Lactic acid solution at pH below 3. Safety and preservative need

Abdullah

I have made this product and 5 people including me have used it for a month daily or once each two days without any problem or side effect. 

It is a 5% lactic acid solution at pH below 3. It is just 5% lactic acid in water with 0.1% tetrasodium EDTA.

My questions are 

1. Is it safe to use 5-10% lactic acid at pH around 2.5? 

2. Does it need preservative if i use only lactic acid and EDTA in water and pH below 3?

3. If it is not safe to use then, is it safe to use this product and then apply a 2% sodium salicylate solution on top of that to increase its pH when on skin or apply a moisturizer lotion on top of that to increase its pH when on skin?

vitalys

1. It is safe to use 5-6%. 10% is used as a pretreatment before stronger skin peels. 
2. It needs a preservative due to high water content. 
3. The neutralization step is not required at concentrations you use. The skin has its own ability to recover pH here. Re: moisturizer. 5% Lactic acid has excellent moisturizing properties. You may formulate the moisturizing lotion with 5% LA. 

Abdullah

@vitalys ; 5% lactic acid at pH 2.5 has 4.8% in acid form and 10% at it's pka as pretreatment which is common in market has 5% in acid form. aren't they almost the same? 

vitalys

It depends on a formulation of the pretreatment product and how much of the free acid has been neutralized, which in turn is defined by the skin types including the determination according to Fitzpatrick system by a dermatologist. They are not the same. Are we discussing free acid or neutralized/partially neutralized acid in the product? 

Abdullah

@vitalys we are discussing the active acid that is working for exfoliation. 
As the salt part don't exfoliant and only the acid part exfoliates the skin so let's talk about it. 

There are a lot of 10% lactic acid at PKa pH which means 5% is in salt form so only 5% in acid form is working as exfoliant. Now if we use 5.2% lactic acid at pH 2.5, there will be 5% in acid form. So will these two products have the same exfoliating power as they both have 5% lactic acid in acid form to do exfoliation? 

vitalys

If 10% solution is 1/2 neutralized  - means that active acid is 5% which is the same with 5% LA solution of free acid. However, this weak solution won't show the pronounced exfoliating power (if any) but efficient moisturizing effect. You need higher% to get some efficient exfoliation or effect. Otherwise you need to use 5% solution for a very long period of time. 

Abdullah

vitalys said:

If 10% solution is 1/2 neutralized  - means that active acid is 5% which is the same with 5% LA solution of free acid. However, this weak solution won't show the pronounced exfoliating power (if any) but efficient moisturizing effect. You need higher% to get some efficient exfoliation or effect. Otherwise you need to use 5% solution for a very long period of time. 

This is what other companies were using. For example The ordinary has 5% and 10% lactic acid pH at PKa so they have 2.5% and 5% in acid form. I haven't seen any stronger acid for home use.

What percentage lactic acid in acid form do you suggest to be effective and safe for home use? 

vitalys

There are plenty of peeling products that contain 50- 70% of the acid for home use. But... There is one trick the manufacturers use - they almost completely neutralize the acid making the product almost inactive. In fact, they sell Sodium lactate or Potassium Lactate, etc. The concentrations they claim is used for marketing. Another story with the preparations for professional use by cosmetologists/dermatologists who use various concentrations from 10 to 70. Those preparations always have highly informative descriptions and TDS almost like descriptions for medications ( in fact, professional peels are medical devices). TDS for those professional use products always declare the % of free acid, pH ( sometimes along with pKa), the recommended neutralizing agent or product, the method of use, etc. 
All those peels for home use can only ensure the moisturizing effect and very mild, almost elusive exfoliation to avoid any complications among users. That is why the range of concentrations in peels for home use rarely exceeds 10% (if we talk about LA). Effective concentrations in skin peeling are always equal to ability to cause the controlled injury and inflammation, which in turn lead to desirable and visible long term skin improvements.

Microformulation

https://www.fda.gov/cosmetics/cosmetic-ingredients/alpha-hydroxy-acids

Abdullah

@Microformulation as i am not in us, i don't have to follow this rule. My question is about safety side. Is it safe to use an AHA lactic acid product with lower pH because there are some products that has lower pH?