Talk:Specific rotation

message: OK, I'm back once more to rant about units. First of all, my thanks go out to User:Orgchemprof for his/her contributions to the page, especially for adding the distinction between pure samples and solutions. But in my opinion, the article has become very confusing. Specifically, regarding the two forms of the equation
:[\alpha]_\lambda^T = \frac{100 \alpha}{l \times c}, where c is in gm/100ml  
:vs.    
:[\alpha]_\lambda^T = \frac{\alpha}{l \times c}, where c is in gm/ml
What is a novice reader supposed to make of the following statement?:
:"Mathematically, the two forms are the same, but chemically they are very different. "
I just don't follow it. If two formulas are mathematically the same, then they are the same.  And the phrase "Because the units [of concentration] are not reported..." frankly points to a larger problem that no Wikipedia article is going to solve. 
Spiel496 23:23, 3 February 2007 (UTC)
replies: text: I did a Google book search, and found Orgchemprof's version of the equation in a couple of places.  They were old books; maybe conventions have changed.  If it is an issue with the rotation not scaling linearly with concentration, perhaps the equation should be dropped completely, as it implies linearity.  The specific rotation could be defined solely with the statement at the top of the page: "the observed angle of optical rotation when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 100 millilitre."  Spiel496 18:11, 23 February 2007 (UTC)
replies: text: Conventions have not changed, people just don't do these measurements as often as they used to, and people are lazy to look up proper definitions outside Wikipedia. It is absolutely necessary to use the 100x equation, because concentration is given in g/100 mL. If you change the equation by removing the 100 x and using c = g/mL, then the value of c (which is given in brackets without units) will be 100 times high! So the problem in those definitions is not that alpha will be wrong (its value comes out the same), but that the value of c will be wrong! 92.194.61.30 (talk) 10:09, 27 March 2014 (UTC)

text: I didn't propose that definition -- it's from the lead paragraph of the article. I think it's consistent with the equations. Spiel496 06:26, 24 February 2007 (UTC)

text: Hmm... Sounds suspiciously like original research.  Seriously, though, I appreciate your comments and recent edits. I have long felt that the handling of units in this article was confusing. Your own (temporary) confusion serves as an illustration of what I'm talking about.  I'll end by pointing out that all confusion could have been avoided if the table of values were listed with their full units, eg. "D-Sucrose: +66.37 deg dm−1cm3 g−1.   Spiel496 (talk) 18:32, 27 June 2014 (UTC)
thread_title: Units Nightmare

message: The equation for solutions uses a concentration in g/100mL for a practical reason: you cannot dissolve 1g of most organic compounds in 1mL of solution. It is however, very common and easy to dissolve 1g of substance in 100mL of solution. Virtually all organic compounds can be dissolved in a solvent at this concentration, and for this reason, the unit of g/100mL is used to report specific optical rotation.
To compensate for the fact that concentration is measured using the unit of g/100mL, the numerator of the solution equation is multiplied by 100 as a correcting constant so that the equation is mathematically comparable to the pure liquid definition.
: No, maybe you cant dissolve grams of organic compounds in 1 mL of solution, but then you dissolve milligrams and put it in the formula as 0,0XX g and you can still use the formel. Nobody is making 100 mL solution to measure the specific rotation, when you just need a few mL to do so. The formula with 100 on both side of the fraction is just stupid, and makes anymore strange then normally. Christian75u (talk) 10:42, 30 July 2009 (UTC)
replies: text: No my friend, it is not stupid with a 100 in the formula. If you change the concentration from g/100 mL into mg/mL then you are using a wrong concentration value! Example: 0.5 g / 100 mL is a concentration of c = 0.5. If you interpret it as 500 mg/100 mL = 5 mg/mL, then the numerical value of c is 10 times high. You cannot remove the 100 and change the concentration unit, since the concentration unit is clearly defined. If you leave it like it is now in Wikipedia, then we will soon have specific rotation values with wrong c values in the literature (because students look it up here rather than in real books).
This solution form of the equation can be found in older textbooks (older than 1980, especially in laboratory texts). What has happened over the years is that new textbooks report only the formal definition (similar to that used for pure liquids). As many textbook authors use as a source of information other textbooks, the situation has been repeated to the point that texts all use the formal form, not the equation that is actually used. 
In the CRC handbook, the equation in the definitions pages uses formal units of g/mL. However if you carefully read the data tables containing optical rotations, you will see that the specific rotations are invariably given using concentrations in g/100mL (with the correcting factor of 100 added to the numerator). A few tables provide the solution form of the equation using the g/100mL convention and 100 in the numerator (it is in the footnotes to the tables). The solution form of the equation can also found in the characterization requirements for chemistry journals like the Journal of Organic Chemistry (http://pubs.acs.org/journal/joceah), see the “Guidelines for Authors”.
User:Orgchemprof|Orgchemprof]] (talk) 19:58, 12 December 2008 (UTC)
thread_title: Units Nightmare Clarified

