Patent ID: 7314582

Claim:
Method of producing a lanthanide-halide based humidity indicator comprising: a. preparing an aqueous hydrohalide solution at least partially saturated with a free halogen; b. mixing said aqueous hydrohalide solution with lanthanide containing compound forming an acidic lanthanide-halide solution; c. evaporating the lanthanide-halide solution until it changes color forming a lanthanide-halide based humidity indicator; d. said evaporated lanthanide-halide based humidity indicator has the formula of AB 3 .X.3CB whereas A is a lanthanide; B is a halogen ion; X is one or more polyhalide ions; and C is a dispersing agent; e. whereby said aqueous hydrohalide is comprised of aqueous HBr and said step of preparing an aqueous hydrohalide solution comprises adding free halogen in a sufficient amount to said aqueous hydrohalide solution whereby said hydrohalide solution has an optical density between 0.1-2.0 per cm of pathlength at 500 nm; f. said lanthanide-halide solution has a pH of four or lower; and further comprising the step of mixing water into the lanthanide-halide solution while maintaining said pH of said lanthanide-halide solution at four or lower.