Ask A Scientist Chemistry Archive

name         Eugene Y.
status       other
age          40s

Question -   I am making cold-process soap (oils and fats plus sodium
hydroxide)as a craft and was told by fellow soap-makers that I should PH
test the soap after curing to make sure that all the sodium hydroxide in
it has been neutralized.  I agree this is necessary to make sure it is
safe to use, especially if I want to give-away soap to friends.

Given that soap is a soft solid, pH test strips do not seem to be a good
choice since I would need to liquify the soap to take the reading which
would result in an improper reading.  I have also read that soap, being a
very strong surfactant, can adversely affect pH readings, though this was
in the context of using the pH test strips.

First, what is an acceptable pH range for hand/body soap?  I have found
numbers anywhere from 7 (I do not see how this would be possible) all the
way up to over 9.

Second, will a 1% phenolpthalein or 1% cresolpthalein test solution
provide a good reading or will it also be adversely affected by the
surfactant nature of soap?  If this does not work effectively, what would
be your suggestion.

Thanks a whole bunch.  I apologize if this is not the correct forum for
these questions but I have been hard-pressed to find anyone else who can
give me a straight answer.

Soap-makers suggest using a *tongue test* whereby the *taste* the soap on
the end of their tongue to see if there is a *sizzle*.  First, this scares
the hell out of me - chemical burns on the tongue are NOT on my top ten
list of things to experience in this life and secondly, this can be a
horribly inaccurate way of judging a soap safe or not.  Besides, I doubt
many people actually like the taste of soap, myself included.

If you cannot answer my questions, maybe you can direct me to the right
place.
------------------------------------------------
My soap making experience has been with fairly large (hundreds to thousands
of gallons at a time) batches.  For testing, try dissolving 10 grams of
soap (about a tablespoon) in 2 ounces of a water/isopropanol solution
[Prepared by mixing 3 parts by volume of 70% isopropyl alcohol (rubbing
alcohol) and one part of distilled or R.O. water.  This 50/50 solution
efficiently dissolves soap and caustic, but not free oils/fats]  Test this
solution with phenolphthalein.  If the color is dark pink and the solution
clear, you have excess caustic - add a little more oil and cook your batch
some more.  If it is colorless to yellowish but clear, your pH will be
fairly neutral.  If it is hazy, you have unreacted oil that you may want to
cook away with caustic.  If it is hazy AND pink, it is not done reacting
yet - cook it a little while longer and retest.  Very pale pink and clear
may indicate good results.  In fact, to conserve your batch and create less
waste, you could shake a few drops (or a pea-sized lump) of your soap in a
test tube half-full of the 50/50 IPA/H2O and a couple of drops of the
phenolphthalein solution for these tests.

Now about pH.  You want to cook the soap until the above test solution is
clear.  pH can then be tweaked a little to suit your need.  You can lower
it by adding citric or phosphoric acid or raise it by adding caustic (it
won't take very much - add slowly.  If you can stomach the math, make 1%
solutions of your adjustment acid or base. add the 1% adjustment solution
to the test solution dropwise, then ratio the amount of undiluted addition
to your batch size.  Then add it a little at a time, testing between
additions).  With a clear solution, you could have a pre-adjustment pH
anywhere from about 5 to about 12 (if you don't oil-down a strong pink
color, it could be much higher!)  For body soap, a pH from 5 to 8 should be
mild to skin.  Higher pH values generally mean better cleaning of greasy
and/or oily soils.  A pH from about 8 to 10 seems fairly optimal for hand
soap.  If the soap is for laundry, a pH of 10 to 12 is acceptable.  If you
use phenolphthalein, it will be colorless in the body soap range, faint
pink in the hand soap range, and deep pink in the laundry soap range.  You
may want to use pH test strips in the pH adjustment stage to dial in your
desired pH level.  Do you see now why many soapers use the taste test they
learned from grandma?  (Excess oil tastes greasy, excess caustic tastes
(too) bitter.)  I hope this helps!

Tim Spry
========================================================
My understanding is that lye soap will generally give you a pH of about 9.
Much above that will mean that not all of the lye has been consumed in the
saponification.

For a cheap, fast, and moderately accurate pH test, your best bet is to use
pH paper.  An indicator solution will only tell you a borderline value - is
the pH above or below the indicator pH?  pH paper will give you an idea of
how far above or below.

What I recommend is putting a little bit of the soap in a drop of water (or
putting a drop of water on the soap) and touching the pH paper to the water
drop.  For your own calibration information, test the color response of the
paper to lye solutions of varying concentration, and to some soap that has
been deliberately spiked with lye (do not ruin a whole batch of soap - just
use a weak lye solution in place of the water drop for the pH strip).  Then
you'll get an idea of what color the paper should be when testing soap.

