pH Testing on various soaps

[McSpin]

You say it is lye free as you've been using it, but say that it might have had lye even when it didn't zap. So it might well still have lye, even though it doesn't burn during use.

True, it may still have a very small amount of lye in it, but with the pH falling as low as it has, I'm betting it would be a barely measurable amount. After 48 hrs, it obviously had lye in it, even though it did not zap. The pH was quite a bit higher and that has to be from the lye. I'll find out at 4 weeks if the pH falls any further. I'm guessing it won't be a difference worth noting.

Have you made a hp batch to control any pH changes during curing and so on?

No, I haven't done any HP testing. I'm not sure what this would test. I don't understand the mechanism that causes the excess lye to dissipate during a CP cure, or how HP affects this reaction. I may experiment a bit with it down the road, but next up is a 30% lye-heavy soap to see what happens.

 

[LBussy]

I'm not sure what this would test. I don't understand the mechanism that causes the excess lye to dissipate during a CP cure, or how HP affects this reaction.

Dr Dunn has been quoted here as indicating excess lye is converted to sodium carbonate (soda ash) via a process involving atmospheric CO2 dissolving in the water to produce Carbonic Acid:

CO2 + H2O <>(is in equilibrium) H2CO3

That weak acid reacts with the lye to form sodium carbonate and water:

2(NaOH) + H2CO3 = Na2CO3 + 2(H2O)

In some other threads (there is a long discussion about high water lye excess Castille) it's further hypothesized that the excess water is not only the transport for the CO2 but the transport for the lye to more freely migrate from inner portions of the soap to the exterior where it may react with the acid.

So there's a fair hypothesis how/why the lye dissipates.

When we have higher lye concentrations (earlier in the cure) there is more "pressure", more lye available to react so that reaction takes place quicker. I'm not a chemist but in thermodynamics heat wants to seek equilibrium. I envision chemicals want the same thing. When there's more lye there's more "pressure" to react with the environment and those reactions go quicker. When there's less the "pressure" decreases and reactions are slower. That likely explains why you see a slower decrease in pH over time - although a titration to determine free OH would likely be a better measure. pH is a product of that other reaction.

A longer cure time may also lower the free OH by allowing it to finally combine with the remaining oils. If we have "leftover" lye, we know we have "leftover" fatty acids since we superfat, then those are free to react as well. That reaction slows as there are less of both available to commingle and react.

Aside from the higher availability of reactive substances, chemical reactions can be sped up by adding energy (heat). When we HP we allow reactions to take place quicker. Certainly saponification is faster in HP, the other reactions may be faster as well.

So, there's a hypothesis for what causes the lye to "dissipate" and how HP would affect the reaction.

 

[MorpheusPA]

0.4 PPM is the accepted limit for tasting pure chlorine in water--so 400 parts per billion. For arsenic, 0.3 PPM.

For good old table salt, you can't easily detect it under 250 PPM (250,000 PPB). Although individual detection limits will certainly vary, and there's a natural ability called "super taster" that enhances what you can detect.

We're not actually tasting the lye, which should be very salty in flavor, so we know our detection is well under 250 PPM. We're detecting it because it creates a salt bridge between our taste sensors, creating a "zap" of electrical potential (or, the same reason we zap on a 9 volt battery). You get the same reaction chewing on a piece of aluminum foil if you have fillings.

So while I don't know, let's just set the detection limit at 0.3 PPM (300 parts per billion). That seems safe for an initial assumption and might be high.

That's still 300,000 PPT (parts per trillion), or three hundred thousand times the amount acceptable for finished soap (which is 1 PPT).

It's not going to take your hide off. It won't render you abraded and bleeding in the shower. But it's not going to be very nice soap and it's high enough to be detectable in a pH test

 

[McSpin]

That's still 300,000 PPT (parts per trillion), or three hundred thousand times the amount acceptable for finished soap (which is 1 PPT).

So, the acceptable limit is 1 part per trillion, interesting. So, the zap test is no where near accurate for determining safety of a soap.

 

[McSpin]

Dr Dunn has been quoted here as indicating excess lye is converted to sodium carbonate (soda ash) via a process involving atmospheric CO2 dissolving in the water to produce Carbonic Acid:

CO2 + H2O <>(is in equilibrium) H2CO3

That weak acid reacts with the lye to form sodium carbonate and water:

2(NaOH) + H2CO3 = Na2CO3 + 2(H2O)

In some other threads (there is a long discussion about high water lye excess Castille) it's further hypothesized that the excess water is not only the transport for the CO2 but the transport for the lye to more freely migrate from inner portions of the soap to the exterior where it may react with the acid.

So there's a fair hypothesis how/why the lye dissipates.

