Diluting Lye Safely
Date: Spring 2009
I have been trying to find some information on lye. My class made
soap on a small scale but we are now trying to make it on a larger scale.
Part of the saponification process is mixing lye in distilled water. When we
did this on the small scale, there were no noticeable odor or fumes. When we
did it on a larger scale, some people claimed to smell something and saw fumes
upon mixing the two. Assuming that the ratio of lye to water was proportional,
were the odor and fumes the appropriate signs of the reaction? If so, is it
dangerous for anyone to be around these fumes? Also, besides not standing directly
over the beaker and breathing in the fumes, are there any safety precautions
that we can take to ensure that no one is hurt?
Lye is also known as caustic soda or Sodium Hydroxide. Its chemical
formula is NaOH.
Caustic Soda dissolving in water is an extremely exothermic reaction - a great
deal of heat is generated as the Caustic Soda is separated into its component ions
On a small scale this means that the water will get warmer. On a larger scale enough
heat can be generated to cause the water to boil - and this can be extremely
When dissolving NaOH in water the following precautions should be observed to
minimise the risk of boiling the solution.
Have Sodium Bicarbonate or vinegar available on hand to neutralise spills.
Use a large container with a wide mouth - (I prefer one that would seem to be
too large -
e.g. a 2 litre container to mix 500mls. The wide mouth prevents the
fountain effect in case you do boil the mixture.)
Use cool water - just less than body temperature - never hot or cold.
Always add flakes to the water - never the other way around.
Add the NaOH flakes a little at a time and stir at every step. DO NOT add all the
flakes at once.
Be aware of the temperature of the mix - if the mix become hot then allow to cool
before proceeding further.
Caustic Soda is extremely corrosive. It should be regarded as more dangerous than
most acid solutions.
If Caustic Soda is splashed or spilled on skin or eyes, wash with copious amounts of
water - up to 30 minutes may be necessary for strong solutions.
The fumes can also be dangerous. If inhaled they can cause irritation of mucous
membranes, or burns in the case of strong exposure. Always mix this in a hood with
proper safety protection including goggles, apron, and gloves.
As a laboratory technician I am regularly called upon to mix chemicals and prepare
solutions. I regard Caustic Soda as one of the most hazardous chemicals in the school
store. A solution of caustic Soda will do significantly more damage to your skin than
a similar solution of most acids - even hydrochloric. Just as it turns your lamb fat
or olive oil into soap, it will convert the oils in your skin and the fats under the
skin into soap - and that will result in major damage, not just in the immediate area
of the spill, but in surrounding tissues as well. Please treat Caustic soda as
Lye is sodium hydroxide or caustic soda. When it dissolves in water the energy released
from the hydration of the Na+ and OH- ions heats up the solution. Although this may not
boil the water as a whole the local heating around the dissolving NaOH can cause
caustic fumes that irritate the lungs and throat if inhaled or can seriously damage
the eyes if you get spray in them.
There are a couple of ways to avoid this. Firstly: add the NaOH (either a solid or a
concentrated solution) to the water, not the other way around. Secondly: you can use
ice/water mix instead of water or place the flask containing the water in an ice/salt
bath to keep it really cold. Finally: there is no substitute for a fume hood, I always
make up NaOH solutions in the hood, sash down with safety spectacles, lab coat and
NaOH is one of the most common chemicals in the lab, but it is also responsible for
some of the worst incidents. Please be careful when using it: NaOH in the eyes poses
a real risk of serious eye damage. To be honest, making up of NaOH solutions from
solid is best performed by your teacher or a qualified chemist.
Let us assume you are working with solid NaOH. Dissolving NaOH in water is an
exothermic process. The solution's temperature increases. If a student puts too
much NaOH into too little water, the temperature can rise to the point where some of
the solution begins to evaporate. I suspect your team mates have been observing this
process. To avoid repeats, just add the NaOH more slowly into a larger volume of
All team members involved at the lab station should be wearing goggles and gloves.
If aprons are available in your lab, put them on. It is best to use a hood. Be sure
to clean up any spills/spatters with lots of water and baking soda.
One of the most dangerous aspects of diluting lye, which is Sodium
Hydroxide (also called NaOH) is that it generates lots of heat. I have
seen the dilution generate enough heat to melt through the bottom of a
5-gallon bucket and cause a dangerous spill.
The fumes you saw were probably related to the heat causing some rapid
evaporation and generation of vapors.
There are many safety precautions that you need to take. First is the
use of protective eyewear, gloves, and aprons for everyone handling the
materials. Second, perform the dilution (and all chemical reactions) in
a well ventilated area. Opening doors and windows is good, but
performing reactions in a hood or outside when possible is much better. Third, to
mitigate the heat problem, dilute the lye into the water a little bit at
a time, letting it cool between each addition. Another method would be
to start the dilution in ice water or to put the reaction vessel into an
You did not indicate quantitatively what "small" and "large" scales were. Chemical
processes often do not scale up linearly, as you discovered. Sodium hydroxide (lye)
reacts vigorously with water generating a large amount of heat. Here are some
precautions to take:
1. Full face shield (Looks like a welder's face shield, but clear and colorless.
Available from any chemical equipment vendor.)
2. Do the experiment in a fume hood.
3. Have sodium bicarbonate on hand to neutralize any spills.
4. Use heavy duty nitrile rubber gloves and a full length rubber apron.
5. Use an ice bath to minimize the temperature rise.
6. Add the sodium hydroxide to the cold water slowly with continuous stirring.
7. Do NOT use any aluminum tools, etc. Aluminum reacts vigorously with sodium
hydroxide. One of the reaction products is flammable hydrogen gas.
8. The odor you detected comes from tiny droplets of water containing some sodium
hydroxide. This is a warning that the temperature is too high. That is, the addition
of NaOH is too fast. Obviously, inhaling these droplets is not a good idea.
9. Ask yourself, "Is a large batch of this reagent really necessary?" If you cannot
find a really valid reason, stick to the small batches. Lye is a dangerous
chemical. I assume yours is a chemistry class. Are your students sufficiently
prepared to handle such corrosive chemicals? There are of course legal liability
10. The mist generated by mixing lye and water should be your "warning bell" that
"new" things can start to happen when you "scale up" a chemical process.
11. My bottom line recommendation is not to try a large batch. Too many nasty
possibilities can be lurking in the shadows.
12. A unit on the safe handling of toxic and/or corrosive chemicals might be a better
But do not actually use a toxic and/or corrosive chemical.
The main issue you will run into in scaling up any reaction is that the
energy involved in carrying out the reaction, such as heat produced, is
proportional to the mass of the reacting materials. The surface area of
the reaction, through which energy is transferred to or from the surroundings,
is usually proportional to the 2/3 power of the mass of the reacting materials.
This means that when the reaction is carried out on a larger scale, it does
not transfer energy to the surroundings as quickly. If the reaction produces
heat, it will tend to get hotter (attain a higher temperature) at large scale
for this reason.
Dissolving lye in water releases much heat. When the reaction is scaled up,
it gets even hotter. Most likely, what your students observed was a mist of
lye solution in the air, produced by the heat from the lye solution.
These fumes are dangerous, because lye in water is dangerous. Their chief
hazards will be from inhalation and eye contact, with some possibility of
injury from skin contact as well. If the dilution is carried out slowly, so
that the temperature of the solution is minimized, the hazards will be slight.
If you are concerned, covering the vessel with a watch glass (or even a piece of
paper or cardboard) will reduce exposure to the mist. Carrying out the dilution
in a fume hood will provide even greater protection, but may be difficult
Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012