Determination of surface tension of liquids by stalagmometric methode

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Laboratory 10
Determination of surface tension of
liquids by stalagmometric methode
Instruction for the Laboratory of Biophysics
Instytut Inżynierii Materiałowej
90-924 Łódź, ul. Stefanowskiego 1/15, budynek A18
tel. 42 631 30 30, fax 42 631 30 38, e-mail: [email protected],
www.mechaniczny.p.lodz.pl
I. THEORETICAL INTRODUCTION
The formation of the droplets, which flows freely from the capillary, is dependent
on the surface tension of the liquid. This phenomenon is used to determine the surface
tension stalagmometric method (method of counting drops). Stalagmometer (Fig. 1a) is
a device made up of a glass bulb with marked above and below the bulb indicators
designating specific volume of liquid, ended capillary. To measure the surface tension of
the test liquid fill of stalagmometer and allow it to flow freely. During the formation of
the drop in volume continuously increases. Thereby growing its weight Q.
At a time where the weight of drops of exceeds values of forces of surface tension
(F) along a drop detached from a capillary(Fig. 1b). Begins the process of forming the
next drop. The cross-section drops its place throat is a circle with a circumference of
radius r l = 2πr. The value of the force F can be expressed as:
F  l   2  r
(1)
a) volume index
b) constriction drops
Fig.1 Stalagmometre with indicators of volume V of the bubble (a) and a drop forming
at the end of the stalagmometre's capillar (b)
Instytut Inżynierii Materiałowej
90-924 Łódź, ul. Stefanowskiego 1/15, budynek A18
tel. 42 631 30 30, fax 42 631 30 38, e-mail: [email protected],
www.mechaniczny.p.lodz.pl
The weight of single drop (Q) can be determined by dividing the weight of the liquid
contained in the volume (V) of the bubble stalagmometre by the number (n) of the formed
droplets:
d V  g
(2)
n
Q
where:
d - density of the liquid
g - acceleration due to gravity
We assume that at the moment separation drop from the capillary stalagmometre, the value of
force F and the weight drops are equal:
2r 
d V  g
(3)
n
Surface tension is:

d V  g
(4)
2r n
Because it is difficult to measure the radius (r) of the throat of drop at the moment of
detachment, the formula (4) does not apply to the determination of the absolute value of the
surface tension.

Stalagmometric method is often used to measure the relative surface tension
, ie. to
o
compare the surface tension of the tested liquid  to surface tension of the standard liquid  o .
Assuming that the throats rays of standard liquid ro and test liquid r are equal, the formula for
the relative surface tension takes the form:
 no  d

(5)
o n  do
where:
no - the number of drops of standard liquid,
n - the number of drops of the test liquid,
to - density of the standard liquid
d - density of the test liquid
The presented considerations that the weight of the drop detachment from the capillary tube,
and its volume depends on the surface tension of the liquid forming droplets.
Instytut Inżynierii Materiałowej
90-924 Łódź, ul. Stefanowskiego 1/15, budynek A18
tel. 42 631 30 30, fax 42 631 30 38, e-mail: [email protected],
www.mechaniczny.p.lodz.pl
III. . INSTRUCTIONS
1. The stalagmometre to put on a tripod
2. Below capillary set the beaker
3. Fill the stalagmometre with distilled water.
4. Count the number of drops flowing from marked volume ofstalagmometre's bubble (no).
Measurement repeated 10 times.
5. Calculate the mean no and the maximum error of no
6. Wash the stalagmometre by test liquid
7. Repeat steps 4 and 5.
8. Calculate the relative value of the surface tension and its error.
9. The calculated value is compared with the data from the physical tables
THE REPORT
1. Purpose of the exercise performed.
2. A short theoretical introduction.
3. Tables with values of measured quantities.
4. Calculate
5. Discussion and conclusions.
Instytut Inżynierii Materiałowej
90-924 Łódź, ul. Stefanowskiego 1/15, budynek A18
tel. 42 631 30 30, fax 42 631 30 38, e-mail: [email protected],
www.mechaniczny.p.lodz.pl

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