With the help of spectrophotometric method, i have determined total nitrogen, nitrate and ammonia value are 17,15 and 14 ppm respectively, my question is how come it is possible if the concentration of nitrate and ammonia will be added up, then the concentration of total nitrogen would be approximately 30 ppm but i am getting only 17ppm.....
Why is it so?
I have followed phenate method for ammonia determination, and 2,6- xylenol method for nitrate and total nitrogen. The sample was first digested (persulfate digestion) for total nitrogen determination then followed by 2,6 xylenol method for determination of T.N.
Why not check then whether your total N determination works properly with a known concentration of let's say (NH4)2SO4. For the first sight it seems that there was a trouble with your total N i. e. with the digestion and you got only NO3 for total. If that is not the case, i. e. total N is OK, you can check then the phenate method with the above mentioned sulfate solution.
I suggest you use total kjeldhal nitrogen method to determine organic-N and ammonium-N together first. This method does involve a digestion process to digest organic-N component.
The total kjeldhal N method will not detect NO3-N hence any colorimetric/autoanalyser method can be used to determine NO3-N and same with assesing ammonium (NH4).
Once total kjeldhal N, NH4-N and NO3-N are all determined separately use the following equations to determine total-N.
Total kjeldhal N = NH4-N + organic-N
Total-N = Total kjeldhal-N + NO3-N
Selva's answer is a good method to follow. Our lab has been doing nitrogen speciation for over a decade, and in a word, its hard. Any time you are using multiple methods of analysis, errors compound. In particular, getting complete digestion of dissolved organic nitrogen is difficult. We use an OI Analytical Autoanalyzer to measure the concentration of total dissolved nitrogen (TDN), and it took two modifications of the method to get it so the numbers added up. In our case, we measure nitrate and nitrite using an ion chromatograph, and use a fluorescence method to measure ammonium. So for us:
TDN-(NO3-N+NO2-N+NH4-N) = DON
Also, what kind of water sample are you analyzing? Having both very high nitrate concentrations and very high ammonium concentrations together is a bit strange to me. High nitrate would indicate oxidizing conditions, and high ammonium would indicate reducing conditions - typically I haven't found high concentrations of both (I've worked with rivers/lakes/wetlands, groundwater, waste-water treatment effluent, and landfill leachate). The redox conditions of your sample environment may shed some light on the problem.
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