jgourlay
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Rochelle Salt Mystery: Homejob vs. Storebought
Gents, I have a frustrating bit of a mystery I'm hoping you can clear up. I need some large (let's say 2" on a side) rochelle salt crystals for a
radio project, and I want to grow these myself.
I found a recipe on the internet for making rochelle salt and I followed those. It worked really well. Getting "seed" crystals was a snap, and
growing the larger crystals was very simple. I would note that making Rochelle salt this way leads to a mildly amber solution which includes a tiny
amount what appears to be cellulosic material. The crystals come out clear/white as does a solution made from these crystals.
However, making Rochelle salt this way is REALLY expensive, and I didn't have enough in the first run to do what I wanted. I suppose the cost of
creaming Tartars has risen with the easy availability of rifles, insurgent war, Amnesty International,and all that. Anyway, the cost of creamed
Tartars is very high.
So, I decided to buy online from B&H photo (photographers formulary). The cost of this is about half or less of what it cost me to use baking
soda and finely powdered Turks. The stuff comes in a jar of crystals smaller than table salt, but not quite a "flour" consistency. Here is where it
gets puzzling.
As you know, the solubility curve of Rochelle salt ramps dramatically with temperature. My homejob rochelle salt demonstrated this property. Even
with a large amount of regent to water, 30 seconds in the microwave was enough to make all it disappear in the water (roughly 300 ml of water).
With the store bought, the reagent dissolves "easily" but doesn't demonstrate this temperature/solubility to nearly the degree my homemade material
did. If I use the "recipe" that "Crystals and Crystal Growing" tells me, there is a little spot of "crusty" material left over that just will not
dissolve. Last night, I put the beaker in a pot of water and raised temperature to 200 degree in the beaker trying to get this last little bit to
dissolve. Finally, I ended up adding water a few ml at a time until it dissolved. I should have recorded how much, but I was in charge of "baby" at
this point, and he kept trying to stick his hand in the boiling water.
Now, the solubility sensitivity to temperature runs both ways. With my homejob Rochelle salt, I would get spontaneous seeds that got big, fast, as
the solution dropped in temperature. With this new stuff, what I mixed last night was still water clear this morning. I poured some into a petri
dish, covered, and sprinkled in some of the tiny crystals from the jar. When I came down from my shower, nothing. So I put the petri dish in the
fridge. Coffee and bagel later, I saw just the tiniest hint of little seeds starting to form, like the head of a pin.
Now, before the run above, I tried this one other time with the new material. In that case I had added too much of the material, and about a 1/4
teaspoon of it wouldn't dissolve. I decided to set this on the windowsill anyway. It took a day for that material to start falling out of solution,
and when it did it fell out in the form of a solid crust of tiny, tiny little crystals on the bottom. Again, VERY different behaviour from the
homemade Rochelle salt AND very different behaviour from what the books say about rochelle salt.
What do you all think is going on?
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ScienceSquirrel
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Try doing a melting point on both.
They should melt at 75C.
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blogfast25
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jgourlay:
Out of interest, what the procedure for synthesis you used? Internet link?
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ScienceSquirrel
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There are plenty of preparations on the internet as it is standard science fair stuff.
Here is one;
http://www.seawhy.com/xlroch.html
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jgourlay
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blogfast:
http://www.seawhy.com/xlroch.html
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ScienceSquirrel
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Casablanca!
Of all the preps in all the world you had to pick the same one as me 
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ldanielrosa
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Thank you, that was a fun one. I got the cream of tartar at the co-op yesterday and cooked the bicarbonate. Tonight I did the second step, only I
weighed the stuff before hand and exactly ran out of washing soda just as the fizzing reaction ceased. Now the liquid is clear (a good sign for
solubility) and I'm filtering it as I type.
I wanted some of this stuff because of some things I read that are (surprisingly) unrelated to electronics. I thought of buying some, but I had the
pleasure of making it for about half the price.
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