WEF Discussion Forums
Laboratory Management and Technical Issues
I have a question about COD and I was wondering if anyone could shed me some light. I use an ISO method for doing the COD analysis, which says that I only need to shake the bottle of my sample before doing the analysis. However,I have a customer who claims that if I use a homogenizator I would have completely different results. We did double checks and it seems that the values I take are higher than then values he takes by using homogenizator. I dont believe that it makes that big difference the homogenizator but I would like a third opinion to be sure. So, what do you think?
The Hach method says to homogenize the sample. If you are pipetting 2 mls into the vial it would be best to homogenize "chunky" samples in order to get a representative sample for analysis. I have experimented with this and gotten higher results with homogenization than not homogenizing. I use a household blender for this.
I use Lovibond vials and I think he is using Hach. The thing is that I take higher results than he is. I check every time my method with CRM and it is always within the acceptable limits. It would be more logical if I was taking lower values, right?
The results of homogenizing the sample depends upon the sample. If your sample contains some volatile compound then homogenizing might expel that organic from the sample thus resulting in lower values.
Most samples will result in higher values as "chunks" and oils are uniformly dispersed in the liquid and a more representative sample pipeted into the vial for analysis. I have seen this with a vegtable oil discharge. Without homogenizing the sample the results were erratic and mostly lower than when homogenized. Homogenized oil is less likely to adhere to the pipet than larger droplets.
I am writing back to give all the feedback I got the last few weeks about the COD homogenization and to make one more question about COD.
We borrowed a homogenizator and made the analysis both ways:with and without homogenization. The result of the homogenized sample was a bit lower than the result of the sample that was just shaken in the bottle, but the difference between the two was not significant.
So now, our client says that the problem is that we make dilution for doing the analysis. How that could be a problem when the ISO method for this analysis says clearly that if it s needed you have to make a dilution?Besides the vials I am using are according to that method.
Give your client a copy of your procedure. If the capacity of the COD vial is exceeded there is no other choice but to make a dilution. Or you could use SM 5220B Open Reflux Method without sample dilution.
You can find ultrahigh COD vials for up to 10,000 mg/L to which you add 0.2 mls of sample instead of diluting sample into lower capacity vials. Pipetting 0.2 mls of a sample presents its own problems however, especially if the sample is nonhomogeneous.