JUNE 2, 2013
CALIBRATION OF VOLUMETRIC GLASSWARE EXPERIMENT # 2
ADORNA JR., JOEMER A. PITAGAN, PAULA JESSIKA C.
MALAYAN COLLEGES LAGUNA
EXPERIMENT # 2
CALIBRATION OF VOLUMETRIC GLASSWARES
I.
II.
OBJECTIVES
To identify different types of volumetric glassware;
To discuss the importance of calibrating volumetric instruments;
To calibrate volumetric glassware; and
To use volumetric glassware properly.
LABORATORY EQUIPMENT / INSTRUMENTS / REAGENTS Equipment/Accessories 25 mL volumetric pipet 10 mL volumetric pipet 50 mL buret Aspirator Analytical balance Iron stand 50 mL beaker 250 mL beaker 125 mL Erlenmeyer flasks Thermometer Parafilm Buret holder Distilled water in wash bottle
III.
Quantity 1 1 2 1 1 1 2 2 2 1 1 1 1
DISCUSSION OF FUNDAMENTALS Introduction In the life of a simple average person, he measures everyday things with a little something called "estimate", especially to liquids, things, and the like. But to scientists, estimation is out of the question. Scientists, especially chemists, do not take second chances when measuring volatile reactive liquids into their reactions. A minute addition or subtraction to it might cause explosions, unexpected or non-visual reactions. This is very critical especially in the pharmaceutical or in the
Experi Exp erimen mentt 2: Cal Calibr ibrati ation on of Vol Volum umetri etricc Gla Glassw ssware are
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food industry. If you accidentally killed a person with a confident estimate, then chances are you are on the verge of your pulling conscience, if you have one. Like the first experiment, measuring volumes are as fundamental as measuring mass. But they are a little more crucial when it comes to measuring liquid volume. Unlike solids, they more significantly respond to changes in pressure and temperature, which is particularly dynamic, even in a laboratory setting. In doing so, volumetric devices for m easurements are used. The type of volumetric device to be used for a particular measurement considers four factors: general goal of the volume measurement, volume or range of volumes to be measured, degree of reliability needed for the measurement and number of measurements to be made. Calibration of these devices is important, as the reasons are stated above.
Theory Volume like mass is another fundamental property of matter that is commonly determined in analytical measurements. For solids, v olume can be obtained through calculations of the object’s dimensions. For liquid materials, volume can be determined by determining the volume of the container the liquid occupies. Most common laboratory glassware like beakers, Erlenmeyer flasks, and test tubes serve as containers for mixing, handling, and heating solutions but are not designed for accurate volume determinations. Volumetric devices used for analytical measurements include volumetric flask, volumetric pipets or transfer pipet, burets, micropipets, and syringes (Hage and Carr 2011). Calibration of volumetric devices is very important especially when the device is recently acquired or when the device will be used at a temperature different from the temperature it was initially calibrated. This is because glassware will contract or expand with a change in temperature. In addition, water expands about 0.02% per degree around 20 C (Christian 2004). Therefore, the true volume is different from the volume that is read from the container. The true volume can be achieved by calculation considering buoyancy effects and measuring the mass of water that is contained by the volumetric device, and then calculating the volume of water that was present using the known density of water at that temperature (Hage and Carr 2011).
Application Practically this experiment must be done first, among all others. This is practically because it main application is to ensure the accuracy of the volumetric glassware, that will be frequently used Experiment 2: Calibration of Volumetric Glassware
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in the proceeding experiments. If this experiment gives out bad values, even though random and systematically errors have been properly avoided and accounted for, chances are that the remaining experiments that will follow onto this will definitely carry on this same type of error, an error that is within the apparatus, not on the user itse lf.
IV.
METHODOLOGY
Glasswares were cleaned with detergent and lots of tap water. They w ere then rinsed with distilled water.
200-mL distilled water was placed in a 250-mL beaker and was allowed to equilibrate in room temperature.
