Hi,
I am doing adsorption tests on nitrates and ammonium in different pH and in Deionised water however the analysis might be delayed. Would freezing the sample be enough to stop any reaction happening in the tubes for few months prior to analysis?
I was thinking putting 2 drops of chloroform to stop microbial reaction (even though in DI water it should be pretty safe).
Dear Peirre:
Several reactions could take place in your samples. Besides there crossed reactions between salts of nitrate and ammonia, consequently it is better if you use fresh samples. Freezing or chloroform are not enough to stop all of these processes. Ammonia could become easily oxidised just in presence of oxygen at several pHs.
I fully agree with Dr. Rosa, you can not keep both nitrates and ammonia solution for a longer time. With the addition of chloroform you may stop partially some microbial growth but it can not stop the oxidation. Therefore, better to use fresh samples.
Thank you for both your answers. It does not sound good then if I have to even wait for few days for the analysis... Then I should exclude chloroform and do the tests right away. I have seen that HACH has a spectrophotometer DR3900 that seems appropriate for this type of analysis. Have you ever done these kind of analyses with it?
Dear Pierre,
Rosa and J.C. offer good advice. The USA reference "Standard Methods for the Examination of Water and Wastewater" (from APHA, WEF, and AWWA) recommends fresh samples, but has allowed storage at 4 C after acidification with H2SO4 (this converts any nitrite to nitrate, though). The instructions are to use 0.8 mL conc. acid/L for ammonium, making sure the resulting pH is 1.5-2; or 2 mL/L for nitrate. I believe our state regulations allow storage for up to 2 weeks (but it might be 1 month) in this case. Good luck with your work!
Peter
Thank you Peter for your answer, the problem residing in the fact that the samples will be frozen for months before analysis and this is a no-no according to the previous comments. I have thought about the issue with nitrite but in a way I wanted to see how much of these species were taken up. If I do my experiment at pH 7 or 9 for example, when I dissolve ammonium Nitrate, only a part will be nitrate in the actual water. THen either I acidify it at the end and check how much of the total nitrate+nitrite has been taken or I measure at this specific pH which will give me only the concentration removed from the water at this pH.
Nitrite form of N is very unstable. It is not present in this form for a long time, unless there is an inhibitor present inhibiting the transformation of nitrite to nitrate. Best is to analyse the nitrite+nitrate fraction and the total mineral-N. Subtracting the former from the total mineral would give you an approximate value of the ammoniacal form of N. Basically, in the aerated conditions, N is predominantly in the nitrate form. The Mulvaney and Bremner's methods could be adopted for your analyses.
- Badges
- Science topic