Pharmaceutical Chemistry Reviewer for PLE Philippines 2026

By LisensyaPrep Team | Last Updated: May 2026 | 11-minute read

Pharmaceutical Chemistry bridges the gap between pure chemistry and clinical pharmacy practice. It explains why drugs behave the way they do in formulations, how they interact with the body at the molecular level, and why storage conditions matter. For the PLE, pharmaceutical chemistry tests your ability to apply chemical principles to pharmaceutical problems, not just recall isolated facts.

Acid-Base Chemistry in Pharmacy

pH and the Henderson-Hasselbalch Equation

The pH of a drug solution affects its stability, solubility, absorption, and compatibility with other ingredients. The Henderson-Hasselbalch equation is the most important equation in pharmaceutical chemistry.

For acids: pH = pKa + log([A⁻]/[HA])

For bases: pH = pKa + log([B]/[BH⁺])

Where pKa is the negative log of the acid dissociation constant, [A⁻] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.

Clinical application: Most drugs are either weak acids or weak bases. Their ionization state at a given pH determines how well they are absorbed. Unionized (non-polar) drugs cross cell membranes more easily than ionized drugs.

Rule of thumb: A weak acid is better absorbed in an acidic environment (stomach). A weak base is better absorbed in an alkaline environment (small intestine).

Buffers in Pharmaceutical Formulations

A buffer resists changes in pH when small amounts of acid or base are added. Pharmaceutical buffers are used to maintain the pH of formulations at the optimal range for stability and compatibility.

Buffer capacity is the ability of a buffer to resist pH change. It is greatest when the ratio of conjugate base to weak acid is 1:1, meaning when pH equals pKa.

Common pharmaceutical buffers:

Drug Stability

Drug stability is one of the most heavily tested topics in PLE pharmaceutical chemistry. An unstable drug loses potency, may form toxic degradation products, or may change in appearance or physical properties.

Shelf Life and Expiration Dating

Shelf life (t90): The time for a drug to degrade to 90 percent of its original potency. Most expiration dates are based on t90.

Zero-order kinetics: Drug degrades at a constant rate regardless of concentration. Amount degraded per unit time is constant.

First-order kinetics: Rate of degradation is proportional to drug concentration. Most drug degradation follows first-order kinetics.

Arrhenius equation: Used to predict stability at different temperatures. A 10°C increase in temperature approximately doubles the rate of degradation (Q10 rule). This is why cold chain storage matters for temperature-sensitive drugs.

Organic Functional Groups in Drugs

Understanding functional groups helps predict drug properties including solubility, stability, and biological activity.

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Solubility and Drug Formulation

Solubility Principles

Like dissolves like: Polar (hydrophilic) drugs dissolve in polar solvents like water. Non-polar (lipophilic) drugs dissolve in non-polar solvents.

Partition coefficient (log P): The ratio of drug concentration in octanol to water. High log P indicates lipophilic drug (good membrane penetration but poor water solubility). Low log P indicates hydrophilic drug (poor membrane penetration but good water solubility).

Salt formation: Converting a drug to its salt form dramatically increases water solubility. Most oral and injectable drugs are formulated as salts. Example: diclofenac sodium, morphine sulfate, amlodipine besylate.

Routes of Administration and Formulation Implications

Oral formulations must withstand the acidic pH of the stomach (pH 1 to 3) and the alkaline environment of the intestine (pH 7 to 8). Enteric coating protects acid-labile drugs.

Parenteral formulations must be sterile, pyrogen-free, and isotonic. pH should be near physiological (7.4). Precipitation in the syringe or IV line is a major incompatibility concern.

Topical formulations must penetrate the skin barrier. Lipophilic drugs penetrate better. Penetration enhancers like propylene glycol increase absorption.

Pharmaceutical Analysis

Pharmaceutical analysis ensures that drug products contain the correct amount of active ingredient and meet purity specifications.

Titrimetric Analysis

Acid-base titration: Determines the content of acidic or basic drugs. Uses a standard solution of known concentration (titrant) and an indicator or pH meter to detect the endpoint.

Redox titration: Used for drugs that can be oxidized or reduced. Example: permanganate titration for ferrous salts.

Complexometric titration: Uses EDTA to determine metal ion content. Used for calcium and magnesium in mineral supplements.

Spectrophotometric Analysis

UV-Vis spectrophotometry: Measures absorbance of UV or visible light. Most drugs absorb UV light due to aromatic rings or conjugated double bonds. Beer-Lambert law: Absorbance = ε × c × l (molar absorptivity × concentration × path length).

Infrared (IR) spectroscopy: Used for identifying functional groups and confirming drug identity.

HPLC (High Performance Liquid Chromatography): The most widely used method in pharmaceutical analysis. Separates drug from impurities and degradation products. Provides both identification and quantification.

Practice What You Just Learned

Pharmaceutical chemistry questions in the PLE combine chemistry principles with pharmaceutical applications. Practice now at LisensyaPrep. No account needed.

Practice Pharmaceutical Chemistry Questions at LisensyaPrep