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Video Abstract

Introduction

Parietal cells in our digestive tract produce hydrochloric acid, a very strong acid, into our stomach cavity. This hydrochloric acid helps break down food and kill bacteria, and also alters the pH of gastric fluid so that digestive enzymes can be active.

A common cause of stomach pain comes from an excess of gastric acid, which is made up of hydrochloric acid and digestive enzymes. Antacids such as Tums and Alka Seltzer ease this discomfort by neutralizing the pH of the stomach. Most antacids are buffers. A buffer is composed of a conjugate acid-base pair and is able to resist change in pH. Buffers resist change in pH by converting a strong acid (in this case, hydrochloric acid) to the weak acid-base conjugate pair through ionization. Excess protons react with the conjugate base, creating more of the weak acid. The amount of strong acid or base that can be added to a buffer solution before causing a significant change in pH is known as buffer capacity.

This experiment aimed to test the buffer capacity of different antacids by testing how much hydrochloric acid could be added before causing a significant change in pH. The antacids tested were Tums and Alka Seltzer. A control group of water with no added antacid was also tested.

Procedure

First, 20 mL of distilled water were measured with a graduated cylinder and then transferred to a 100 mL beaker. The recommended serving size of an antacid (2 tablets of Tums or 2 tablets of Alka Seltzer) was ground into a fine powder using a mortar and pestle. The powder was then added to 20 mL of water in a 100 mL beaker and mixed, first with a wand and then with the spinning plate (Fig. 1, 2). 30 mL of hydrochloric were added 2 mL at a time to the solution while the solution was spun. The mixture was spun to ensure that the hydrochloric acid was mixed evenly throughout the solution (Fig. 1). 2 mL increments of hydrochloric acid were measured with a pipette. The pH of the solution was measured using a Vernier pH probe each time 2 mL of HCl was added, 30 seconds after the HCl was added to the solution. Preliminary testing suggested that 30 seconds was enough time to allow the pH of the solution to be relatively stable. The mixture was spun to ensure that the hydrochloric acid and the antacid powder were mixed evenly throughout the solution. When the mixture was not spun during preliminary test, the powder settled at the bottom of the solution and the antacid did not react with the hydrochloric acid as quickly.

The Alka Seltzer solution was mixed for longer than the Tums solution prior to initial measurement of pH and addition of hydrochloric acid. This was due to the presence of bubbles that were a result of adding the powdered Alka Seltzer to distilled water (Fig. 2). Although stirring the solution did not get rid of the bubbles completely, it decreased their size and quantity.

A group without an antacid was tested as a control group. 30 mL of hydrochloric acid was added 20 mL of distilled water, 2 mL at a time. Here, the procedure was the same as it was for the other two groups, but no antacid was added.

Results

Results suggested that both Tums and Alka Seltzer were successful, at least initially, in resisting a significant change in pH upon the addition of hydrochloric acid (Fig. 3). Tums seemed to have a slightly higher buffer capacity than Alka Seltzer (Fig. 3). A significant change in pH occurred after 25.3 mL of HCl were added, on average, to the Tums solution (Fig. 3). The active antacid in Tums is calcium carbonate.

Alka Seltzer, which has active ingredients of anhydrous citric acid and sodium bicarbonate, demonstrated a significant change in pH after just 17.3 mL of hydrochloric acid were added (Fig. 3). Sodium bicarbonate (Alka Seltzer) has a shorter duration of action than calcium carbonate because it reacts more quickly with hydrochloric acid. (1)

Water, which was tested as a control, showed a significant change immediately after the first addition (2 mL) of hydrochloric acid (Fig. 3).

Although this data suggests that Tums has a higher buffer capacity than Alka Seltzer, it is not necessarily the most effective antacid. Often, antacids also relieve stomach pain by inhibiting the enzyme Pepsin, a factor that was not qualified in this experiment. Additionally, a further experiment might include testing the time during which antacids work best. In this experiment, time was a control. The solution was mixed for 30 seconds each time hydrochloric acid was added before the pH was measured. Designating time as a controlled variable allowed for consistency in this experiment, but it is possible that a different antacid might have been more successful at resisting change in pH if the antacid had been given more time to neutralize the pH of the solution.

Sources

1. Antacid - an overview | ScienceDirect Topics (10:40 am, 4/26/22)

2. Why don't our digestive acids corrode our stomach linings? - Scientific American (10:40 am, 4/26/22)

3. ​​How does the stomach work? - InformedHealth.org - NCBI Bookshelf. (10:40 am, 4/26/22)

4. 14.6 Buffers - Chemistry 2e | OpenStax (10:40 am, 4/26/22)

5. Antacids - StatPearls - NCBI Bookshelf (10:40 am, 4/26/22)

6. Neutralization - Chemistry LibreTexts (10:40 am, 4/26/22)