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Water Hardness: Details, Recipes, and Experimental Results on Lather Quality

I'm curious if there is a difference in the interaction of these minerals in soaps produced with Sodium Hydroxide vs. Potassium Hydroxide. Other than the softness of the bar itself is one better than the other for "hard" water?

Sodium cations (Na+) and potassium cations (K+) are interchangeable when reacting with hydroxide anions (OH) in forming sodium hydroxide (NaOH) and potassium hydroxide (KOH). Sodium and potassium hydroxide make harder and softer soap, respectively, but since sodium and potassium ions are interchangeable in that they both have charges of +1, they are interchangeable in the formulas for the salts formed in soap. With calcium ions (Ca2+) and magnesium ions (Mg2+) both having charges of +2, calcium and magnesium are interchangeable here, too. Overall, when soap is added to water, the calcium and magnesium ions present in the water react in an interchangeable way with the sodium and potassium salts in the soap and form calcium and magnesium salts, better known as "soap scum". There is no fundamental difference in the interactions of water hardness compounds having calcium and magnesium with soap salts having sodium and potassium.
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