___University of Puget Sound
Department of Chemistry
Chem 230
EXP. 1–STATISTICAL ANALYSIS OF VOLUMETRIC TOOLS
OF THE LABORATORY
LABORATORY OBJECTIVES AND ASSESSMENTS
1. Understand the difference between accuracy and precision.
a. Determine the precision of a set of measurements.
b. Determine the accuracy of a set of measurements.
c. Describe whether careless lab technique leads to large uncertainty (low
precision) of the measurement, inaccurate result, or both.
d. List possible sources of systematic error, and describe how much each would
contribute to inaccuracy of a reported measurement.
2. Understand the proper use and choice of different laboratory tools for measuring
volume of liquids.
a. Describe the difference between how burets/pipets and volumetric flasks are
designed to measure volume.
b. Describe why the analytical balance is the best tool for measuring the volume
of a liquid.
c. Calculate the volume of water from its mass and density.
d. Demonstrate proper use and reading of a buret, a micropipet, and a
volumetric pipet.
e. Demonstrate calibration of a buret and construct a correct calibration plot.
f. Demonstrate proper choice of volumetric tool based upon its accuracy and
precision.
3. Understand the function of a laboratory notebook.
a. Maintain a clear, complete account of laboratory procedure and all results in
the laboratory notebook.
b. Prepare and record data from laboratory measurements into a table in the
laboratory notebook.
c. Record clearly in the notebook sample calculations for each type of
calculation.
4. Understand how to use a spreadsheet to record and analyze data from
experiment.
a. Use calculated fields and common spreadsheet tools, i.e., Fill commands,
formatting tools, etc.
b. Use spreadsheet functions to calculate results of experimental data and apply
fundamental statistical analysis of results.
INTRODUCTION
Analytical methods based upon the measurement of volume are called volumetric
or titrimetric methods. They are widely used because they tend to be rapid and
convenient. Compared to some other methods, they are also more accurate.
Harris, Exploring Chemical Analysis, has an excellent description of tools chemists
use to measure volume (pp. 41-54) and a description of the use of spreadsheets (pp.
74-78).
Measuring Volumes
Both burets and pipets are calibrated to deliver a specified amount of solution.
Volumetric flasks, on the other hand, are calibrated to contain the specified amount of
solution.
Harris lists the precision and/or accuracy of various pieces of analytical glassware.
We will compare the published values by measuring water using a buret, a volumetric
(transfer) pipet and a micropipette. We will use an analytical balance to evaluate the
c230 Exp. 1 -Volumetric Analysis and Calibration of Glassware
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volume delivered or contained by the above tools. All tools have imprecision in the
last digit of the measurement. An analytical balance measuring 5 mL of water will
have 5 significant figures, 5.0000. The imprecision of any measurement should be
contained in the last decimal place. Mass should not be a temperature-dependent
measurement and thus, the analytical balance can be used to evaluate the volumetric
methods.
CALCULATIONS AND REPORT
Since liquids have density that change with temperature, the true volume
delivered will be calculated by converting the weight of the water obtained for the
increment using Table 2-5 (p 54, Harris).
mL true volume = gramsmeasured x
The difference between this true volume and that measured by the buret (or
expressed by the pipet) is the correction for that volume increment. The temperature
of your water will most likely not fall on the table in Harris thus you will need to
interpolate this value. (p 270, Harris)
=
Example calculation
Given that a 10.00 mL increment of water, as delivered by the buret, weighed 10.01 g at
22.3°C. What is the actual volume of the water?
From Table 2-5, one gram of water at 22.0°C should occupy 1.0033 mL and at 23.0°C
should occupy 1.0035; therefore the extrapolated volume should be:
=
y = 1.00336
(wt. of 10 mL H2O at 22°C) x (Volume of 1 g H2O at 22.0°C) = true volume delivered
(10.01 g) x (1.00336 mL/g) = 10.04 mL water
STATISTICAL ANALYSIS
You will repeat each measurement and as with any repeated measurement, statistical
analysis is necessary for reporting your results. In this experiment, you will use the built-in
functions in Excel to determine the average and the standard deviation of your data. A
mathematical description of mean and standard deviation can be found in Harris, pp 84-85,
and will be discussed in lecture.
®
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Figure 1. Sample Excel spreadsheet. Each cell in Row 4 with an asterisk is the Header to a column of
calculated cells. These calculated cells will contain equations, e.g., the cell E6 should contain the
equation "=D6-C6" (without quotes).
EXPERIMENTAL PROCEDURE
WORK INDIVIDUALLY
Every volume reading and weight must be recorded in your laboratory
notebook throughout this lab.
PART 1-CALIBRATION OF THE BURET
Preparing your Buret
1. Each student will be issued a Class A 50-mL buret. Mark your buret
with your name and laboratory section. Apply the label just above the
stopcock but below the lowest graduation. When the label has been
c230 Exp. 1 -Volumetric Analysis and Calibration of Glassware
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2.
3.
applied, wrap a piece of Scotch tape completely around the buret to
protect the label. Since the burets will be stored with the tips up, you
may want to place the label “upside-down” so that you can read them in
the cupboard.
