Suppose I have prepared initial concentration of 10ppm(100 ml). After maintaining pH (2-6) by 1M NaOH and 1M HCl , it's original concentration decreases to a great extent, so what can be the possible reason behind it? Kindly give me some proper suggestions.
Assuming your reagents, water, etc. are all pure -- you might try some of the suggestions in https://www.researchgate.net/post/What_is_the_standard_method_for_preparation_of_a_stable_ferrous_ion_solution_I_want_to_know_what_the_best_reagents_ratio_for_this_preparation_are and try addiing sacrificial dithionite or ascorbic acid.
Assuming you have pure (NH4)2Fe(SO4)2 dissolved in water, you cannot adjust such unbuffered solution using NaOH or HCl, the pH is very unstable and will depend mostly on a little bit of carbon dioxide (so H2CO3 in water) present.
Depending on the purpose of such dilited Mohr's salt soilution start with a buffer in pure water, e.g. at a concentration of 0.01 mol/L to get a steady pH. If you can use it, citric acid sodium salts cover the pH range of about 2 till 6 and are very cheap.
Yeah, I wondered what the "using NaOH and HCl" was all about. If you're adding strong NaOH you're likely to locally precipitate your Fe as hydrated iron oxide/hydroxide. I'd assume that NH4+ should provide some acidity anyway, if the Mohr's salt is anhydrous, i.e. fresh?
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