Determination of the Aqua Surface Tension by Determining

the Biggest Vacuole Pressure

退出

ContentContent

Purposes and Demands Purposes and Demands PrinciplePrinciple Apparatus and ReagentApparatus and Reagent procedureprocedure Data Records and Processing Data Records and Processing QuestionsQuestions AAttentionttentions s DemonstratedDemonstrated

1.Determine surface tension of ethanol 1.Determine surface tension of ethanol

liquor in the terms of concentration. Figure liquor in the terms of concentration. Figure

out surface absorption capacity of ethanol out surface absorption capacity of ethanol

and area of cross-section of ethanol and area of cross-section of ethanol

molecule.molecule.

Purposes and DemandsPurposes and Demands

2.Understand surface tension, surface free ener2.Understand surface tension, surface free energy and the relationship between surface and agy and the relationship between surface and absorption.bsorption.

3.Command the technique and principle of the 3.Command the technique and principle of the method using to determine the surface tensiomethod using to determine the surface tension.n.

PrinciplePrinciple

1. Surface free energy1. Surface free energy

Surface tension shows whether liquid Surface tension shows whether liquid

surface tends to shrink automatically. The surface tends to shrink automatically. The

value depends on concentration and value depends on concentration and

component of liquid, temperature and component of liquid, temperature and

atmosphere.atmosphere.

2. Surface absorption2. Surface absorption The lowest energy principle says, if surface tensThe lowest energy principle says, if surface tens

ion of solvent can be reduce by a solute, the coion of solvent can be reduce by a solute, the concentration of surface solute is bigger than intncentration of surface solute is bigger than interior, contrarily, if a solute make surface tensierior, contrarily, if a solute make surface tension of solvent increase, the concentration of suon of solvent increase, the concentration of surface solute is lower than interior.rface solute is lower than interior.

The phenomenon that concentration of solute in exterior and interior are not identical is called surface absorption. Among them, there are a well-known equation Gibbs isotherm we find:

where Γ is the surface absorption

quantity; T, C, and R are, respectively,thermodynamics temperature, concentration, and gas constant.

Measurements have shown that in dilute solutiMeasurements have shown that in dilute solution the surface tension increases linearly with con the surface tension increases linearly with concentration. Draw a graph of surface tension oncentration. Draw a graph of surface tension toward concentration. Make tangent to an arbitoward concentration. Make tangent to an arbitrarily point on the curve and the corresponditrarily point on the curve and the corresponding slope σ/dci. Then figure out all the valves of ng slope σ/dci. Then figure out all the valves of and draw the graph of Γ against c. and draw the graph of Γ against c.

3. The biggest vacuole pressure 3. The biggest vacuole pressure Fill liquid in a measuring tube until the liquid Fill liquid in a measuring tube until the liquid

level outside is at or slightly above the zero level outside is at or slightly above the zero point of the capillary tube. The liquid level ipoint of the capillary tube. The liquid level inside the capillary tube would rise. Start thnside the capillary tube would rise. Start the pump and there is a pressure degression ie pump and there is a pressure degression in the measuring tube. n the measuring tube.

For the pressure dispatch among the measuring tube and capillary, liquid level in the capillary tube would be pressed down to nozzle and air bubble is shaped. Curvature radius of the air bubble increases until it is equal to radius of capillary tube. According to the Laplace formulae, the maximal pressure dispatch the air bubble endure can be expressed:

also, it can be expressed by the high dispatch in the U manometer：

pma x=gh

K’, apparatus constant, can be determined base on known surface tension. (Surface tension of water and ethanol liquor can be looked up on appendix)

Apparatus and ReagentApparatus and Reagent

Measuring detector of surface tension 1Measuring detector of surface tension 1 Abbe refractometer 1Abbe refractometer 1 Beaker(200ml) 1Beaker(200ml) 1 Thermostatic water bath 1Thermostatic water bath 1 Spheric aspirator Spheric aspirator Enthanol(analytically pure) 1Enthanol(analytically pure) 1

ProcedureProcedure 1.1.  Determination of instrumental constantDetermination of instrumental constant Regulate the thermostatic bath at the temperaturRegulate the thermostatic bath at the temperatur

e of 30±0.1e of 30±0.1ooC.C. If the instruments have been recently cleaned, imIf the instruments have been recently cleaned, im

merse ethanol liquor to measuring tube until the merse ethanol liquor to measuring tube until the liquor level outside is at or slightly above the zero liquor level outside is at or slightly above the zero point of the capillary tube, then mounted the instpoint of the capillary tube, then mounted the instrument in the thermostatic bath for 10 min.rument in the thermostatic bath for 10 min.

