Have you also encountered such a dilemma when doing COD rapid detection: strictly following the digestion steps, but the final data fluctuates like a roller coaster, with a difference of tens or even hundreds of mg/L? Repeatedly checking the operation process, but unable to find the root cause of the problem, can only waste reagents and time.

In fact, the vast majority of people fall into these four easily overlooked "hidden misconceptions"! Today, we will break it down for you one by one, master these key points for avoiding pitfalls, and make the COD experimental data stable and accurate.

Misconception 1: Blindly selecting dissolution tubes

Many experimenters, for the sake of convenience, use ordinary sealed glass digestion tubes instead of specialized colorimetric tubes, unaware of the hidden dangers that have been left:

Insufficient heat resistance and easy deformation during the digestion process;

Uneven wall thickness and inconsistent light transmission effect directly affect the accuracy of colorimetric results;

The sealing performance of the pipe cover is poor, which can easily lead to gas and liquid leakage, and may also adsorb organic matter in the water sample.

Correct operation: A specialized 16mm sealed digestion colorimetric tube must be selected to ensure the reliability of experimental data from the source.

Misconception 2: Insufficient mixing of water samples

The core of COD detection is to determine the total oxygen consumption of "dissolved substances+suspended solids". If the water sample is not thoroughly shaken before sampling and only the upper clear liquid is taken for testing, the oxygen consumption contribution of suspended solids will be missed - the final result will be more than 10% lower! Even if the subsequent operation is precise, it will only be useless on the unqualified samples.

Correct operation: Before sampling, be sure to shake the bottom of the water sample well to ensure that the sample taken has sufficient representativeness.

Misconception 3: Misconception in Controlling Cooling Time

After the digestion is completed, there are two common types of incorrect operations:

In a hurry to achieve results, directly take advantage of the heat for testing: high temperatures can change the density of the solution, resulting in changes in the actual thickness of the colorimetric solution and causing distortion of the absorbance data;

Long cooling time: Excessive waiting can seriously reduce overall experimental efficiency.

Correct operation: Cool the digested sample to room temperature (about 25 ℃) before testing on the machine, which can ensure accurate results and balance experimental efficiency.

Misconception 4: Adding unnecessary details before testing

This is the most easily overlooked misconception: many people are accustomed to shaking the solution in the colorimetric tube before testing, but COD specific colorimetric tubes contain insoluble substances that are resistant to chlorine. Once shaken, these insoluble substances will disperse in the solution, directly interfering with the detection process and leading to data deviation.

Correct operation: Do not shake or shake the colorimetric tube before testing. Wait until all insoluble substances in the tube have settled to the bottom of the tube before conducting machine testing.

The instability of COD experimental data is often not due to errors in core operational steps, but rather due to inadequate handling of these details. Remember the above 4 key points, you can avoid detours and achieve ideal results in the next experiment!

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