University of Puerto Rico
Mayagüez Campus
Chemistry Department
CALIBRATION OF AN ANALYTICAL BALANCE
Elena M. Flores Vélez
Analytical Chemistry Laboratory
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Abstract/Summary
When performing a data analysis, an important factor for reliable results is the balance’s
accuracy. Several factors can interfere when measurements are being taken from a balance
reading. Some factors can be classified as random factors (ground movement or wind change);
others can be classified as systematical factors (unclean balance or direct hand contact). In this
report several techniques, such as measuring an object several times, are presented to see if the
reliability of the readings obtained is increased, providing an accurate balance calibration.
An example of a balance calibration results is also presented. It states that the chances of
obtaining linearity (precision) in the results increase if the balance is well cleaned and if several
measurements are taken for the analysis. Furthermore, other studies have shown that if an object
is touched with bare hands, the natural grease found in hands will affect the balance’s reading;
therefore it is recommended the use of gabblers to enhance precision. By using appropriate
techniques, the analyst can obtain desired results and thus, assure the accurate calibration of a
balance.
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Table of Contents
1. Introduction: Validation of an Analytical Balance…………………...…….4
2. Experimental Procedure.………………….……..……………………….......5
3. Results and Discussion (Example)…..………………………………………..5-7
4. Conclusion (Example)...………………….……………………………………7-8
5. References..……………………………….……………………………………8
6. Supporting Information…………………….…………………………………8-9
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1. INTRODUCTION
1.1 VALIDATION OF AN ANALYTICAL BALANCE
When working in an industry, a standard operating procedure is vital for the quality of results
obtained in a determined procedure. Depending on the procedures and techniques used to handle
substances and instruments in the laboratory, the results obtained can be used for future
references or not. Some unpredictable as predictable factors can be present that might affect the
analytical procedure and thus significant errors may appear on the data. Ground movement,
unclean areas and improper use of instruments are some of the factors than an analyst must be
alert to if her or him wants to obtain the correct measurements. In the use of instruments, bad
measurements taken from an analyst balance are some of the most common sources of where
errors in the data might come from. An analyst balance measures masses within 0.0001 g and
they are used for high degree precision. [1] The weighing plate of the balance must be inside a
transparent enclosure with doors so dust or the air in the room can not affect the reading
process.[2]
To ensure an analytical instrument like the balance can be used in the laboratory or industry,
validation studies must be made. One of the steps taken in these studies is to determine the
following parameters:[3]
 Precision
 Bias
 Linearity
 Detection limit
 Robustness
 Selectivity
The study must show that these parameters can be well obtained in the instrument by the
utilization of some method.
For the studies to be valid, they must present a realistic survey of “the number and range of
effects operating during normal use of the methods”.[3] Also, the studies must be compared from
other laboratories realizing the same procedure with the same method. This is called traceability
and it is defined as: “The property of the result of a measurement or the value of a standard
whereby it can be related to stated references, usually national or international standards, through
an unbroken chain of comparisons all having stated uncertainties.”
As part of a validation process some analysts prefer, for example, to take several samples of a
substance (lets say, blood), introduce it into an instrument and take measurements several times.
Then, these results are compared with another machine that has experienced the same process.[4]
This procedure can be realize specifically for the validation of an analytical balance.
Analytical balances have been improved over the years as results of this kind of test and, thus
precautions must be taken for its utilization. When using an analytical balance for the first time, it
is recommended by the United States Pharmacopeia [USP][5]:
 To keep the balance away from pressure and temperature gradients
 The balance must be clean and the weighing pan must be free of corrosion
 To tare the mass of the empty pan with all the windows closed
Another fact is that the substance being measured must be handled with the proper instrument
since other substances from our body can affect the reading. (This will be discussed in the
RESULTS AND DISCUSSION section)
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2. EXPERIMENTAL PROCEDURE
Before starting experimentation, it is very important to assure that the analytical balance
was clean. This must be done with the purpose of obtaining measurements with minimal
errors as explained in the introduction section.
PART 1:
To test the accuracy of the analytical balance under study, seven measurements must be taken and
read from the balance utilizing a pre-weighed (100g) standard mass. First, after cleaning the
balance, place with a grappler the standard mass at the center of the balance’s plate. Then record
the measure of the balance (weight). After that, remove the weigh (with the grappler) and press
the TARR bottom once until “0.0000” is seen on the balance screen, indicating that the balance is
ready for the next measure. Continue with the calibration repeating the procedure for additional
six times.
NOTE: If the TARR bottom is not pressed after each weight taken, the measurement will not be
reliable since it might start measuring from, for example, 0.0001 instead of 0.0000 accurately. If
that happened, then the measurement is not valid for the calibration.
PART 2:
After verifying the balance is clean from last experiment, placed three different pre-weighted
masses inside the balance; the first mass is placed inside at the center of the balance plate for
better results, as the other experiment. Then, after pressing the TARR bottom, a measurement was
taken. The mass is taken out, the TARR bottom is pressed again and after “0.0000” appears in the
balance’s screen, the same mass is placed again inside the balance. This was done three times
with each mass.
After recollecting the data, averages and standard deviations are meant to be done in order to
determine and analyze the result and determine of the balance is well calibrated. Below it is
presented an example of data organization and results interpretation.