Distilled water bottle was allowed to equilibrate to room temperature. It was used for the calibration of the burette. Figure 2.1. Glassware and sample preparation
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A clean and dry Erlenmeyer flask with a stopper was weighed in an analytical balance. The weight was recorded. Actual/ true volume was taken using: Mass of H 2O with buoyancy effect = mass of H2O * buoyancy correction (eq. 1) Burette was filled with distilled water up to the 0.00-mL mark. It was ensured that
True volume = (corrected mass of
no leakages or bubbles were found.
H2O)/(density of H 2O at specified temperature). (eq. 2)
Initial volume was read to the nearest 0.01 using a meniscus reader.
Correction value for each apparent volume was taken using: Correction value = true volume –
About 10-mL of water was transferred
apparent volume (eq. 3)
from the burette to the preweighed Erlenmeyer flask. Delivered water = apparent volume. Entire procedure was repeated with 20 aliquots of water per delivery. Same was done for 30-mL, 40-mL, and 50-mL A stopper was put on the flask with
aliquots of water.
water. Mass was recorded. Step was repeated until water in buret was discarded until 50-mL mark carefully. Values were plotted: correction values on y-axis and apparent volume on x-axis. Differences in mass between two
Figure 2.2. Calibration of 50-mL buret
consecutive mass weighings were taken as water delivered.
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For 25-mL pipet: A clean and dry 125-mL Erlenmeyer flask with stopper was weighed on an analytical balance. Recorded as: initial mass.
Difference in mass between each set of two consecutive mass measurements
For 10-mL pipet: A clean, dry 50‐mL beaker with parafilm was weighed on an analytical balance. Recorded as: initial mass.
were obtained to determine mass of water delivered in each trial.
Get true volume using eq. 1 25-mL/10-mL water was transferred from the pipet, allowing water to run out, with the tip of the pipet touching the side of the beaker. It was allowed 7-10 seconds to drain. The mass of the beaker and
Do entire procedure using 10-mL pipet.
added water was recorded. Recorded: water delivered = apparent volume.
The following were calculated for calibrated pipets: a. the mean volume, b. This addition was repeated three more
standard deviation, c. relative standard
times without discarding water in beaker
deviation (%RSD), d. 95% confidence
or flask. Recorded for every transfer:
interval, e. % relative error (theoretical
apparent volume and mass of container
volume assumed was exactly 25-mL and
plus added water.
10-mL for pipets)
Figure 2.3. Calibration of 25-mL and 10-mL pipets
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V.
DESCRIPTION OF THE APPARATUS / SET - UP
Figure 2.4. Volumetric flask.
Figure 2.5. Volumetric pipet
Figure 2.6. Burette (buret)
A volumetric flask is an instrument that is used to contain an accurate amount of liquid. A typical volumetric flask that is used in our laboratory measures 500 mL of liquid, which has a Experiment 2: Calibration of Volumetric Glassware
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tolerance of ±0.2 mL. This tolerance equates to a relative uncertainty of about 400 ppm (parts per million). A volumetric pipet is an instrument that is used to transfer accurate amounts of liquid. There are many types of volumetric pipets, among which are used in this laboratory where the 10-mL and the 25-mL ones. The volumetric pipet has only one graduation, and in doing so it can only deliver one accurate measure at a time. A burette is a laboratory apparatus that is mainly used for quantitative chemical analyses of liquids. It consists of a long, graduated glass tube with a stopcock (in a liquid burette’s case, on the bottom) that is handled by a burette clamp, which is connected to an iron stand. The volume that the burette dispenses is controlled by the stopcock, and is accurately measured by the graduations of the glass tube.
VI.
DATA SHEET
I.
