I want to make jelly from a mixture of 50% fresh apple juice and 50% juice of another fruit. The resulting pH must be less than 4.6, otherwise the jelly will not be safe from C. botulinum.

Based on my research, apple juice generally has a pH between 3.3 and 4.0 (Let's say 4.0 to be conservative), so it's safe for home canning. The other juice is less acidic (a pH of ~ 6.0).

I plan on mixing equal parts (by volume) of the two solutions (pH 4.0 and 6.0); I need to know if the resulting pH will be less than 4.6.

I found a claim that if you mix equal parts of 2 acids, the resulting pH will always be the lower pH plus 0.3. So if that's true, the answer to my question is 4.3 (safe from botulism, but I need to know if this claim (and my subsequent result of pH 4.3) is correct!

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    $\begingroup$ You can't reliably calculate pH this way. There is too much going on at once. The claim you found might be a good approximation when working with pure individual compounds, which the juices are not. $\endgroup$ – Ivan Neretin Nov 27 '17 at 7:54
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    $\begingroup$ This. You have most of the periodic table in a glass of juice, literally thousands of reactions. Just prepare a sample and measure the pH. $\endgroup$ – SF. Nov 27 '17 at 8:13
  • $\begingroup$ I agree with @SF , there is nothing like measuring the pH; and different batches of juice can have different pH's to start with, so there is no universally valid formula. However, the concept of adding 0.3 sounded intriguing, so I did some calculations, and I don't think the rule is correct. For instance, if you assume that the solution with the lower pH contains the strongest acid, and ignore all weaker acids, then doing a 1:1 mix of the two solutions is like halving the concentration of the strongest acid. But that would result in a pH increase of about 0.15, not 0.3... Bizarre... $\endgroup$ – user6376297 Nov 27 '17 at 18:00
  • $\begingroup$ Here's an 'exact' calculation. Juice A is a 0.3 M solution of an acid with pKa = 3. Juice B is a 0.3 M solution of an acid with pKa = 5. pH of A = 1.774. pH of B = 2.763. pH of the 1:1 mixture of A and B = 1.927. pH of the 1:1 mixture of A and water = 1.930. 1.927 - 1.774 = 0.153. It would be interesting to see the effect of different initial concentrations of the two acids, of their difference in strength, of the presence of salts of the acids (in cases buffers should form...). Not obvious at all. $\endgroup$ – user6376297 Nov 28 '17 at 16:57