Cross-polarized crystal of 'Floralife Crystal Clear'

rjlittlefield · Post by **rjlittlefield** » Sun Jul 23, 2017 4:03 pm

Cross-polarized at 10X on Canon T1i, somewhat cropped, frame width here 1.41 mm. Short stack at 5 microns, Zerene Stacker DMap.

Normally an image like this would appear in Photography Through the Microscope, but this was shot using a macro setup because that's what I happened to have handy. Illumination was from a single Jansjö LED, reflecting light bounced off an index card up through a polarizer and the subject. Optics above that were a Mitutoyo 10X objective, another polarizer, and Raynox 150 as tube lens, with bellows to the camera body. ISO 100, 1/8 sec, incandescent color balance. The polarizers were just cheap circular polarizers, the bottom one flipped to put the linear faces together. The image has not been curves or levels adjusted -- these two polarizers just happen to give a nice dark extinction that the camera sees as almost pure black.

I presume you're wondering what chemical this is. Me too. It formed on evaporation of a solution of "Floralife Crystal Clear". But I can't find anything like a detailed ingredients list for that stuff.

The manufacturer just says Floralife Crystal Clear® Flower Food contains three main ingredients: nutritional supplement, pH adjuster, and stem absorption enhancers. [ref]

I did track down an MSDS, but the only ingredient it lists is "Hydroxycarballylic acid 1-5%". I did not recall ever hearing of that compound, so I tried to find more information. To my surprise, web search found only 35 results(!), all traceable to Floralife or related mixtures. Hhmm...a typo, perhaps?

But wait, the MSDS also lists a CAS Number for that compound: 77-92-9. So I looked that up by CAS search. The CA Index Name is "1,2,3-Propanetricarboxylic acid, 2-hydroxy-". The name "Hydroxycarballylic acid" did not actually appear in the entry, but there's a long list of other names: 2-Hydroxy-1,2,3-propanetricarboxyic acid; 2-Hydroxy-1,2,3-propanetricarboxylic acid; 2-Hydroxypropan-1,2,3-tricarboxylic acid; 3-Carboxy-3-hydroxypentane-1,5-dioic acid; and so on.

Still quite confused, I kept reading. Finally, in the middle of the list, I saw a name I recognized: "citric acid". Ah, the pleasures of marketing!

Now, if I'm reading other materials properly, citric acid is not optically active. So I guess we can rule out citric acid for this particular crystal.

What else might it be? I have no idea. And I have to confess, I'm only mildly curious. All I really wanted was some reasonably attractive image that I could use as a handle to tell the "Hydroxycarballylic acid" story. I think this image works OK for that, so I'm happy.

I hope you find this amusing. If anybody has more info, I would be interested to hear.

--Rik

ChrisR · Post by **ChrisR** » Sun Jul 23, 2017 6:20 pm

Searching does find references to optically active hydroxycarboxyllic acid compounds. It's widely used (found a "soil conditioner"), could be that. Not surprising that asymmetric isomers are chiral I suppose.

Pau · Post by **Pau** » Mon Jul 24, 2017 1:48 am

...citric acid is not optically active. So I guess we can rule out citric acid for this particular crystal

AFAIK the term optical activity usually refers to the chemical in solution. The effect is pretty weak and to demonstrate it with sugars at the classroom without a proper polarimeter I needed to make really concentrate solutions.

The birefringence due to optical anisotropy of crystals is a different phenomenon much easier to spot in cross pol. All transparent crystals except those of the cubic system do show it. Citric acid crystallizes in the Monoclinic system, so it must be birefringent when solid.

rjlittlefield · Post by **rjlittlefield** » Mon Jul 24, 2017 2:09 am

Pau, thanks for the information. I'm always happy to get educated.

Googling for "citric acid crystals" images, I find https://www.dreamstime.com/stock-photos ... ge10216613 which indeed looks very much like what I have. But I suppose lots of other things would crystallize into similar forms too.

--Rik

ChrisR · Post by **ChrisR** » Mon Jul 24, 2017 4:09 am

I'd read previously of "optically active" in relation to solids. I hadn't seen it as an unusual adjective? https://www.fiberoptics4sale.com/blogs/ ... -materials . Optical activity (rotation of the plane of polarization) doesn't imply birefringence.