GLASSWARE AND SAMPLE PREPARATION
Table 1. Water temperature Container
Temperature ( C)
250 mL beaker
24C
Distilled water bottle
24C
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II. CALIBRATION OF 50 mL BURET Table 2. Calibration of 50 mL buret Conditions
Trial 1
Trial 2
113.79
117.52
Final volume, mL
9.9
10.0
Initial volume, mL
0.00
0.00
Volume used, mL
9.9
10.0
123.19
127.57
9.41
10.05
Final volume, mL
20.0
20.0
Initial volume, mL
9.9
10.0
Volume used, mL
10.1
10.0
Mass of flask + 10 mL water, g
133.2
137.59
Mass of water, g
10.05
10.02
Final volume, mL
30.0
30.0
Initial volume, mL
20.0
20.0
Volume used, mL
10.0
10.0
143.28
147.54
Mass of Erlenmeyer flask, g st
10-mL delivery (1 )
Mass of flask + 10 mL water, g Mass of water, g nd
10-mL delivery (2 )
rd
10-mL delivery (3 )
Mass of flask + 10 mL water, g
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Mass of water, g
9.96
9.95
Final volume, mL
40.0
40.1
Initial volume, mL
30,0
30.0
Volume used, mL
10.0
10.1
Mass of flask + 10 mL water, g
153.28
157.61
Mass of water, g
10.02
10.07
Final volume, mL
50.0
50.0
Initial volume, mL
40.0
40.1
Volume used, mL
10.0
9.9
Mass of flask + 10 mL water, g
163.27
167.51
Mass of water, g
10.01
9.9
Mass of Erlenmeyer flask, g
113.79
117.52
Final volume, mL
19.9
20.0
Initial volume, mL
0.00
0.00
Volume used, mL
19.9
20.0
Mass of flask + 20 mL water, g
137.37
137.36
Mass of water, g
19.85
19.84
th
10-mL delivery (4 )
th
10-mL delivery (5 )
st
20-mL delivery (1 )
nd
20-mL delivery (2 )
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Final volume, mL
40.0
40.2
Initial volume, mL
19.9
20.0
Volume used, mL
20.1
20.2
157.47
157.58
20.1
20.22
117.52
117.52
Final volume, mL
30.0
30.1
Initial volume, mL
0.00
0.00
Volume used, mL
30.0
30.1
147.62
147.59
30.1
30.07
117.52
117.52
Final volume, mL
40.1
40.0
Initial volume, mL
0.00
0.00
Volume used, mL
40.1
40.0
Mass of flask + 40 mL water, g
157.58
157.51
Mass of water, g
40.06
39.99
Mass of Erlenmeyer flask, g
117.52
117.52
Mass of flask + 20 mL water, g Mass of water, g Mass of Erlenmeyer flask, g st
30-mL delivery (1 )
Mass of flask + 30 mL water, g Mass of water, g Mass of Erlenmeyer flask, g st
40-mL delivery (1 )
st
50-mL delivery (1 )
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Final volume, mL
50.0
50.0
Initial volume, mL
0.00
0.00
Volume used, mL
50.0
50.0
Mass of flask + 50 mL water, g
167.21
167.46
Mass of water, g
49.69
49.94
Trial 1
Trial 2
Apparent volume , mL
9.9
10
Mass of water, g
9.4
10.05
Corrected mass, g
9.41
10.07
True volume, mL
9.44
10.10
Correction value, mL
-0.46
0.10
Apparent volume , mL
10.1
10
Mass of water, g
10.13
10.02
Corrected mass, g
10.15
10.04
True volume, mL
10,17
10.06
Correction value, mL
0.07
0.06
Table 3. Volumes for the 50 mL Buret Conditions st
10-mL delivery (1 )
nd
10-mL delivery (2 )
rd
10-mL delivery (3 )
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Apparent volume , mL
10
10
Mass of water, g
9.96
9.95
Corrected mass, g
9.98
9.97
True volume, mL
10.00
10.00
0
0
Apparent volume , mL
10
10.1
Mass of water, g
10
10.07
Corrected mass, g
10.02
10.09
True volume, mL
10.04
10.12
Correction value, mL
0.04
0.11
10
9.9
Mass of water, g
9.99
9.9
Corrected mass, g
10.01
9.92
True volume, mL
10.03
9.95
Correction value, mL
0.03
0.05
Apparent volume , mL
19.9
20.0
Mass of water, g
19.85
19.84
Corrected mass, g
19.88
19.87
Correction value, mL th
10-mL delivery (4 )
th
10-mL delivery (5 )
Apparent volume , mL
st
20-mL delivery (1 )
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True volume, mL
19.93
19.92
Correction value, mL
0.03
-0.08
Apparent volume , mL
20.1
20.2
Mass of water, g
20.1
20.22
Corrected mass, g
20.13
20.25
True volume, mL
10.19
20.31
Correction value, mL
0.09
0.11
Apparent volume , mL
30.0
30.1
Mass of water, g
30.1
30.07
Corrected mass, g
30.14
30.12
True volume, mL
30.23
30.2
Correction value, mL
0.23
0.1
Apparent volume , mL
40.1
40.0
Mass of water, g
40.06
39.99
Corrected mass, g
40.12
40.05
True volume, mL
40.23
40.16
Correction value, mL
0.13
0.16
nd
20-mL delivery (2 )
st
30-mL delivery (1 )
st
40-mL delivery (1 )
st
50-mL delivery (1 )
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III.
