Skip to main content

Surface Tension Determination

Free pharmacy material

Surface Tension Determination

INTRODUCTION
Surface tension is a phenomenon in which the surface of the liquid is in contact with the gas. Surface tension is defined as the force acting at right angles to a line of unit length present in the surface. This is denoted by ‘γ’.
W = F × d
where W = work; F = force; d = distance.
Therefore,
(γ) Gamma = F/d
The SI units for the surface tension are newton/meter or dyne/cm.
At a liquid surface, the molecule is not completely surrounded by other molecules.
Surface tension phenomenon
Surface tension phenomenon
The surface tension of a liquid is the energy required to increase the surface area which increases the stability of the solution and increases the solubility rate.
Examples of surface tension are drops of water, insects walking on water and paper clips floating on water.
An important measure for the characterization of surfactants which reduces surface tension is the critical micelle concentration (CMC). These surfactants contain both hydrophobic and hydrophilic groups attached to the surface. CMC is defined as the concentration of the surfactant at which the micelle formation starts.
The main cause for the surface tension is van der Waal's force which draws the molecules together.
FACTORS AFFECTING SURFACE TENSION
  • The stronger the intermolecular force, the higher the surface tension
  • Increased temperature decreases surface tension
  • Surfactants present in the solution reduce surface tension
  • Increased molecular motion increases surface tension
METHODS FOR DETERMINING SURFACE TENSION
There are different methods for the determination of surface tension, as follows:

Du Nouy Ring Method

This method was first proposed by the Pierre Lecomte du Nouy in 1883. The basic principle involved in this method is lifting a ring from the surface of a liquid. Then the slow lifting of the ring which is madeup of platinum from the liquid surface is observed. The force required to raise the ring from the liquid surface is measured.
γ = (dial readings in dynes/2 X ring circumference) (correction factor)
Du Nouy ring apparatus
Du Nouy ring apparatus

Drop Shape Method

Surface tension is determined by fitting of the shape of the drop to the Young–Laplace equation:
ΔP = γ(1/R1 + 1/R2)
where ΔP = pressure difference; γ = surface tension; R1 and R2 = surfaces radii of curvature.

Wilhelmy Plate Method

This method was first proposed by Ludwig Wilhelmy. A Wilhelmy plate is a thin plate used to measure equilibrium surface tension between the air–liquid or liquid–liquid interfaces.
γ = F/l cosθ
where l = wetted perimeter of plate; θ = contact angle between the interfaces of liquid and plate; F = force.

Stalagmometric Method

The stalagmometer consists of the glass capillary which is having the marks. The stalagmometer is filled with the sample solution and measures the weight of the drops and the number of drops. Based on this method, there are two other types:
  1. Drop weight method
  2. Drop number method

Drop weight method

The stalagmometer is filled with water and then allowed to run from Mark A to Mark B. The method follows the stalagmometer method and is rinsed with water. The weigh bottle is taken and the weight of the empty bottle is noted as W1. Then the collected water is taken into the weigh bottle and the weight is noted as W2. Then the stalagmometer is filled with the sample solution and is allowed to run from Mark A to Mark B. Then collect in the weigh bottle and take the weight as W3. Then the surface tension is determined by the following equation:
γ = W2 − W1 W3 − W1

Drop count method

The procedure is the same as for the drop count method. However, the number of drops is counted instead of the weight. Then the number of drops is counted from the liquid flown from Mark A to Mark B. Then the surface tension is determined from the following equation:
γ = (n2/n1) × 100
Stalagmometer apparatus
Stalagmometer apparatus
APPLICATIONS
  • Used in the determination of different surfactant critical micelle concentrations
  • Used in the removal of dust particles
  • Used in the soap and detergent manufacturing industry
  • Used in the determination of surface tensions of the liquid–liquid or air–liquid interfaces
REVIEW QUESTIONS
  1. Define surface tension.
  2. What are the factors affecting surface tension?
  3. What is the principle involved in the du Nouy tensiometer?
  4. Write the equations for the drop count and drop weight methods.
  5. What is the principle involved in the Wilhelmy plate method?
  6. What are the applications of surface tension determinations?


Labels:

Comments

Post a Comment

Popular posts from this blog

Nephelometry and Turbidimetry

Free pharmacy material Nephelometry and Turbidimetry INTRODUCTION This is mainly used to determine the scattering of the light by the suspended particles present in the sample solution. The instruments used for the measurement of the scattering are called nephelometer and turbidimeters. The choice between the nephelometry and turbidimetry depends upon the fraction of light scattered. This light scattering by the particles which are present in the colloids is known as the Tyndall affect. Nephelometry is the measurement of the scattered light by the suspended particles at right angles to the incident beam. This method is mainly used for the determination of the low concentration suspensions. Turbidimetry is the measurement of the transmitted light by the suspended particles to the incident beam. This is used for the determination of the high concentration suspensions. PRINCIPLE Light scattering is the physical character of the sample which will depend on the following:
3 comments
continue reading

Diazotization Titrations

Free pharmacy material Diazotization Titrations INTRODUCTION The diazotization titration is nothing but the conversion of the primary aromatic amine to a diazonium compound. This process was first discovered in 1853 and was applied to the synthetic dye industry. The reaction mechanism was first proposed by Peter Griessin. In this method, the primary aromatic amine is reacted with the sodium nitrite in acidic medium to form a diazonium salt. This method is first used in the determination of dyes. PRINCIPLE The principle involved in this method is that the primary aromatic amine present in the sample reacts with the sodium nitrite in the presence of acid such as hydrochloric acid to obtain a diazonium salt. R − NH 2 + NaNO 2 +HCl R − N + ≡ N − Cl − + NaCl + H 2 O Sodium nitrite is added to the solution of amine in the presence of acid at 0–5 °C. The amine reacts with the nitrous acid to form nitrosamine, which is followed by the tautomerisation and the water mol
163 comments
continue reading

Electrogravimetry

Free pharmacy material Electrogravimetry INTRODUCTION Luckow first discovered the electrogravimetry for the determination of the copper. Then Alexander Classen first published the paper on the electrogravimetry in 1881. After that, Gibb's was the first founder of the electrogravimetry for the deposition of the metals on the mercury cathode. Electrogravimetry is a method for the separation of the metal ions by using the electrodes. The deposition takes place on the one electrode. The weight of this electrode is determined before and after deposition. This gives the amount of the metal present in the given sample solution. PRINCIPLE The main principle involved in this method is the deposition of the solid on an electrode from the analyte solution. Electrogravimeter The material is deposited by means of potential application. The electrons are transported to electrode by the following mechanisms: Diffusion Migration Convection THEORY A metal
Post a Comment
continue reading