Animated Solution for Chemistry - s and p-Block Elements: Dihydrogen of high purity ($>99.95\%$) is obtained through

Have you ever wondered how we get hydrogen gas that is almost perfectly pure? I'm talking about purity levels exceeding $99.95\%$. In the world of chemistry, achieving such high purity is no small feat. It requires a method that not only produces the gas efficiently but also ensures that absolutely no volatile impurities tag along for the ride. Let's embark on a journey to understand why one specific method stands head and shoulders above the rest.

When we first think about producing hydrogen, our minds often jump to the classic laboratory setup: reacting a metal with an acid. For instance, reacting zinc with dilute hydrochloric acid ($\text{HCl}$).

It's a straightforward reaction:

While this definitely gives us hydrogen, there is a major catch. Commercial zinc is rarely pure; it contains trace amounts of elements like phosphorus, sulfur, and arsenic. When these impurities react with the acid, they form gases like phosphine (${\text{PH}}_3$), hydrogen sulfide (${\text{H}}_2\text{S}$), and arsine (${\text{AsH}}_3$). These gases mix right in with our hydrogen, completely ruining any chance of achieving high purity.

What about the electrolysis of acidified water? Using platinum electrodes, we can split water into hydrogen and oxygen.

This is a much cleaner method. However, the hydrogen produced here can carry tiny droplets or traces of the acid used (like sulfuric acid) as an aerosol spray. While it's relatively pure, it still doesn't hit that ultra-high purity mark of $99.95\%$.

Then there's the chlor-alkali process—the electrolysis of brine (aqueous $\text{NaCl}$). Here, hydrogen is obtained as a byproduct at the cathode, while chlorine gas is evolved at the anode. Because hydrogen is merely a byproduct in an industrial setup focused on producing sodium hydroxide and chlorine, the gas collected is not optimized for the highest purity.

Warm Barium Hydroxide

So, how do we achieve the ultimate purity? The answer lies in the electrolysis of a warm aqueous solution of barium hydroxide (${\text{Ba(OH)}}_2$) using nickel ($\text{Ni}$) electrodes. This is the commercial golden standard for ultra-pure dihydrogen.

Let's visualize the electrolytic cell. We use warm barium hydroxide because it acts as a strong electrolyte, providing an abundance of ions to conduct electricity. But why barium? The beauty of the barium ion (${\text{Ba}}^{2+}$) is that it has a very low discharge potential compared to hydrogen ions (${\text{H}}^{+}$). This means that when the current is applied, the barium ions happily stay in the solution and don't interfere with the reactions at the electrodes.

When we pass a direct current through the solution, the water dissociates into hydrogen and hydroxide ions.

At the negatively charged cathode, the positively charged hydrogen ions migrate over and gain electrons. This process is called reduction. Pure hydrogen gas bubbles out at the nickel cathode.

Simultaneously, at the positively charged anode, the negatively charged hydroxide ions migrate over and lose electrons. This is oxidation. Oxygen gas evolves at the nickel anode.

Why is this specific setup so incredibly pure? It boils down to two main reasons.

First, the nickel electrodes are completely inert under these alkaline conditions. They do not react, dissolve, or degrade, ensuring no metallic impurities enter the gas stream.

Second, barium hydroxide does not form any volatile impurities. Unlike the acid in acidified water or the impurities in commercial zinc, there is nothing in this solution that can vaporize and mix with the hydrogen gas.

It is a beautifully clean and efficient process, perfectly designed to give us dihydrogen of the highest possible purity.