Initial Operation
7
Note: The air-hydrogen atmosphere does not react chemically, because the temperature is
much lower than needed to supply the activation energy of the oxyhydrogen gas reaction.
Even if it would react, the theoretic maximum of the reaction enthalpy (the amount of energy
which would be set free by the reaction of hydrogen with the oxygen of the air) is about 0.1
kJ and therefore harmless.
1.4
The Reagent
Calcium hydride reacts with water to calcium hydroxide (hydrated lime, agricultural hydrate),
which is a weak base and quite harmless. Nonetheless, take care calcium hydride does not get
into contact directly with (liquid) water. The reaction heat together with the oxygen of the air
could ignite the developing gas. The small amount of reagent which is needed per
measurement averts this danger. Always close the reagent bottle directly after use. Because
the reagent is always added in surplus, it will be still reactive after the measurement. Avoid
the contact with inflammable, especially wet materials and leave the used reagent exposed to
the air, where it can react slowly with the air humidity. An ideal container is an open tin can.
After a few hours calcium hydride is completely transformed to calcium hydroxide. If left even
longer exposed to the air, calcium hydroxide reacts to calcium carbonate (CaCO3, chalk) with
the carbon dioxide of the air and can be disposed without any problems.
