In winter cold chain transportation, freezing during COD reagent transportation can easily cause component deterioration, inaccurate detection, and even packaging damage risks. In the face of this common transportation problem, one must not blindly thaw and use it. This article will combine the characteristics of COD reagent components to explain in detail the scientific processing flow after freezing, from preliminary investigation, standardized thawing to performance verification, to ensure the accuracy of detection and safe use throughout the process.

1. Clarify the composition and type of COD reagent

COD reagents are usually divided into two categories, with significant differences in their effects after freezing.

·Prefabricated tube type COD reagents: mostly sealed mixed reagents (containing potassium dichromate/potassium permanganate, concentrated sulfuric acid, silver sulfate, etc.), some also contain mercury salt masking agents. Freezing of these reagents may cause the volume of the reagents inside the tube to expand, the tube body to break, or the components to crystallize locally, but the components are relatively stable. Generally speaking, thawing according to scientific methods is not a problem.

·Bulk reagents (such as oxidants and catalysts in separate bottles): If they are single component reagents (such as concentrated silver sulfate catalysts), the risk of freezing is relatively low; If it is a mixed buffer system, freezing may cause component stratification.

2. Preliminary inspection and thawing after freezing

·Appearance inspection: First, check whether the reagent packaging is damaged or leaking. If the prefabricated tube ruptures or leaks, seal it directly and discard it (due to the presence of strong corrosive and toxic components); Thaw the packaging intact.

·Scientific thawing: It is strictly prohibited to use hot water baths or open flames for heating. It is preferred to slowly thaw at room temperature (20-25 ℃) or gradually thaw in a refrigerator at 4 ℃ to ensure uniform heating of the reagents and avoid local overheating that may cause deterioration of the reagents.

During the thawing process, do not shake vigorously. You can gently invert the pre made tube or shake the reagent bottle (loose). After the WAN is completely melted, observe whether there are any residual crystals. If there are, let it stand at room temperature for 1-2 hours to confirm whether the crystals are completely dissolved.

3. Performance verification after thawing

This is a key step to ensure the accuracy of COD detection results, which needs to be verified through comparative experiments.

·Appearance confirmation: Observe whether the reagent is clear, without obvious layering or precipitation. If it appears turbid, discolored (such as potassium dichromate reagent changing from orange red to dark green), or the crystal cannot dissolve, it indicates that the components have deteriorated and should be discarded directly.

·Accuracy verification: Use thawed reagents to detect COD standard samples of known concentrations (such as 50mg/L, 200mg/L, 1000mg/L standard solutions), and compare the deviation between the detection results and the standard values.

If the deviation is within the allowable error range of the reagent manual (usually ≤± 10%), it can continue to be used; If the deviation exceeds the range or the repeatability is extremely poor, it indicates that the reagent has failed and needs to be replaced with a new reagent.

4. Follow up processing

If the reagent is verified as qualified, it should be used as soon as possible to avoid repeated freezing and thawing caused by refrigeration and freezing again; If the verification is not qualified, it needs to be disposed of according to hazardous waste requirements (COD reagents contain heavy metals and highly corrosive components and cannot be discarded arbitrarily).

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