COD Test Tubes: Instructions & Precautions Guide

The aperture of most digestion instrument colorimeter on the market is made into 16mm, and the tolerance of the tube glass bottle is mostly ±0.2mm, so the diameter of the test tube is mostly 15.75±0.2mm.

The height of the test tube is generally 90, 100, 110, 125, 130, 150mm. According to the different reagent ratios of each manufacturer, the height is also different. Among them, the most commonly used is the 100mm test tube.

COD Test Tubes Materials

The materials of the COD test tube are generally low borosilicate, neutral borosilicate, and high borosilicate. They can be used when no colorimetry is required. When water quality digestion and colorimetry are required, the selected material is neutral borosilicate.

There are three types of materials for the lid: white polypropylene for one-time use; transparent polycarbonate for repeated use; black phenolic resin for repeated use of test tubes above 110mm.

For more information on COD test tubes and their applications in water analysis, refer to this article: "How the COD Test Tube is Used in Water Analysis."

Digestion tube precautions

1. Use a digestion instrument that matches the bottom of the tube. Round bottoms are matched with round bottom digestion instruments, and flat bottoms are matched with flat bottom digestion instruments. Otherwise, uneven heating will cause the tube to rupture. When placing the test tube, it should be placed slowly to avoid rupture caused by violent collision of the digestion tube with the digestion hole. According to past experience, some digestion tubes burst due to improper operation.

2. Avoid using digestion tubes and lids with mismatched threads. Mismatches will not seal and leak. It mainly depends on whether it is smooth when twisting. Smooth and smooth and the lid is not too crooked to meet the requirements.

3. Avoid too many scratches. The tubes and tubes will have scratches when rubbing against each other. Cleaning the bottle is also easy to cause scratches. The scratches are the weak points of the tube's strength.

4. Avoid using damaged bottles and lids. Due to manual intervention during transportation and production, such products are inevitable. Do not use bottles and lids with defects.

5. Do not digest water samples directly. You must have the corresponding reagents. The boiling point of water is very low. The more water there is, the greater the pressure in the bottle will be, and the lid will not be able to hold up. If you need to digest more water samples, please use a harder and more heat-resistant lid.

6. Taking the determination of COD index with water samples as an example, since concentrated sulfuric acid and other chemical reagents need to be added to the water sample before digestion, and the specific heat capacity and density of concentrated sulfuric acid are too different from those of water, if it is not mixed evenly, it may cause the sample to boil during digestion and cause the digestion tube to rupture. Therefore, the water sample must be mixed evenly before digestion to prevent the digestion tube from bursting.

Comparison Precautions 

1. Shake well before comparison. Before placing the test tube into the photometer, wipe the test tube clean with a test tube cloth.

2. Choose a calibrated instrument and a pipetting device with a small error to avoid more errors caused by labor and equipment.

3. If there is a scratch, do not align the scratch surface with the comparison position.

4. In the rapid digestion spectrophotometry, the requirement for the colorimetric tube is that the absorbance error is within ±0.005, that is, within 10 units, so the colorimetric tubes of the same batch must be within the error range as much as possible. If 20% of the tubes in a batch exceed the error range, the colorimetric tubes of this batch are unqualified.

5. Please use pure water to test the tube error.

[Tips for colorimetric comparison]

When colorimetric comparison, do not test only one angle, because it is easy to have errors in the maximum and minimum values, or it may just happen that the surface you are testing has scratches or dust. It is best to test three surfaces with the same tube. If the values of the three surfaces are within ±0.005, use these three values to take an average as the test result. This avoids errors caused by scratches and avoids taking the maximum value of the tube error. If the same tube is taken more than 6 times and still cannot get three values within ±0.005, then there is a problem with this tube (uniformity is not good).

For an in-depth understanding of how COD vials operate in water testing, please refer to this article: "The Working Principle of COD Vial."