Measuring pH in cp soap

[katz2711]

Hi,

I just came across this article about measuring pH values correctly.

http://www.millersoap.com/phtome.html

It kind of blew my mind that there are such differences in how you measure.

Does anyone have input on this? Is the article trustworthy in what it states?

I just bought myself pH strips and my cp soaps measure btw 7 and 8. I also have used phenolphthalein before but thought it was not enough to just know if it is above 10 or not.

So how do you guys handle this? Any input is most welcome! :smile:

 

[shunt2011]

You really can't test CP soap with ph strips as they are not accurate. Soap will be 8.5-11 ph. The best way to check your soap to see if it's safe is to zap test it. If it doesn't zap you are good to go lye wise. That's what you will find most of us do.

 

[sassanellat]

You really can't test CP soap with ph strips as they are not accurate. Soap will be 8.5-11 ph. The best way to check your soap to see if it's safe is to zap test it. If it doesn't zap you are good to go lye wise. That's what you will find most of us do.

Hmmm. That makes no sense to me at all. If you use distilled water and dilute, pH strips should work perfectly. There are also pH meters out there that will do the same job (even better, but they are more expensive). As a lab scientist, there really isn't much in life that's going to get me to stick part of my chemical reaction in my mouth to check it for completeness. I like my undamaged tongue quite well. And yes, I know that's the everyday person test, but even so I'll stick to my pH monitoring tools. The hydroxides dissolve readily in water, and even in the presence of buffering fatty acids should readily show in the pH of the diluent.

The article above seems to support this. Don't go with the roll of paper stuff, get the plastic strips with the five to seven 'tiles' on them.

 

[LBussy]

I have a meter because some strips are not going to do well with soap. I also know a CP soap is not going to come out at 7-8 unless it's been neutralized with something. All that being said it's more than enough to know there's no zap as a final sanity check. A person can measure to the hills and back but I think for the average Joe it's a waste of time. There's such a large difference in unreacted vs reacted soap that anything else seems a true waste.

I get the lab background, don't hurt your tongue thing. But, I think that's just a mental hurdle, not a true issue.

 

[sassanellat]

I have a meter because some strips are not going to do well with soap. I also know a CP soap is not going to come out at 7-8 unless it's been neutralized with something. All that being said it's more than enough to know there's no zap as a final sanity check. A person can measure to the hills and back but I think for the average Joe it's a waste of time. There's such a large difference in unreacted vs reacted soap that anything else seems a true waste.

I get the lab background, don't hurt your tongue thing. But, I think that's just a mental hurdle, not a true issue.

I understand and respect your opinion, but to me, it's not a 'mental hurdle' to go with the actual science: the chemical test safely shows the truth every time. I have a device (strips or meter) that requires no opinion and doesn't have a bad day, so my sanity check (as a not everyday person) is the same objective one required by law for all commercial products. One of the most basic rules of chemical (or biological) safety is to never put anything in your mouth. You wear gloves and goggles when using lye for a reason, and a 'zap' is your taste buds dying on part of your tongue.

Of course, cost is always a consideration, it bothers some people more than others, your tongue heals, and the zap test more or less works. It's just something that I'm never going to do, or recommend. YMMV.

 

[katz2711]

Thanks for the responses everyone.

I'm not 100% comfortable to rely on a zap test when someone else is supposed to use the soap I made. So overall it looks like a pH meter is a good investment.
Does anyone have a recommendations regarding pH meters?

It is kind of ironic because I purchased those pH strips at a known soap supply store and was wondering about the results testing almost neutral, which is why I looked into this topic in the first place. Not happy about the wasted money...

 

[shunt2011]

By all means do what works best for you. However, this topic has been discussed many times so if you do a search you will find more information. I stand by my first statement. Zap testing works best for me. I've been doing it for 4+ years and my tongue is no worse for wear. Only been zapped once.

 

[Seawolfe]

When I hear that someone's CP soap is measuring pH 7-8, my bet is on the test failing - regardless of if its strips or meters.
A meter is only as good as its last calibration, and the proper dilution of the soap - and that still only gives you pH, not whether or not theres any excess lye or lye pockets. A zap test WILL tell you that.

I've never heard of anyones tongue actually being damaged by a zap test, but I suppose anything is possible.

 

[DeeAnna]

Katz and Sassanellat -- The pH measurement of soap DOES require interpretation whether one looks for a phenolphthalein "blush", pH strip color change, or a meter reading.

The pH of a neutral soap (no excess alkali, no excess fatty acid) will vary depending on the types of fatty acids in the soap and can range roughly from 8.5 to 11. Stearic and oleic acids (olive oil, lard, palm, tallow) have the highest pH and pKa values of the fatty acids commonly found in soap. The pH and pKa are lower for the shorter chain saturated fatty acids (myristic, lauric from coconut oil, PKO) and for the longer chain unsaturated fatty acids (linoleic, linolenic from sunflower, safflower, etc.).

