77. For H2S detection, sensitized tapes indicate H2S presence by means of discoloration of an exposed spot on the tape. The shade of the color on the spot depends upon the concentration of H2S and which of the following factors?

The discoloration of sensitized tapes, indicating H2S presence, is determined by the duration of exposure because the chemical reaction is cumulative over time. This reaction produces a darker spot proportional to the dose of H2S, which is the product of concentration and exposure time. Air pressure, temperature, and humidity are not primary factors influencing the discoloration.