Every drop of an eye solution, every injection, and every oral syrup depends on one silent but powerful factor—pH control. If the pH is too high or too low, drugs may degrade, irritate tissues, or lose effectiveness. This is why pharmacists rely on buffers and isotonic solutions to ensure medicines remain stable, safe, and comfortable for patients.
UNIT 5 focuses on these critical concepts, explaining how hydrogen ion concentration, buffer systems, and isotonicity guide modern pharmaceutical formulation.
Understanding pH: The Foundation of Acid–Base Chemistry
Sorensen’s pH Scale
The pH scale, introduced by Søren Sørensen, measures the acidity or alkalinity of a solution. It is defined as:
pH = –log [H⁺]
The scale typically ranges from 0 to 14:
- pH < 7 → acidic
- pH = 7 → neutral
- pH > 7 → basic
Even small changes in pH represent large changes in hydrogen ion concentration, which can strongly influence drug stability and solubility.
Why pH Matters in Pharmacy
Correct pH ensures:
- Drug stability
- Maximum solubility
- Reduced irritation
- Better absorption
For example, eye drops and injections must match physiological pH to avoid discomfort.
Methods of pH Determination
Accurate pH measurement is essential during formulation and quality control.
Electrometric Method
The electrometric method uses a pH meter with a glass electrode. It is highly precise and widely used in laboratories and industries.
Advantages include:
- High accuracy
- Quick readings
- Suitable for colored or turbid solutions
Colorimetric Method
This method uses indicators that change color depending on pH.
Key features:
- Simple and inexpensive
- Suitable for rough estimation
- Less accurate than electrometric method
Both methods play important roles depending on the required precision.
Buffer Solutions: Resisting pH Changes
What Is a Buffer?
A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It usually consists of:
- A weak acid and its salt
or - A weak base and its salt
Buffers are essential for maintaining constant pH during storage and administration.
Applications of Buffers in Pharmacy
Buffers are widely used in pharmaceutical preparations.
Major Applications
- Stabilizing drug solutions
- Reducing tissue irritation
- Enhancing solubility
- Controlling drug release
- Maintaining biological compatibility
Without buffers, many medicines would degrade quickly or cause discomfort.
Buffer Equation: Henderson–Hasselbalch Relationship
Understanding the Equation
The buffer pH can be calculated using the Henderson–Hasselbalch equation:
pH = pKa + log (salt/acid)
This equation helps pharmacists:
- Predict pH
- Select suitable buffer components
- Adjust formulation accurately
It provides a scientific basis for designing stable drug products.
Buffer Capacity: Strength of a Buffer System
What Is Buffer Capacity?
Buffer capacity refers to the ability of a buffer to resist pH change. It depends on:
- Concentration of buffer components
- Ratio of salt to acid
- Temperature
Higher buffer concentration generally means greater resistance to pH changes.
Importance in Formulation
Strong buffer capacity is required for injections and ophthalmic solutions where pH stability is critical.
Buffers in Biological Systems
Natural Buffer Systems in the Body
The human body naturally maintains pH using several buffer systems:
- Bicarbonate buffer
- Phosphate buffer
- Protein buffer
These systems keep blood pH around 7.4, ensuring proper enzyme and metabolic activity.
Pharmaceutical Relevance
Drug formulations are often designed to match these physiological buffers to minimize irritation and maximize compatibility.
Isotonic Solutions: Comfort and Safety in Drug Delivery
What Is Isotonicity?
An isotonic solution has the same osmotic pressure as body fluids such as blood or tears. This prevents cell swelling or shrinking.
If a solution is:
- Hypertonic → cells shrink
- Hypottonic → cells swell
- Isotonic → cells remain normal
Maintaining isotonicity is crucial for parenteral and ophthalmic preparations.
Buffered Isotonic Solutions in Practice
Where Are They Used?
Buffered isotonic solutions combine pH control with osmotic balance.
Common examples include:
- Eye drops
- Intravenous fluids
- Nasal sprays
- Injectable drugs
These preparations ensure both chemical stability and patient comfort.
Practical Calculation and Adjustment of Isotonicity
Pharmacists use several methods to adjust isotonicity:
- Sodium chloride equivalent method
- Cryoscopic method
- White–Vincent method
These calculations ensure formulations match body fluids precisely.
Real-World Impact of pH and Buffer Control
In modern pharmaceutical industries, improper pH or isotonicity can lead to:
- Drug degradation
- Reduced efficacy
- Pain during administration
- Product rejection
Careful buffer design and isotonic adjustment prevent these problems and ensure safe therapy.