How do you lower the freezing point of water to -12C?

Question

want to know how much and of what substance to add to 1 liter of water to lower it's freezing point to about -12C - ideally it would be non toxic - but preferrence is on ease to mix the substance in and cost (table salt doesn't work for my application) - the end solution must be clear. Thanks

Answer 1

I would suggest you Calcium chloride

effectively CaCl2 ---> Ca++ +2Cl- (1 molecule = 3 ions)
and 1 mole of CaCl2 gives 3 osmoles

1 osmole lowers the melting point of water by 1.85°C

To lower the melting point by 12C you need = 6.48 osmoles

in molarity of CaCl2 it is equal to 6.48/3 = 2.16molar solution

A mole of CaCl2 = 40+2*35.5= 111g

this corresponds to 111*2.16 = 240g/L

Answer 2

Instead of using table salt, try using Magnesium Chloride, this will get you around -15 deg. C.
Check this out for a list of more common salts.
http://chemistry.about.com/cs/howthingswork/a/aa120703a.htm