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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: 2r d V g (3) n Surface tension is: d V g (4) 2r 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