Sodium water softening advantages

Caustic soda will convert the magnesium salts into the still less soluble hydroxide and thus soften the water more effectively.1 If sufficient free carbon dioxide is present, dissolved in the water, caustic soda, or a mixture of sodium carbonate and milk of lime, may with advantage be added instead of sodium carbonate, as it is transformed into this latter salt by the carbon dioxide, and then reacts according to the above equations. This fixes the dissolved carbon dioxide and prevents it from converting into bicarbonates the normal carbonates precipitated when sodium carbonate is first used, and thereby converting a permanently hard water into one possessing temporary hardness. 

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