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Because of the variety of the soap recipes available, I would like to pour the lye/alkali/base solution in the oil mixture slowly and periodically check for pH so I can eyeball it with safety.

As an indication that I've put in too much lye, would you recommend using pH paper? As a countermeasure, I can neutralize it with more oil or vinegar/lemon juice.

What alternative indicators I can use in this case as well?

Due to inexperience I will use the process that requires no heat.

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Unfortunately, this strategy won't be much help. pH is affected by both how active the alkali is and its concentration. When the concentration goes down by a factor of 10, the pH goes down by 1. Even when the reaction is complete, soap will still register as alkaline because it is made from a strong base and a weak acid; it never gets to neutral even though all of the free ions are bound with each other. At best, pH is a crude measure for this purpose, not really accurate enough to measure what's going on in the mixture.

It's also complicated by the fact that the reaction can take days to complete and the soap changes consistency. The reaction also slows down as the unreacted ions become scarcer. Measuring pH at the time you mix the ingredients doesn't give useful information. You can't measure the concentration of the unreacted oil to know what's left. Because the reaction takes a long time, trying to add lye a little at a time over the course of days presents the same problem as putting it in at the beginning, but then you're also contending with the evolving soap mix.

If you go too far with the lye, you can't easily neutralize it with acid because soap will never be neutral and still be soap. The acid changes its properties even when the soap is still a net alkaline. Adding more oil puts you back in the situation described before.

Experience will bring more familiarity with what the soap, itself, is telling you. Aside from additives that don't substantively affect the soap reaction, a big reason the recipes are different is that different kinds of oil require differences in the lye. The best strategy is to start with a proven recipe and don't make any substitutions, or look up the lye requirements for the specific oil you want to use, and make a batch. If that batch doesn't turn out perfect, tweak the recipe the next time.

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