I wonder if I may produce CP homemade soap with tap water

[NcDon]

Has anyone experienced this?

In my home, tap water filtered through the BRITTA filter cup has TDS 28 ppm and PH 6.5. The PH of water decreases if it is kept in an open container because it interacts with the oxygen in the air, which lowers its PH. Distilled water with TDS 3 ppm only has PH 7 when it is produced. After that, its PH drops to 5.6 due to the oxygenation reaction with the oxygen in the air and it becomes more acidic.

 

[DeeAnna]

The point of using distilled water (or reverse osmosis water or demineralized water from reputable systems) is to remove metals from the water that can contribute to soap scum and/or rancidity -- calcium, magnesium, copper, iron, and others.

The recommendation to use these types of purified water for soap making (as well as for lotion making and other B&B products that use water) has nothing to do with the pH of the water. That's easily fixed if needed.

Brita filters or other filters designed to polish drinking water do not remove these dissolved metals. The only processes that do remove dissolved metals are distillation, reverse osmosis, or demineralization.

If you're making liquid soap, it's super important to use this type of purified water due to the amount of water needed to make and then dilute the soap. If you're making bar soap and either don't mind rancidity or are using an effective chelator, it's probably less critical to use this type of purified water if you have a real reluctance to use it.

The problem is you don't know your soap is going to go rancid until it happens, which is usually weeks or months down the road. You then have to try to pin down the problem that is triggering the rancidity. Using distilled water up front eliminates one source of possible contamination.

 

[NcDon]

The point of using distilled water (or reverse osmosis water or demineralized water from reputable systems) is to remove metals from the water...

If you're making bar soap and either don't mind rancidity or are using an effective chelator, it's probably less critical to use this type of purified water if you have a real reluctance to use it.

Thank you @DeeAnna for your in-depth explanations. I will test in the future a small batch of CP for solid soap using a chelator (anhydrous citric acid 3% ppo) premixed with water into which I will pour NaOH. Of course, with extra lye added accordingly for a water:lye ratio of 2.068:1 and total weight of oil.

 

[DeeAnna]

...Of course, with extra lye added accordingly for a water:lye ratio of 2.068:1 and total weight of oil....

I don't understand this sentence very well so I hope you can explain in more detail.

It sounds like (a) you're adding solid NaOH to modify the water:lye ratio, or (b) you're calculating water based only on the NaOH needed for saponification and then back calculating a water:lye ratio based on the total NaOH for the batch (the total would include the NaOH for the citric acid).

Either way, it's making your job more complicated than it needs to be.

Calculate the amount of solid NaOH needed to saponify the fats. Calculate the additional weight of solid NaOH needed to react with the citric acid. Add those two weights together. Then choose a water:lye ratio. Calculate the amount of water based on the total weight of NaOH you plan to use and your desired water:lye ratio. That's all that's necessary.

While it's mathematically fine to use a water:lye ratio calculated to 3 decimal places, it's also true that this level of precision is wasted on soap making. A water:lye ratio of 2.1 is sufficiently accurate enough.

How does "total weight of oil" relate to the other parts of this sentence? This really has me stumped.

 

[NcDon]

I don't understand this sentence very well so I hope you can explain in more detail.

In the recipe formulator from Lye Calculator & Recipe Creator for Soap Making | LyeCalc there are some recommended additives for advanced soapmakers.

For citric acid powder (anhydrous) %1 ppo in my recipe (8.10 g) -> extra Lye for C.A. %1 NaOH (purity 99%): 5.11 gram(s). For different total oil weight, lye purity and water:lye ratio, the calculator recommends a slightly different amount of extra lye.

I don't know, but this soap recipe calculator seems to be more sophisticated than others. I produced soap with the help of lyecalc.com and it came out perfectly every time.