Be sure to keep the capillary tube vertical. Be sure to keep the capillary tube vertical.

Assemble the instruments following the Assemble the instruments following the

Sketch-maps of Equipments. Turn on the Sketch-maps of Equipments. Turn on the

pump for beginning. Note that the bubbles pump for beginning. Note that the bubbles

shape at a steady speed, usually likely for shape at a steady speed, usually likely for

8-12 per min. Mark down three times both 8-12 per min. Mark down three times both

the highest and lowest reading of liquid the highest and lowest reading of liquid

level in two arms of U manometer. Figure level in two arms of U manometer. Figure

out the average number and Δh.out the average number and Δh.

2.Determination of surface tension of ethanol s2.Determination of surface tension of ethanol solutionolution

Repeat the same procedure, measure Δh in terRepeat the same procedure, measure Δh in terms of concentration ethanol solution which bems of concentration ethanol solution which begins with high concentration solution to low ogins with high concentration solution to low one. Before each determination measuring tubne. Before each determination measuring tube and capillary tube must be washed three time and capillary tube must be washed three times with liquid that will be determined. es with liquid that will be determined.

Sketch-maps of Sketch-maps of EquipmentsEquipments

Data Records and ProcessingData Records and Processing

1. Room temperature:1. Room temperature:

Atmosphere pressure:Atmosphere pressure:

σσH2O H2O ==

hhH2OH2O = =

Instrumental constant:Instrumental constant:

2. Fill the table with experiment data:2. Fill the table with experiment data:

C enthanol Δh=h 上 -h 下 /cm σ/N·m-1 Γ/mol·m-2

C（M） 1 2 3

average

   

             

             

             

             

             

             

             

3.3.    Draw the graph of surface tension σ tDraw the graph of surface tension σ toward concentration.oward concentration.

4.4.    Draw the graph of dσ/dci against conDraw the graph of dσ/dci against concentration.centration.

5. Draw the graph of Γ against concentra5. Draw the graph of Γ against concentration.tion.

QuestionsQuestions

1.1.  What’s the meaning of surface tension and surface fWhat’s the meaning of surface tension and surface free energy? What’ the relationship between surface teree energy? What’ the relationship between surface tension and temperature?nsion and temperature?

2.2. How to understand about positiveabsorption and neHow to understand about positiveabsorption and negative absorption?gative absorption?

What variable did the experiment using Cibbs isotherm tWhat variable did the experiment using Cibbs isotherm to determine, and what data else need to know?o determine, and what data else need to know?

44 Why should we determine instrumentaWhy should we determine instrumental constant? l constant?

5. What’s the result if fill the manomet5. What’s the result if fill the manometer ethanol liquor instead of water?er ethanol liquor instead of water?

6. What’s the most important factor effe6. What’s the most important factor effect the experiment?ct the experiment?

AAttentionttentionss

Make sure that the liquor level Make sure that the liquor level

outside is tangency with capillary.outside is tangency with capillary.

Air bubble form steadily.Air bubble form steadily.

Abbe refractometerAbbe refractometer The figure shows the best state of visual field that has is a sharp boundary between light and dark fields.

1.350

1.360

The reading is n ＝ 1.3515

Surfactant molecule can lie low on surfacSurfactant molecule can lie low on surface of solution when the concentration is ve of solution when the concentration is very small.ery small.

On the surface of concentrated solution, On the surface of concentrated solution, polar group of surfactant molecule point polar group of surfactant molecule point to the interior and apolar group tend to to the interior and apolar group tend to stay on the exterior. stay on the exterior.

When the concentration comes to a When the concentration comes to a

certain value, surfactant molecule certain value, surfactant molecule

hold fully on surface and form a hold fully on surface and form a

saturated absorption layer.saturated absorption layer.