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Acids, Bases & Buffers: Isotonic Solutions
Comprehensive exam notes on Buffer Equations (Henderson-Hasselbalch), Buffer Capacity, and Measurements & Methods for adjusting Isotonicity.
Definition: A Buffer solution is a solution that resists any drastic change in its pH when a small amount of strong acid or strong base is added to it.
- Acidic Buffer: Mixture of a Weak Acid and its salt with a strong base (e.g., Acetic acid + Sodium acetate). pH < 7.
- Basic Buffer: Mixture of a Weak Base and its salt with a strong acid (e.g., Ammonium hydroxide + Ammonium chloride). pH > 7.
The Henderson-Hasselbalch Equation
This equation is used to calculate the pH of a buffer solution and to prepare a buffer of a desired pH.
(For an Acidic Buffer)
Buffer Capacity (Van Slyke's Concept)
Definition: Buffer capacity (β) is the magnitude of the resistance of a buffer to pH changes. It is defined as the ratio of the increment of strong base (or acid) added to the small change in pH produced.
Where: ΔB = gram equivalents of strong base/acid added, ΔpH = change in pH.
When is Buffer Capacity Maximum?
A buffer has its maximum capacity to resist pH change when the concentration of the Salt is equal to the concentration of the Acid ([Salt] = [Acid]). At this point, according to the Henderson-Hasselbalch equation, pH = pKₐ.
Preparation Steps
- Step 1: Select a weak acid whose pKₐ is close to the desired pH (within ± 1 unit).
- Step 2: Calculate the ratio of [Salt] to [Acid] required using the Henderson-Hasselbalch equation.
- Step 3: Weigh and mix the exact quantities, then adjust final pH using a pH meter.
Stability & Factors
- Temperature: pH of acetate buffers changes slightly with temperature, but basic buffers (like ammonia) are highly temperature-sensitive.
- Dilution: Moderate dilution does NOT change the pH of a buffer, but it decreases its buffer capacity.
Definition: A solution that has the same osmotic pressure as specific body fluids (like blood plasma or tears) is called an Isotonic solution.
Clinical Importance of Isotonicity
If an IV injection or eye drop is Hypertonic (high conc.), it draws water out of the RBCs/tissue cells, causing them to shrink (Crenation). If it is Hypotonic (low conc.), water enters the cells, causing them to swell and burst (Hemolysis). Therefore, ophthalmic and parenteral solutions MUST be made isotonic with blood.
Two major methods are utilized in pharmacy to calculate the amount of adjusting substance (like NaCl) needed to make a hypotonic drug solution completely isotonic.
1. Freezing Point Method
Based on the fact that an isotonic solution (like 0.9% NaCl) lowers the freezing point of water to exactly -0.52°C.
W = Weight (g) of NaCl added per 100ml.
a = Freezing point depression of 1% drug.
b = F.P. depression of 1% NaCl (0.576°C).
2. NaCl Equivalent Method
The Sodium Chloride Equivalent (E-value) is the amount of NaCl that produces the same osmotic effect as 1 gram of the drug.
This formula tells exactly how many grams of NaCl are required to make 100ml of the drug solution isotonic.