In exchange process, $H_{2}S$ gas is passed through cold water, the hydrogen atoms in $H_{2}S$ exchange with deuterium from $D_{2}O$ present in water. Thus, $H_{2}S$ becomes $D_{2}S$.
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In exchange process, $H_{2}S$ gas is passed by successive bubbling through ice cold water and boiling water, a liquid containing about $2\%$ $D_{2}O$ is obtained which is then concentrated by fractional distillation and electrolysis. In the reaction, the hydrogen atoms in $H_{2}S$ exchange with deuterium from $D_{2}O$ present in water.