[Back to the main article: Functional Ingredients: Hydrating Substances]
Before you read further: 60% sodium lactate solution can be found in all cosmetic ingredient supplier shops, and it’s not even one of the super expensive functional ingredients. If you are not feeling confident with chemistry yet, and in particular with acids and bases, don’t prepare this product at home. Do it only if you already have some confidence with sodium hydroxide (maybe because you make soap) and if you already know something about chemistry and acid-base reactions.
Sodium lactate is formed by neutralisation of lactic acid with sodium hydroxide (NaOH).
This is the reaction:
The molar ratio between acid and base is 1:1.
To calculate the amount of each component, we need to know the molecular weight of each reagent.
- Lactic acid: 90,08 g/mol
- NaOH: 40 g/mol
For example, to make the reaction of 0,2 mol of lactic acid with 0,2 mol of sodium hydroxide, we will need 18 g of lactic acid and 8 g of NaOH.
How do you know? The calculation is a pretty simple proportion: for lactic acid, 90,08 g : 1 mol = x g : 0,2 mol; x g = 0,2 * 90,08 / 1 = 18 g. Same calculation for NaOH.
Now, if we carry the reaction with these quantities, we will obtain 0,2 mol (22,4 g) of sodium lactate, + 0,2 mol (3,6 g) of water. As you can see, we cannot obtain a 100% sodium lactate solution: water will inevitably form.
In addition, we must consider that we cannot make the reaction between 100% pure lactic acid and NaOH solutions: we will have some solvent (water) to take into account.
Specifically, lactic acid is never provided as 100% pure solution: it is typically provided as 80% or 90% w/w solution.
- If we have the 80% solution, to have 18 g we will need 22,5 g of this solution (18 * 100/80).
- If we have the 90% solution, to have 18 g we will need 20 g of this solution.
NaOH is typically sold as pellets and we must dissolve them in water in order to carry the reaction. NaOH is very soluble in water and we can play a bit more on the quantity of water to be used, depending on the final concentration of sodium lactate that we desire.
Sodium lactate is typically sold as 60% w/w solution, so let’s try to do the same.
The amount of water we have – without considering the NaOH solvent – is:
- 3,6 g of water generated by the neutralisation.
- 4,5 g of water present in the 80% lactic acid solution, OR 2 g of water present in the 90% lactic acid solution.
- In total, we have 8,1 g of water (if we use 80% lactic acid), or 5,6 g of water (if we use 90% lactic acid).
To reach a 60% concentration, the 22,4 g of sodium lactate that we produced should be dissolved in 14,9 g of water. This means that we must add 6,8 g of water (if we use 80% lactic acid) or 9,3 g of water (if we use 90% lactic acid). This is the amount of water that we can use to dissolve NaOH.
So, in summary:
- If you use 80% lactic acid solution:
- 22,5 g of 80% lactic acid solution
- 8 g of NaOH dissolved in 6,8 g of water
- Yields 22,4 g sodium lactate + 14,9 g water (3,6 g generated by reaction, 4,5 g present in lactic acid solution, 6,8 g used to dissolve NaOH)
- If you use 90% lactic acid solution:
- 20 g of 90% lactic acid solution
- 8 g of NaOH dissolved in 9,3 g of water
- Yields 22,4 g sodium lactate + 14,9 g water (3,6 g generated by reaction, 2 g present in lactic acid solution, 9,3 g used to dissolve NaOH)
Let’s see how to do this in practice:
- Protection equipment: wear gloves and protective eyewear. Be sure that you can make this process in a place where free air circulation is allowed. I do it outside on the balcony.
- Beakers & stuff: you will need a small polypropylene or borosilicate beaker for solid NaOH; two bigger polypropylene or borosilicate beakers for the water and for lactic acid; two glass rod (to stir the mixtures); tools to measure pH (pH strips are fine).
- Prepare an ice bath (a bowl with some ice in it will work).
- Prepare the lactic acid: transfer 22,5 g (80% solution) or 20 g (90% solution) in a beaker and place the beaker in the ice bath.
- Prepare the sodium hydroxide (pellets): in a small beaker, scale the NaOH (8 g).
- Prepare the sodium hydroxide (solution): in another small beaker, scale the water for NaOH (6,8 g if 80% lactic acid solution, 9,3 g if 90% lactic acid solution). Then go outside and add the NaOH pellets to the water and stir with a glass rod. Don’t breath the fumes.
- Now, slowly add the sodium hydroxide solution to the lactic acid solution. Do this dropwise and stir with the glass rod each time. You have to do this dropwise, as the acid-base reaction can turn pretty violent. The ice bath is there to try to keep the temperature of the reaction down, but it will inevitably go up. Be extra-careful in this passage and, again, add the base slowly to the acid: you don’t want the reaction mixture to spash on your face.
- Once you added the whole NaOH mixture, check the pH value. It should be around neutral. Pay attention here: if you exceed with NaOH, the pH will skyrocket and you will have to adjust it with some more drops of lactic acid. Just be careful when you scale the NaOH in first place and everything will be fine.
So this is how one can make sodium lactate. Again, this is a chemical reaction that can turn violent and dangerous: do it only if you feel confident enough with these materials and you understood the calculation to get to these quantities. I trust in your common sense.