Desalting Water by Freezing

If i were to try and desalt saltwater by freezing it, what would i have to do? What materials do i need? How can i measure how much salt has been removed afterwards?
- Anonymous

I haven't directly tried this, but I think it is easy. First, you want a way to measure the salt concentration. For a first try, not very quantitative, taste isn't bad. To get a little more quantitative, you could use a cheap volt-ohm meter to measure the electrical resistivity, which goes down as more salt is dissolved. You’d want to use two electrical contacts with fixed geometry (maybe glued to the side of a cup) and just measure the electrical resistance across the water. This also isn’t extremely accurate, unless you’re very careful with cleaning the electrodes and perhaps switching their connections to the ohm meter, taking the average of the two readings, but it’s a good start. You can also evaporate the water (boil it if you’re in a hurry) and weigh the salty residue, but you may need a very sensitive scale or balance for the salt you might find in an ice cube after the de-salting process. There are also commercially-available water salinity meters. A quick web search indicates that you may be able to find these where fancy fish and aquariums are sold. Some fish need just the right amount of salt in their water and so a salinity meter is just right to have around. They mostly rely on the electrical conductivity principle described above, but a few take advantage of the different refractive index saltwater. Others can measure the density very precisely and compare that against unsalty water.

For the de-salting, try putting the water in an ice-cube tray and let most of it freeze. If it’s very salty, and your freezer won’t freeze any of it, you may need to use a styrofoam cooler with some dry ice in it instead of the freezer. Try to arrange it so that the freezing is slow, to avoid getting saltwater trapped in little pockets in the ice. Then pour off the liquid. Melt the ice in a clean cup. You should find that the salt concentration is very much lower. You can repeat the procedure with the new liquid to get even less salty water, in case too much salt got trapped in those pockets the first time.

mike w (and Tom J.)

(published on 10/22/2007)

If you freeze saltwater into ice would that technically mean you can eventually drink the melted ice as if it was pure water? If so do we do that today to help the water promblems.
- Anthony (age 15)
Yonkers,new York, United States

People have been getting water from snow in cold climates for a long time. Sometimes people talk of using ice as a source of pure water in warm places. The specific plan I've heard mentioned was to tow ice from Antarctica to some hot, dry places that really needed the fresh water. 

In most cases, purifying water by deliberately freezing it isn't very practical. It takes a lot of energy to run a heat-pump that can freeze much ice in warm climates. Unless there were some other use for the heat dumped out by the pump, that's inefficient compared to other methods of getting pure water. Reverse osmosis, in which the water is forced through tiny pores that tend to block salt ions, is one of the common methods. Distillation is another method that can be used in how regions just using energy from sunlight.

Mike W.

(published on 04/20/2014)