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3. RESULTS AND DISCUSSION (EXAMPLE)
In the results seen in table 1, techniques like measuring mass severable times can be effective
when it comes to reducing analytical errors. According to the data from the experiment where a
100g mass was tested, the more a mass is measured, the less chance is there to obtain significant
errors. The standard deviation was + 0.0002g and relative error was 0.0535g, confirming what
was said before.
Table 1: Calibration Results
Precision
Qualification (PART 1)
Read
out
1
2
3
4
5
6
7
Standard
(100 g)
100.0532
100.0534
100.0537
100.0537
100.0536
100.0535
100.0533
Calibration Qualification (PART 2)
Sample
Table 2: Calibration Analysis
Stdandard
Sample
Mass (g)
Blank
0.0000
S1
12.3044
S2
8.3012
S3
16.4505
Std. 100 g
100.0000
Blank
S1
S2
S3
Stdandard
Mass (g)
0.0000
12.3044
8.3012
16.4505
Avg. Exp.
Mass (g)
0.0000
12.3112
8.3043
16.4578
100.0535
Exp. Mass 1
(g)
0.0000
12.3114
8.3042
16.4576
Exp. Mass 2
(g)
0.0000
12.3112
8.3043
16.4582
Exp. Mass 3
(g)
0.0000
12.3110
8.3043
16.4576
s (g)
%RSD
%RE
0.0000
0.0002
0.0001
0.0003
0.0002
N/A
0.0016
0.0007
0.0021
0.0002
0.0000
0.0553
0.0369
0.0444
0.0535
For the three masses, the similar results as for the 100g mass are seen. Again, weighing a mass
several times can be effective since the standard deviation for sample 1 (12.3044g), for example,
was + 0.0002 meaning that the measurements are around 0.0002 over or under the true value. As
for the relative standard deviation and the relative error the results were 0.0016g and 0.0553g.
These results are very good since they are telling that there is minor difference between the
standard and average numbers (RSD), and that the average of the masses are near the true value
(RE). All of this confirms that the technique used was successful. In the graph 1, a calibration
curve (Average mass vs Standard mass) was made were the data from the three masses are
shown. As it can be observed line was obtained and the correlation factor (R^2) was one,
meaning that the data is near the true value (if not, the R will not be near one). Another detail that
shows the technique’s success in the experiment is the well aliened data points through the line. If
the data points were under or upper the line, then the data could be farther from the true value.
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Graph 1
4. CONCLUSIONS (EXAMPLE)
The technique of measuring a substance several times was proved to be successful. Results show
that when a substance is measured several times, probability of minimizing significant error
increases. Calculations such as standard deviation and average results are helpful for the analysis
since they are used to classify the data obtained into good or bad. This way, a user can tell if the
analytical balance being used can be trusted for future experiments or not.
Though results were not very clear, several precautions must always be taken when handling
substances to be weigh. In other words, a substance must be handle with a grappler just to be sure
no unnecessary systematical errors are obtained from the measurements and thus, making future
data unusable.
Note that even with all the precautions taken, an error range will always be present: not all the
measurements are the same or trustworthy. But the systematical or random errors can be
controlled with the right use of techniques and instruments.
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5. REFERENCES
1. ChemLab. “Analytical Balance”.
http://www.dartmouth.edu/~chemlab/techniques/a_balance.html
2. Unknown. “Weighing scale”. http://en.wikipedia.org/wiki/Weighing_scale
3. Unknown. “Analytical measurement and uncertainty”.
http://www.measurementuncertainty.org/mu/guide/analytical.html
4. Ernesto Hernández Pérez, Halima Dawood, Ulenta Chetty, Tonya M. Esterhuizen, Maresce
Bizaare
“.Validation of the Accutrend® lactate meter for hyperlactatemia screening during antiretroviral
therapy in a resource-poor setting”. International Journal of Infectious Diseases, Volume 12,
Issue 5, September 2008, p.p. 553-556
5. Marco A. De Jesús; J.I. Padovani; M. Vera; Héctor Areizaga; “Analytical Chemistry 1
Laboratory Manual QUIM-3025”, University of Puerto Rico, Mayagüez, 2008, 17.
6. SUPPORTING INFORMATION
For this experiment, the average, standard as relative standard deviation and relative error
formulas were used.
AVERAGE FORMULA
N = number of measurements taken
xi = individual measurement (x1, x2…)
Example:
(100.0532 + 100.0534 + 100.0537 + 100.0537 + 100.0536 + 100.0535 + 100.0533) / 7
= 100.0535
STANDARD DEVIATION FORMULA
N = number of measurements taken
xi = individual measurement (x1, x2…)
x = average value
Example:
45.5264
45.5266
45.5262
(xi – x)^2
0.00000004
0.00000004
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x = 45.5264
= 0.0002
RELATIVE ERROR FORMULA
µ = true value
x = average value
** p can be any number but if set for 2, it means that the result will be expressed in percentage
form.
Example:
RE = (8.3043 – 8.3012)/ 8.3012 * 100 = 0.0373%
RELATIVE STANDAR DEVIATION FORMULA
s = standard
x = average value
** p can be any number but if set for 2, it means that the result will be expressed in percentage
form.
Example:
RSD = (0.0002)/(12.3112) * 100 = 0.0016
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