Reversible detector for gaseous carbon dioxide

An indicator substrate to be coated with a coating composition to make a reversible color carbon dioxide detector, the indicator substrate being made by a process comprising

• preparing an indicator solution comprising a solvent and a dye component, said dye component being present in said solvent at about 10% concentration by weight;
• said solvent being prepared by mixing about equal parts by volume of acetone, ethanol, diethyl ether and acetonitrile;
• and said dye component being prepared by mixing about 50% by weight of N-methyl-4-(phenylazo)benzenamine, with about 50% by weight of a mixture prepared from approximately equal parts by weight of the following dyes:
• o-cresolsulfonephthalein (Cresol-Red),
• thymolsulfonaphthalein (Thymol-Blue),
• p-benzenesulfonic-azo-dimethylaniline (Methyl-Orange),
• o-carboxybenzene-azo-dimethylaniline (Methyl-Red),
• amino-dimethyl amino-toluphenazonium chloride (Neutral-Red),
• benzene-azo-dimethyl-amine (Methyl-Yellow),
• 2,4,6-trinitrophenyl-methyl-nitro amine,
• p-nitro benzene-azo-salicylic acid (Alizarin-Yellow), and
• thymolphthalein;
• and
• applying said indicator solution to a porous substrate comprising pH neutral material to make an indicator substrate to be coated with a coating composition to make a reversible color carbon dioxide detector.