Sciencemadness Discussion Board

Troubles crystallizing my homemade Rochelle salt

alancj - 27-5-2007 at 01:43

As I have read, potassium sodium tartrate is supposed to be very easy to grow large crystals with. It is fitting, of course, that when I try to do it… it doesn’t work at all.

I made some Rochelle salt by neutralizing cream of tartar with sodium carbonate (made by roasting sodium bicarb) and filtered insoluble and then proceeded to evaporate the solution to saturation and crystallize. The first time I think I removed too much water and made a smooth transition from the salt in solution to just the molten salt. When it cooled I got what is shown in the pictures below. Note that in one of them it looks like my beaker is cracked… it is just a funny illusion.

I re-dissolved this, filtered again since there was some dust that had collected, and then tried not to evaporate the solution as much this time. I did this on a hot plate from an old coffee maker that keeps the solution hot but not boiling. I would evaporate, say 50 ml, then let it cool to room temp and put it into the freezer… then do it again after nothing happened, and then again… etc.

So, I never got it to make nice crystals or anything. Right now my solution is evaporated to a honey-like consistency and still nothing. I really am at a loss as to what is wrong.

Ideas, anyone?


Rochelle salt_bottom.jpg - 22kB

alancj - 27-5-2007 at 01:46

Here is the other picture.
How do I attach more than one file or image in a single post, btw?

Rochelle salt_top.jpg - 30kB

not_important - 27-5-2007 at 03:16

1) You could have used the bicarbonate directly, and kept the putity up.

2) from the colour you have some impurities in there.

Slow cooling gives larger crystals, rapid cooling of saturated solutions tend to give clusters of small crystals, which is what it looks like you have in the photo from above.

Get your salt back into solution in hot water. Filter with real filter paper, not coffee filters. Adding rubbing alcohol, say twice as much as the volume of solution. Let cool to room temperature. Filter to collect the purified salt.

Use a waterbath, a large pot of water, heated to 75 C for heating a small container that will hold your solution. Roughly weigh your salt, put a little less water than needed to dissolve that amount at 75 C into the smaller container, add the salt, and let sit for 10 or 15 minutes with occasional stirring. Stop stirring, let undissolved salt settle out, pour the clear solution into another container that has been pre-warmed in the same waterbath (you can fill it with water to keep it from floating, pour out the plain water just before pouring the solution in). Put it back into the waterbath, turn off the heat, cover the salt solution to keep dust off, go do something else for a half day.

If you want big crystals, it's better to make up a saturated solution, and let some of thar slowly evaporate over several days (keep dust out) to get seed crystals. Put several of the seed crystals into the rest of the saturate solution, or tie a thread around a crystal and hang it in the saturated solution. Keeping dust out, let the solution evaporate on its own over a period of days.

[Edited on 27-5-2007 by not_important]

12AX7 - 27-5-2007 at 08:11

Ohhh, I hate growing crystals of organic acid salts.

Lead acetate almost always does that to me. I rarely get colorless bulky crystals from the stuff.


Magpie - 27-5-2007 at 09:35

I have made Rochelle salt and a photo of my crystals are in another thread. Here's the procedure I used according to my record:

"Weighed out 39g Cream of Tartar and added to 49 mL of water. Placed slurry in 250mL beaker in water bath using Corelle bowl on stirrer-hotplate. Heated until bath water at simmer. Added washing soda (Na2CO3) carefully with spatula until no more bubbles formed. Filtered hot solution into evaporating dish. ~2 hours later 7 large crystals of Rochelle salt had formed."

UnintentionalChaos - 27-5-2007 at 12:02

I've done something similar before. Even food grade cream of tartar is rarely pure and has some organic impurities that make it that tan color. Dissolve it in as little as possible boiling water and allow to cool. Rochelle salts are known for giving large, fairly perfect crystals so crystal forming should not be much of an issue. You can clear out the color with some finely powdered actived charcoal, then filter that out if you really want. That may even improve the ability to crystallize.

alancj - 27-5-2007 at 13:20

Originally posted by not_important
1) You could have used the bicarbonate directly, and kept the putity up.

2) from the colour you have some impurities in there.

Slow cooling gives larger crystals, rapid cooling of saturated solutions tend to give clusters of small crystals, which is what it looks like you have in the photo from above.

