So I sat on what you said DeeAnna, and quite frankly, after doing a little more reading, it didn't compute with me. One moment I had the "A-Ha! I get what she means now!" And next, I was stumped all over again. So, today I had a conversation with a good friend of mine about it. As she herself said, she's no "soap doctor". But she loves her science, and does have 7 years of under graduate and graduate study in science and biology. She was majoring in biomedical engineering. She explained to me what you were getting at and after a half hour chat, between her 7 years, with my bit of experience in biology and medicine, plus soap making, we came to some very definitive conclusions, and I'd like to see what everyone's thoughts are.
To start, the intrinsic argument seems to be a bit misrepresented here. While yes, soap does have it's own, based on what kind of soap it is. This pH is brought down to a more neutral level through soap making. In the end, the intrinsic pH doesn't have a huge effect, and it just means we are starting with a product with a higher pH, that needs to be brought down. However it may not be brought down as low a preferable. That being said, the pH is still a solid, measurable number that means the same thing, no matter what is going on inside. a pH of 11 is still dangerous and can kill your skin cells. No matter what. A higher pH number just means that there are more basic atoms (atoms that have extra electrons) than acidic atoms (atoms that need extra electrons). A neutral pH means they effectively cancel each other out. Put simply, 11 is not skin safe, no matter what, and it should be tested adequately. It's a common concensous that soap cannot attain a pH below 9 without falling out of solution. So that it what we attempt to attain through proper soap making methods and testing.
Now lets look at the testing methods. We al know that the 'zap' test is the universal method in soap making for determining if the finished batch is 'lye heavy' or excessively alkaline (high pH). If it zaps, it MUST mean it's lye heavy, correct? Not true. There are several factors that can effect this test quite easily and can give plenty of false positives/negatives at any given point. Let's start with the basics behind the test.
The reason soap would zap your tongue is because it is making a battery. You're mouth's pH is more acidic than your skin because of enzymes in your saliva that break down food. So, it may be true that the soap may zap your tongue but may not immediately irritate your skin, because there is LESS OF a pH difference between your skin (5 or 6) than the difference between your tongue(3 or 4) and the lye (10-12). However, using any product on your skin that is significantly different from your skin's natural pH level (while it may not be immediately apparent) could still cause damage to your cells. If the drops are turning very pink, that means the pH is still much higher than 7. While you should expect it to be a bit higher than 7, you definitely don't want it to be too high, tongue zap or not. Which is why, again, we attempt to attain a 9. So now, i've covered the science behind the zap test, let's look at why i can be inaccurate more often than not. If the soap doesn't zap, that just means the difference in pH isn't large enough to feel the electrical charge. It will only zap when the pH is a certain amount higher than the pH of the tongue. There has to be a big enough difference. So, while a zap says theres a big difference, it doesn't say how much. It isn't a quantifiable answer as to whether the soap is truly lye heavy or has a high pH. To put it in another perspective, back to one of my previous arguments, everyone's tongue is different. And it can be different on any given day. The mouth is already naturally acidic. It shouldn't ever be basic, unless you just drank something like milk of magnesia. Now when i mention the variables between each person, person A's mouth may be naturally more acidic than person B. And if they both do the zap test on the same bar of soap, person A is more likely to feel the zap than B, if B feels it at all. Also, we don't exactly know what the threshhold is for a zap to occur between the acidic mouth, and basic soap, so the soap could very well have a pH of 9. But it still zapped person A, because their mouth is more acidic. Now add in day to day factors like, eating or drinking, and that definitely changes things. A mouth with a present pH of 2 or 3 testing a soap with a pH of 9 or 9.5, is a huge threshhold/ gap and will most definitely zap. Me persoanlly, I do drink soda, which is acidic. Therefore, my mouth is going to be more acidic than the person who doesn't. My soap will more likely always zap me. Unless something else of variying pH was in my mouth prior to testing my soap, ie, water, food, coffee, alcohol, whatever. Now if a person's mouth were naturally a little higher on the pH scale, closer to neutral, or even neutral for that moment, testing the same soap, it won't zap. And again, because we don't know the exact threashold between an acid and base before the zap occurs, the pH of a soap could be a 10 or 11 and not zap either. But because the general procedure is, "no zap no lye excess/high ph", the soap maker won't accurately know the true pH of their soap, nor be able to take corrective measures.
To summarize: The closer in pH a person's tongue is to the pH of their soap, the less likely it will zap. The further away in range, the more likely. And without knowing the threshhold in Ph differences, at when the soap begins to zap, we can't definitively prove the accuracy of the zap test. It's results are not quantifiable to indicate 'skin safety" because there are too many factors that can effect when the soap will zap. But testing for pH using modern laboratory means, ie phenol drops, strips or a meter, are all quantifiable, in that you will get a number to work with. While it is understood that the zap test was the best way to determine doneness for some time, we do have more accurate means for testing safety.
As a scientist, my friend does not recommend the zap test considering there are more appropriate methods. As a soap maker, who soaps for her family first, and who would like to someday sell her goods, safety is of upmost improtance to me and based on my conversation with my friend, I am now more than reaffirmed that the zap test is outdated; that it is not safe. This -->>
http://en.wikipedia.org/wiki/Phenolphthalein, is more accurate in regards to testing pH of my soap, than a test that has many variables that can affect it's results. Even then, there are better methods, and I may just as well upgrade. A pH meter really isn't expensive at all. With that, there should be no reason to discourage any soap maker from using proper testing methods, no more than is should be to discourage one from using a preservative in their soap. But the latter is a whole different subject I'm not getting into at this time.
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