酚试剂法原理是甲醛与酚试剂反应生成嗪，嗪在酸性溶液中被高铁离子氧化形成蓝绿色化合物，颜色深浅与甲醛含量成正比，该化合物在630nm处摩尔吸光系数ε可达7.0×104，该法对甲醛的测定非常灵敏最低检测限为0.015mg/L。方法的缺点是乙醛、丙醛的存在会对测定结果产生干扰，存在二氧化硫时测定结果偏低，反应受温度限制，室温低于15，显色不 完全，20～35时15min显色最完全，放置4小时，吸收情况稳定不变。
The principle of phenol reagent method is that formaldehyde reacts with phenol reagent to form zine, which is oxidized by high iron ion in acid solution to form blue-green compound. The color depth is in direct proportion to formaldehyde content. The molar absorption coefficient ε of the compound can reach 7.0 × 104 at 630nm. The minimum detection limit of this method is 0.015mg/l. The disadvantage of the method is that the presence of acetaldehyde and propionaldehyde will interfere with the determination results. The determination results are low when sulfur dioxide is present, the reaction is limited by temperature, the room temperature is lower than 15, the color development is incomplete, the color development is the most complete at 20-35 15 minutes, and the absorption is stable when it is placed for 4 hours.