Re: recrystallization

[The Home Scientist]

Yes, recrystallization can be used to purify most of the solid chemicals used in the book. Before you attempt each chemical, look up the solubility by temperature. Chemicals that have large differentials by temperature are generally pretty easy to recrystallize in high yield. For example, a chemical may have solubility of 400 g/L at 100C and only 20 g/L at 0C. If you produce a liter of solution that's saturated at 100C and then cool it to 0C, you'll get back about 380 g of the 400 g that you dissolved at boiling. Conversely, another chemical may have 100C/0C solubilities of, say, 200 g/L and 180 g/L. In that case, you'll get back only 20 g of pure compound from the 200 g that you started with. In cases like that, you can do multi-pass recrystallization by getting the first 20 g, reheating the mother liquor and adding crude compound until it's again saturated, and then cooling again. You get back only 20 g per pass, but that original 180 g that stays in solution at 0C isn't "wasted" because you use the mother liquor over and over again.

Also be aware that recrystallization conditions can affect the hydration state of the recrystallized compound. For example, if you cool the mother liquor only slightly and hot-filter it, you may get crystals of the monohydrate. If you cool the mother liquor quickly to 0C, you may instead get crystals of (say) the pentahydrate.

On Sunday, January 13, 2013 10:02:14 AM UTC-5, chris_...@yahoo.com wrote:

A quick background, I'm 25+ years out of H.S. which did not offer a chemistry course. I believe chemistry to be magic. After aquiring my master chemistry kit at HMS Beagle (where John has been quite helpful) and Robert Bruce Thompson's book, I am finding that the volume of chemicals are not all sufficiant for the labs as noted. So I have been acquiring hardware store chemicals and would like to purify them. Without a lot of chemistry experience, Recrystallization (lab 6.3) seems a good option. However pre-reading other labs, I believe that the purity of recrystallation can only be calculated at 99.00%. This happens to be what I bought the product at. Also does this production method work for most dry chemicals in this book? I really am trying to start a thread of buying good chemicals at local stores, purifying them correctly and sharing this information with others in my situation. P.S. to Robert Bruce Thompson, your videos on you tube inspired me to purchase this kit. with regards to "The Golden Book of Chemistry for Boys" supplied with the HMS Beagle kit. There are a lot of labs/experiments that produce new compounds/chemicals. Can these be purified to a reasonably good purity? And would they be a good source for experiments in your book? 

[chris_...@yahoo.com]

If I understand correctly, the compounds/chemicals created in "the Golden Book" need to be purified and then would be an acceptable chemical for the labs in your book?

I would also like to confirm, that after recrystallization that I could deduce a monohydrate or (say) pentahydrate by making a mass to volume solution and doing the appropriate math to confirm the mono or (say) pentahydrate compound?

[The Home Scientist]

I haven't looked at the Golden Book for a long time, but as I remember it didn't require materials of any great purity. Anything you recrystallize should be fine for the experiments in the Illustrated Guide chemistry book.

Yes, you can determine the hydration state of a recrystallized compound by titration, gravimetric analysis, or other means. In general, you'd want to dry your product either by air-drying or on low heat in an oven, depending on the product. Once you've removed any free water you can do the analysis.