Get your salt back into solution in hot water. Filter with real filter paper, not coffee filters. Adding rubbing alcohol, say twice as much as the volume of solution. Let cool to room temperature. Filter to collect the purified salt.
[Edited on 27-5-2007 by not_important]

I can use sodium bicarbonate :o? Why would the instructions I found on the net tell me to use sodium carbonate, made from baking soda? Any reason to use the carbonate?

I did use "real" filter paper, actually. I suction filtered it a few times through the same paper since that usually gives better filtration after the first round has plugged up the pores with crap.

Does a pure solution of Rochelle have no color? Maybe I'll try filtering with activated carbon like UnintentionalChaos mentioned. I don’t have any however. I started with food grade cream of tartar, made a suspension of it in hot distilled water and added carbonate till no more bubbles formed. I was too lazy to weigh anything :(

I will try the procedure you gave. Is the water bath for slower cooling? Is 70% isopropyl concentrated enough? How concentrated of a solution of Rochelle salt should this be added to?

What I don't understand is how my solution can be saturated enough to be exactly like warm honey, and still not solidify. The salt is supposed to be easy to crystallize because of its strong temperature dependence on solubility, but the stuff still stays the same (perhaps thickens a little) if I put it in the freezer.

Does anyone have a solubility table or graph for this salt?

Thanks for the help guys,

JohnWW - 27-5-2007 at 13:33

Rochelle salt - K Na tartrate, KNaC4H4O6.4H2O - is, when crystallized, pyroelectric. Is this the reason for your interest in large single crystals of it, Alancj and Magpie? Pyroelectricity is when certain opposing crystal planes of a single crystal develop opposite charges on being heated. Another, and more durable, pyroelectric crystalline compound is the mineral tourmaline (an aluminium borosilicate, often black in color due to the presence of iron). I wonder if it could be a means of converting heat directly into electrical current, like metallic thermocouples.

As well as being used as cream of tartar in baking powders, it is also used in silvering mirrors, and medically as a laxative (Seidlitz powder). The stuff comes in enantiomers due to optically active asymmetric C atoms in it, and crystallizes in left- and right-handed crystals - in fact, I think that it was one of the first compounds, along with sodium ammonium tartrate, to be separated into its enantiomers by manual separation of large single left- and right-handed crystals by Pasteur in 1848.

[Edited on by JohnWW]

Eclectic - 27-5-2007 at 16:11

You don't want to take the temperature much over 180F. You may be degrading the tartrate into other compounds and polymers. Also, the solution can become so supersaturated and viscous that it will cool to a glass without crystallizing, much like hard sugar candy. The molecules need to be able to move around a bit and find their place in the crystal structure. So the water bath is both to prevent overheating, AND to aid slow cooling to get large crystals.

It might be easier to start over and don't let it get so hot...
Go with Magpie's recipe, roughly 1.25 parts water for one part cream of tartar, or a 50% solution of Rochelle salts.

If you just use bicarbonate, you will give off twice as much CO2, but end up with the same thing in solution. Also, any excess would be less likely to dissolve.

[Edited on 5-27-2007 by Eclectic]

UnintentionalChaos - 27-5-2007 at 19:00

Rochelle salt crystals are piezoelectric as well, although in home conditions, It may be hard to manipulate them to this purpose since they would have to be used in dry conditions. I will try Magpie's procedure and report back since I once had a batch do exactly what alancj's is doing. I just dumped mine since it was only @25g worth of salts.

alancj - 27-5-2007 at 21:51

Originally posted by JohnWW
Rochelle salt - K Na tartrate, KNaC4H4O6.4H2O - is, when crystallized, pyroelectric. Is this the reason for your interest in large single crystals of it, Alancj and Magpie?
[Edited on by JohnWW]

Actually, I’m not really interested in large crystals, but I just wanted to crystallize the product for purities sake. Bigger crystals, however, are easier to wash, but I’m not going for any records. That’s interesting info on its pyroelectric properties, might be something to investigate in the future, though that is not what I want it for right now. I mainly just want it for use in a silver electroplating experiment on PbO2. See Pat # 3,318,794.

Now that I’m looking at the procedure again, it begs the question… can I use the tartaric acid I just bought? It calls for two solutions to be made:

Solution A: 15g of silver nitrate in 1500 ml solution at a pH of 10, using aqueous ammonia to raise the pH.
Solution B: 1 gram silver nitrate and 0.83 grams Rochelle salt in 500 ml water, boiled for 20 minutes and filtered.