When we have higher lye concentrations (earlier in the cure) there is more "pressure", more lye available to react so that reaction takes place quicker. I'm not a chemist but in thermodynamics heat wants to seek equilibrium. I envision chemicals want the same thing. When there's more lye there's more "pressure" to react with the environment and those reactions go quicker. When there's less the "pressure" decreases and reactions are slower. That likely explains why you see a slower decrease in pH over time - although a titration to determine free OH would likely be a better measure. pH is a product of that other reaction.

A longer cure time may also lower the free OH by allowing it to finally combine with the remaining oils. If we have "leftover" lye, we know we have "leftover" fatty acids since we superfat, then those are free to react as well. That reaction slows as there are less of both available to commingle and react.

Aside from the higher availability of reactive substances, chemical reactions can be sped up by adding energy (heat). When we HP we allow reactions to take place quicker. Certainly saponification is faster in HP, the other reactions may be faster as well.

So, there's a hypothesis for what causes the lye to "dissipate" and how HP would affect the reaction.

Thanks for the explanation of the theory, Lee.

 

[Dorymae]

So, the acceptable limit is 1 part per trillion, interesting. So, the zap test is no where near accurate for determining safety of a soap.

I disagree, as he said it will be safe. It won't harm you, although it may not be great soap.

You will never have great soap without a good cure. The cure is what is the great equalizer. Just time. However it would be safe.

 

[MorpheusPA]

So, the acceptable limit is 1 part per trillion, interesting. So, the zap test is no where near accurate for determining safety of a soap.

Acceptable for sale, anyway. A zap test would be unacceptable for determining whether a soap is acceptable for sale, but that's why we cure for four to six weeks minimum and super-fat.

As far as safety is concerned, lye isn't that much of an issue. We use it to make bagels crispy and pretzels hard and crunchy. If you eat lutefisk (I avoid it like the plague as I simply don't care for it), it undergoes a lye soaking.

Excessive amounts can burn, but if you can't detect it on a zap test, it's not going to damage your skin. There's no danger. Your skin is far less sensitive than your tongue is. Will it be a bit drying? Sure. Will it hurt you? No.

We're using a biological apparatus that simply didn't evolve to detect lye, it just happens to do so as a spandrel (biological definition). It's not surprising that it's not completely accurate, just as your tongue can't tell you if water is completely safe.

Advanced testing can tell you exactly how much free lye is in your soap, but these would be difficult to do at home for most of us.

 

[LBussy]

We're using a biological apparatus that simply didn't evolve to detect lye, it just happens to do so as a spandrel (biological definition).

Okay, I'll admit it - I looked that one up.

 

[McSpin]

Acceptable for sale, anyway. A zap test would be unacceptable for determining whether a soap is acceptable for sale, but that's why we cure for four to six weeks minimum and super-fat.

As far as safety is concerned, lye isn't that much of an issue. We use it to make bagels crispy and pretzels hard and crunchy. If you eat lutefisk (I avoid it like the plague as I simply don't care for it), it undergoes a lye soaking.

Excessive amounts can burn, but if you can't detect it on a zap test, it's not going to damage your skin. There's no danger. Your skin is far less sensitive than your tongue is. Will it be a bit drying? Sure. Will it hurt you? No.

We're using a biological apparatus that simply didn't evolve to detect lye, it just happens to do so as a spandrel (biological definition). It's not surprising that it's not completely accurate, just as your tongue can't tell you if water is completely safe.

Advanced testing can tell you exactly how much free lye is in your soap, but these would be difficult to do at home for most of us.

This make sense to me, but it also reinforces my thoughts on zap testing being pretty much worthless (pH testing a known formula isn't much better in my opinion). Since, I'm waiting six weeks or more before using the soap, what possible purpose could putting my tongue on it serve? I already know that the soap will not look or feel right if it was a lye-heavy soap at the beginning. None of us can test to the less 1/ppt standard, so why bother with any of it? Wait six weeks and if the soap looks and cuts normal, then use it. If it looks like it might have been lye-heavy, then re-batch it or use it as a laundry soap.

 

[McSpin]

Okay, I'll admit it - I looked that one up.

I was a biology major and I don't remember that one coming up, but then again, it was over 40 years ago.

 

[DeeAnna]

Just a caution... In industry and science, "ppt" means parts per trillion. In Dunn's Scientific Soapmaking book, "ppt" means parts per thousand.

Yes, really.

So be careful with numbers in his book if they're stated in "ppt".

 

[LBussy]

so why bother with any of it? Wait six weeks and if the soap looks and cuts normal, then use it. If it looks like it might have been lye-heavy, then re-batch it or use it as a laundry soap.

