It's been a while since I had my Hanna out, but I don't recall using a syringe tip for stirring or for dispensing both water and reagent from one syringe as being part of the instructions...........
Utmost to the argument of returning unused reagent to the bottle is good laboratory practices. HERE is a reference, page 109 (which should be the first page you see), bottom of the page, item 5…..”Never return any excess chemical or reagent to a bottle.” Again, it’s just the right thing to do to totally eliminate the chance of contamination of the mother reagent.
Secondly, I wasn’t talking about Hanna’s, which I believe are handheld spectrophotometers whereby you empty a small packet into a cuvette, where there is no returning of unused material to the mother source. I’m talking wet chemistry titration test kits where you withdraw reagent from a bottle and slowly add this titrant to a mixing bottle which contains tank water and typically a second “coloring” reagent. Here is a photo of the Red Sea mixing bottle and you can clearly see that the blue syringe tip is within the mixing bottle. That blue tip is “contaminated” with tank water, coloring reagent and reacted titrant.