Dear Sir. Concerning your issue about the best acid digestion to analyse the NPK of wastewater by flame photometry. Nitric acid will digest most samples adequately (Section 3030E). Nitrate is an acceptable matrix for both flame and electrothermal atomic absorption and the preferred matrix for ICP-MS.4 Some samples may require addition of perchloric, hydrochloric, hydrofluoric, or sulfuric acid for complete digestion. These acids may interfere in the analysis of some metals and all provide a poorer matrix for both electrothermal and ICP-MS analysis. Confirm metal recovery for each digestion and analytical procedure used. Use Table 3030:I as a guide in determining which acids (in addition to HNO3) to use for complete digestion. As a general rule, HNO3 alone is adequate for clean samples or easily oxidized materials; HNO3-H2SO4 or HNO3-HCl digestion is adequate for readily oxidizable organic matter; HNO3-HClO4 or HNO3-HClO4-HF digestion is necessary for difficult-to-oxidize organic matter or minerals containing silicates. Although dry ashing is not generally recommended because of the loss of many volatile elements, it may be helpful if large amounts of organic matter are present. I think the following below links may help you in your analysis:

http://kpatco.com/Download/SM-Metals_0408.pdf http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/15/030/15030051.pdf http://ijates.com/images/short_pdf/1471866540_ijates_147.pdf

Thanks

Thank you very much. Now i have some clear idea.

The best thing to do is to perform optimization. Analyze NPK at varying volume ratio of the combined acid.