The tongue test probably will not hurt you unless you test soaps for hours on
end or neglect to rinse your mouth afterwards.  Soap tastes lousy (as anyone
who's ever had his mouth washed out with soap knows), so you are not likely
to leave soap in your mouth long enough to cause injury.  But, like the pH
paper method, you would have to do some calibrations - how would you know 
if the
soap has too much lye unless you know what too much lye tastes like?

The effect of concentration or surfactant on accurate pH readings would only
be a concern if you are trying to get reading accurate to with 1/2 a pH unit
or so.  For that, you would be using a glass electrode pH meter, 
calibrating it
with standard buffers, and worrying about things like ionic strength.
That is not your situation.  You just want to know if you have excess lye.
You will see from your calibration experiments that even a small
concentration of lye gives a very high pH reading, so it will be easy to
detect.

Ask again if you run into problems.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
========================================================
Eugene,

It is practically impossible to remove enough of the sodium hydroxide from
home-made soap to render it safe to use -- or to offer as a gift. It cannot
be neutralized -- only "salted out" (at best, an unreliable removal
process).

I agree, the tongue test is dangerous. The alkali would make short work of
one's tongue.

I offer the following safe home concoction that makes a nice gift. Following
are the details of a lab experiment I developed for use in my organic
chemistry classes as an end-of-semester reward. I hope you find it to have
potential. Alas, the "composition table did not format into columns -- but I
think you can figure it out.

===

Preparation of a Simple Hand Cream

Introduction:

Although cosmetics are used to make us look, feel, and smell better, no one
disputes the fact that a not inconsiderable amount of money is spent on body
chemistry. Cosmetic scientists are learning more about the interaction of
chemicals with the human skin. As a result, perfumes and colognes have been
designed to react with individual body chemistries to produce aromas that
are (more-or-less) unique to the person who wears the essence. Like the
clothes that appeared a different color under the store lighting than they
do in daylight, one's body chemistry and its interaction with cosmetic
chemicals can sometimes cause surprising, and not always desirable, effects.
Given the complexity of human skin chemistry it is remarkable that cosmetics
perform as well as they do.

Certainly, in a practical sense, no matter how you may feel about the
benefits and efficacy of cosmetics, they are only helpful when they assist
the skin in carrying out a normal function -- that is, in elimination of
wastes, cooling the body by radiation (thus aiding in maintenance of normal
body temperature), and protecting it from moisture loss and sun damage. Any
skin-care product that clogs the pores with a heavy insoluble material, or
restricts sweating by astringency, is detrimental to health. Think about it.
How many cosmetic products you know of meet those simple criteria without
failing in one or more other respects?

Skin creams contain a variety of ingredients. Each is selected to give a
specific effect on the skin or to produce a specifically desired product
characteristic such as adding thickness or color, improving ease of
application, raising or lowering the melting point, adjusting the pH,
emulsifying the oil/water components, softening the skin, or providing
various medicinal properties.

In this classic activity you will prepare a simple hand cream from five
ingredients. You will study the function of some of the parts by leaving one
of them out in subsequent preparations to observe the change in physical
properties that results from its omission -- thereby deducing its purpose.

Composition:

Prepare the creams as described in the procedure and table. Be sure to read
the Special Note before proceeding with the preparation.

Ingredient Mix #1 Mix #2 Mix #3 Mix #4 Mix #5

stearic acid** 5.0 g 5.0 g 5.0 g 5.0 g none
lanolin** 3.5 g 3.5 g 3.5 g none 3.5 g
mineral oil 5.0 g 5.0 g none 5.0 g 5.0 g
triethanolamine 1.0 mL none 1.0 mL 1.0 mL 1.0 mL
water 24 mL 24 mL 24 mL 24 mL 24 mL
perfume/color* ?? ?? ?? ?? ??

** Refer to Special Note below
  *optional ingredients (see part 4)


Procedure:

Special Note: The masses of ingredients in the table above are correct.
However, because the lanolin is so sticky and difficult to measure out, you
will portion it by using the (lanolin) labelled spoon.

One full spoon of melted lanolin is just about the 3.5 grams required.
Likewise, apportioning the granular stearic acid is facilitated by using the
(stearic acid) labelled spoon instead of the lab balances. One heaping
spoonful of stearic acid is just about the 5.0 g required.

Triethanolamine, TEA, is soluble in water; mineral oil is not. Please do
make a special effort to ensure that the TEA medicine droppers are kept with
the beakers marked TEA and are used for triethanolamine only. Note: one
millilitre (1.0 mL) of TEA is about twenty drops.

Use the cylinders marked "MO" for the mineral oil only. Anything put in
these cylinders with the mineral oil will make a mess that must be washed
out with a test tube brush and detergent. If you accidentally foul the
cylinder, ask your instructor for assistance on how to proceed.

Please clean up spills as soon as they occur. The lab detergent works very
effectively in the cleanup of your glassware.

