Please check this researchgate link.

https://www.researchgate.net/post/how_to_estimate_the_a_mount_of_NA_K_and_Cl_fro_plant_leaves

Plants should be separated into stem, roots, branches and leaves. The stem will further be separated into two parts upper and lower halves, while leaves should be separated into three groups, basal, middle and upper leaves for complete understanding of the pattern of ion partitioning under imposed salinity stress. The finely ground plant material of each part should be digested in conical flasks separately with 10 ml of 30% HNO3. The digested material should be cooled, diluted with distilled water and filtered through Whatman No. 42 filter paper. The content of K and Na in the samples and standards should be estimated using K and Na-specific filters in a flame photometer. Three replications should be used for measuring ion concentrations, which will calculated as mg g−1 dry weight.

You can consult any book on Plant Analysis .This involves digestion of whole plant samples or any other index plant parts in diacids , mostly followed , then the plant extracts to be diluted according to the standards used , and later subjected to their analysis on flame photometer as broad steps involved....  

please check the link

https://www.researchgate.net/post/how_to_estimate_the_a_mount_of_NA_K_and_Cl_fro_plant_leaves