The upshot is the pH that is perfectly fine for one particular soap is going to indicate excess alkali for another, so pH is really not a reliable method of ensuring a skin-safe soap. See attached -- the info is for commercial soaps, but the point is still valid for handcrafted soaps as well.

The industry accepted test for excess alkalinity (too much lye) is not pH; it is an acid titration for alkalinity -- in other words, the test is a direct measure of excess alkalinity. This test tells the maker exactly how many milligrams per liter of free alkali (lye) there is in the soap. It can be done, with some care and practice, by the average person in the kitchen. Scientific Soapmaking by Kevin Dunn describes how to do it. In lieu of that, the zap test is actually a good empirical measure of excess alkalinity and it has been used by soap makers for centuries.

Having spent plenty of time in a research lab and having a degree in chemical engineering, I know all about the reservations about putting unknown chemicals in one's mouth. In the lab, I would respect that utterly because I really cannot know what's in that beaker or test tube. When it comes to soap that I personally have made, that is a quite different situation. I'll do a zap test without reservation and I trust the zap test to tell me more about excess alkalinity more than I trust pH measurements to do the same.

 

[katz2711]

DeeAnna: thank you for taking the time to put together that information. That was very helpful and much appreciated! I'll give the zap test a try.

shunt2011: I entered "pH" in the forum search, but nothing came up.

 

[DeeAnna]

The problem with most soapers who use pH testing ... and I'm not including you, Sass, in this criticism... is that they want the cozy feeling of a "safe pH number" rather than truly understand what they're doing and why. When they don't get the pH result they want, they jump through all kinds of major hoops to "fix" their soap so it has an "acceptable" pH. And then they wonder why their soap is soft or separating or whatever. <sigh> IMO, pH testing has ruined far more soap than has been saved with all the fiddling about.

I do want to correct:
"...free alkalinity is directly correlated to pH in aqueous solutions..."

This would be correct given a fixed fatty acid composition. That means for a given recipe, you most certainly can correlate pH with excess alkalinity. If you make recipes with widely varying fatty acid content, there will be no accurate correlation between the pH measurements for the various soaps vs. excess alkalinity.

 

[DeeAnna]

For better search results, go to your favorite search engine (google, for example) and type the key words you want to search for. Follow them with:

site:soapmakingforum.com

and you will get some decent results. Example:

pH testing site:soapmakingforum.com

 

[shunt2011]

Thanks DeeAnna I was just going correct myself and state this.

 

[IrishLass]

Thank you once again DeeAnna. You explain things so well and so thoroughly. If I haven't said so before, I truly value your contributions here. I've got so many of your posts bookmarked now. lol


IrishLass

 

[Susie]

I have a file on the computer named DeeAnna's Nuggets of Knowledge, and one named IrishLass' Soap Info. You both rock!

 

[Meganmischke]

I am not judging you for your decision not to zap test your soaps but I think you might be over complicating this. To my knowledge taste historically has been a valuable and valid chemical property. Your tongue too is an important chemical sensor. Personally, for me there is no more simple or effective way.

 

[IrishLass]

Yes, the context of not ever sticking a potent base in your mouth ever... as in international standard context. :wtf:

If I were touching the raw, undiluted base to my tongue, then that would be different, and under those terms I have no dispute with the 'international standard context', but the zap test is far different from touching undiluted base to one's tongue.

In a zap test, the soap (which is greatly diluted by water and oils) is lathered with water, and then the resulting suds are briefly touched to the very tip of the tongue for one or two seconds at most. If the soap has any unreacted lye in it, the tongue will readily detect it without the tongue being damaged.

For what its worth, chemist Dr. Kevin Dunn has a 2.5 page chapter regarding the tongue test in his book, 'Scientific Soapmaking' (pages 76-78 ), and while he makes a point in the chapter to acknowledge the 'international standard context', he considers the tongue test to be a legitimate test for detecting unreacted lye in soap.

IrishLass

 

[Seawolfe]

Im a biologist too, and I sometimes eat the study of my subjects in the field and raw! Being an ichthyologist does have its perks

 

[LBussy]

Im a biologist too, and I sometimes eat the study of my subjects in the field and raw! Being an ichthyologist does have its perks

You study Icky things and then eat them?!

 

[Sinful7]

My goodness lots of "measuring" going on today. You are both super smart, can we go back to soaping now? If you want to lick it go ahead, and if you don't, don't neither side will ever convince the other .. No matter how many PhDs are involved.