PHYSICAL CHEMISTRY OF MILK

Module 6. Collegative properties

Lesson 11
COLLIGATIVE PROPERTIES OF DILUTE SOLUTION-01

11.1 Introduction

A dilute solution is one in which the amount of the solute is very small in comparison to the amount of the solvent. The dilute solutions show more or less ideal behavior as the heat and volume changes, accompanying the mixing of solute and solvent, are negligible for all practical purposes. Dilute solutions containing non-volatile solute exhibit some special properties which depend only upon the number of solute particles present in the solution irrespective of their nature. These properties are termed as colligative properties. Historically, colligative properties have been one of the means for determining the molecular weight of unknown compounds.

11.2 Definition

Colligative properties are properties of solutions that depend on the number of molecules in a given amount of solvent and not on the properties/identity (e.g. size or mass) of the molecules or those properties of solutions that depend on the number of dissolved particles in solution, but not on the identities of the solutes.

Colligative properties are the physical changes that result from adding solute to a solvent. Colligative Properties depend on how many solute particles are present as well as the solvent amount, but they do NOT depend on the type of solute particles. For example, the freezing point of salt water (solution) is lower than that of pure water, due to the presence of salt dissolved in it. It does not matter whether the salt dissolved in water is sodium chloride or potassium nitrate; if the molar amounts of solutes are the same and the number of ions is the same, the freezing points will be the same. For example, (AlCl₃) aluminium chloride and potassium phosphate (K₃PO₄) would exhibit essentially the same colligative properties, since each compound dissolves to produce four ions per formula unit.

The four commonly studied colligative properties are freezing point depression, boiling point elevation, lowering of vapor pressure, and osmotic pressure. Since these properties yield information on the number of solute particles in solution, one can use them to obtain the molecular weight of the solute.

Colligative properties are the properties of dilute solutions, that these properties are related to one another. Thus, if one is measured, the other can be calculated. The importance of these properties lies in the fact that they provide methods for the determination of molecular masses of dissolved solutes. The results are excellent if the following three conditions are satisfied.

• The solution should be very dilute
• The solute should be non-volatile
• The solute does not dissociate or associate in solution

11.3 Colligative Properties

A colligative property is one that depends only on the number of particles in solution rather than the type of particle. Molecular solutes have only one particle per formula, but ionic materials come apart into their ions and have almost as many particles in the solution as there ions are available. Sometimes re-association of the ions decreases the number of particles. The effect of the ionic pair depends upon the properties of the species dissolved and the concentration of solute. The more concentrated the solute, the greater percentage of ion pairing takes place.

The colligative properties of solutions are;

1) The solution shows an increase in osmotic pressure between it and a reference solution as the amount of solute is increased. A semi permeable membrane is one which allows only water from the solution but not the solutes. When two solutions are separated by such a membrane osmotic pressure will develop across the membrane, consequently due to the process of osmosis the water from the solution having lesser solute will pass through the semi permeable membrane and dilutes the solution with more solutes. Semi permeable membranes are an important part of any living thing. Cell membranes are semi permeable. The membranes on the outside of eggs are semi permeable. Trees pull up water from their roots by osmosis.

2) Solution of a solid (non-volatile) solute in a liquid solvent shows a decrease in vapor pressure above the solution as the amount of solute is increased.

3) The solution shows an increase in boiling point as the amount of solute in it is increased. The boiling point of a liquid is just the point at which the vapor pressure of the liquid equals the surrounding pressure. If the vapor pressure decreases, it will take a greater temperature to boil the liquid.

4) The solution shows a decrease in melting point as the amount of solute is increased.

5) It may be observed that the dissolved materials block the water molecules from attaching onto the rest of the water in the crystal or possibly that the dissolved material holds onto the water molecules more tightly than the water in the crystals. Whatever the cause, you have seen this in action in the making of home made barrel ice cream. The barrel on the outside of the ice cream container has ice and salt (sodium chloride) in it. The ice melts (grabs up the heat) at a temperature lower than the usual melting point of ice. Just ice in the barrel would not work, because it does not get cold enough to freeze the ice cream inside that has dissolved materials in it itself.

6) Refractive index also called index of refraction is the measure of the bending of a ray of light when passing from one medium into another. If i is the angle of incidence of a ray in vacuum (angle between the incoming ray and the perpendicular to the surface of a medium, called the normal), and r is the angle of refraction (angle between the ray in the medium and the normal), the refractive index n is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction; i.e., n = sin i / sin r. Refractive index is also equal to the velocity c of light of a given wavelength in empty space divided by its velocity v in a substance, or n = c/v. Measurement of this bending gives a direct measure of refractive index, n = sin i / sin r . Where ‘i’ is the angle of the ray to the surface as it approaches (incidence) and ‘r’ is the exit angle (refraction). Since the refractive index varies with the sample temperature and the wave length of the light these must be critically controlled and specified. Thus n²⁰_D refers to the index at 20 °C with D line of the sodium spectrum (589.0 and 589.6nm). The refractive index of water is n²⁰_D = 1.33299, the value of n²⁰_D for cow’s milk generally falls in the range of 1.3440 to 1.3485. Buffalo milk is similar to that of cow’s milk while human, goat, and ewe milk appear to have higher refractive index values. Since refractive index increments contributed by each solute in a solution are additive, much consideration has been given to the possible use of refractive index as a means of determining total solids or added water in milk. The refractive index of milk itself is somewhat difficult to determine because of the opacity but by using a refractometer such as Abbe’s instrument which employs a thin layer of sample, it is possible to make satisfactory measurement particularly of skim milk products and sweetened condensed milk.

Fig. 11.1 Abbe's refractometer

The relation between solids content (on the basis of weight per unit volume) and refractive index is linear and the contributions of the several components are additive. The individual components of milk differ in specific refractive increment Δn(ρc) where ‘ρ’ is the density of the sample and ‘c’ is the weight /weight concentration specific refractive increment 'n') of the component. Thus the relation between percent solids and refractive index will vary between lot of milk. Studies by the researches have revealed the following specific refractive increments (mn g^-1).

• Casein complex 0.207
• Soluble proteins 0.187 and
• lactose 0.140

The total contribution to the refractive index for milk containing 2.34% casein complex, 0.83% serum proteins, and 4.83% lactose becomes 0.00500 +0.00159 + 0.00695 or 0.01354. The residue 0.95% contributes 0.00166 to the total difference between the refractive indices of water and the milk. The refractive index of milk fat is 1.4537 to 1.4578 at 40°C. Fat does not contribute to the refractive index of whole milk because refraction occurs at the interface of air and the continuous phase. Sterilization and storage does not alter the refractive index.