When the buret is clean it should drain completely with no drops of
water forming on the sides of the buret as the water moves down the
buret. Drain the water to be sure the buret is clean, but leave enough
water in the buret to keep the stopcock and tip filled with no trapped air
bubbles.
If needed clean the buret with soap and water. Brushes with long
handles are provided in the laboratory. If you use them, be sure not to
scratch the buret with the wire. Rinse the buret and fill it with distilled
water. Allow the water to pass through the stopcock so that the
stopcock, barrel, and tip are completely filled, with no air bubble.
Draw about 250 mL distilled water into a large beaker and cover it with
a watch glass. This water will be used to calibrate the buret and you
must measure the temperature to the nearest degree Celsius. Refill and
zero buret.
To “zero” the buret, fill the buret to above the 0 mL line. Drain water
from the buret so that the meniscus is adjusted to about 0.1 mL below
the zero level! It is not good practice to adjust the meniscus to zero, or
0.10 exactly, because this wastes time. Read the buret to the nearest 0.01
mL, and record the reading to the nearest 0.01 mL. Touch off the
hanging drop from the tip of the buret into a waste liquid container.
Check for leaks
4. After waiting for a period of time (~5 minutes) check the initial buret
reading to be sure it is the same. If it isn’t you may have a leak at the
stopcock. Check with your instructor if this occurs. Check for air bubbles
forming during use. This usually indicates a leaky seal between the
Teflon stopcock and the glass buret.
Initial Buret Reading
5. Measure and record the temperature of the water Check the buret
reading and if needed re-zero the buret. Touch off the hanging drop
from the tip of the buret into a waste liquid container.
Initial Mass Reading
6. Measure and record the mass of a 50-mL Erlenmeyer flask on the
analytical balance to the nearest 0.0001. At this level of precision it is not
a good idea to touch the glassware to be weighed as you can transfer oils
from your fingers that can actually affect the mass. Fold a paper towel as
demonstrated by your instructor and use to hold and carry your flask.
Calibrating Buret
7. When the buret is ready for use and you have recorded the initial
reading, drain about 10 mL of water into the weighed Erlenmeyer flask.
Touch the tip to the wall of the flask. After the buret has settled for a few
seconds, read the final volume. Weigh the flask and water recording the
mass to the nearest 0.0001 gram. Record this second weight, along with
the buret reading to the nearest 0.01 mL.
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8.
9.
10.
11.
Deliver from the buret another 10 mL of water to the flask so that you
have added approximately a total of 20 mL to the flask. Record the
volume reading of the buret to the nearest 0.01 mL and weigh the flask,
recording the mass to the nearest 0.0001 gram. This is your final Volume
at 20 mL. With Excel you will use this with your initial volume from
Step 8 to calculate ΔV.
Continue the above procedure to obtain data for a volume of 30 mL.
Make sure that you are recording all of your data in your lab notebook
and on the Excel file. Repeat for 40 mL and 50 mL volumes. Do Not
ever drain below the 50 mL mark.
If there is no room in the flask for the next increment volume dump out
the water in the flask and refill the buret to near the 0.10 mL mark.
Reweigh the empty flask. NOTE: It does not need to be dry on the
inside.
Repeat steps 7-10 two more times until you have 3 measurements at
each volume increment.
Part 2-STATISTICAL ANALYSIS OF VOLUMETRIC TOOLS
Volumetric Pipet
13. Using a 1-mL volumetric pipet, add 1 mL of water to a preweighed 50mL Erlenmeyer flask. Record the mass to the nearest 0.0001 g. Repeat
this measurement 4 more times as above.
14. Using a 5-mL volumetric pipet, add 5 mL of water to a preweighed 50mL Erlenmeyer flask. Record the mass to the nearest 0.0001 g. Repeat
this measurement 4 more times as above.
Micropipet
15. Using a p1000-micropipette (1000µL), add 1 mL (1000 µL) of water to a
preweighed 50-mL Erlenmeyer flask. Record the mass to the nearest
0.0001 g. Repeat this measurement 4 more times as above.
16. Harris states that the precision of a micropipette changes throughout its
deliverable range. Adjust the volume of the p1000-micropipette to 200
µL, pipet and measure and weigh 200 µL. Record mass and repeat 4
more times.
Graduated Cylinder
17. Using a 10-mL graduated cylinder, measure 5 mL of water and record
the volume reading in your table. Pour the water into the preweighed
50-mL Erlenmeyer flask and record the mass in your notebook. Record
the mass to the nearest 0.0001 g. Repeat this measurement 4 more times
as above recording in appropriate location in your notebook for transfer
to the spreadsheet.
Data Analysis and Plotting
18. Complete the transfer of entries from your notebook to the Excel
spreadsheet. Be sure the appropriate values are being calculated
correctly.
19. From the Buret Calibration data, construct a buret calibration plot. Put
five values on the x-axis, 10 mL, 20 mL, 30 mL, 40 ml and 50 mL. The yaxis will be mean volume correction difference (column K). Plot your
c230 Exp. 1 -Volumetric Analysis and Calibration of Glassware
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19.
mean of the five-volume difference for the 10 mL, 20 mL, 30 mL, 40 ml
and 50 mL values.
