The Brinicle

The normal saline we use in the hospital contains 9 grams of sodium chloride per liter of water. If you look on the bag you will see 0.9% which is the mass concentration expressed as g/dl (0.9 grams per deciliter of water - as there are 10 deciliters in a liter there are 9 grams of sodium chloride in the one liter bag).

The molecular weight of sodium chloride is 58.4 mg/mmol (23.0 mg/mmol from sodium and 35.4 mg/mmol from chloride) hence there are 154 mmol of sodium chloride in the one liter bag of normal saline.

9000mg/58.4mg/mmol = 154 mmol

As there is one molecule of sodium and one molecule of chloride in each molecule of sodium chloride (commonly referred to as salt) there are 154 mmol of sodium in the one liter bag or 3542 mg (well above the daily recommended 2300mg).

Now some craziness: average sea water is 3.5% sodium chloride (there are multiple other ions at much lower concentrations) meaning 35 grams of salt per liter of water!

This leads to some interesting phenomena. Check out this recent BBC time lapse video of something called a "brinicle."



In arctic waters the formation of surface ice forces solutes out of the newly formed solid and into "brine channels" which contain super concentrated sea water. As the concentration of solutes rises the freezing point of the solution drops and the density increases.

This cold dense super concentrated sea water descends towards the sea floor. The distribution of the brine channels can be such that the super concentrated sea water comes out in columns. As it does less concentrated water with a high freezing point around it freezes. When the column reaches the ocean floor the water flows with gravity to the lowest point, freezing everything in its path including animals.

So what sort of salt concentration is this "super concentrated sea water" at? The lowest recorded temperature of sea water I could find was negative 2.6 degrees C. Calculating backward from the freezing point depression equation and assuming all solute in sea water is sodium chloride...

Change in freezing point of solution = KF · m · i

KF = the cryoscopic constant, which is dependent on the properties of the solvent = 1.853 kg/mol for water

m = mol of solute per kg of solvent

i = the van 't Hoff factor (number of solute particles per mol) = 2 for NaCl

So...

2.6 = 1.853 * m * 2

2.6 = 3.706 * m

0.702 = m

0.702 mol/L or 702mmol/L of sodium chloride

58.4 mg/mmol * 702mmol/L = 40,997mg/L or 41g/L or 4.1% sodium chloride

That super concentrated sea water makes our 3% "hypertonic saline" look like weak tea!