[roundar]

Assuming that a minor fluctuation in pH is acceptable how would I go about determining the required amount of HCl?  Would it be 1:1 HCl : NH3?

[typhoon2028]

You definitely would want to have dilute HCl.

[roundar]

Yeah, figured I'd get 1:500 HCl.  If someone else can verify it's use/amount.  Will HCl react w/ Sodium thiosulfate?

[typhoon2028]

A simple search:

"Thiosulfate anion characteristically reacts with dilute acids to produce sulfur, sulfur dioxide and water:[1]

Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H2O

This reaction is known as a "clock reaction", because when the sulfur reaches a certain concentration the solution turns from colourless to a pale yellow. This reaction has been employed to generate colloidal sulfur. When the protonation is conducted at low temperatures, H2S2O3 (thiosulfuric acid) can be obtained. It is a somewhat strong acid with pKas of 0.6 and 1.7 for the first and second dissociation respectively."