6
$\begingroup$

My 7th grader is embarking on a long-term science project and I'm on tap to "help." He is showing a strong interest into finding ways to get drinking water to the masses. His idea is that we need to find inexpensive and efficient desalination processes. So, we are exploring current desalination processes to see what makes it "expensive" and "inefficient."

However, we live in the Rocky Mountains and don't have access to proper sea water. We started to explore simply adding salt to tap water and researching how much. Then, we discovered "Artificial Sea Water" or "ASW" is a common creation in science. Because sea water contains more than just salt. It contains chemicals like fluoride, strontium, boron, bromide, inorganic carbon, potassium, calcium, sulfate, magnesium, chlorine and of course, sodium. I am not a chemist, so this is starting to look daunting.

So, here are my questions to counsel my son:

  • To experiment with desalination, is adding salt to tap water sufficient or do we need to investigate finding official ASW?
  • How do we test the salinity of water? Do I really need to invest hundreds of dollars on a salinity meter?
$\endgroup$
5
$\begingroup$

If you look at the (average) composition of seawater:

       enter image description here

you will find that NaCl is the main solute of this complex solution. Its concentration is 10 times higher than that of the next constituents (magnesium and sulfate). So, it seems to me like a simple NaCl solution is a good approximation for your experiments.

Regarding testing salinity of water, I don't have a definite answer on that, I do have two suggestions to offer:

  • Measure salinity by electric conductance of the solution, e.g. by making a simple conductometer and testing it with solutions of known concentration of salt.
  • Measure salinity by the quantity of salt (solid residue) once all water is evaporated. You can speed the evaporation by heating up.
$\endgroup$
  • $\begingroup$ Thank you for your answer. Can you provide some guidance of building a "conductometer?" I have some familiarity with electronics. Am I measuring resistance (ohms)? Can I just use an ohmmeter? Also, It seems that temperature will affect density and thus "conductance"? Will I need to ensure that all measurements are at the same temperature? $\endgroup$ – mawcsco Oct 3 '13 at 14:22
  • 1
    $\begingroup$ @mawcsco you want to measure resistance between two electrodes in solution, with fixed geometry and (yes) fixed temperature. You can find DIY guides on the internet for such devices, e.g. here $\endgroup$ – F'x Oct 3 '13 at 14:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.