Add y-uncertainty bars to your points. Click on any data point on the
chart, and “Format  select the data series”. In “Format data series”
window, open the “y error bars” tab, select “Display  Both”, “Error
amount  Custom”. Then, by clicking on the data selection icon, you
can select the data in the “Stdev of V” column for both the “+” and the
“-“ error amounts.
WHAT TO DO
Before Lab: Read this experiment and the readings in Harris, pp. 41-54 and pp.
74-78. Fill in the “Title bar” for every page of the experiment.
You should include the following information:
• Completed title bar.
• Purpose.
• Disposal instructions for all reagents.
• Using the incomplete Sample Table (see Figure 2 below) prepare a
complete table in your notebook for summarizing the calibration of your
glassware. You will need to expand the sample table to include the
measurements for the volumetric and micropipets and the grad. cylinder.
• Following the example given in Figure 1. Sample Excel spreadsheet., p. 3,
generate an Excel spreadsheet file. Your spreadsheet will need to contain
the formulas to calculate the values for the volume and mass differences as
well as the values for the “mean volume correction” and “Std. Dev. Corr”.
Submit a printed copy of this empty spreadsheet as prelab.
For use in lab, you must bring a copy of this spreadsheet on a flash drive or
email yourself an electronic copy of this file, so that you will not need to
regenerate it during lab. There are only enough computers for 1 per pair so
if you would like to speed up your time in lab you are welcome to bring
your own laptop to do calculations.
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Water Temperature: ___°C
*Volume of 1.0000 g =________ mL
*(see table 2-5 page 54 of Exploring Analytical Chemistry, Harris)
Volume Readings from Buret (mL)
Increment Trial
1
Calibration
increment
Vi
Vf
Mass Flask + water (g)
Massinitial
Massfinal
0 - 10 mL
10-20 mL
20-30 mL
30-40 mL
40-50 mL
2
0 - 10 mL
10-20 mL
20-30 mL
30-40 mL
40-50 mL
3
0 - 10 mL
10-20 mL
20-30 mL
30-40 mL
40-50 mL
Vol. Pipet1 .00mL
1
2
Figure 2.
1.00
Etc.
1.00
Incomplete example of data table to be prepared in lab notebook.
During the Lab:
Please do not bring your calculators to lab since all
calculations will be performed automatically by Excel. Carry out the procedure as
written. Modify your procedure if you find it necessary, but be sure to record the
modifications. Record all observations and measurements in your laboratory
notebook as you do the experiment. The data collected should be written in the table
in your laboratory notebook and in the Excel spreadsheet. You must have your 1st row
of buret values in your Excel Spreadsheet and checked by your instructor before
leaving lab.
To Be Completed after lab: Complete the transfer of data to the Excel™
spreadsheet. Answer the questions on a separate page at the end of the experiment in
your lab notebook. After completing the spreadsheet prepare a plot showing the
results of your buret calibration with the appropriate error bars.
To be Turned in Next week at the beginning of lab- turn in the copies of the
pages from your notebook, the question answer sheet, and a copy of the print out of
your excel spreadsheet and plot. NOTE: Specific pages and order of assignment to be
turned in will be state on the Moodle page for the next experiment.
QUESTIONS ON NEXT PAGE.
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QUESTIONS
(ANSWER ALL QUESTIONS ON ONE (OR MORE) SEPARATE PAGE AT THE END OF THE
LAB IN THE NOTEBOOK. USE THE PROVIDED FORMAT FOR QUESTIONS 4B AND 4C.)
Q.1.
Calculate the total volume delivered from your 5 trials by your 1 mL glass
transfer pipet. If you needed to transfer 5.00 mL of a solution and you had a
1 mL and 5 mL transfer pipet, which would be the more accurate option? Use
your data to justify your answer.
Q.2.
Based on your data, which pipet technique, volumetric vs. micro, would you
use to deliver 1.00 mL of water in regards to both precision and accuracy? Use
your data to justify your answer.
Q.3.
Based on your data, which technique, pipet or graduated cylinder, would you
use to deliver 5.00 mL of water in regards to both precision and accuracy? Use
your data to justify your answer.
Q. 4. a. Examine your uncertainty bars on your buret calibration plot. Are these
error bars indicative of absolute or relative error?
b. Calculate the standard deviation and relative error at both the 10 mL and
30 mL points. If you were to use a buret, what volume would you rather
deliver based on your data?
Relative error - Buret
10 mL ________________Mean
30 mL __________________Mean
________________Std Dev
__________________Std Dev
________________Rel Err
__________________Rel Err
c. Calculate the standard deviation relative error for both 200 µL and 1000
µL delivered by the p1000-micropipet. Based on your calibration curve for the
micropipette, would you tend to use this pipet for 200 µL or 1000 µL
applications?
Relative error - Micropipet
200 µL ________________Mean
1000 µL ___________________Mean
________________Std Dev
___________________Std Dev
________________Rel Err
___________________Rel Err
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