Solution B, I suppose, just needs to be silver tartrate. There appears there would be some excess silver nitrate present, though. If it was Stoichiometric, you would need 1.21 grams of Rochelle salt. So, I’m guessing that if I wanted to use my taratic acid, I would need to dissolve AgO, instead. Doing it this way, I wouldn’t have any NaNO3 or KNO3 hanging around, and I would need to up the solution A amt. of silver nitrate by 0.32g, which is probably insignificant. I wonder if the nitrates play any role in the electroplating. What does the ammonia do, do you guys suppose? Neutralize any nitric acid generated? The plating is done with a silver wire as the anode, so it should keep the silver ion concentration constant.

Anyway… I think I will take the solution I have, add alcohol to it, and see if I can get anything to come out, If not, I will just start over and implement not_important’s and/or magpie’s suggestions. I have had this stuff sitting in the beaker for months… putting-off trying to figure out what the problem is. There is probably only about 50 grams that would go to waste if I were to toss it (the beaker unavailable for use for 4 months is the bigger waste). Is it likely that my possibly overheated and rather old stuff could be polymerized or decomposed into something else by now like eclectic said?


not_important - 28-5-2007 at 01:35

Pure Rochelle salt is colourless, yellow-brown tints are suggestive of 'gorp' - complex mixtures of organics, possibly formed through condensation reactions. Such mixtures can interfere with crystal formation, as solvent soluble polymers often do.

If you have access to Na2CO3 then use it, otherwise NaHCO3 and gently boiling the fairly dilute solution works well RCO2H + NaHCO3 <=> RCO2(-)Na(+) + "H2CO3" = > H2O + CO2 (gas)

You might first try heating to say 60 C, in a waterbath, add water to get full solution, turn off the heat and let cool to room temperature in the bath, and see what happens. If you get decent crystals, then try chilling or slow room temperature evaporation to increase the yield.

By the way, when crystallised quickly the salt tends to form rounded clusters of crystals, which looks as if what happened to you.

Isopropanol 70% should work OK, but you'll lose a couple of percent more of the salt in the solution. You want at least 1/3 of the total liquid volume to be alcohol, the higher percentage the better. You might warm the beaker to 75-80 C in a waterbath, to avoid local overheating, add just enough water at that temperature to get full solution, and then pour the solution into the proper amount of alcohol with strong stirring. The colour should stay with the alcohol, distill that to recover the alcohol as the azeotrope.

Attached is some data on solutions of Rochelle salt

[Edited on 28-5-2007 by not_important]

Attachment: je60068a005.pdf (257kB)
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unionised - 28-5-2007 at 06:57

I think you have got the salt you are after but it hasn't crystalised well. If I were you I would disolve it in as little cold water as possible then let the water evaporate slowly. I have done this long ago and I have a crystal that's about 2 inches by 1.5 (only about1/4 inch thick).

prole - 7-7-2007 at 15:21

Can anyone recommend a suitable brand of tartar sauce as a source of potassium ditartrate? I've been to all the local grocery stores doing some good ol' label reading, and can only find tartaric acid listed on a select few brands. No other brand lists tartaric acid or its salts in the ingredients. I'm almost ready to just buy potassium ditartrate and proceed from there.

alancj - 7-7-2007 at 16:24

You want cream of tartar which is a powder, not tartar sauce which is just a... sauce. It may or may not have an ingredients list. There are some brands of "cream of tartar" which are not potassium bitartrate (potassium hydrogen tartrate) at all.


cnidocyte - 22-3-2011 at 12:57

I made some Rochelle salt which I'm gonna need for making biuret reagent. I ended up with a big solid chunk of it too since I when I boiled off the water I was left with molten rochelle salt. Mine is tinted yellow too. I had no luck recrystallising it from water, anyone know a good solvent to recrystallise rochelle salt?

ScienceSquirrel - 22-3-2011 at 13:32

There are recipes for Rochelle salt on the internet.
It is a standard experiment for big crystal competitions / science fairs.
It is easy to make and crystallise by following standard published procedures.

cnidocyte - 22-3-2011 at 14:04

Actually on closer inspection, my Rochelle salt is colourless. I had some serious trouble weighing out 6g of the stuff for making biuret. I could have filed down the brick of it I had but it would have taken all day so I melted it instead and weighed out the molten salt. I suppose crystallising is the easiest way to get the stuff in powder form.