EXPERIMENT 4
CALIBRATION OF VOLUMETRIC
APPARATUS
Structure
4.1
Introduction
Objectives
4.2
43
4.4
,
Principle
Requirements
Procedure
Calibration of a Pipette
Calibration of a Burette
Calibration of a Volumetric Flask
4.5
Observations
!
Calibration o a Pipette
Calibration o a Burette
4.6
Calculations
Calibration of a Pipette
Calibration of a Burette
4.7
Summary
4.1
INTRODUCTION
You havestudied the use of various volume measuring devices such as
measuring.cylinders, burettes, pipettes and volumetric flasks in Unit 2, Block 1
of this course. Measuring cylinders are used for approximatemeasurement of
volume for various purposes and do not require any calibration.
Burettes, pipettes and volumetric flasks are used for accurate quantitatiye
measurements. When using apparatus of standard quality. it is not desifable to
attempt calibration of such apparatus for three reasons. Firstly. the process of
caliltration is time consuming. Secondly, due to possibility of introduction of
errors in calibrating the apparatus, the apparatus thus calibrated by an
inexperienced beginner may be less accurate than the apparatus calibrated by
the manufacturer. Thirdly, the error permitted in a determination may take care
of any inaccuracies in the calibration of the apparatus. However, if the
apparatus being used is not of standard quality. it is advisableto check the
calibration of the apparat& before performing any measurements with such an
apparatus. In this experiment. you will perform the calibration of a pipette, a
burette and a volumetric flask.
Objectives
After performing thjs experiment. you should be able to:
justify the need for calibration of apparatus,
state the steps involved in the calibration of volumetric apparatus,
calibrate burettes,
and volumetric flasks.
1
Basic Experimmts
in Chemistry
4.2
PRINCIPLE
Calibration of a burette and pipette is carried out by measuring the mass of a
known volume of water delivered by the burette or the pipette. By knowing the
temperature and the density of water at that temperature, the true volume
delivered by the burette or the pipette is calculated. This will be the volume
delivered by the burette or the pipette and not the one which is etched on the
apparatus. The volumetric~flasksare calibrated by filling them with a
calibrated pipette.
4.3
REQUIREMENTS
Apparatus
Analytical balance
Beaker
Burette
Conical flask
Cork
Pipette
Thermometer
Volun~etricflask
4.4
1 No.
1 No.
1 No.
4 No.
4 No.
1 No.
1 No.
1 No.
Chemicals
Distilled water
PROCEDURE
4.4.1 Calibration of a Pipette
Take a clean 100 cm3 conical flask.fitted with a cork or stopper. Weigh the
conical flask with cork accurately on an analytical balance to the nearest
milligram and record its mass in your notebook. Pipette out 10/20/25 cm3
(equal to the capacity of the pipette) of distilled water from a beaker into the
conical flask and weigh the flask with its contents. Record the temperature of
the water with a thermometer. Calculate the mass of water delivered from the
difference of the two readings. Repeat the procedure 3-4 times. Take average
mass and divide it by density of water at room temperature to calculate the
actual capacity of pipette. Values of density of water at different temperatures
are given in Table 4.1.
Table 4.1 : Density of water.
4.4.2
Calibration of a Burette
Take a clean 100 cm3 conical flask fitted with a cork. Measure its mass
accurately to the nearest milligram on an analytical balance and record the
mass. Clean the burette thoroughly and grease the stop-cock properly. Fill up
the burette with distilled water and note the room temperature. Ad-just the
water level to the zero mark. Touch the tip of the burette to the wall of a beaker
to remove any adhering drop. Take the initial reading of the water level in the
burette.
I
Run 10 cm3 of distilled water into the conical flask. Touch the tip of the burette
to the wall of the flask. Cork the flask and weigh it accurately on an analytical
balance and record the mass. The difference between this mass and the initial
mass of the flask gives the mass of water delivered. Calculate the volume of
water actually delivered by dividing the mass by density of water.
Repeat the process with 20 cm3, 30 cm3, 40 cm3 and 50 cm3 water, thus
calibrating the burette over its entire length.
4.4.3
Calibration of a Volumetric Flask
Volumetric flasks may also be calibrated in a manner similar to that described
for burettes and pipettes. But their large volume requires the use of an
oversized analytical balance. Therefore, the volumetric flasks are calibrated by
the procedure described below.
Take a clean 250 cm' volumetric flask. Using a calibrated 25 cm3 pipette,
carefully transfer 25 cm' of distilled water 10 times in succession, to the flask.
Tf the level of water in the flask does not correspond with the mark on the neck
of the flask. paste a label with its upper edge on a level with the bottom of the
meniscus of water. This will represent the correct calibration of the flask. You
can similarly calibrate the volumetric flasks of other capacities, if required.
If you want to find the true volume of the water in the flask upto the graduation
mark shown on the flask, you may do so with the help of a calibrated burette
by transferring water exactly up to the graduation mark.
4.5
OBSEK\rATIONS
4.5.1
Calibration of a Pipette
i)
Mass of the empty corked conical flask = FYIg
ii)
Mass of the corked conical flask + distilled water
iii)
Temperature of water . . .. . . . . . . . ... = t°C
iv)
Density of water at t°C
4.5.2
=d
Calibration of a Burette
i)
Temperature of water = t°C
ii)
Density of water at roc= d
iii)
Mass of the empty corked conical flask = w lg
-
W2g
Basic Experiments
in Chemistry
iv)
Mass of the corked conical flask + 10 cm3 disti1le.d water = r t t 2 g
v)
Mass of the corked conical flask + 20 cm3 distilled water = MI, g
vi)
Mass of the corked topical flask + 30 cm' distilled water = w 4 g
vii) Mass of the corked conical flask + 40 cn13 distilled water = w;g
viii) Mass of the corked conical flask + 50 cn~histilledwater = ns6g
I
e
4.6
CALCULATIONS
4.6.1 Calibration of a Pipette
Mass of water delivered by the pipette = (W? - CYI)g
wcT
= x cm3
d
Hence, the pipette is calibrated to deliver = x cm3
Volume of water delivered by the pipette =
'
4.6.2 Calibration of a Burette
Mass of 10 cm3 water delivered by the burette
= (wz -
Volume of 10 cm3 water delivered by the burette
Error in volume
wl)g
-
MJ2 M', =
= --- xl
= (x
d
cm 3
- 10) cm'
% Error
Similarly calculate the calibrated volume atid error in burette for 20 cm5,
30 cm3, 40 cm3 and 50 cm3 and tabulate the results as given below.
Table 4.2: Calibration of a burette
( S.No.
1
2
3
4.7
Volume of
Water delivered
10 cm'
20 cm3
30 cm3
Volume of water as Error in cm3
found by calibration
XI cm3
( 1 1 - 10)
( x 2 - 20)
x2 cmJ
x2 cm'
(.r3 - 30)
% Error
I OO(x - I O)/.r,
1OO(x2 - 20)/.rz
1 OO(x; - 3 O)/x,
SUMMARY
In this experiment, you would have performed the following:
calibrated a pipette,
calibrated a burette,
calibrated a volurnetric flask.