[Procyon]

I need advice on the advantages and disadvantages of each protocol to measure thiosulfate levels, in order to incorporate one of the protocols.

Probably the spectrophotometric determination by oxidation with iodate or HPLC.

I can't find the protocols approved by EPA. Which one is it?

[Arkcon]

http://lmgtfy.com/?q=thiosulfate+epa+analytical+method

[Procyon]

http://lmgtfy.com/?q=thiosulfate+epa+analytical+method

Thank you. Typing "google it" or "use google" would have been less work.

The main question was about incorporating an spectrometric or HPLC based methodology.

[Arkcon]

http://lmgtfy.com/?q=thiosulfate+epa+analytical+method

Thank you. Typing "google it" or "use google" would have been less work.

The main question was about incorporating an spectrometric or HPLC based methodology.

Oh, I understood what you wrote, and the little Google animation wasn't just to remind you you can Google for methods.  Because when you wrote:

I need advice on the advantages and disadvantages of each protocol to measure thiosulfate levels, in order to incorporate one of the protocols.

OK, someone who's done them all may offer some input, if they've really studied the topic.

Probably the spectrophotometric determination by oxidation with iodate or HPLC.

OK, you can Google for those specifically.

I can't find the protocols approved by EPA. Which one is it?

You can if you follow my Google cartoon.  You seem to be asking, without saying the words, (so I feel like I have to do it for you,) that you want an EPA approved protocol, using the methodology that you want to perform.  It is possible for that not to be the case.  The EPA (or any other regulatory body) can possibly only approve one method.  You might be able to prove equivalency between your chosen method and the EPA approved method.  But that's up to you, and your work -- and the EPA will want to see it done in your facility.  And will want to see your plan beforehand with regard to what you plan is if it turns out your method fails at an inopportune moment.