Emergency tap water softening & ZNSC faux sea water in one step

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Notorious Lyear
Jan 14, 2021
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This is not meant as general advice. This is a pure pen&paper recipe, absolutely untested for suitability or safety. If possible, use store-bought distilled water, reverse-osmosis, or deionised water. Consider rain water/molten snow, or dehumidifier wastewater first. It's a proof of concept, a stoichiometry finger exercise – no more. Read on on your own risk.

Disclaimer 2
I will be using metric units. In my head (and throughout this post), ZNSC faux sea water is the solution of 17 g/L of each NaCl and NaHCO₃ in water. That is 0.5 mol/L sodium Na⁺ (about 3% of the total amount of sodium in the final soap), as well as 0.3 mol/L chloride Cl⁻, and 0.2 mol/L of bicarbonate HCO₃⁻ (to some arbitrary degree of precision). Feel free to convert numbers into whatever unit system you prefer, but don't count on my help for that. Water hardness units are enough of a mess already (see below).

Motivation & Idea
Be it the next shortage of distilled water bottles, be it bad timing with shopping, or zero-waste ambitions. Store-bought distilled water, while it is the liquid of choice for most soapmaking projects for good reasons, is not always the best option, or just has run out. The major points why we prefer distilled water over tap water or mineral water, is not that it doesn't contain any of the many ions, but is very low in some specific solutes. Namely calcium & magnesium (water hardness), and trace amounts of heavy metals (e. g. from plumbing – there is a reason why the chemical name of lead is Pb), as well as an acid/base imbalanced (pH≠7).

But there are ways to get rid of (most of) those with household means. The carbonates of all the multivalent cations are highly insoluble – a thing that everyone who regularly has to descale kettles, coffee machines etc. is painfully aware of. How convenient that a main ingredient of ZNSC faux sea water readily assists with this reaction! We just have to raise the bicarbonate dosage, so that it precipitates the water hardness, and we will end up with a solution that will have things like monovalent cations (Na⁺, K⁺) from the water still present, along most of the anions (sulfate, chloride) – but all those are no issue with soap!

The numbers
So, how much bicarb do we need?
This is the point where it gets hairy for several reasons. For one, bicarbonate is not heat-stable, so later on (when we will boil the water), it will decay into regular carbonate – just as it would in contact with NaOH for making lye, so we actually will anticipate this step and incorporate it into the instructions.
And then of course, the water hardness itself determines how much we will have to add to compensate it. If you use tap water, ask your waterworks for the numbers. You can also use bottled mineral water with a mineral analysis printed onto the label. There are also test strips for a rough hardness level determination.
Unfortunately, there is no globally agreed standard, so there is a zoo of conversion factors:
For each litre of water, when the label says X [unit], use that many grams of NaHCO₃:
MineralUnit XMolaritySodium bicarbonate
CalciumX mg/L49.9 · X µeq/L4.19 · X mg/L
MagnesiumX mg/L82.3 · X µeq/L6.91 · X mg/L
German degree, dGHX°dH357 · X µeq/L30.0 · X mg/L
Molarity Ca²⁺ & Mg²⁺X mmol/L2000 · X µeq/L168 · X mg/L
ppm CaCO₃X ppm (= X mg/L)19.98 · X µeq/L1.68 · X mg/L
French degreeX°fH199.8 · X µeq/L16.8 · X mg/L

Strictly speaking, only the permanent hardness has to be considered, since the carbonate hardness (limescale) takes care of itself during boiling. But with numbers based on the total hardness, we are on the safe side in any case, have some headroom to scavenge minor metals (Fe, Cu, Zn, Mn, Pb, Al …), and the worst thing to happen is that we have a slightly elevated amount of unreactive soda ash dissolved in our lye water/soap batter.

Implicit here is the point that bicarbonate is a very weak, but strong enough acid to react with NaOH: bicarbonate will, later on when preparing the lye solution, instantly react with hydroxide to form regular carbonate, and upon this use up some 0.5% of the NaOH. @Zany_in_CO designed the recipe at formal 0% lye discount, but there is actually that half percent superfat bonus, as wiggle room to avoid a lye-heavy product.

Example 1
The mineral water I'm currently drinking says it has 293 mg/L Ca²⁺ and 51.6 mg/L Mg²⁺, that makes (293×4.19+51.6×6.91) mg/L = 1.58 g/L of extra sodium bicarbonate needed to fully knock the earth-alkaline metals out of solution.
Example 2
According to the water analysis of my local waterworks, the tap water here has 14.8°dH. The equivalent amount of baking soda is (14.8×30.0) mg/L = 0.44 g/L.

These amounts are small compared to what the faux sea water will calls for, but we now can account for them nonetheless.

