Physical Chemistry of Milk

Lesson 25. ELECTROLYTE EQUILIBRIUM AND pH INDICATORS

Module 9. Buffers

Lesson 25
ELECTROLYTE EQUILIBRIUM AND pH INDICATORS

25.1 Introduction

Electrolytes when added to water will dissociate in to charged particles and can pass electric current while nonelectrolytes although dissolve in water but cannot pass electic current. The number of dissociated and undissociated ions attain equilibrium which necessary for the stability of the solution.

25.2 Dissociation of Ions in Solutions

Some of the compounds like sodium chloride (NaCl), hydrochloric acid (HCl), acetic acid (CH₃COOH) etc dissolve in water by dissociating into ions. To begin this process the ions require some energy which they obtain by the process known as hydration.

25.3 Hydration

This is a process in which opposite charges attract one another. In the present example the partial positive charge on the hydrogen molecule of H₂O is attracted to the anion and the partial negative charge on oxygen is attracted to cations. In this way during hydration each ion is surrounded by a group of opposite charges there by lowering the overall energy of the solution. It may be mentioned that these substances in their pure state are not separated ions, are electrically neutral and do not conduct electricity.

25.4 Conductivity

The molecular compounds such as pure acetic acid do not conduct electricity. Under the influence of water and during the process of hydration these compounds can be broken down into ionic species due to which the solution conducts electricity. Ionic compounds are also poor conductors of electricity when not in solution.

25.5 Reversible Reactions and Equilibrium

Hydration is an example of a reversible reaction. Most chemical reactions are the equilibrium of the hydration process is largely influenced by the type of electrolyte i.e, strong or weak electrolyte. Weak electrolytes upon introduction into water begin to dissociate into ions but most of the particles remain in molecular form. But the tendency of the particles in the molecular form to dissociate is same as those molecules that have already dissociated into ions. What could be the reason for these molecules to continue to exist in their undissociated form? In fact, these molecules have also been dissociated but they immediately get associated into their molecular form. In this way, the total number of ions and molecules stays the same while each is constantly becoming the other. In the case of strong electrolytes the molecules dissociate to a great extent than weak electrolytes and the equilibrium shifts to one side of the reaction where there are more ions and no molecular particles are found in the solution. The situation or the stage in which the number of ion pairs and molecular particles stay the same is called Equilibrium. Equilibrium should not be thought of as the end of a reaction but rather a stage or point at which no net change occurs.

25.6 Kinds of Equilibrium

Many kinds of equilibrium could be observed in solutions

• Physical equilibrium can be reached between a solid salt and a saturated solution of that salt

• Physical equilibrium between liquid water and gaseous water in a closed container
  

• Physical equilibrium can be reached between molecular particles and ions in solution

• Chemical equilibrium can be reached between reactants and products
  

25.7 Non Equilibrium System

Some reactions have an end point and due to the formation of precipitate. These reactions are not considered equilibrium because a product formed by this reaction does not become available to react in the reverse direction, a precipitate is insoluble and will not react with ions in solution a gas escapes to the atmosphere and is no longer even physically present to react.

25.8 Le Chatelier's Principle

Reversible reactions will reach equilibrium which is a point at which no net reaction is occurring or the reaction is progressing backward at the same time it is progressing forward. if it is left undisturbed. It is possible to disturb this equilibrium causing the reaction to progress more in one direction than another until a new equilibrium is reached. There are many conditions which will disturb this equilibrium which are

· Change in temperature i.e., any addition or subtraction of energy from the system
· A change in pressure
· The removal or addition of a product or reactant

As mentioned by Le Chatelier “any change in one of the variables that determines the state of system in equilibrium causes a shift in the position of equilibrium in the direction that tends to counteract the change in the variable under consideration

25.9 pH Indicators

A pH indicator is a chemical compound that is added in small amounts to a solution so that the pH (acidity or alkalinity) of the solution can be determined easily. A pH indicator is a chemical detector for protons (H⁺). Normally, the indicator causes the colour of the solution to change depending on the pH, of the solution.

pH indicators themselves are frequently weak acids or bases. When introduced into a solution, they may bind H⁺ (Hydrogen ion) or OH^- (hydroxide) ions. The different electronic configuration of the bound indicator causes indicator's colour to change.

Since determination of colour change is subjective in nature the results obtained by using pH indicators are most likely approximate and not precise. However, for applications requiring precise measurement of pH, a pH meter is frequently used.

pH indicators are frequently employed in titrations in analytic chemistry and biology experiments to determine the extent and magnitude of a chemical reaction.

Tabulated below are several common laboratory pH indicators. Indicators usually exhibit intermediate colour at pH values inside the listed transition range. For example, phenol red exhibits an orange colour between pH 6.6 and pH 8.0. The transition range may shift slightly depending on the concentration of indicator in solution and on the temperature at which it is used.

Table 25.1 Common laboratory pH indicators

(Source: en.Wikipedia.org)

Universal Indicator is a blend or mixture of different indicators that exhibits several smooth colour changes over wide range of pH values.Anthocyanins are a class of compounds that occur in many different plants; they appear red in acidic solutions and blue in bases. Extracting anthocyanins from red cabbage leaves to form a crude acid-base indicator is a popular introductory chemistry demonstration.