The limit test I'm referring to is USP method II. In this procedure you need to dropwise add 6N of NH4OH to the sample until it the pH is alkali. After that you need to dilute and add HOAc until pH 3-4. Why do you need to add NH4OH if you're going to turn the pH to acid almost immediately? My first thought was to form a NH4OH/NH4Cl buffer because the step prior the addition of NH4OH was adding HCl, but I have no explanation for what the use of that could be.
Please look at the following below links which may help you in your analysis:
http://www.dimmittsulfur.com/MSDS/Ammonium_Hydroxide,_Solution_NH4OH_SDS_NA+MEX_081914_FINAL2.pdf
https://www.cir-safety.org/sites/default/files/ammoni122017rep.pdf
https://www.pharmaguideline.com/2011/08/limit-test-for-heavy-metals.html
https://www.who.int/docs/default-source/medicines/norms-and-standards/current-projects/qas18-769-heavymetals.pdf?sfvrsn=fd51b69c_2
Thanks
NH4OH is added as a neutralization substance/chemicals.
NH4OH is used as a neutralization substance/chemicals, but its not always preferred due to cost and safety reasons. Instead sodium hydroxide is used.
https://www.uspnf.com/sites/default/files/usp_pdf/EN/USPNF/revisions/231-rb.pdf
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