Natural tartaric acid is l-tartaric.  It is rare in nature to have anything other than stereospecific molecules.  This is a consequence of enzymatic synthesis.

There is a method for separating dl rochelle salts based on their solubility.. but maybe this is one of those things that would be worth ordering from another country.

!wiki:

"Naturally occurring tartaric acid is chiral, meaning that it has molecules that are non-superimposable on their mirror-images. It is a useful raw material in organic chemistry for the synthesis of other chiral molecules. The naturally occurring form of the acid is L-(+)-tartaric acid or dextrotartaric acid.

"A rarely occurring optically inactive form of tartaric acid, DL-tartaric acid is a 1:1 mixture of the levo and dextro forms. It is distinct from mesotartaric acid and was called racemic acid (from Latin racemus - "a bunch of grapes")."

Much of that assumes the source was produced by biosynthesis.

It might be generally safe to assume samples consist of a single isomer.

From what has been said, many probably do.

A studious bee would be advised to find a way to ensure purity. It would be a shame for a bee to waste many hours and much of his meager discretionary income on pricey supplies only to flush it away with an assumption. Pure science does not like assumptions.

I would look into the process in which tartaric acid is manufactured, if it is perhaps by fermentation, than it should be good, and if it is by some chemical means it is bad...
So what way is it normally produced for the food industry?

That probably makes too many assumptions regarding what tartaric acid is out there, if it may have been blended, where it came from, and how it was made, at least for a general rule to be established.

You could always buy a small amount at first and test its optical purity by getting two dirt cheap polarized lenses either from a 3D movie house, or pop some out from some sun glasses used by fly fisherman. I did this with karo syrup and you could see light changing, etc because of the sugar and angles of the lenses. if it were DL you would see no difference, I think.
I believe that would allow you to determine if it is optically pure or not, but not inform you of the D or L unless you wanted to get more into it. right?

I'm thinking something like this:

1. Buy a quantity of tartaric acid. It is dirt cheap and readily available, so it makes sense to start off with a large enough quantity.
2. Tartaric acid is optically active. Test purity by examining optical rotation through a polarized lens. If it appears to be pure, then there is at least a substantial enantiomeric excess of one or the other, and further testing is warranted.

Now this is where things become tricky. If the unknown tartaric acid has a substantial excess, but not enantiomeric purity, then there will be a problem.

The procedure could be:

3. Use tartaric acid in substantial excess to resolve a small, accurately weighed quantity of a sample.
4. Bioassay the result. The results of the bioassay on a resolved sample will determine the configuration of the bioassayed sample, allowing one to backcalculate the nature of the tartaric acid which produced that sample.

However, this would neither establish enantiomeric purity, nor would it accomplish much beyond the optical test, although this can be viewed as an alternate procedure avoiding the need for physical examination of its optical properties.

(4) would also be dependent on the sample. (3) and (4) would only be applicable if a subjective bioassay can reliably distinguish the difference.

Perhaps:

5. Calculate yield of resolved sample compared to unresolved input.

If (5) yields a mass in line with what would be expected from the resolution procedure, then the enantiomeric purity of the unknown tartaric acid can be deduced. If the yield exceeds that published in the literature, or goes substantially over 50% (or whatever is appropriate for the sample input), then the tartaric acid lacks purity, for otherwise it could not precipitate out more of one configuration than existed in the input sample.

If all the tests are passed, then it should validate the suitability of the tartaric acid. Tartaric acid being cheap, a bee would have been wise to order enough to avoid frequent repetition of this time consuming procedure.

This should give both the identity of the unknown tartaric as either predominately D-tartaric or L-tartaric as well as a reasonable basis for its enantiomeric purity.

I like the simplicity so far because it requires no equipment or reagents beyond what was already required to get to the sample point.

Any mistake so far?

But doesn't that assume pure D, pure L, or a balanced DL?

What is readily available is food grade. We don't know where it came from, what process(es) were used to make it, or if it was blended somewhere along the chain. For all we know, the mom and pop wine supply store switched suppliers at some point and is shipping a haphazard blend.

The problem is food grade tartaric acid is fungible for food use but not for laboratory use.

Any large commercial lab or industrial customer would be buying large lots of exactly what they need, knowing exactly what it is, not improvising as hobbyists are forced to.

Suppose you are dealing with 90% D and 10% L. That becomes a problem.

Despite improvisation in sources and some methods, there is still something to be said for doing things the 'right' way and taking an interest in the science and technique behind it all. I feel that is what separates this place's mindset from some of the others.

I don't know, it just feels wrong to mix in grab bag tartaric acid with complete ignorance of composition after many hours spent carefully paying attention to process and quality. I guess it is the juxtaposition which is so bothersome. After so much precision, so much procedure, and so much extra work making OTC items into lab grade supplies, and trying to work like a real scientist (not a "cook" - I hate that mentality), it seems rather anticlimactic to use mystery tartaric acid.

This is my hobby. I find it interesting and intellectually stimulating when one can use one's mind to work with skill and precision despite a social structure which tells us all we need is available on a store shelf. I am not nearly as knowledgeable as I would like to be, or as many of the people here, which is why I was hoping to formulate a proper procedure.


That being said, any idea how tartaric acid behaves during recrystallization if it is somewhere between a pure D/L and a balanced DL?

You just simply aren't going to run into d-tartaric OTC in any usable amount.  Have you looked at the price differences between the 2?  If for some crazy reason an industrial process produced a mixture of both they would separate the d-tartaric and sell it at a huge profit.  Count out any process related to wine or organic sources as it was stated about that only the natural l-isomer is produced in those industries.   What I was saying is that you should find DL-potassium sodium tartrate and use the solubilities to separate those and covert back to tartaric.
p.s. interesting fact, uncle fester claimed to have been visited long ago by "chemical scrutinisers" after ordering rochelle salts, doesn't mention which configuration.