Phosporic acid is the most common preservative in fertilizers. The other 2 are either sulfuric acid or formic acid. All 3 not safe. Over time will cause disease when crops are consumed. Eating or smoking. Your body absorbs those harmful chemicals and can't purge them out so they build up and cause disease. There's several others too. Like ehylenediaminetetraacetic acid which is used as a chelating agents in most bottled nutes.
Phosphoric acid is known for causing skin cancer, eye disorders, kidney disease, and several respiratory and / or lung disorders. It's also in many processed foods and soda.
Phosphoric acid is not used as a preservative. It's used to provide phosphorus and also reduce the ph of the solution, as a 5.8-6.2 ph is usually best.
Phosphoric acid is actually used in a lot of sodas. Legally of course.
Sulfuric acid and formic acid are the only two on that list that are, for the most part, more harmful than phosphoric acid. But they can still be used responsibly.
Are you having the fertilizer analyzed to reverse engineer? That's totally cool. I think that's one shortcoming in my spreadsheet method. I just go by the labels. I get the impression the labels aren't accurate because they're minimum guarantees.
For example, some of the more flim-flammy multi-bottle products might completely mis-state those numbers to protect their proprietary "formula." I.e., people aren't buying the product for nutrient content--the quantity of K you get for you dollar. People buy it for the name-recognition, the schedule.
What does that cost to have it analyzed? (I've got one in mind I'd like to do.).
Contrary to popular belief, fertilizers can't have a proprietary and unknown blend. All the information on what the elemental makeup of a blend is easy to find. Either at the dept of ag or on the bottle. They are required to list this information, by law. The accuracy of these numbers are typically pretty good.
But yes, you are right that people buy it for name recognition. What I'm doing here is showing that name recognition is pointless.
The cost to get a water sample analyzed ranged from $50-100 or so, depending on what you want tested. A lot of universities do it. What bottled blend are you thinking about getting analyzed? I'd happily look into it.