How do you prepare a 10 mg L-1 solution of NaCl in water?

You probably put 10 mg of common salt inside of one liter volumetric flask and poor water until the etched ring graduation mark is reached ensuring that there is not trace of the crystaline solid inside the flask. In this way you have prepared nicely one liter of a precise 10 mg L-1 NaCl solution. In this way we were teached to prepare in a standard way our solutions reported using molarity units.

Lately, in the rush of the daily days, you could probably catch me taking 1 L of water and spreading a fast weighted spoon with 10 mg of salt to prepare the same not-so-accurate solution than above. Or maybe not, it depends obviously of the accuracy required later by the experiment.

But now you imagine that you need to prepare 1 mL of a 467 mg mL-1 solution of PbI2 in DMF. On other words, the famous 1 M solution of PbI2 in DMF. Do you weigh the quantity of PbI2 and follow a similar procedure as first described above filling with solvent a volumetric flask up to its mark on the glass? or, do you pour 1 mL of DMF solvent on 467 mg of PbI2?

The IUPAC Gold Book established clearly the definition of amount concentration: molarity takes in consideration the mixture volume not the volume of solvent poured. In my hands, the former correct procedure using graduated volumetric flasks gives a ~ 42% w/w PbI2 DMF suspension assuming that later or sooner the PbI2 dust particles will dissolve without a significant volume change of the mixture. On the other hand, the “fast” and “wrong” method gets really a 33.1% w/w of PbI2 in DMF as the amount concentration. It is a huge difference, 42 vs 33%, on reporting experimental procedures for solution preparation. In fact, the source of this mess is that for highly diluted concentrations, as the example entliting this post, the mismatch is negligible. Instead, these highly concentrated solutions labelled wrongly as 1 M will lead a significant error for others because researchers with standard education in Chemistry would be aware that “solution” volume could be a different quantity that “solvent” volume.

Noterworthy, this problem could be easily avoided reporting these high concentrated solutions using the most appropiated solute/solvent % w/w units instead of molarity units.

20/09/20018. Update: Be aware, after almost five years of writing this post entry, take it for granted: if you read the experimental section of an article on the topic of hybrid perovskite that they labeled as 1 M their solutions, they actually almost very probably poured X milliliters of solvent into X mmols of solute.