0, 5, 10, 20, 50 or 100 are filled in by the formaldehyde calibration solution(8.5.4.4) ml in a 100ml measuring cylinder and with distilled water up to mark.10ml of solution is photometrically analyzed with the same procedure, as that of8.5.3. The optical densities are applied against the formaldehyde concentrations(between 0 and 3 µg/ml) on standard curve. The upward gradient is determinedgraphically or is calculated.