Where Did the Electricity Go? Uncover the Emotional “V-Shaped Graph” of Neutralization! (Barium Hydroxide × Sulfuric Acid Experiment)

I’m Ken Kuwako, your Science Trainer. Every day is an experiment!

Today, I want to introduce one of the most dramatic and visually satisfying experiments in the chemistry curriculum: the neutralization reaction between barium hydroxide and sulfuric acid. This isn’t just your average chemical reaction. It’s a fascinating process where you can actually watch ions “disappear” from a solution through the movement of an ammeter needle. When you plot the data, it creates a stunningly beautiful V-shaped curve. From preparation tips to the thrilling conclusion, let’s dive into the science behind this “vanishing act”!

Preparation: The Secret is in the “Supernatant”!

Let’s start with the setup. Precise preparation is the first step toward a safe and successful experiment.

1. Preparing the Barium Hydroxide Solution

Once your lab day is set, dissolve plenty of barium hydroxide in water and let it sit undisturbed for a few days. You only want to use the clear supernatant (the liquid at the top). Since barium hydroxide is a strong alkali, please make sure to wear safety goggles. It reacts easily with carbon dioxide in the air to form a white film (barium carbonate), so don’t make it too far in advance. Freshly prepared for each experiment is best. Keep your beakers covered with plastic wrap during storage.

Recommended Amounts (for 4 classes): Adding about 10g of barium hydroxide octahydrate to 220mL of water creates a saturated solution. For a group of 8 students, you only need 5mL x 8 groups = 40mL. For four classes, 160mL is plenty (this calculation includes a bit of a buffer).

On the left is a freshly made batch; on the right is one that sat for about a week. It becomes transparent with a thin film on the surface.

2. Diluting the Sulfuric Acid (The Golden Rule!)

Next, prepare the sulfuric acid. There is one absolute rule here: “Never pour water into concentrated sulfuric acid.” It generates intense heat and can cause the liquid to splatter violently. Always follow the order of “adding concentrated sulfuric acid to water” in small amounts.

Dilution Guide (for 4 classes): Carefully add 3mL of concentrated sulfuric acid to 51mL of water, then dilute this 10-fold to make a total of 540mL (approx. 0.1 mol/L).

Put about 15mL in a test tube and set it out with a pipette to make it easy for students to measure. Also, prepare beakers containing 5mL of the barium hydroxide solution for each group.

Once the prep is done, it’s finally time to start the experiment!

The circuit looks like this. I used the Petit-X mini power supply, which worked great, but any standard power supply will do.

Power On! Setting the Voltage

Connect the power supply and set the voltage to 5V. At this stage, the solution is full of ions, so the current flows strongly.

I used a Petit Meter as the ammeter. Very handy!

The Drop-by-Drop Countdown

Now for the main event. Add the sulfuric acid 1mL at a time, recording the ammeter reading after each drop.

The Magic of the V-Graph: Why Did the Electricity Vanish?

When you plot the data on a graph, a surprising result awaits.

As you add sulfuric acid, the current steadily drops until it hits nearly zero at one specific point. But keep adding acid beyond that point, and the current starts to climb again!

Why do we get this perfect V-shape? The secret lies in the formation of an “insoluble salt.” When barium hydroxide and sulfuric acid meet, they react to form barium sulfate—a white precipitate that does not dissolve in water.

The ions that were busy carrying electricity through the liquid bond together to form solid particles (the precipitate) and sink to the bottom. Since the “delivery drivers” of the current (the ions) are gone, the electricity stops flowing. However, as you continue to add more sulfuric acid, you introduce an excess of new ions, which allows the current to flow once again.

In a reaction between hydrochloric acid and sodium hydroxide, the resulting salt is sodium chloride (table salt). Because sodium chloride stays dissolved in water as ions, this “zero-current” phenomenon doesn’t happen. The fact that the salt is insoluble is the key to this experiment.

Wrapping Up: Science is Found in the “Change”

Watching the current steadily march toward zero is a thrill no matter how many times I see it. Rather than just reading “ions decrease” in a textbook, seeing the ammeter needle stop with your own eyes gives you a real sense that “something is happening in the invisible microscopic world!” This experiment is a perfect blend of chemical beauty and data-driven discovery. I hope you’ll try it out and experience the scientific drama hidden within the valley of that V-shaped graph!

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