Ask A Scientist Chemistry Archive

name         Karen H.
status       educator
age          40s

Question -   My question pertains to Chemistry. In studying different chemicals to balance pH levels we are looking at the chemical, Potassium Hydroxide.
In of itself we had learned that it is a very corrosive chemical, but have learned that it is used in soaps and face creams as a pH adjuster. How can this be? Is it safe at this point?
Can potassium hydroxide is reacted and converted to a potassium salt, which is neutral? What is this process?
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Karen,

Even though potassium hydroxide in pure form is very corrosive and merits caution in handling, its solution in water may be diluted to an extent that it will have lost much of its capacity to do harm.

In addition, potassium hydroxide is a base. As such, it can be neutralized with an acid. For example, potassium hydroxide may be neutralized with acetic acid as shown here:

KOH    +    HC2H3O2    ===>    KC2H3O2    +    H2O

The compound KC2H3O2 is potassium acetate the salt of potassium hydroxide. It is far less corrosive than potassium hydroxide.

Regards,
ProfHoff 639
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You are exactly correct.  Potassium hydroxide, KOH, reacts with acids to form salts, which may or may not be neutral.  It all depends on how basic the "conjugate base" of the acid is.  The reaction goes like this:

  KOH + HA --> KA + HOH (water),

where HA is some acid.  If the system is in solution in water, KA does not really exist; instead, the K+ and A- aren't directly bonded to each other, and they only are found near each other to maintain an average neutral electric charge.  A- is the "conjugate base" of the acid HA.  It is called a base because it can consume a proton,

 H+ + A- --> HA.

Stronger acids (ones that most readily give up their protons) have weak conjugate bases (ones that less readily consume protons), and weaker acids (ones that less readily give up their protons) have stronger conjugate bases (ones that more readily consume protons).

So when KOH is used to neutralize an acid, the acid's conjugate base A- is produced.  The OH-, which is the strongly alkaline part of KOH, is converted to water.  That is why KOH can be safe when it is combined with other substances.

A little bit more goes on when KOH or NaOH (sodium hydroxide, or lye) is used to make soap, but the end result is the same: the caustic OH- is consumed, and the sodium or potassium salt of an acid is produced.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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1.)
KOH (potassium hydroxide) is very similar to NaOH.  Either one of them can be used in the manufacturing of soaps, a process referred to as saponification.  Some sort of triglyceride, or generically speaking, a fat is hydrolyzed by the strong anion (OH-).  The ester (triglyceride) is broken up into a "3-legged" alcohol and a "poor mans" soap.  Here is a related link:

http://chemistry.about.com/library/glossary/bldef825.htm

Yes it is VERY safe at this point.  Provided that the reaction has been carried out to its completion.  Or maybe even a little excess triglyceride to ensure full consumption of the STRONG BASE, NaOH or KOH.

2.)
Yes KOH can be reacted with HX (Where X = Cl, Br), as follows;

KOH + HCl --> H(2)O + KCl  (salt water)... safe if pH = 7 (stoichiometry precisely followed in a rxn)

Similarly,

NaOH + HCl --> H(2)) + NaCl  (salt water)...safe if pH = 7 (stoichiometry
precisely followed in a rxn)

If you want to try this reaction I recommend you use 1M (1 Molar) solutions of each acid and base.  The reason for this is because this reaction will give off quite a bit of heat at high acid / base concentrations.  If you dont know what one molar is than I do not recommend you do the experiment. Also, it might help to have some litmus paper standing by to check the pH of the solution.

Regards,
Darin Wagner
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 Both acids and bases may or may not be corrosive depending upon: 1. The concentration 
 and 2. What other substances are present. Most dilute acids and bases are not 
 inherently corrosive. Vinegar is about 5% acetic acid, which is pretty safe; however, 
 50% acetic acid is quite corrosive. The same is true for bases. If the soap/face cream 
 has a pH of say 2.5 during preparation it might be desirable to increase that pH to 
 say 6.5 (of course pH=7.0 is neutral). This adjustment could be done with KOH provided 
 quality control was being monitored so that the pH didn't end up at 9.5 which would be 
 very alkaline and probably not very easy on the skin. The toxicity / hazard of 
 chemicals is determined by its inherent toxicity -- somethings you just do not want to 
 have around -- but also by its concentration. In the case of KOH it is being used to 
 neutralize excess acid produced in the formulation.
 
Vince Calder
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