"Oil & Vinegar" sodium acetate in soap experiment

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topofmurrayhill

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An experiment to see if we can use sodium acetate derived from a common household product in a batch of soap and obtain any of the benefits of sodium lactate. It was inspired by seeing references in soapmaking literature to either sodium lactate OR sodium acetate making harder soap with better lather, particularly when using soft oils. We normally use sodium lactate, but sodium acetate can be made by substituting white vinegar for some of the water in a recipe.

Due to a chemistry brain seizure, there is an error in the experiment. I intended to produce 10 g sodium acetate, which in baker's measure would have been 2% of the 500 g oil amount. What I actually produced is 8.2 g sodium acetate (plus 1.8 g water), which is 1.64% of the oil amount.

I made one soap containing sodium acetate and another that is identical but has no additive. I have a little custom 1/2 lb mold that I use for testing, but there is only one of those and I wanted to make both soaps simultaneously, so I dusted off my very first soap mold instead -- a 2 lb HDPE log from 2007-ish. I made two 1/2-length liners and inserted them end to end with a piece of cardboard in between to divide the mold into 2 sections.



I used no color or fragrance. I calculated no lye discount, but my grade of lye is 97% pure and the container has been open a while. I don't expect there to be excess caustic.

The plain soap is formulated as follows:

125 g coconut oil
125 g palm oil
250 g olive oil
74 g sodium hydroxide
150 g water

The sodium acetate soap is formulated as follows:

125 g coconut oil
125 g palm oil
250 g olive oil
78 g sodium hydroxide
120 g vinegar
36 g water

The vinegar is Heinz Distilled White Vinegar with 5% acetic acid. That is 6 g acetic acid, which reacts with the extra 4 g NaOH to produce 8.2 g sodium acetate and 1.8 g water.

I made the lye for the plain batch as usual. The temperature peaked at 155 F. Then I added the sodium hydroxide to the vinegar water and stirred, ready to run. The temperature peaked at a bit over 200 F. That is an important thing to note. It's quite possible to boil your lye if you do this wrong! The odor of vinegar quickly disappeared. Sodium acetate is colorless, but presumably due to impurities my lye turned a very light straw color.



I soaped at about 115 F. The batch with sodium acetate traced faster than the plain batch. The difference was noticeable but not dramatic. I did more stirring of the plain batch to keep them even with each other. I then poured both simultaneously and at a very similar consistency -- a little past light trace. There was a minor color difference between the two batches. The sodium acetate soap is on the left.



I poured the soap up to the rim of the mold and didn't cover it, so I could easily see what was going on. The log was placed in an electric convection oven at 150 F for 2 hours.

At 1 hour, the plain soap appeared to be fully gelled. The sodium acetate soap took an extra 15 minutes or so. While it did seem to reach a uniform darker color, it never looked quite as dark as the plain soap. Possibly it was gelled but more opaque. The surface of the plain soap was covered in little bubble-like blemishes that are typical of leaving the soap uncovered in the oven. The sodium acetate soap had fewer of these by far.

That's all I have to report for now. I'm thinking I will unmold the soaps and compare at about the 24 hour mark.
 

topofmurrayhill

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Both soaps were firm and dry when unmolded after 24 hours. They look very different! The surfaces of the soaps responded differently to being uncovered in the 150 F convection oven, but the biggest difference is that the sodium acetate soap is much more opaque, making it look lighter and more creamy than the plain soap. With these oils and color, it actually looks much better.



Subjectively, the two soaps seemed equally easy to cut with a wire cutter. Both cut cleanly, with no crumbling or other problems. However, the consistency of the soaps is different. When I took a thick trimming from each soap and tried to bend it, the plain soap bent and the sodium acetate soap broke.



I decided to plane and bevel the soaps to see how the different textures responded to hand work. When I try to plane fresh soap, even if it seems quite firm, I find it often gums up the planer blade after several slices, so I have to keep stopping to remove the stuck soap. I had that exact experience with the plain soap.



The sodium acetate soap planed cleanly, no matter how many slices. It was very easy to work with.

 

topofmurrayhill

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At this stage, my observations are that the sodium acetate soap has a stiffer, less gooey consistency and a more opaque, creamy look. I would also tentatively say that sodium acetate and sodium lactate seem to have a very similar effect -- so far. Looking back at threads where people asked about sodium lactate alternatives because they have trouble getting it, this could end up being the most direct substitute.

Sodium acetate soap on the left:



Sodium acetate soap hand worked for round edges:

 
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Steve85569

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This thread got me wondering. What would happen if ascorbic acid and vinegar were combined to neutral and then used in soap. would the citrate resulting be similar to the sodium citrate more commonly used?

