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 pH and pKa

The pH of a solution indicates its acidity or alkalinity, whereas the pKa of a substance indicates the pH at which it is half ionized. If the pH is lower than the pKa, the substance is predominantly in its non-ionized form. If the pH is higher, the substance is mostly in its ionized form.

Step 2: Evaluate conditions in the stomach

The pH of gastric juice is between 2 and 3, which is lower than the pKa of aspirin (3.5). This lower pH means aspirin will remain mostly unionized in the stomach as it tends to stay in its non-ionized form when pH < pKa.

Step 3: Evaluate conditions in the small intestine

The pH in the small intestine is around 8, which is higher than the pKa of aspirin (3.5). In this case, aspirin will be predominantly in its ionized form as the pH > pKa.

Step 4: Determine the correct situation

Based on the conditions analyzed, aspirin is mostly unionized in acidic conditions like the stomach and is mostly ionized in basic conditions like the small intestine.

Key Concepts

Gastric Juice pH

In the human body, gastric juice plays a vital role in digestion. It is highly acidic, with a typical pH range of 2 to 3. This low pH is crucial for breaking down food, killing harmful bacteria, and activating digestive enzymes. The acidity of gastric juice is comparable to that of lemon juice or vinegar, making it very effective at beginning the process of digesting proteins.

This environment is not only essential for digestion but also affects how certain medications, like aspirin, behave. When we consider the pKa of a substance, such as aspirin, which is 3.5, it provides insight into how the drug will behave at different pH levels. A substance with a pKa near the pH of the stomach will likely stay mostly uncharged and unionized. This means the drug stays in its neutral form, which can influence how it is absorbed and transported within the body.

Ionization of Aspirin

Aspirin, or acetylsalicylic acid, is a common medication with a pKa value of 3.5. This means that at a pH of 3.5, aspirin is exactly 50% ionized. Ionization refers to the process where a molecule gains or loses an electron to form ions. In acidic environments like the stomach (pH 2-3), aspirin remains mainly unionized or neutral because the pH is lower than the pKa.

When the pH is higher than the pKa, aspirin becomes more ionized. In the small intestine, where the pH is around 8, aspirin turns mostly into its ionized form. This conversion is important as it affects how and where in the body aspirin is absorbed.

• Unionized aspirin = neutral, non-ionized form
• Ionized aspirin = charged form, more water-soluble

Ionization can impact a drug's solubility and absorption, influencing its efficacy and duration of action.

Acid-Base Chemistry

Acid-base chemistry involves the concepts of pH, pKa, and how molecules behave in different acidic or basic environments. The pH scale ranges from 0 to 14, where lower values represent acidic conditions and higher values indicate basic conditions. pKa is a value that signifies the pH at which a given molecule is 50% ionized. Understanding these concepts helps in predicting how a molecule like aspirin will react in various parts of the body.

The relationship between pH and pKa:

• If the pH < pKa, the environment is more acidic, and molecules are mainly in the neutral, unionized state.
• If the pH > pKa, the environment is more basic, and molecules tend to ionize.

This principle is essential in the field of pharmacology to understand how drugs will act, where they will be absorbed, and their overall effectiveness.

Small Intestine pH

The small intestine is a crucial part of the digestive system where nutrients from digested food are absorbed into the bloodstream. It has a pH of approximately 8, which is a basic or alkaline environment. This higher pH is necessary to neutralize the acidic chyme (partially digested food mixed with gastric juices) that comes from the stomach.

In this alkaline setting, substances like aspirin become ionized, because its pKa (3.5) is much lower than the intestinal pH. The ionized nature of aspirin in the small intestine leads to increased solubility in the intestinal fluids.

Role in Digestion:

• Neutralizes stomach acid
• Provides optimal conditions for digestive enzymes working in this part of the digestive tract
• Facilitates the absorption of drugs that are ionized

Thus, the small intestine's pH plays a key role in determining how and where the body absorbs medicines and nutrients, and it highlights the importance of understanding the relationship between pH and drug ionization for effective drug delivery systems.