 lot of overkill here, and my experience as a bench chemist informed me that small changes in the non-tomato ingredients in a salsa that is predominantly tomatoes will have little effect on the acidity of the salsa. Surely someone with some math skills and a bit of common sense can keep their salsa at a safely acidic level.
I found a study called Safe acidification of salsa for home boiling water canning from the Foods and Nutrition department at the University of Georgia. For some reason this group seemed to believe that it was necessary to bring the pH of the salsa to around 3.8, which requires 4 tablespoons of lemon juice per pint. Some quick calculations on the old Texas Instruments scientific calculator told me that a pH of 3.8 is over six times as acidic as a pH of 4.6, the threshold pH. I guess they wanted to make sure that the salsa would be sufficiently acidic no matter what ingredients were used. Their recipe called for 200 g tomatoes, 200 grams other ingredients and 60 milliliters lemon juice (4 tablespoons) which works out to about a pint of salsa. Their research showed that you could use the ingredients in any combination, even leaving out the tomatoes, and the addition of that much lemon juice produced a pH of about 3.8. In other words, nearly all the acidity in the salsa came from the lemon juice.
This publication by the North Dakota State University extension, Why add lemon juice to salsa before canning? has a lot of useful information. First of all they tested the acidity of different varieties of crushed tomatoes. The pH ranged from 4.95 to 5.20, but most tomatoes were about pH 5. Then they tested the pH of salsa made from the same varieties before and after the addition of lemon juice. Guess what, the pH of the salsa before adding lemon juice was actually slightly lower than the pH of the tomato pulp. Not much lower but lower, indicating that the additional ingredients really have little effect on the acidity (onions and peppers are also slightly acidic but not as acidic as tomatoes). The addition of lemon juice brought the pH down to a range between 4.14 to 4.30, well below the 4.6 threshold. It looks to me like the 4 tablespoons of lemon juice per pint used by the U of Georgia group is gross overkill.
But there’s a fly in the ointment here. The NDSU paper never said how much lemon juice was added in the study. They recommend adding one tablespoon per pint, but their own recipe in the same paper which makes 16 pints of salsa calls for 2 cups of lemon juice which works out to 2 tablespoons per pint.
I went back to the acidification curves from the Georgia paper, where they added lemon juice in increments to tomatoes, peppers and onions and then tested the pH. Interestingly they never did an acidification curve for the salsa, just the components so I considered the tomato pulp the same as salsa. To compare apples to apples I converted everything to percentages. A tablespoon of lemon juice per pint works out to 3.12%. In the Georgia study 5 mL lemon juice in 100 mL Roma pulp, the lowest amount tested, works out to 4.8% lemon juice, about 1.5 tablespoons per pint. That amount produced a pH of 4.2. A salsa with a pH of 4.2 is 2.5 times as acidic as a salsa with a pH of 4.6. Looking at the titration curves and estimating the lines between the points it looks like a tablespoon per pint gets the acidity to a pH of about 4.4. To be solidly on the safe side it seems to me that a minimum amount of lemon juice should be 1.5 tablespoons per pint. The take-home message is this: there is considerably more acidity in a tablespoon of lemon juice than in a pint of tomatoes.
That leaves one other question: Do you really have to use the bottled lemon or lime juice? The canned salsa recipes stress that the bottled version has a consistent acidity, whereas real lemons or limes will not, so you should always use the bottled lemon juice, according to them. Of course real limes or lemons will taste better. To answer that question there is a post Real Lemon versus ReaLemon by Linda Ziedrich, who has a chemistry background. She