem. Show the calculations on the reverse side.
     What would be the calculated volume of your pipet if the buoyancy correction was not
     applied? What is the relative error in parts per thousand of the volume of your pipet
     when the buoyancy correction is not applied?

        Volume of pipet ________________                Relative error, ppt _______________

Calibration of Volumetric Equipment                                                       7
Calibration of Volumetric Equipment   8

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