Now we know how much baking soda we will need to soften the water.
We weigh this amount, plus double the amount of the original recipe: 34 g/L aka 2 tbsp/L (in my examples, this would be 35.58 g/L or 34.44 g/L, resp.).
We can now measure the table salt too.
If we would just dissolve this into the water, nothing remarkable/helpful would happen. So the next (and probably most important) step is to boil the water. Only once it starts to simmer, we sprinkle in the bicarbonate+salt in small steps, because it will foam up vigorously. You might notice that the water becomes cloudy and/or limescale will precipitate as whitish flakes/crusts (if they are brown, you should check your piping, don't ask why I know that). No need for a full boil, it'll only cost energy and you'll lose water to evaporation.
Just wait until the bicarb-fizzing has stopped, and let cool down. Decant or filter off the scale flakes; let it settle for hours…days if you have a stubborn residual turbidity. The faux sea water is now ready to use.

Superfat caveat hot vs. cold FSW
This product is not the same as the original faux sea water. It is considerably alkaline (from excess sodium carbonate aka soda ash, that had formed when the bicarbonate decomposed), and has the first step of the ZNSC lye preparation already done (you remember: the 0.5% superfat crumple zone) – so if you're after a faithful ZNSC recreation, increase the amount of oils by 0.5%.

Note that these boiling&bubbling instructions aren't a side effect of the water softening process, but a general property of aqueous bicarbonate solutions. So the doubling & SF adjustment step is, strictly speaking, also necessary for a precise HP adaption of ZNSC.
Thanks for doing these calculations!

To check my understanding 🙂

For Example 1:
Weigh out 34 g/lL baking soda, which is 2x the amount in the faux seawater recipe, plus the amount calculated for the Example 1 water, which is 0.44 g/L. The total baking soda needed is 34.44 g/L for Example 1 water.

When added to simmering water, the baking soda will cause the transition metals (and alkaline earth metals?) to precipitate(?) The precipitate (lime scale) will eventually settle (quickly or slowly). The treated water should be decanted or filtered to remove the precipitated scale. The recipe can be adjusted by adding 0.5% superfat to account for the remaining alkalinity in the treated water.

Zany’s recipe uses sea water and you‘re using table salt in your method. If the alkaline earth metals precipitate out of solution when using this treatment method, is there any reason to use sea salt? Does the potassium remain in solution? Based on the periodic table, I’m guessing it will. If so, is it likely to influence the soap?

I need to go to work now, which is before I feel confident that I understand why you are using 2x of the baking soad as a starting point 😩

I have previously pondered the source of the precipitates in the lye water when making ZNSC. Is the bicarbonate-driven reaction the answer?
Thank you Resolvable Owl :winner:
Last edited:
Wrt the double amount of baking soda: I haven't explained it above (to make the post not even longer than it is anyway).
In the classic ZNSC procedure, bicarbonate is exposed to an excess of hydroxide. Every HCO₃⁻ ion is converted to CO₃²⁻, i. e. all the carbon stays in solution, but a bit of lye is “wasted” to form sodium carbonate.

Now, in my above hot-brine detour, bicarbonate is added to hot water. It will decompose, half into CO₃²⁻, half into CO₂ that is bubbling away. To end up with the same amount of carbon, I have to start with double the amount of NaHCO₃. BUT it is in carbonate form, and doesn't eat up NaOH when the lye solution is prepared. You don't have to take care of the part of CO₂ (aka carbonic acid) that is lost to bubbling.
As a bonus, there comes some extra sodium with the extra baking soda. This is not an error, but tells you that you can (should) add more oil – or, if you want so, that you can make mind-boggling 0.5% more soap out of the same amount of NaOH if you boil the faux sea water beforehand – at the cost of that one lousy tablespoon of baking soda.

Zany’s recipe uses sea water and you‘re using table salt in your method.
Good catch. I usually don't care which type of salt I'm using (except that it shouldn't contain anticaking agents like silica or ferrocyanide) – more often than not it is sea salt in fact. I call it “table salt” to make clear it's >90% NaCl. Which FWIW is all that matters. Impurities like calcium, potassium or sulfate from the sea salt occur in really negligible amounts in the final dilution. But maybe enough to effectively add some hardness to the brine/lye, so that it becomes clear? Good point.
salt without any added iodine would be the best option to use
Is this the case? Serious question. Too many of the “iodine conspiracy busters” have nothing but diffuse anti-scientific frustration up their sleeves. It is everyone's own decision if this outweighs the well-proven effect of iodine for the thyroid. But I'm uncomfortable to copy iodine FUD without at least quoting independent sources.
It is true. But as I said it is clearly on the label and salt without iodine is also available (but not always at the same price and brands etc) - Europe has the after effects of a certain Soviet power station which had a bit of an issue back in the 80s, so the iodine is for the thyroid and so on.

It's something I mention because it doesn't always come to mind for many people here that it might be an issue at all - they just grab the regular salt that they always use from the cupboard, not thinking about the iodine and what it might do. For example, I have to make sure that my salt for salt bars has no iodine.

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