Question

The pKa of acetyl salicylic acid (aspirin) is \(3.5\). The 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.

Short answer

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

Step-by-step solution

Understanding pKa and pH

The pKa is a measure of how easily a compound donates a proton (H+) and becomes ionized. The pH is a measure of the acidity or basicity of a solution. If pH < pKa, the substance is largely in its non-ionized (protonated) form. If pH > pKa, the substance is largely ionized (deprotonated).

Analyze Aspirin in the Stomach

The pH of the stomach ranges from 2 to 3. This is below the pKa of 3.5 for aspirin, which means in the stomach, aspirin will mostly exist in a non-ionized form. Non-ionized substances often have better absorption in biological membranes.

Analyze Aspirin in the Small Intestine

The pH in the small intestine is about 8, which is much higher than the pKa of 3.5 for aspirin. This implies that aspirin will be mostly ionized in the small intestine. Ionized substances are less likely to be absorbed compared to their non-ionized forms.

Determine Correct Answer

Given the behavior of aspirin in both environments, we see that it is unionized (protonated) in the stomach and ionized (deprotonated) in the small intestine. Therefore, the correct option describing aspirin's behavior is (a): Ionized in the small intestine and almost unionized in the stomach.

Key concepts

Aspirin Ionization

Aspirin ionization plays a crucial role in its absorption and efficacy within the body. This process describes how aspirin transforms between an ionized and non-ionized state depending on its surroundings.
The ionization state of a molecule like aspirin is guided by the pKa and the pH of its environment. Specifically, the pKa of a substance signifies the pH at which the molecule is half ionized and half unionized. Aspirin, with a pKa of 3.5, will behave differently in environments with varying pH levels.
Understanding these concepts is essential because the ionized and non-ionized states have different biological properties. - A non-ionized aspirin molecule can easily pass through cell membranes, facilitating absorption. - Conversely, the ionized form is less permeable and is absorbed to a lesser extent. The knowledge of aspirin's ionization is vital in anticipating where and how effectively aspirin will work in the body.

Gastric Juice pH

Gastric juice is a digestive fluid found in the stomach, with a highly acidic pH ranging from 2 to 3. This environment is conducive for breaking down food particles, but it also significantly affects drug absorption.
For aspirin, the importance of gastric juice pH cannot be understated. At a pH lower than its pKa of 3.5, aspirin remains largely non-ionized in the stomach. This non-ionized state enhances its ability to cross the stomach's lipophilic membrane.
As the non-ionized form is more capable of penetrating biological membranes, aspirin is efficiently absorbed when it is in the stomach. This property of gastric juice keeps aspirin effective shortly after ingestion, as it allows rapid penetration through the stomach lining. - The low pH in the stomach reduces aspirin ionization. - Non-ionized aspirin is better absorbed and remains effective in this acidic environment.

Small Intestine pH

In the small intestine, the environment transforms with a pH around 8. This significant difference from the stomach's pH requires aspirin to adapt its state.
At this higher pH, which is far above aspirin's pKa, most aspirin molecules become ionized. The ionization of aspirin in the small intestine means that it often struggles to penetrate the thicker membrane layers found there.
Ionized forms of aspirin in the intestine do not absorb readily, leading to a reduced concentration entering the bloodstream compared to when it is in the stomach. - The high pH in the small intestine promotes aspirin ionization. - Ionized aspirin is less efficiently absorbed, impacting its effectiveness when it reaches the intestine. Thus, understanding the pH difference between the stomach and the small intestine is crucial for optimizing aspirin's absorption and functionality.

Acid-Base Equilibrium

Acid-base equilibrium is the balance between the acidity and basicity in a solution. For drugs like aspirin, this equilibrium determines how effectively it transitions from one state to another in the body's different environments.
The principle of acid-base equilibrium explains why aspirin behaves variably in stomach and intestinal conditions. The equilibrium enables the prediction of drug behavior by merging knowledge of pKa and environmental pH.
In acidic settings, such as the stomach, aspirin remains largely non-ionized due to equilibrium favoring the protonated form. This non-ionized state is beneficial for absorption. - Equilibrium shifts towards a protonated (unionized) state when pH < pKa. - In basic settings like the intestine, equilibrium favors the deprotonated (ionized) form, reducing absorption. Understanding the acid-base equilibrium not only aids medication design but also assists in anticipating drug response times and effectiveness in different regions of the digestive system.