Crystallization from water solutions is usually done using 2 methods:
- Using temperature solubilty curve which means cooling saturated solution or heating it (in case of salt with inverse solubility curve), for most salts it means cooling hot concentrated solution, so for convenience we will call it “Crystallization from hot solution“. This method is simple and fast, but causes rapid growth of many small crystals with higher chance of inclusions or other impurities getting into the crystal.
- Evaporation method, that is letting the solvent to evaporate, forcing the dissolved salt to crystallize. This method is much slower than the first one, but yields significantly higher quality crystals.
The method we will talk about in this article is basically a combination of the two, with the intention to use their advantages, while trying to limit their cons. Even though I found out this method myself, I later saw some other people using it. But I still believe it can be useful for many.
In short it consist of these steps:
- Crystallization from hot solution to get the seed crystal.
- The seed crystal is attached to nylon thread.
- The solution is then reheated and more salt is added until the solution is concentrated.
- Seed crystal is placed inside the solution and the solution is let to cool down again. But this time, proper thermo-insulation is used around the beaker, to slow down the cooling process in order to aid crystal quality.
- Seed crystal is removed again and steps 3-5 are repeated until crystal of desired size is acquired.
Both of the methods mentioned prior in this article have their pros and cons, and this method is of no exception.
In the experimental part we will be try to cover different circumstances where this method is or is not beneficial and why.
For the experiment, we will use magnesium sulfate to demonstrate the impact of different crystallization methods on the final result.
- First distilled water (50mL) was mixed with anhydrous magnesium sulfate (25g) and heated until it dissolved fully.
The beaker was then let to cool down freely.
the photo below shows the product of the first recrystallization.As you can see, it´s composed of quite nice monocrystals of magnesium sulfate heptahydrate. Their dimensions aren´t ideal though (the longest one is cca 3cm long). Apparently, this method alone is not usable for growing large monocrystals. but it can and will be used as a source of seed crystals for the next experiments.
- The second experiment involved two crystallization batches which were used to compare traditional evaporation process and the modified method proposed in this article.
In both cases, the same 250mL beakers were used as containers for the crystallization and the same amount of solvent (H2O, 200mL) were used to ensure objective comparison. Also constant ambient temperature (20C) was kept during the entire experiment.
In the container no.1 70g of anhydrous MgSO4 were dissolved and the seed crystal attached to a nylon thread was submerged into the solution. The beaker was covered with an air permeable tissue to eliminate dust and other impurities contaminating the solution.
The seeds were both approximately 1.5cm long and weighted 0.19g and 0.20g.
The second container was heated to 50C and 105g of anhydrous MgSO4 was added, which is about 5g more than what will really be dissolved. That ensures the solution is really saturated. the undissolved magnesium sulfate was removed and seed crystal was submerged into the solution. then the solution was let to slowly cool down in a thermo-insulated container. The solution was reheated and cooled down multiple times as described in the theoretical part of this article during the span of the experiment.
The experiment had been finished after 7 days and the resulting crystals were compared.
Using the repeated cooling has been shown to be very beneficial in case of pottasium dichromate too. It’s caused by its sharp difference in solubility at the room temperature and when heated. It wasn’t used to grow monocrystal in this case though. This method was also used to grow a crystal of Copper sulfate pentahydrate in a timeframe of 2weeks with these results:
The crystal has grown really fast, but that also caused formation of many parasitic crystals which were, unlike the MgSO4, very hard to remove.
The experimental part has shown this method has a promising potential to accelerate the growth of crystals, while keeping the quality acceptable. In some cases, it even makes the crystallization process easier than the traditional evaporation method, by avoiding issues like Creeping of saturated salt solutions phenomenon, as it’s actually dependent on the evaporation process . It is also useful when the crystal needs to be grown in a certain temperature range (due to formation of different polymorphs, or low solubility at room temperature) and proper temperature maintaining device is not available. The main Issue has been shown to be the formation of parasitic crystals, which have to be removed manually.
 The Creeping of Saturated Salt Solutions. T. H. Hazlehurst, H. C. Martin and L. Brewer The Journal of Physical Chemistry Vol. 40: , Issue. 4, : Page 444