Why does adding more salt to an already saturated brine solution cause TDS to rise?

[BrandonWhite]

I've been doing some water softening experiments and have discovered something that has me scratching my head.

My assumption all along has been that if you add salt to water you will reach a point where no more salt can be dissolved and so any additional that you add will simply remain solid and thus not increase the TDS of the brine solution. Well it turns out that is completely wrong. Or at least my experiment is indicating as such. But I'm hoping that someone with more chemistry knowledge can explain why this is.

The experiment: I filled two containers with roughly equal parts of RO water. In container A I added rock salt until the undissolved salt was above the water line. In container B I added salt until there was a significant layer of undissolved crystals at the bottom, but nowhere near the top. I then left them for approximately 36 hours.

The hypothesis: TDS measurement would be the same in both solutions. After all, the water in both has dissolved all that it can given the starting TDS and temperature, right? Wrong.

The result: The TDS of liquid extracted from container A (packed full of salt) was over 3 times higher than container B (undissolved salt, but not packed).

So what gives?!

 

[JoyfulNoise]

You probably need to repeat your experiment in a more controlled and careful manner. Repeatability is the hallmark of a proper scientific experiment.

Salt has an incredibly high solubility in water at almost 360g/L (~ 3lbs per gallon). Also, dissolution of salt in water is driven primarily by concentration difference and exposed surface area. Your solutions were likely not saturated and had different salt concentrations in them.

A better way to test your hypothesis would be to boil the water first and create a supersaturated solution of salt and water by adding excess salt to the hot water until it can not dissolve anymore. Then let it cool off. Once it's cool you can add some salt crystals to precipitate the excess salt out of the cooled solution (since there will be excess salt in solution at room temperature). You then use the fully saturated water/salt solution to check and see if any further changes occur to TDS when you add solids to it.


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