8.5.4.3 Calibration curve0, 5, 10, 20, 50 or 100 ml are filled in by the formaldehyde calibration solution(8.5.4.2) in a 100ml measuring cylinder and with distilled water up to mark. 10ml of solution are photometrically analyzed with the same procedure, as that of 8.5.3.The optical densities are applied against the formaldehyde concentration (between 0 and 15 µg/ml) on standard curve. The upward gradient is determined graphically or is calculated.