As the core consumable for carrying high-temperature and high-pressure reactions in water quality testing, the cleanliness of COD digestion tubes directly affects the accuracy of testing results and the lifespan of the instrument. The following is a systematic cleaning process and key points analysis:

1、 Pre treatment and preliminary emptying

Immediately after the experiment, remove the digestion tube from the heating module and place it on a high-temperature resistant ceramic tray to cool naturally to room temperature. Wear heat-resistant gloves before unscrewing the screw cap and slowly release internal pressure to avoid splashing. When dumping waste liquid, the tube body should be tilted at a 45 ° angle, and the ear wash ball should be used to blow away residual liquid droplets on the tube wall to ensure that there is no visible residue retention.

2、 Graded soaking treatment

（1） Acidic pre soaking

Prepare a 10% dilute nitric acid solution (analytical grade), immerse the digestion tube in the acid solution, and cover the threaded part of the tube with the liquid level. Soaking in a 60 ℃ water bath for 2 hours can effectively dissolve iron manganese oxides and calcium magnesium salt crystals. If stubborn mercury residue is encountered, it can be soaked in aqua regia (hydrochloric acid: nitric acid=3:1) for a short period of time.

（2） Alkaline deep degreasing

For oil and fat pollutants, a saturated solution of sodium hydroxide (about 500g/L) is used for secondary soaking. Invert the digestion tube into the alkaline solution container to ensure that the inner wall is fully wetted. Maintain a constant temperature of 70 ℃ for 1 hour, and oscillate every 15 minutes during this period to break down the organic polymer film layer.

（3） Special treatment for special pollutants

-Sulfide deposition: Boil hydrogen peroxide solution (30%) for 10 minutes to decompose black sulfides through oxidation;

-Metal complex: EDTA disodium salt solution (0.1mol/L) was sonicated for 30 minutes to disrupt the metal chelation structure;

-Pigment pollution: Activated carbon suspension circulates and flushes, adsorbing chromophores.

3、 Mechanical cleaning process

Use a long handled nylon brush (diameter adapted to the pipe diameter) to scrub back and forth in a spiral manner along the axial direction, with a focus on cleaning the scale line area and the bottom groove. Rinse the threaded part of the pipe mouth with a small dental toothbrush and deionized water. It is prohibited to use hard materials such as steel wire balls to avoid scratching the surface of quartz/glass and causing stress concentration.

4、 Precision rinsing and drying

（1） Multi stage water washing

Passing through a three-stage flushing system of tap water, deionized water, and ultrapure water in sequence. Before each water change, it is necessary to shake the pipe body to generate vortices, ensuring that the detached particles are discharged with the water flow. Finally, immerse and rinse with anhydrous ethanol to evaporate and remove trace amounts of water.

（2） Temperature controlled drying

Invert in a constant temperature blast drying oven, set the temperature to 85 ℃, and continue for 2 hours. Pay attention to placing the pipe mouth downwards to facilitate the flow of condensed water. It is strictly prohibited to bake upright to prevent water droplets from flowing back and forming water stains.

5、 Quality verification and storage

After cleaning, perform a light transmission inspection: hold the digestion tube facing the fluorescent lamp and observe that the inner wall should present a uniform and transparent state, without any foggy spots or rainbow patterns. Only after passing the leak test can it be put back into use. When storing, it should be placed vertically on a dust-proof bracket to avoid deformation of the pipe mouth caused by stacking.

Precautions

1. Different batches of digestion tubes need to be processed separately to prevent cross contamination;

2. When exposed to acid and alkali, it is necessary to wear a protective face mask and acid and alkali resistant apron;

3. Waste washing solution should be classified and collected according to the "Laboratory Hazardous Waste Management Regulations";

4. Backwash the cleaning equipment weekly to prevent pipeline blockage.