What is the difference between standard and Mercury Free COD vials? Can I switch to Mercury Free vials?

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What is the difference between standard and Mercury Free COD vials? Can I switch to Mercury Free vials?

COD tests are vital in many industries for the monitoring of effluent and discharge consent.

The most common test method involves using COD vials – sample is pipetted into the vial where it mixes with the reagents already present in the vial. This is then reacted at high temperatures, producing a colour change. This colour change is analysed in a colorimeter, which reads out the result of COD in mg/L.

COD Vials contain hazardous reagents, including Mercury compounds


One of the major problems that users face is the highly hazardous reagents present in these COD vials, including mercury compounds.

To reduce health and safety risks and their environmental impact, many users want to switch to mercury free COD vials. However it is important to find out if these will be suitable for the samples being tested before making the switch.

We’ve taken a look below at what the differences are, and how you can see if they would be suitable for you.


Why do COD vials contain Mercury?

Standard COD vials contain Mercury Sulphate (aka Mercury Sulfate or Mercuric Sulphate) to remove any Chloride in the sample. Chloride is the main interferent when measuring COD with vials, so it is important to remove this in order to achieve an accurate COD reading.


So why are there Mercury Free vials?

If your samples do not contain any chloride, or only very low levels, then the mercury free vials can be used.

They remove a hazard from the vials which is an improvement from both a health and safety and an environmental standpoint. It should be noted though that COD vial still contain sulphuric acid so are still hazardous and they contain Potassium dichromate which is also a Substance of very high concern.

Both kids of vials require specialist disposal through a licensed waste contractor –  we can office this service in the UK See COD disposal


How do I know if I can switch to Mercury Free COD vials?

The simplest way to do this is to conduct your own tests in house to see if results on the Mercury Free COD vials are comparable to those in the standard COD vials.

These should be done at your site, with your samples. Since the results will be different depending on the exact composition of your samples.

You will need both standard and mercury free COD vials, then run the same samples on both vial types. Comparing the results will let you see if there is any significant difference between the standard any mercury free vials.

Tabulate the results for a quick comparison; Sample No. / COD result in standard vial (mg/L) / COD result in Mercury Free vial (mg/L).

You may want to formally analyse the differences via statistical tests to confirm if the differences are significant.

Hach recommend that mercury free COD vials should only be used if Chloride and Ammonia levels do not exceed those stated below;

  • Low range, 0-150 mg/L – Chloride 0 – 100 mg/L – Ammonia 0 – 50 mg/L
  • High range, 0-1500 mg/L – Chloride 0 – 200 mg/L – Ammonia 0 – 50 mg/L
  • Ultra High range, 0 – 15,000 mg/L – Chloride 0 – 2000 mg/L – Ammonia 0 – 500 mg/L

It should also be noted that Ammonia usually only causes interferences when Chloride is present also.


What other options are there to avoid Mercury compounds in COD measurement?

If Mercury Free COD vials are not suitable for your samples, there is an other option you can consider;

Pre-treatment to remove chloride – using Hach’s kit for Chloride elimination you can remove chloride from samples, and then use the Mercury Free COD vials. This system uses Silver Oxide instead of Mercury Sulphate. You should note that it may cause low COD readings in some samples, so running your own comparison study (as described above) is recommended before making the change.

Hach’s Chloride Elimination kit uses Silver Oxide instead of Mercury Sulphate to remove Chloride from samples prior to analysis


For more information just follow the links above, or contact us with any questions about COD analysis;

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You mention that if Mercury Free COD vials are not suitable for my samples, there are two other options, but explain only one. What is the other?
David Miller
Hi Rafael, the other alternative was a new technology that is no longer available I have updated the blog