I'm not a chemist so I ask.

Steve
 

The Efficacious Gentleman

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This is very interesting.

In the interests of science, I might well make my batch of Castile with hp using 3% sodium acetate to see how it helps the pour. If it goes horribly wrong, it's Castile which in many people's eyes is ruined from the very start [emoji39]
 

topofmurrayhill

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This is very interesting.

In the interests of science, I might well make my batch of Castile with hp using 3% sodium acetate to see how it helps the pour. If it goes horribly wrong, it's Castile which in many people's eyes is ruined from the very start [emoji39]
That would be great because I don't normally do HP and won't have time for it. Since using SL for pourability is a common application, somebody should try it with the SA. I'm starting to feel some confidence that it will work and would look forward to your report.

Soon I will try to write up straightforward instructions for how to use vinegar to add sodium acetate to a recipe, in terms of both math and safety, though I know you can figure out the numbers from what I've already posted.

Safety concerns center on the extra heat of the reaction. My recipe included additional water to get a 33% lye. The highest concentration you can get is with straight vinegar, which in this recipe would have been about a 40% lye. I don't know if that would have boiled from the heat. Stronger than 5% vinegar poses additional risk. It might be a good general rule to add the caustic more gradually and monitor the temperature, possibly allowing some cooling time.

At first blush, it seems as if the usage rates for SL and SA may be similar, though we don't know exactly. SL recommendations are usually stated in terms of a 60% solution. The 1.64% of oil weight I calculated for sodium acetate in this recipe is based on dry weight. In terms of a 60% solution it was actually 2.73%, close to the amount you want to try. For 3% of oil weight in your HP castile you only need 1.8% dry weight. Or maybe try a bit less in case it turns out to be more potent than SL, because we're up near the high end of the range.
 
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lionprincess00

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On a side note, adding vinegar caused it to heat just below boiling. Higher concentration vinegar and you may be looking at a volcano. This experiment shows why adding vinegar to lye burns or spills is NOT a safe method compared to plain ole water. Something most of us here aleady know yet many new comers still believe.
 

Soapmaker145

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One more thank you to add to the list.

I think I'm going to try it too. I plan to neutralize the vinegar to around pH 8 with NaOH a day before I need it. This way I can cool it or freeze it to be ready to make the lye solution.
 

topofmurrayhill

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On a side note, adding vinegar caused it to heat just below boiling. Higher concentration vinegar and you may be looking at a volcano.
That's a hazard I've had in mind all along the way, but to be accurate there was actually a good safety margin in this particular experiment. A 33% lye solution boils at something like 250 F.
 

lionprincess00

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That's a hazard I've had in mind all along the way, but to be accurate there was actually a good safety margin in this particular experiment. A 33% lye solution boils at something like 250 F.
Well i figured you with all you're precise calculations probably did take the hazzard into account. I did (as a side note) want to remind newbies or those following the wives tale like advice of vinegar for lye spills why it isnt the best choice, and i think youre experiment contributes to the issue for it shows the exothermic reaction between acid and strong base.
Great experiment, looking forward to the observations to come!
 

lsg

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Thank you for sharing your experiment. I definitely want to try the vinegar approach, using your calculations.
 

topofmurrayhill

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Here are instructions for calculating white vinegar and sodium hydroxide if you want to experiment with sodium acetate in your soap. For now I would suggest trying usage rates similar to sodium lactate, meaning 1% to 3% of oil weight based on a 60% solution.

The instructions will use the following recipe numbers as an example:

1000 g oil
300 g water
148 g sodium hydroxide
2% sodium acetate

Substitute the recipe numbers from your lye calculator and the sodium acetate usage rate you want.

---

1. Calculate how much sodium acetate you need.

Multiply the oil weight by the percentage of sodium acetate you want, then multiply that by 0.60 to put it in terms of a 60% solution.

1000 * 0.02 * 0.60 = 12.00 g sodium acetate needed.

---

2. Calculate how much vinegar you need.

Vinegar is our source of acetic acid to make sodium acetate. Multiply the sodium acetate amount by 0.73 to get the required amount of acetic acid.

12.00 * 0.73 = 8.76 g acetic acid needed

Divide by the percentage strength of your vinegar (5% in this example) to get the amount of vinegar. Round off as needed for weighing.

8.76 / .05 = 175 g vinegar needed

---

3. Calculate how much extra sodium hydroxide you need.

We neutralize our acetic acid with sodium hydroxide to produce sodium acetate. This requires some extra NaOH to avoid changing the superfat. Multiply the sodium acetate amount by 0.49 to get the required amount of NaOH.

