CHEMICAL QUALITY ASSURANCE

Module 3. Chemical analysis of milk and milk products

Lesson 5
TERMINOLOGY OF STANDARD SOLUTIONS AND REAGENTS

5.1 Introduction

Standard solutions and reagents are the core of any chemical analysis. The accuracy, repeatability and precision of chemical analysis of food or dairy products depend upon the accuracy of the reagents and solutions used in the analysis. Therefore, it is very important that the reagents and the solutions should have the correct strength and should be checked from time to time before use. To express the concentration or strength of solutions and reagents different terms are used in the analytical chemistry or in chemical analysis of food article. The proper understanding of the terminology involved is very much desired before the start of any analysis.

5.2 What Is Meant by a Reagent?

A substance which can react with another substance or an agent capable of producing a chemical reaction.

5.3 What Is Meant by Solution?

A solution is a homogeneous mixture of two or more substances (or components).

The component present in higher proportions is called solvent.

The component present in smaller proportions is called solute.

We generally say that solute is dissolved in solvent to form a solution.

1. A solution in which small amount of solute is present is called dilute solution.
2. A solution in which large amount of solute is present is called concentrated solution.

Fig.5.1 Solution

3. A solution which contains the maximum amount of solute can be dissolved in a given amount of solvent at a particular temperature is called a Saturated solution.

5.4 Standard Solution

A solution whose strength or concentration is known is called as Standard solution.

Concentration or strength of solution can be expressed in different ways:

• Normal solution
• Molar solution
• Molal solution

5.4.1 Normal solution

A solution containing 1gram equivalent (i.e. Eq. wt expressed in gms) of the solute in one litre of the solution is called a normal solution.

5.4.1.1 Normality

The number of gram equivalents of a solute dissolved per litre of the solution. Solute may be acid, base or salt. It is represented by ‘N’.

Eq. wt. of solutes i.e. acids, bases & salts is calculated as follows

5.4.1.2 Calculation of eq. wt. of acids

eq. wt. of an acid is generally calculated by dividing the mol wt by its basicity.

Eq. wt of an acid = Mol. Wt/Basicity

By the basicity of an acid we mean the number of replaceable hydrogen atoms present in one molecule of the acid.

Table 5.1 Property of solutions

5.4.1.3 Calculation of eq. wt. of alkalis

The eq. wt of an alkali is often calculated with the help of the expression
Eq. wt of an alkali = Mol. Wt/Acidity

By the acidity of a base we mean the number of hydroxyl groups present in one molecule of the alkali.

Table 5.2 Properties of solutions

5.4.1.4 Calculation of eq. wt. of salts like carbonates and bicarbonates

The eq. wt. of salts like carbonate or bicarbonate can be calculated from the equation of its reaction with an acid.

For example, see the equation given below

Na₂CO₃ + 2HCl = 2NaCl + H₂O + CO₂

106 2x36.5

This equation shows that two gram equivalents of HCl react with one gram molecule of Na₂CO₃

Eq. wt. of Na₂CO₃ = Mol. Wt/2 = 106/2 = 53

Similarly the eq. wt. of K₂CO₃, NaHCO₃ and KHCO₃ can be calculated from their equations

Table 5.3

In general, the Eq.wt. of salts

For example,

Sodium in Na₂CO₃ has total valency of 2
Sodium in NaHCO₃ ------------------------------------ 1
Potassium in K₂CO₃ -----------------------2
Potassium in KHCO₃ ----------------------1

5.4.1.5 Mole

The quantity of a substance which contains one gram formula weight (or molecular weight or formula mass or molecular mass) of the substance.

5.4.2 Molar solution

A solution which contains one mole (Mol. wt., if the substance is molecule or At. wt., if the substance is atom) of a solute per litre of the solution is called a Molar Solution.

5.4.2.1 Molarity

The number of moles of solute dissolved per litre of the solution. It is denoted by M
M = mole of solute/volume of solution in litre

5.4.3 Molal solution

A solution which contains one mole solute dissolved in a Kilogram of solvent. The total may be more than or less than one litre, depending upon the density of a solvent.

Solvent = 1000gm (1Kg), Solute = 1 mole

Molality = the number of moles of solute dissolved per 1000gm of solvent. It is denoted by ‘m’
m=(No.of moles of solute)/(wt.of solvent (K.g)

Note Normality and molarity are temperature dependent, where as molality is temp independent.