Why is sodium gluconate used?
Sodium gluconate (SG) is a chelator. It offers two benefits when used in soap and other personal care products -- It increases the shelf life of your soap by chelating (binding up) certain metals that can cause DOS (dreaded orange spots, also known as rancidity). It also reduces the amount of sticky soap scum formed when lye-based soap is used in hard water.
Sodium gluconate, tetrasodium EDTA, and citrate all do similar jobs. If one does not appeal or is not available to you, then consider using one of the others. SG appears to be as or more effective than EDTA according to the manufacturer (1) as well as from anecdotal reports from soap makers.
Sodium gluconate is manufactured as a powder at 98-99% purity and as a water-based liquid containing up to 60% SG by weight as well as a preservative. Suppliers selling small amounts of SG are offering it only in the powder form at the time of this writing.
Can I make a sodium gluconate masterbatch solution?
Yes, but I strongly advise against this due to how fast SG biodegrades in the presence of bacteria and fungi and the impossibility of knowing if a sodium gluconate solution is still effective just by looking at or smelling it.
The best way to ensure your sodium gluconate stays as effective as possible is to use the powder directly. It dissolves easily in water-based liquids.
A compromise would be to make only enough SG solution needed for the current day of soap making and discard any leftover solution at the end of the day. Sodium gluconate is highly soluble in water, so it would be reasonable to make a 50% masterbatch -- 50 grams of SG dissolved in 50 grams of distilled water. It might also be wise to also add a broad-spectrum, water-based preservative to discourage bacteria and fungi.
How much sodium gluconate to use in soap?
Soap makers who use sodium gluconate typically use 0.5% to 1% SG powder based on total batch weight. This is 5 to 10 grams SG powder per 1000 g of total batch weight (oil weight + water weight + alkali weight).
A dose of 0.5% seems to be a commonly used rate for soap. (2, 3)
One cosmetics supplier recommends using SG from 0.1% to 1.0% in personal care products, which include soap, lotion, shampoo, etc. (1) so a 0.5% dose seems reasonable.
How much lye does sodium gluconate neutralize?
None! Do not adjust your lye weight for sodium gluconate.
How should I add sodium gluconate to my soap?
Mix the sodium gluconate powder or solution into any water-based liquid -- lye solution, extra water, aloe, beer, vinegar, milk, etc.
(1) Sodium gluconate USP. Making Cosmetics. https://www.makingcosmetics.com/Sodium-Gluconate-USP_p_1103.html Version viewed 6 October 2020.
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