Problem 68 - A Complete Resource Book in Chemistry for JEE Main

Question

The pKa of acetyl salicylic acid (aspirin) is \(3.5 .\) The \(\mathrm{pH}\) of gastric juice in human stomach is about \(2-3\) and \(\mathrm{pH}\) in the small intestine is about 8. Aspirin will be (a) ionized in the small intestine and almost unionized in the stomach (b) unionized in the small intestine and in the stomach (c) completely ionized in the small intestine and in the stomach (d) ionized in the stomach and almost unionized in the small intestine.

Step-by-Step Solution

Answer

(a) Ionized in the small intestine and almost unionized in the stomach.

Step 1: Understanding the Concept

To determine whether a substance is ionized or unionized, we can use the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \). In this equation, \([\text{A}^-]\) represents the concentration of the ionized form of the acid, and \([\text{HA}]\) is the concentration of the unionized form.

Step 2: Evaluating Aspirin in the Stomach

Given that the pH of gastric juice in the stomach is \(2-3\) and the pKa of aspirin is \(3.5\), we see that the pH is lower than the pKa. According to the rule, when pH < pKa, the substance is mostly in its protonated, unionized form. Thus, aspirin is almost unionized in the stomach.

Step 3: Evaluating Aspirin in the Small Intestine

In the small intestine, the pH is about \(8\), significantly higher than the pKa of aspirin, which is \(3.5\). Applying the same rule, when the pH > pKa, the substance is mostly in its ionized form. Therefore, aspirin will be ionized in the small intestine.

Step 4: Interpreting the Result

After evaluating both environments, we conclude that aspirin is mostly unionized in the acidic environment of the stomach and mostly ionized in the more basic environment of the small intestine.

Key Concepts

pH and pKa Relationship

The pH and pKa relationship is pivotal in understanding how acids behave in different environments. pH is a measure of how acidic or basic a solution is, while pKa refers to the acid dissociation constant, which indicates the strength of an acid. Notably, the Henderson-Hasselbalch equation provides the link between these two parameters: \[ \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \]This equation helps predict whether an acid is more likely to exist in its ionized or unionized form.

• When \( \text{pH} < \text{pKa} \), the environment is more acidic, pushing the equilibrium towards the unionized (protonated) form.
• Conversely, when \( \text{pH} > \text{pKa} \), the environment is basic, favoring the ionized (deprotonated) form.

This balance is crucial in drug delivery and absorption mechanisms, affecting where a drug is best absorbed based on its chemical properties.

Ionization of Acids

Ionization is the process by which an acid releases hydrogen ions, transforming from an unionized to an ionized state. This process depends heavily on the pH of the surrounding environment relative to the acid's pKa.

• In an environment where the pH is lower than the pKa, acids are less ionized because they tend to hold onto their protons.
• In higher pH environments, acids tend to lose their protons more readily, increasing the concentration of their ionized form.

Consider aspirin with a pKa of 3.5. In the human stomach, where the pH ranges between 2 and 3, aspirin is mostly unionized, allowing it to be absorbed directly through the stomach lining. In the small intestine, however, where the pH is around 8, aspirin becomes ionized, making it less readily absorbed but allowing for interaction with different biological functions. Understanding these dynamics is essential for pharmacology and drug formulation.

Gastric and Intestinal pH Levels

The pH levels within the digestive system play a major role in how substances like drugs are processed. The stomach's pH is characteristically acidic, typically around 2 to 3. This low pH facilitates the breakdown of ingested materials and activates digestive enzymes. Such an environment means substances like aspirin will remain largely unionized, slowing down their absorption. As we move to the small intestine, the pH drastically increases to about 8, which is more alkaline. This change in pH signifies an environment where substances like aspirin become more ionized. Ionization increases solubility but can potentially decrease passive absorption, as ionized molecules do not easily cross nonpolar cell membranes.
The transition from stomach to intestine demonstrates why certain medications are designed specifically for absorption in differing parts of the digestive tract, thus enhancing their effectiveness with these pH conditions in mind. Knowing these gastric and intestinal pH levels helps healthcare professionals predict drug behavior, ensuring medications are effective and delivered where they're most needed.