The indicator tube method developed by the National Bureau of Standards, USA, is based upon a colour reaction
with palladous salt and ammonium molybdate. Highly purified silica gel impregnated with ammonium molybdate
and a solution of palladium-or palladium oxide digested in sulphuric acid forming palladium silicomolybdate is
exposed to carbon monoxide. A morybdate blue is formed, the depth of colour in the detector tube varying from
faint green to blue in proportion to the amount of carbon monoxide present~in the air being tested. The indicator
tubes being available now commercially, the carbon monoxide can be determined quickly and accurately.