Mix # 1 is called the "control" preparation because it contains all of the
ingredients, each in the proper amounts. This will be the only successful
preparation because a different ingredient will be purposely left out of the
preparation for each of the other mixes.

The class will be divided so that each group of (about four) students will
prepare one control sample and one of the other preparations. Share your
results with all the other groups so they and you may complete the data
necessary to the conclusion section below.

1.  Use the quantities listed in the table (see Special Note on the previous
page). Here is how to prepare Mix #1: Place the stearic acid (one labelled
spoonful), lanolin (one labelled spoonful), and 5 mL of mineral oil in a
clean dry 150 mL beaker. Heat it on a boiling water bath until the contents
have melted into a homogeneous syrup. Set this beaker aside on a paper towel
so that it stays melted while you prepare the TEA/water in the next step.

Note: Cosmetic ingredients should never be heated with an open flame because
they tend to char or decompose when overheated.

2.  Place 24 mL of distilled water into a separate beaker, add twenty drops
(1.0 mL) of TEA and one drop of food coloring (if you want a colored
product), then heat the solution of  TEA/coloring/water on the water bath.

3. After those ingredients have been warmed for about 5 minutes, be sure
that the materials you melted together in step #1 are still liquid, then
slowly pour the oily materials from step #1 into the triethanolamine/water a
little at a time while stirring constantly. If you pour too fast or fail to
stir it uniformly, the emulsion obtained will be lumpy and rather
unattractive.  Continue stirring the warmed mixture until a smooth, uniform
paste is obtained.

4.  Should you wish to prepare a perfumed cream, add a drop of your favorite
essence to the cooled product and stir it well.

Report Date __________   Name __________________________
Lab Section ______    Partner/s __________________________



Preparation of a Simple Hand Cream


1.  Describe the function of each ingredient based on the properties of the
products you prepared.

stearic acid:

lanolin:

mineral oil:

triethanolamine:


2.  See note below. Try a small dab of Mix # 1.  Is it absorbed easily by
your skin?

3.  Why should the perfume (if desired) be added to the cooled mixture
rather than at the time when all the other ingredients were blended?

4.  Does the un-perfumed product smell all right?

A final note:  Some people are sensitive to cosmetic chemicals. Test any you
wish to use on the inside of your arm just above your elbow. Place a tiny
drop on your skin, cover the area with a band-aid overnight. If no redness
is produced, the cosmetic should be reasonably safe for you to use.
Discontinue use of any cosmetic that causes any kind of irritation.

====

Regards,
ProfHoff 394
========================================================
After reading your additional description of the cold process, I think I
understand a little better what you are doing.  I agree about the pH
adjustment - it must be done AFTER the reaction is complete, or it gets in
the way. The IPA/water solution will certainly be cloudy at a 50:50 dilution
with that much excess fat/oil.  It works pretty well when making high-purity
soap by the kettle-cooking method.  When I was in that business, we had a
few gallon-size glass jugs with black marks about half-way up.  We would add
anhydrous IPA to the first mark, water to the neck, about 3mL of
phenolphthalein solution and then 0.1N NaOH dropwise while stirring until
the solution was a faint pink color.  This way, the natural (relative)
acidity of the alcohol would not cause the test solution to produce false
results.
One way to find a dilution better suited to your process is to flake a bar
or so of high-purity commercial soap and then add the appropriate amount of
your fat/oil blend to get the amount of excess fatty acid you normally try
to achieve (a sort of artificial standard - you might want to do this for
each of your levels of free FA).  If this preparation dissolves to clear in
the rubbing alcohol, then add water (dropwise, stirring between) until it
develops a very slight haze (text read through sample is less sharp than
when read through water at equal volume/level in container of same
geometry).  (Then note to add just less so it will be clear for good
product.)Keep track of additions, because you will want to scale this up for
a stock solution.  It's no fun to reblend every time, plus you get the
benefit of equal performance from test to test.  If the preparation is hazy
in rubbing alcohol, the try starting with the 99+% stuff, probably available
at the hardware store.  If not, your local chemical distributor likely has
containers as small as 5 gallons.
You are correct about the previous dilution numbers.  I was trying to
propose something easy-to-blend from easy-to-obtain materials, but still
close enough to a known item that it would still work.  The test must work
well enough to provide useful information.  That is what we are going for
here.  As I recall, rubbing alcohol is in fact a 70% v/v solution as
regulated by the United States Pharmacopaeia.  Since IPA has a density of
about 0.787 compared to water, it is approximately a 65% solution w/w.  BTW,
solutions of IPA this strong are quite flammable (low flash point) so having
a clean gasoline-grade storage container for it may be a good idea.  Since
your product is designed to be quite a bit long on fat/oil, you might also
consider running a free fatty acid titration.
If you're interested, I'll dig it out of my files for you.  You may find it
easier.  Does this help?
=========================================================