What is Concentration?

Concentration is the measure of the amount of solute (the substance being dissolved) present in a given amount of solvent or solution.

• Solute → The substance being dissolved (e.g., NaCl, glucose, aspirin).

• Solvent → The medium in which solute dissolves (e.g., water, ethanol).

• Solution → A homogeneous mixture of solute and solvent.

In simple words:
Concentration tells us how strong or how dilute a solution is.

Why Do We Need Different Expressions?

Not all experiments or formulations require the same unit of concentration.

• In formulations, we prefer percentage concentrations (w/w, w/v, v/v).
• In quantitative analysis, Molarity and Normality are most useful.
• For biological fluids and trace impurities, we use ppm or ppb.
• For thermodynamic studies, we rely on molality and mole fraction.

Thus, understanding all methods of expressing concentration ensures accuracy in pharmaceutical practice.

Methods of Expressing Concentration

1. Percentage Solutions

Percentage is one of the simplest ways of expressing concentration. It relates solute amount to 100 units of solution.

(a) % w/w (Weight by Weight)

Defined as the grams of solute per 100 grams of solution.

Formula:

Example:
Prepare a 10% w/w NaCl solution.

• Solute = 10 g NaCl
• Solution weight = 100 g (10 g NaCl + 90 g water)

Thus, 10 g of NaCl dissolved in 90 g water = 10% w/w solution.

Applications:

• Used in ointments and creams.

(b) % w/v (Weight by Volume)

Defined as the grams of solute per 100 mL of solution.

Formula:

Example:
A 5% w/v glucose solution = 5 g glucose in 100 mL solution.

Applications:

• Used in syrups, injections, and oral solutions.

(c) % v/v (Volume by Volume)

Defined as the millilitres of solute per 100 mL of solution.

Formula:

Example:
A 70% v/v alcohol solution = 70 mL ethanol + 30 mL water = 100 mL.

Applications:

• Used for alcoholic solutions, antiseptics, perfumes.

2. Molarity (M)

Molarity expresses the moles of solute present in one litre of solution. It is one of the most widely used concentration units in pharmaceutical and analytical chemistry because it directly relates to chemical reactions and stoichiometry.

General Formula:

But since we often weigh substances in grams and prepare solutions in millilitres, molarity can also be expressed as:

This formula is very practical for pharmaceutical preparations, where the mass of solute and volume of solution are known.

Example:

Prepare 0.5 M NaCl solution in 500 mL.

• Mass of solute = ?
• Molar mass NaCl = 58.5 g/mol
• V = 500 mL

So, dissolve 14.625 g NaCl in water and make up to 500 mL solution.

Applications:

• Used in titrations, assays, and chemical reactions.

Limitation:

• Molarity depends on temperature (since volume changes).

3. Normality (N)

Normality expresses the gram equivalents of solute per litre of solution. It is very important for acid-base and redox titrations.

Formula:

Or in practical terms:

Example 1: Preparation of 1 N HCl

• Molecular weight of HCl = 36.5 g/mol
• n-factor = 1 (HCl donates 1 proton)
• Equivalent weight = 36.5 g/eq

So, for 1 N solution in 1 L → 36.5 g HCl.

Example 2: Preparation of 1 N H₂SO₄

• Molecular weight = 98 g/mol
• n-factor = 2 (two protons can dissociate)
• Equivalent weight = 98 ÷ 2 = 49 g/eq

So, 49 g H₂SO₄ in 1 L solution = 1 N H₂SO₄.

Applications:

• Used in titrations (acid-base, redox, precipitation).
• Convenient when reactions involve equivalents rather than moles.

4. Molality (m)

Molality expresses the moles of solute per kilogram of solvent.

Formula:

Example:
10 g NaOH dissolved in 200 g water.

• Moles = 10 ÷ 40 = 0.25 mol
• Solvent = 0.2 kg

Applications:

• Used in colligative property studies (osmotic pressure, freezing point, boiling point elevation).

Advantage:

• Independent of temperature (since mass is constant).

5. Mole Fraction

Mole fraction is the ratio of moles of one component to total moles of all components.

Formula:

Example:
20 g NaOH in 180 g water.

• Moles NaOH = 20 ÷ 40 = 0.5 mol
• Moles water = 180 ÷ 18 = 10 mol
• Total = 10.5 mol

Mole fraction of NaOH = 0.5 ÷ 10.5 = 0.047.

Applications:

• Used in thermodynamics and vapor pressure calculations.

6. Parts per Million (ppm) and Parts per Billion (ppb)

Used for very dilute solutions such as impurities in drugs or contaminants in water.

Formula:

Example:
0.002 g NaCl in 1000 g water.

Applications:

• Used in toxicology, environmental monitoring, trace impurities.

• ring.

Comparison of Concentration Units

Method	Definition	Formula	Example	Applications
% w/w	g solute per 100 g solution	(w/w ×100)	10 g NaCl in 90 g water	Creams, ointments
% w/v	g solute per 100 mL solution	(w/v ×100)	5 g glucose in 100 mL	Syrups, injections
% v/v	mL solute per 100 mL solution	(v/v ×100)	70 mL ethanol in 100 mL	Alcoholic preps
Molarity	Moles/L solution		40 g NaOH in 1 L	Titrations, assays
Normality	Equivalents/L solution		49 g H₂SO₄ in 1 L	Acid-base titrations
Molality	Moles/kg solvent	moles ÷ kg solvent	10 g NaOH in 200 g water	Colligative properties
Mole Fraction	Mole ratio	X = nA ÷ ntotal	NaOH = 0.047	Thermodynamics
ppm	Parts per million	(solute/solution) ×10⁶	2 ppm NaCl	Impurities
ppb	Parts per billion	(solute/solution) ×10⁹	Trace metals	Toxicology

Practical Importance in Pharmaceutical Analysis

• Standardization of reagents → Normality and molarity are key.
• Formulation science → % solutions are used for syrups, injections, creams.
• Clinical pharmacy → mEq/L helps in IV fluid therapy and monitoring electrolytes.
• Quality control → ppm and ppb detect impurities in drugs.
• Thermodynamic studies → Molality and mole fraction are used in colligative property analysis.

Conclusion

Understanding the methods of expressing concentration is essential in pharmaceutical sciences. Each method has its own formula, applications, and limitations. Students must be comfortable in switching between different units depending on the requirement.

From % solutions used in formulations, to Molarity and Normality for titrations, to ppm and mEq/L for trace analysis and clinical monitoring, concentration expressions form the backbone of pharmaceutical analysis.