Apparent volume , mL
50.0
50.0
Mass of water, g
49.69
49.94
Corrected mass, g
49.77
50.05
True volume, mL
49.9
50.15
Correction value, mL
-0.10
0.15
CALIBRATION OF 25 mL AND 10 mL VOLUMETRIC PIPET
Table 4. Calibration of 25 mL volumetric pipet and 10 mL measuring pipet Conditions Mass of Erlenmeyer flask, g
25-mL pipet 117.52
25-mL delivery
water, g
30.45
st
trial)
st
Mass of flask + 25 mL
10-mL pipet
Mass of beaker, g
10-mL delivery (1
(1 trial)
Volume delivered, mL
Conditions
25.0 142.45
Volume delivered, mL Mass of beaker + 10
10.0 40.35
mL water, g
Mass of water, g
24.93
Mass of water, g
9.9
Corrected mass, g
25.02
Corrected mass, g
9.91
True volume, mL
25.04
True volume, mL
9.94
25-mL delivery
10-mL delivery (2 trial)
nd
(2 trial)
Volume delivered, mL
nd
25.0
Volume delivered, mL
Experiment 2: Calibration of Volumetric Glassware
10.0
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Mass of flask + 25 mL water, g
Mass of beaker + 10
167.33
30.29
mL water, g
Mass of water, g
24.88
Mass of water, g
9.94
Corrected mass, g
24.96
Corrected mass, g
9.96
True volume, mL
24.99
True volume, mL
9.98
25-mL delivery
10-mL delivery rd
(3 trial)
rd
(3 trial )
Volume delivered, mL Mass of flask + 25 mL water, g
25.0
Volume delivered, mL Mass of beaker + 10
192.21
10.0 60.28
mL water, g
Mass of water, g
24.88
Mass of water, g
9.99
Corrected mass, g
24.97
Corrected mass, g
10.01
True volume, mL
24.99
True volume, mL
10.03
Statistical Analysis Average volume
25.01
Average volume
9.96
Standard deviation
0.029
Standard deviation
0.05
RSD
0.12%
RSD
95% confidence interval % relative error
25.01±0.028 0.04%
95% confidence
0.54% 9.96±0.049
interval % relative error
Experiment 2: Calibration of Volumetric Glassware
0.4%
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VII.
SAMPLE CALCULATIONS
For Table 3
( )() VIII.
RESULTS AND DISCUSSIONS This experiment is all about calibration of laboratory glassware. Calibration is very much needed for the instruments, since it is one of the primary processes used in maintaining the instrument’s measuring accuracy. Calibration, hence, means the process of configuring an
instrument (and accepting whatever environmental factors might be affecting it, even in a controlled laboratory setting) to provide certain results from a sample within an acceptable range (which is known as the tolerance). The table showing the tolerances of common Class A glassware is shown.
Table 1. Tolerance for Class A Laboratory Glassware* Tolerance of Class A burets Buret volume Smallest (mL) Graduation (mL) 50 0.1
Tolerance (mL) ±0.05
Tolerance of Class A transfer pipets Volume (mL) Tolerance (mL) 10
25 *Quantitative Chemical Analysis (6th ed). NY: W.H.Freeman and Company.
