If you wanted to be precise about it, how could one determine the optimal ratio of Sodium Benzoate to Potassium Sorbate in a solution? For example, I've seen 0.35% Na-benz to 0.20% K-sorb as a good starting point. Is Na-benz higher only to compensate for reduced disassociation?
Estimating dissociation at 50% and 75% respectively at my target pH of 4.2, that gives 219 ppm benzoate (438 ppm Na-benz) and 180ppm sorbate (240ppm K-sorb). Would having benzoate and sorbate at equal molarity be more antimicrobial than not?
If I start from 0.35% Na-benz and set K-sorb such that sorbate is equal to benzoate, then I end up with 0.24% K-sorb (by mass) instead of 0.20%. Is there any way to know if it's better or worse other than getting it challenge tested?
As an aside, the two research papers I've found that look into synergy with sodium benzoate and potassium sorbate turned up negative results. Is this effect even real? Do both of them together provide significantly more effect than using one by itself? (Which is to say, inadequate.)