Osmolarity

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adnza Posted: 9 Jul 2009 8:05

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How can i convert Osmolality / Kg to Osmolarity / L (concersion formula).

Problem: I got 129 mOsmol / Kg of Iron Sucrose @ 2% solution (recommended by manufacturer) how can i convert it to mosmol / L

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nsol replied on 9 Jul 2009 11:44

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I'm not an expert on this subject but I believe I know how to answer your question.

When the concentrations are low enough, the osmolality and osmolarity of the solutions can be considered the same. Also, the osmolarity of a solution is temperature dependent as the volume of the solvent can change with temperature.

The way to convert from one to another is not straight-forward and can be done using the following equation:

 Osmolarity = Osmolalty(mOsmol/kg) x [ρ(g/ml) - W%(g/ml)]

Where:       ρ = Density of the solution at a specified temperature in g/ml

                  W% = Is the weight percentage of the solution, g/ml

In your case, you mentioned that an Iron Sucrose (2%) solution with and osmolality of 129 mOsmol/kg

• I am assuming the density of the solution at room temperature is 1.1 g/ml (a 3
• little more dense than water)
• It is a 2% weight solution means there is 2g/L of iron = 0.002g/ml of iron

We can now calculate this:

Osmolarity = 129(mOsmol/kg) x [ 1.1(g/ml) - 0.002(g/ml)] = 141.6 mOsmol/L

I hope that helps. I used some links to solve this so you might want to have a look:

http://sitemaker.umich.edu/tutorial/files/handout_milliequiv.pdf

http://en.wikipedia.org/wiki/Osmolarity

and example 6-17 in the below link:

http://books.google.co.uk/books?id=nt-crAJEtVYC&lpg=PA159&dq=convert%20between%20osmolarity%20and%20osmolality&client=firefox-a&pg=PA159