Calibration of Volumetric Glassware
Jonniel Vince Cruz
College of Science, Pamantasan ng Lungsod ng Maynila
Received: 23 July 2014
ABSTRACT
This experiment aimed at calibrating volumetric glassware (Mohr pipette) by comparing
its volume capacity with the calculated volume of liquid delivered. Theoretical volume was
determined by dividing the mass of liquid by its reference density at a specified temperature. A
correction factor of -0.0070 ± 0.1000 mL was obtained, which meant that the glassware
delivered lesser actual volume than it was expected to transfer, creating a positive error for every
reading. Hence, calibration methods were proven to be useful producing reliable, accurate and
repeatable measurements. From the uncertainties in the procedure, it was recommended to track
visual biased judgments, variations in the drainage time, and drafts which cause small variations
in the balance readings.
INTRODUCTION
A very important part of all
analytical procedures is the calibration and
standardization
process.
Calibration
determines the relationship between the
analytical response and the analyte
concentration. This relationship is usually
determined by the use of chemical
standards. The standards used can be
prepared from purified reagents, if available,
or standardized by classical quantitative
methods. Most commonly, the standards
used are prepared externally to the analyte
solutions (external standard methods).
(Skoog et al., 2013)
Glass wares are commonly calibrated
using a liquid of known, specific density,
and an analytical balance. The procedure is
to determine the mass of liquid the
glassware will hold, and to divide this mass
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of liquid by the reference density of the
liquid at specific temperature, obtaining the
corresponding volume of liquid. (Lee, 2013)
Calibration is accomplished by comparing
the experimental value with a standard
response until
Sexperimental + Scorrection = Sstandard
Where Sexperimental is the volume reading of
the glassware, Sstandard is the volume
calculated determined constant, and Scorrection
is called the correction factor, a value that
adjusts the experimental so that determinate
errors were at minimum.
Density is affected by temperature,
(UCMP Berkeley Contributors, 2014), so it
is necessary to measure the liquid
temperature and look up appropriate density
values.
In calibrating a mass-measuring
instrument, an electronic balance often
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includes an internal calibration weight for
routine calibrations, as well as programs for
calibrating with external weights. In either
case, the balance automatically adjusts
Sexperimental to match Sstandard. (Harvey, 2000)
In this experiment, volumetric glass
wares such as pipette will be calibrated by
determining the mass by differences in filled
and empty vessel to correct determinate
errors rooted from chemical and thermal
changes in the morphology of the glass
METHODOLOGY
Dry
and
stoppered
100-mL
volumetric flask was weighed in an
analytical balance. Ten milliliters of
temperature-equilibrated
water
was
transferred from the pipette to the
volumetric flask, and was weighed with a
stopper. The mass of water transferred was
calculated by subtracting the final and initial
readings, and the volume of water delivered
was calculated using the density of water at
the measured temperature. Correction factor,
which is volume calculated minus volume
capacity was determined in the glassware.
RESULTS AND DISCUSSION
In the experiment, the pipette was
calibrated for its volume capacity, and the
following data was obtained:
Table 1.Data in the calibration of a volumetric
glassware
Glassware: Mohr pipette
Weight of empty
51.9791 (±0.0001)
glassware (g)
Weight of glassware
61.9345 (±0.0001)
+ water (g)
weight of water (g)
90.9554 (±0.0001)
ambient temperature
28 °C
density of water at
ambient temperature
volume of water
delivered
volume capacity
glassware
correction factor
0.99623659 g/mL
9.9930 (±0.0001)
mL
10.0(±0.1) mL
-0.0070(±0.1000)mL
After calculating the data by using
the constant, density of water at 28 °C, the
correction factor, was calculated by
subtracting the volume capacity of
glassware from the volume of water
delivered. It was found out that the Mohr
pipette used, with a glass tolerance of (±0.1)
mL, had a correction factor of -0.0071±0.0001
mL (Table 1), which meant that the glassware
delivered lesser actual volume than it was
expected to transfer. With the different
measurements caused by temperature and
humidity, differences in mass and volume
readings might cause an error in the
experiment.
Sources of random uncertainties in the
calibration of pipette include (1) visual
judgments, such as level of the water with
respect to the marking on the pipette and the
mercury level in the thermometer; (2)
variations in the drainage time and in the angle
of the pipette it drains; (3) temperature
fluctuations, which affect the volume of the
pipette, the viscosity of the liquid, and the
performance of the balance; and (4) variations
and drafts that cause small variations in the
balance readings. (Skoog et al., 2013)
Undoubtedly, there are many other sources of
random uncertainty in the calibration process,
such as the purity of the water used which
changes its density, some buoyancy and
humidity
corrections
which
changes
physicochemical properties of water. The
cumulative influences of all variables were
responsible for the observed changes in the
results of experiment.
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CONCLUSION
Volumetric glass wares could be
calibrated by determining the correction factor
from a standardized set of signal – density of
water at specified temperature. From a
standardized volume, and calibrated balance
differences between the standard signal and
analyte’s signal (volume), more accurate
results can be generated for the instrument
being analyzed, such as volumetric glass
wares like pipette. In the experiment, a pipette
was calibrated, and yielded a correction factor
of -0.0070 ± 0.1000 mL, which meant that the
glassware delivers lesser actual volume than it
was expected to transfer. Thus, the measure
volume without calibration is actually greater
than the actual volume, creating a positive
error. Hence, calibration is an important
procedure in producing reliable, accurate and
repeatable measurements.
REFERENCES
1. "Exploring the Big Ideas About Density."
Exploring the Big Ideas About Density.
N.p., n.d. Web. 22 July 2014.
2. Harvey, David. "Calibrating the Signal
(Stotal)." Analytical Chemistry 2.0.
Boston: McGraw-Hill, 2000. Electronic
version.
3. Lee,
Terrence.
"Calibration
of
Glassware." N.p., n.d. Web.
4. Skoog, Douglas A., Donald M. West,
and F. James. Holler. "Distribution of
Experimental Results; Standardization
and Calibration." Fundamentals of
Analytical Chemistry. Fort Worth:
Saunders College Pub., 2013. Web.
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