±0.02 ±0.03
This is done to minimize or readily eliminate the inaccurate me asurements that can be done by the instrument, and is one of the instrumentation design’s fundamental aspects. With this done, an
instrument that has assigned its proper measurement can now be called a standard. Also, calibration is done when the environment’s temperature setting is not in tune with the one from
which the instrument was originally made, or formerly calibrated, since density and buoyancy factors change the slightest of a single measurement, even though the estimated significance of the
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measurement is certain (because you are using a graduated measuring instrument). The table showing the corrections of density and buoyancy correction is shown below.
Table 2. Density of Water at Various Temperatures** Temperature ( C) 10 11
Density (g/cm3) 0.9997026 0.9996084
Buoyancy correction mdisplay ∙ 1.00152 mdisplay 1.00152 ∙
12
0.9995004
mdisplay 1.00152
13
0.9993801
mdisplay 1.00152
14
0.9992744
mdisplay 1.00152
15
0.9991026
mdisplay ∙ 1.00152
16
0.9989460
mdisplay 1.00152
17
0.9987779
mdisplay 1.00153
18
0.9985986
mdisplay 1.00153
19
0.9984082
mdisplay 1.00153
20
0.9982071
mdisplay ∙ 1.00153
21
0.9979955
mdisplay 1.00153
22
0.9977735
mdisplay 1.00154
23
0.9975415
mdisplay 1.00154
24
0.9972995
mdisplay 1.00154
25
0.9970479
mdisplay ∙ 1.00155
26
0.9967867
mdisplay 1.00155
27
0.9965162
mdisplay 1.00155
28
0.9962365
mdisplay 1.00156
29
0.9959478
mdisplay 1.00156
30
0.9956502
mdisplay ∙ 1.00156
∙
∙
∙
∙
∙
∙
∙
∙
∙
∙
∙
∙
∙
∙
∙
** All of the densities shown for pure air -free water at a pressure of 101.325 kPa (1 atm). The -3
3
buoyancy corrections assume that the air density is 1.20 x 10 g/cm and the density of the reference 3
weight is 8.00 g/cm . ** Analytical Chemistry and Quantitative Analysis. New Jersey: Pearson Prentice Hall. We started the experiment off by thoroughly cleaning the glassware. They are clean as it seems, but maybe the students that used it before us didn’t thoroughly clean it through, so we have to do it again for assurance that the calibration will be very much accurate. The final rinsing will be Experiment 2: Calibration of Volumetric Glassware
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the distilled water, which is free of most ions present in tap water, and in effect the water will not be attracted to the sides of the glassware, creating a free flowing liquid inside. Since the distilled water that we used in the wash bottle already equilibrated to room temperature, we just read this temperature which will be used for computations later on. We started calibrating the 50-mL buret by filling the buret with distilled water. Then, by getting the mass of the Erlenmeyer flask (which will be used throughout this whole procedure) we now transfer an appropriate amount (10-ml aliquots, adding 10 to the preceding one until we have 50mL aliquots, for two trials) to the flask and weigh the flask. The difference between the last reading and the initial will be the mass of the water. After all trials and different aliquots have been done, we go to the computation step. By having the apparent mass of the water, we now compute for the corrected mass of water by the equation:
( )( ) . (Eq 1) By getting the mass, we can get the corrected/true volume by the equation:
. (Eq 2) By having the true volume, we compute for the correction value as follows:
. (Eq 3) After this, we will graph the correction values vs. apparent volume. By inspection, you can see that there are negative and positive values for the correction value. This is mainly because of the difference in the mass weighed to the volume that you have estimated to the burette’s significant
figures (even with the help of the meniscus reader). The slope of the line will serve as the relative error of the calibration process, and if it goes by the specific tolerance, then the glassware has been properly calibrated. For the calibration of the 10-mL and 25-mL pipet, a proper container for delivery is allocated for the different measuring instruments. A 125-mL E. flask is used for the 25-mL pipet and a 50-mL beaker with parafilm for the 10-mL pipet. By getting the respective weights of the containers, for three trials we transferred distilled water to its appropriate container, and weighing it to get the difference, which will serve as the apparent mass of water. After all trials have been made, the computation process will be the same as the 50-mL buret computations, i.e. getting the correction Experiment 2: Calibration of Volumetric Glassware
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value. After this has been made, the data that we have computed and determined will be compared using statistical analysis. By getting the means of the true volumes of the different pipets, we computed for the standard deviation by the equation:
∑( ) ̅ √
.