Because it takes 2 seconds, because things do go wrong and they are not safe at 6 weeks (meaning it's a bad batch at that point).

Because most soaps are zap-free after 48 hours, testing at that point as a relative indicator has value for an experienced soaper I would think. If every other time your soap was zap-free at 24 hours and this one is not at 48, you know something changed. That's good info.

The funniest (and most disgusting) analogy I ever heard was from a rather colorful individual I know. He said "you look at a pile of poop sitting there" (he used a different term) "and it's just poop. You look at YOUR poop every day, you know what's normal and what's not." In the same way licking one bar of soap may tell you nothing. Doing it to all your batches does give you information.

 

[McSpin]

Doing it to all your batches does give you information.

Just as doing a pH test to all your batches will give you information, and a lot more than a lick will. Though, the "only takes 2 seconds" does have merit.

 

[MorpheusPA]

This make sense to me, but it also reinforces my thoughts on zap testing being pretty much worthless (pH testing a known formula isn't much better in my opinion). Since, I'm waiting six weeks or more before using the soap, what possible purpose could putting my tongue on it serve? I already know that the soap will not look or feel right if it was a lye-heavy soap at the beginning. None of us can test to the less 1/ppt standard, so why bother with any of it? Wait six weeks and if the soap looks and cuts normal, then use it. If it looks like it might have been lye-heavy, then re-batch it or use it as a laundry soap.

It's a useful test for people who aren't sure, or where a recipe might be just slightly lye-heavy without showing the characteristic crumbling.

In my case, I mostly consider it insurance. No zap? It won't hurt me, guaranteed, and hand-wash testing can begin any time I like.

Excess lye should neutralize with the air or excess fats during cure, so by week 8 even severely lye heavy soaps should be approaching usability (assuming you used enough water to transport the lye through the structure).

 

[The Efficacious Gentleman]

For hp, it can mean cook longer or not, but that is a different thing

For cp, if it zaps when it shouldn't it can prompt you to double check something and if you need to rebatch for some reason (forgot an oil or something) then it might be easier to chop the softer soap up for melting rather than grating the hard soap.

 

[MorpheusPA]

Just a caution... In industry and science, "ppt" means parts per trillion. In Dunn's Scientific Soapmaking book, "ppt" means parts per thousand.

Yes, really.

So be careful with numbers in his book if they're stated in "ppt".

How lovely.

I'd find it surprising if 1 part per thousand were acceptable for soap; that would still give a pH of 11 even in pure water without any other considerations. And given how reactive sodium hydroxide is, there shouldn't be a part per thousand of the stuff left in any reasonable time period. Certainly not after four to six weeks of cure.

 

[Susie]

Why zap test every time?

Because this soap is going to people I love. It takes me just a couple of seconds, and I KNOW, beyond any shadow of a doubt, it is safe. If it zaps at unmolding and cutting, it needs a rebatch. It is also free.

I also get to troubleshoot right away what went wrong with that batch. I learn more from the mistakes if I know about them quickly when I can still recall interruptions and such.

 

[McSpin]

What has stuck with me most during this experiment was that there was no zap after 48 hours on the 10% lye-heavy soap, yet visually, the soap was so different that I instantly would have known something was not right (if this wasn't an experiment). The soap was so hard and so brittle that it was obviously not what would have been expected. I'm guessing that all soap is safe after a cure, but that doesn't mean it's a good soap.

To me, the most reliable information came from my eyes, not my tongue. In the end, no one really knows if they have a good soap until they use it, and for more than one test wash. I can see where a zap test may give a raw beginner somewhere to start, but a beginner with 50 batches (me) under their belt, should be able to easily detect a problem by sight with anything that would have created a zap at 48 hrs.

 

[DeeAnna]

Morpheus -- Your comparison doesn't apply -- we're not dealing with water; we're dealing with soap. Apples and oranges. Soap chemistry is closer to protein chemistry, not water.

Dunn wrote his book for freshman chemistry students and handcrafted soap makers. He knows this audience cannot work to the level of accuracy and precision typical of an industry or research lab, so his data is presented accordingly. Parts per thousand is decent accuracy for kitchen chemistry.

Lastly ... it works. You can speculate and theorize and criticize all you want, but real-life experience says otherwise.

 

[DeeAnna]

McSpin -- Have you split a bar and tested the center? My experience shows the exterior may be fine after a relatively short time, but the proof of full cure for a lye-heavy soap is when the center is fine too.

And, no, some soaps do remain lye heavy more or less indefinitely. One soaper in the 2014 "superlye" castile experiment made the lye-heavy soap but used a "normal" amount of water. The bars are still lye heavy in the interior according to recent comments she's made.