To measure the 'spot concentrations' of cyanide. After carrying out the ten strokes the reagent ampoule must be broken, the liquid transferred onto the white separating layer and carefully drawn through with two strokes in cyanide free air using the pump. The indicating layer must not become moist.

• Acid gases indicated with different sensitivities
• A certain portion of the cyanide reacted with the CO₂ in the air through hydrolysis
• Impossible to measure cyanide in the presence of phosphine

Cross sensitivity: Free hydrogen cyanide indicated already before breaking the ampoule.

One stroke is equivalent to a volume of 100 ±5 ml of gas passing through the tube.