Question

We use methylene iodide (MI, CH2I2) as a heavy liquid for sink-swim mineral separation. We use acetone to wash the MI off of the minerals and glassware after we're done. MI is pretty expensive, so we save that MI/acetone mix to later recover the MI. This is done by adding lots of water to the mix. The solution starts off very cloudy, but with enough water (probably something like a 10:1 water-acetone ratio), the solution become clear, and a well-defined blob of MI is present at the bottom of the container.

My question is if it is better to use deionized water for this, or if it's better to use hard tap water that's high in calcium bicarbonate? I thought the hard tap water might actually be better for this, because of all the dissolved ions interacting with the water, limiting the space for MI to stay dissolved to the water and acetone, similar to a salting out precipitation reaction.

Answer 1

Deionized water is generally preferred for recovering methylene iodide (MI) from an MI/acetone mix as it avoids complications that could arise from the presence of ions in hard tap water, ensuring a more predictable and effective separation process.

Step-by-step explanation:

In the context of recovering methylene iodide (MI) from an MI/acetone mix using water, using deionized water is generally preferable to hard tap water high in calcium bicarbonate. The rationale behind this preference is that deionized water is free from ions that could potentially compete with MI for solubility.

The presence of calcium and magnesium ions in hard water could indeed lead to the formation of complex ions with acetone or methylene iodide, or they might precipitate as salts with other anions present, thus complicating the separation process. Furthermore, deionized water will prevent the potential formation of an insoluble layer of calcium or magnesium acetate on the glassware or minerals that could result from the reaction with acetone.

On the other hand, salting out is an effect observed when ions increase the ionic strength of a solution, leading to reduced solubility of organic compounds.

However, in this scenario, the additional complications that hard water ions could introduce outweigh the potential benefits of the salting-out effect on MI recovery. In a controlled laboratory setting, employing deionized water will likely result in a more predictable and effective separation of MI, thus avoiding unwanted reactions or precipitates.