Q For The Chemists.

[LeeH]

Why does star san not drop the pH of softened water very well.

I have a house wide softener due to the water close to 400ppm, so generally use RO water to fill up a 25L batch of sanitiser. The pH drops to 2.5 with the recommended dose of 10ml/5L. However, when using tap water which has a TDS of <30ppm, the pH only drops to 3.8pH.

Both have a starting pH of 7.

 

[Agentgonzo]

Water softeners (ion exchange, which is the normal type) work by replacing calcium ions with sodium ions. However, the carbonate ions remain unchanged and in solution.
The benefit to your home is that sodium carbonate is much more soluble in water than calcium carbonate, so you don't get the white buildup.

However, in water chemistry, the concentration of carbonate ions remain untouched. And it is these carbonate ions that act as a buffer and prevent the pH dropping as much.

Your RO water has filtered out these carbonate ions so they can't act as a pH buffer.

 

[LeeH]

Simple when you explain it. Thanks!