If the stock solution is prepared in NaOH, I would expect to use this stock to work under pH 7-14. If you are going to work in HCl, i.e. acidic solution, it sounds logically to make the stock solution in HCl. BUT, the electrochemical conversion and therefore stripping is often sensitive to pH. It means, if stripping works out in basic solutions, it doesn"t always mean, that it will be the same in acidic. The electrochemistry of arsenic species is pH dependent.

Thank you Nataliya.  I have been using a 0.5M HCl throughout to conduct the experiments.  So can I take as the pH is fixed (constant) across?  Even I felt it is logically OK to use just HCl to prepare the solutions.  But I was stuck when I have to use 0.1M and 1mM HCl for the experimentation.  I mean, if I were to prepare 100ml of 0.1M (or 1mM) HCl, there is no way I can dissolve arsenic trioxide in the amount of HCl required to prepare 0.1M (or 1mM) HCl.  Generally what would be the approach in this case?  I do not want to prepare 1000 ml of the solution just to increase the amount of HCl required.  Moreover, I would end up using just 20 to 40 ml of the prepared solution for experimentation.  Thank you again for your time.

http://www.bucksci.com/catalogs/EPA%20Method%20-%20206_2%20-%20Arsenic%20AA%20-%20Furnace%20Technique.pdf

You can better verify with vogel inorganic qualitative analysis book. pls see the attachment

It dissolve easily with NaOH then the pH can be adjusted using HCl OR Sulphuric acid

For a related RG discussion, see: https://www.researchgate.net/post/Is_there_any_way_to_make_ArsenicIII_oxide_soluble_in_water