12.00 * 0.49 = 5.88 g additional sodium hydroxide needed

Add that to the number in your lye calculator. Round off as needed for weighing.

148 + 5.88 = 154 g total sodium hydroxide

---

4. Calculate how much water you need.

Some of our water amount is coming from the vinegar. The percentage of water in the vinegar is 100 minus its strength, so our 5% acetic acid vinegar is 95% water. Multiply the vinegar amount by that to get the amount of water.

175 * 0.95 = 166.25 g water from the vinegar

Subtract that from the number in your lye calculator. Round off as needed for weighing.

300 - 166.25 = 134 g additional water needed

---

Here is our updated example recipe:

1000 g oil
175 g vinegar
134 g water
154 g sodium hydroxide

---

Extra Credit

The reaction of acetic acid and sodium hydroxide produces sodium acetate and water. We did not take into account the water produced because it would have very little effect on the recipe, but you can calculate it by multiplying the sodium acetate amount by 0.22.

12.00 * 0.22 = 2.64 g water produced

If you like, you can subtract it from the amount of water to add.

300 - 166.25 - 2.64 = 131 g additional water needed

Also notice how the numbers add up:

8.76 g acetic acid + 5.88 g NaOH = 14.64 g reactants

12.00 g sodium acetate + 2.64 g water = 14.64 g products
 

topofmurrayhill

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Please see the previous post for how to do the numbers if you want to experiment. Here are a few notes about heat.

So far I've made two sodium acetate batches. The first was an average recipe with a 33% lye, no discount, and an SA usage rate of 2.7%. The second was a coconut oil soap (which requires more caustic) with a 35% lye, no discount and an SA usage rate of 1.5%. Usage rates are in terms of a 60% solution, for comparison with sodium lactate. The starting temperature of the materials was in the mid to high 60s F.

For both of those initial batches, the temperature of the solution topped out pretty close to 200 F from dissolving the lye and neutralizing the acetic acid. The boiling point of lye is on the high side:

25% 234 F
30% 240 F
33% 245 F
40% 262 F
50% 288 F

I think this technique is fairly safe from boiling over across many typical recipes, lye concentrations, sodium acetate amounts and environmental conditions. However, it's impossible to anticipate every combination of variables and every mistake that could be made. Also, using a stronger vinegar allows for more concentrated lye and/or greater amounts of sodium acetate, which increases the hazard.

Use cold water and vinegar. You can even put them in the fridge. If you are doing anything that could possibly be outside of the envelope, don't add all the caustic at once. Allow cooling time between additions.
 

CaraBou

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Great math job and process documentation! I followed most of it (except why the 60% sodium acetate and 73% acetic acid solutions. Maybe it was explained in previous posts, which I read earlier; I didn't go backward). If I follow this right, it looks like one would not be able to use a 1:1 masterbatched NaOH solution in this example without exceeding the desired amount of water (approx. 2:1). Water is of course flexible in a recipe but that is something to keep in mind before plunging in, if you use MB lye.

Thanks for sharing. I would never have gone through that kind of exercise on my own to figure it out.
 

topofmurrayhill

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Great math job and process documentation! I followed most of it (except why the 60% sodium acetate and 73% acetic acid solutions. Maybe it was explained in previous posts, which I read earlier; I didn't go backward). If I follow this right, it looks like one would not be able to use a 1:1 masterbatched NaOH solution in this example without exceeding the desired amount of water (approx. 2:1). Water is of course flexible in a recipe but that is something to keep in mind before plunging in, if you use MB lye.

Thanks for sharing. I would never have gone through that kind of exercise on my own to figure it out.
Those oddball numbers 0.73, 0.49 and 0.22 are just ratios of the molecular weights of the reactants and products.

For instance, one acetic acid molecule reacts with sodium hydroxide to produce one sodium acetate molecule. The molecular weight of acetic acid is 60 and the molecular weight of sodium acetate is 82. So if you know the weight of sodium acetate you want, the amount of acetic acid you need is that times 60 / 82 = 0.73.

Doing this with the vinegar method imposes some constraints, such as being stuck with having to add the amount of water in the vinegar. If you run the numbers with 8% vinegar strength, you might find that it's possible to use a your 50% lye. Of course you could just buy anhydrous sodium acetate, but in that case you could also just buy sodium lactate since the soap suppliers sell it.

That leads us to the 60%. When you buy sodium lactate for soaping, most suppliers have it as a 60% solution. Even if you were to buy it as a powder, you'd be best off making your own 60% solution. So when people talk about a percentage of sodium lactate in a batch, that's what they are usually referring to. I decided to express the usage rate for sodium acetate in terms of a 60% solution to make it comparable with sodium lactate.
 
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