A note on 6 of the 7 structures under Crystallographic Origins of Birefringence: http://hyperphysics.phy-astr.gsu.edu/hb ... biref.html

Pau · Post by **Pau** » Mon Jul 24, 2017 4:24 am

I'd read previously of "optically active" in relation to solids...

Me too. This is why I wrote AFAIK and usually. I've read several Biochemistry and Mineralogy textbooks and articles over the years and I refer to just the normal use of the terms I've found.

ChrisR · Post by **ChrisR** » Mon Jul 24, 2017 4:55 am

I've read just a few, but recently. I find it a bit of a minefield of mixed terminology and partial information.

mtuell · Post by **mtuell** » Mon Jul 24, 2017 9:52 am

Pau is essentially correct. I'm not sure about the solid/liquid aspect, but I can't think of an example of a chiral solid at the moment. Certain liquid crystals are chiral, notably our friend cholesterol (a cholesteric liquid crystal).

Optical activity as a term is generally reserved for substances with circular birefringence, which rotates the plane of polarization. Linear birefringence, on the other hand introduces a phase shift between linear components, turning the polarization state (in general) elliptical.

You can tell the difference if you rotate the sample between crossed linear polarizers. Linear birefringence will have a pair of axes (fast and slow), which if the input linear polarization is aligned with one of them, no change is evident in the (mostly blocked) light going through the analyzer. In other words, you will see colors come and go as you rotate the specimen. On the other hand, if you are looking at optical activity between crossed polarizers, rotating the specimen won't have any effect since it is still rotating the plane of polarization the same way.

Mike

ChrisR · Post by **ChrisR** » Mon Jul 24, 2017 1:11 pm

This was one of the somethings I read
https://www.chemistryworld.com/news/chi ... 80.article
"100,000 out of 130,000" non biological crystalline solids are chiral. Really?

I really should read less.

Edit - I've since found that very ordinary solid Quartz is Optically Active in the C direction. It rotates the polarization one revolution every several mm, strongly dependent on wavelength.

It's also birefringent normal to that axis, but not strongly.

Roel Wijtmans · Post by **Roel Wijtmans** » Wed Jul 26, 2017 8:35 am

As was mentioned above, optically active is a different thing and only applies to compounds in solution. Citric acid is an achiral compound and thus optically inactive, but its crystal structure does give rise to birefringence, which is what you see here.

I've done a whole bunch of cross-polarized shots on citric acid and I find it to be one of the most interesting of the compounds I tested. Below is one picture with citric acid.

rjlittlefield · Post by **rjlittlefield** » Wed Jul 26, 2017 7:35 pm

Thank you all for the observations and explanations. I have learned much!

As a reward(?), let me offer another view of the crystal as it continued to grow, unconstrained by a cover slip and sort of sitting on the edge of a pool of sticky non-crystallizing stuff. (The pool is perhaps some sort of sugar?)

This is just the left side of the original crystal, rendered in cross-eye stereo. The stereo separation is artificially large, resulting in about 4X exaggeration of the depth relief. I thought the structure was interesting, especially the on-edge plates at upper right.

--Rik

mtuell · Post by **mtuell** » Wed Jul 26, 2017 10:33 pm

Very nice, Rik!

Thanks for sharing,
Mike

Roel Wijtmans · Post by **Roel Wijtmans** » Thu Jul 27, 2017 7:26 am

Wow, it never even crossed my mind to do a stereo of such a scene. Looks great!

rjlittlefield · Post by **rjlittlefield** » Thu Jul 27, 2017 12:33 pm

Roel Wijtmans wrote:Wow, it never even crossed my mind to do a stereo of such a scene. Looks great!

Thanks! To clarify, this is Zerene Stacker synthetic stereo, computed as +-3% shift from a stack of 41 frames at 5 microns, so total depth 200 microns, with a frame width of 2.23 mm (22.3 mm sensor width at 10X). Following the calculations HERE, a total of 10 degrees separation (+-5 degrees) would require a shift of only +-0.78%. 3% is about 4 times larger than 0.78%, hence my comment about exaggerating the depth by about 4X.

I tried it with the non-exaggerated shift, but the result was about what you'd expect -- not nearly as interesting.

--Rik

Sumguy01 · Post by **Sumguy01** » Sat Jul 29, 2017 5:15 pm

The stereo shot is amazing.
Thanks for sharing.