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EXPERIMENT 4 CALIBRATION OF VOLUMETRIC APPARATUS

Structure

4.1 Introduction Objectives 4.2 Principle

43 , Requirements 4.4 Procedure Calibration of a Calibration of a Burette Calibration of a 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, 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 Chemicals 1 No. Distilled water Beaker 1 No. Burette 1 No. Conical flask 4 No. Cork 4 No. Pipette 1 No. 1 No. Volun~etricflask 1 No.

4.4 PROCEDURE

4.4.1 Calibration of a Pipette Take a clean 100 cm3 conical flask.fitted with a cork or . 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.

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.

I 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 = FYI g ii) Mass of the corked conical flask + distilled water - W2g

iii) Temperature of water ...... = t°C

iv) Density of water at t°C = d

4.5.2 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 = wlg Basic Experiments iv) Mass of the corked conical flask + 10 cm3 disti1le.d water = rtt2g in Chemistry v) Mass of the corked conical flask + 20 cm3 distilled water = MI, g

vi) Mass of the corked topical flask + 30 cm' distilled water = w4g 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

Volume of water delivered by the pipette = wcT = x cm3 ' d Hence, the pipette is calibrated to deliver = x cm3

4.6.2 Calibration of a Burette

Mass of 10 cm3 water delivered by the burette = (wz - wl) g

MJ2 - M', = 3 Volume of 10 cm3 water delivered by the burette = --- xl cm d

Error in volume = (x - 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. Volume of Volume of water as Error in cm3 % Error Water delivered found by calibration 1 10 cm' XI cm3 (11 - 10) I OO(x - I O)/.r, 2 20 cm3 x2 cmJ (x2 - 20) 1OO(x2 - 20)/.rz 3 30 cm3 x2 cm' (.r3 - 30) 1 OO(x; - 3 O)/x,

4.7 SUMMARY

In this experiment, you would have performed the following: calibrated a pipette, calibrated a burette, calibrated a volurnetric flask.