Sciencemadness Discussion Board

Determining concentrations in solutions

ZIGZIGLAR - 6-2-2014 at 18:33

Hi,

What I'm wanting to do is determine the concentration of 3 different chemicals in a solution.

Here's how I calculate two, take Methanol/Ethanol:
Code:
x = unknown volume of Methanol (1-x) = unknown volume of Ethanol Then x + (1-x) = 1 litre Specific Gravity of Methanol = 0.7915 Specific Gravity of Ethanol = 0.7905 (x) (0.7915) + (1-x) (0.7905) = SG of solution Solve for X (x) (100%) = concentration of Methanol (1-x) (100%) = concentration of Ethanol


How on earth can I calculate the concentrations of three chemicals, such as adding water to the above scenario?

Cheers

[Edited on 7-2-2014 by ZIGZIGLAR]

Metacelsus - 6-2-2014 at 18:44

You can't (too many variables in your equation).

Even with two chemicals, there is a problem: Volume may not be conserved. For example, mixing 100 mL ethanol and 100 mL water results in 192 mL of solution (https://en.wikipedia.org/wiki/Ethanol#Physical_properties). If the change in volume is unknown and nonzero, your calculations will fail.

ZIGZIGLAR - 6-2-2014 at 19:04

What if you first diluted the ethanol/methanol/water solution to the ethanol/water azeotrope, so that you knew the percentage of water?

Assuming methanol and ethanol result in no change of volume, we could figure that variable out at least. I still have no idea what kind of formula to apply though - or if I'm just chasing my own tail here.

Metacelsus - 6-2-2014 at 19:17

First of all, what are the expected ranges of methanol and water concentration in your ethanol?

(I will proceed assuming this is related to your wanting to purify denatured alcohol.)

You can remove the water by distilling over CaO. If you want to know the amount of water, you can weigh the CaO before and after. Besides, you will want to remove the water anyway.

Once you have just methanol and ethanol, you can measure the density and calculate the proportions. If you want to be extra accurate, you can look up the density in a table (that adjusts for volume changes) and see what proportions it corresponds to.

ZIGZIGLAR - 6-2-2014 at 20:33

Thanks for your reply. Yes this is a continuation of my thread concerning denatured alcohol. I thought that this was enough of a side venture to justify its own thread.

The removal of water by distillation over CaO can yield a maximum concentration of 99.5% ethanol. While 0.5% doesn't sound like much, it will probably have an impact on the accuracy of a density comparison when the resolution of the math is at 0.0001.

Perhaps I should look up how to make a ghetto GC system?! :/