(Eq 4)
After that, we computed for the coefficient of variation (%relative standard deviation) to see how precise our measurements are. We got a low value; therefore our precision is very high. Also, the %RSD can be used as comparison for the tolerances that is in Table 1. From what we got, we are inside the bounds of the tolerance values, therefore the values are within range. By getting the confidence interval, we computed for its actual tolerances, assuming that this is the working temperature for this glassware throughout the whole term. Lastly, we determined the %relative error, using 25-mL and 10-mL measurements as the theoretical volume. This can be compared to the tolerances, and from what we got, we are within range.
10-mL aliquots (Correction values vs. Apparent volume) 0.12
y = 2x - 19.92
0.1
0.1
0.08
Correction values
0.06
0.06 Linear (Correction values)
0.04 0.02 0 -0.02
9.94
0 9.96
9.98
10
Experiment 2: Calibration of Volumetric Glassware
10.02
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20-mL aliquots (Correction values vs. Apparent volume) 0.1 0.09 0.08
y = 0.3x - 5.97
0.06 Correction values 0.04 0.03
Linear (Correction values)
0.02 0 19.85 -0.02
19.9
19.95
20
20.05
20.1
20.15
-0.03 -0.04
By cleaning up everything and putting everything to what it was back before, we conclude the experiment to a finish.
IX.
SUMMARY AND CONCLUSIONS This experiment verified the concept involving the how to’s of calibrating glassware, such as
burettes and volumetric flasks. The glasswares were already previously calibrated during manufacture by the Quality Control (QC) department, but calibrating the instruments would be needed to ensure that no errors have been made by the QC, if ever. Even in a controlled laboratory setting, a random error can always happen. Certain factors that can affect the precision and accuracy of measurements of these glasswares are quite a few. Linear expansion would b a thing, but since the difference in the temperature in the laboratory would be very low, this can be way much disregarded. A difference in temperature, from what is said earlier, however, can affect the density of the liquid being contained or transferred. Density is the mass to volume ratio of a substance i.e. liquid, which is significant since for every degree of change, there would be a notable difference in the density. This is because the mass of
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EXPERIMENT # 2
CALIBRATION OF VOLUMETRIC GLASSWARES
the water is affected by buoyancy. With this changes, plus environmental factors, will cause a significant change between the volume obtained, and the true volume. Overall, by getting the relative difference through statistical methods and comparing it to the standard values (with the tolerance values as a guide for maximum allowable error), if it is within the bounds, then we can conclude that the glassware are calibrated to accuracy, and can be used for future purposes.
X.
POST LAB QUESTIONS 1. What is the maximum allowable error for the respective volumetric glassware that you calibrated? Based on our data, the maximum allowable error is about 0.41%, since the average tells this much difference, as to the computation o the relative error. 2.
Are your errors within the tolerance volumes for the Class A glassware? What are the systematic or random errors that have occurre d? Yes. If there are errors, what have possibly occurred would be: the reading of the lower meniscus, which always results to estimation; and possibly the water drops left inside the glassware due to the surface tension inside.
XI.
REFERENCES Christian, Gary D. 2004. Analytical chemistry (6th ed.). John Wiley and Sons Inc. Hage, David S. and James D. Carr. 20 11. Analytical chemistry and quantitative analysis. New Jersey: Pearson Prentice Hall. th
Skoog, Douglas et. al. 2004. Fundamentals of Analytical Chemistry (8 ed.). Singapore: Thomson Learning.
Experiment 2: Calibration of Volumetric Glassware
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