Dihydrogen

To determine the method that yields high purity dihydrogen (>99.95%), we must analyze the options provided:

1. Reaction of zinc with aqueous alkali

This process typically involves the reaction:

The hydrogen produced may carry impurities from the alkali and zinc used. Therefore, it is generally not considered a method for producing high purity dihydrogen.

1. Electrolysis of acidified water using platinum electrodes

Electrolysis of water, though a clean method, often involves impurities due to the nature of the acids used for acidification and the resultant oxygen gas which can recombine with hydrogen, resulting in less purity.

1. Electrolysis of warm aqueous barium hydroxide solution between nickel electrodes

This method is specifically used to produce high purity hydrogen. The electrolysis of barium hydroxide using nickel electrodes typically avoids the introduction of impurities, and the setup under controlled conditions yields hydrogen of purity >99.95%.

1. Reaction of zinc with dilute acid

This is a common method for producing hydrogen, depicted by the reaction:

The hydrogen produced may again have impurities from unreacted acid or other byproducts, and thus it is not the preferred method for high purity hydrogen.

After evaluating all the options, the correct answer is the electrolysis of warm aqueous barium hydroxide solution between nickel electrodes. This method is designed to yield high purity dihydrogen (>99.95%) due to controlled electrolysis conditions and the non-contaminating nature of the substances involved.