message: Imagine if you were to encounter the following while trying to learn about "velocity"...
:The specific velocity is given by:
: v = \frac{d}{t} where d is in miles and t is in hours
:However because it is impractical to observe motion for a whole hour we use this formula:
: v = \frac{60d}{t} where d is in miles and t is in minutes 
:The formal units of velocity are "miles per hour" but usually speeds are reported simply as "miles".
Unnecessarily confusing, right?  However, that is how the specific rotation article treats units. Because I know zilch about organic chemistry, I've been reluctant to push my point of view here. However, I invite you to hop over to some articles that I am qualified to judge, Faraday effect and Verdet constant, and see how they approach what is a very similar phenomenon. These articles also describe how the plane of polarization is rotated. Instead of concentration, the rotation depends on the strength of a magnetic field, but otherwise the situations are very similar. The Verdet constant is the material property, and it is analogous to the specific rotation.
The Faraday effect article isn't perfect, but it achieves something I wish Specific Rotation article could: The Verdet constant is defined via one unambiguous formula without getting tied up in knots about the units. Here it is (after an algebraic rearrangement to make the connection to this article more clear):
: \mathcal{V} = \frac{\beta}{B d} 
:where β is the angle of rotation, B is the magnetic field and d is the path length.
This formula is true, regardless of the units used for β, B and d.  There is no need for a second formula in case, for example, B is in Gauss instead of Teslas. How is this magic achieved? It's simply that when one quotes the value of Verdet constant for a material, one spells out the complete units.  For example, the Verdet constant for TGG is −40 rad/(T m) = −0.40 rad/(T cm) = −23°/(T cm) = (etc.).   One does NOT say the Verdet constant is simply 23° and then append a paragraph defining the standard magnetic field and the standard length.  Doing so would be cumbersome and would lead to confusion.  
I do not dispute that reliable sources quote specific rotation simply as "degrees" without remaining units. I do not dispute that text books insert that factor of 100 in the formula so that it can be canceled out with the phrase "c is in g/100mL". However, Wikipedia does not need to parrot this strange convention. What I propose is, that we follow the convention of other articles on Wikipedia and treat the units as described in Units of measurement. The specific rotation of sucrose should be stated as "66 deg dm−1cm3 g−1". We should provide one formula (without the factor of 100) for [\alpha]. Finally, we let the reader know that if a piece of literature quotes the specific rotation as "66 deg",  they really mean "deg dm−1cm3 g−1". Spiel496 (talk) 06:14, 17 January 2011 (UTC)
replies: text: Wikipedia can't take the approach you suggest, and it's even almost exactly for a reason you even mentioned: reliable sources quote specific rotation simply as "degrees" without remaining units. Per policy, wikipedia can only say what is already said and in use by others, we can't push an improvement ourselves that isn't already in reliable sources. DMacks (talk) 22:20, 17 January 2011 (UTC)
replies: text: As this article currently has no references at all, it seems premature to start quoting the reliable sources doctrine.  Anyway, I'm not seeking to "Right a great wrong" in the field of organic chemistry. I'm simply trying to help a Template:technical-tagged article to make more sense to people outside the field.  This means removing jargon, and it appears that "66 deg" is jargon for "66 deg dm−1cm3 g−1".  So that's my proposal. Doing nothing is the wrong choice. I'm curious to hear other solutions.   Spiel496 (talk) 23:56, 17 January 2011 (UTC)
replies: text: Yikes, the article really is poor! I added a ref that specifically supports use of both the weird combination of units in the component measurement (dm, g/mL, etc) and reporting as just an angle-measurement. It may be jargon, but it's the jargon that's used. DMacks (talk) 15:47, 22 January 2011 (UTC)
replies: text: Okay, it's still confusing, but can accept that's "the way it's done". Thanks for looking into it.  Spiel496 (talk) 00:58, 25 January 2011 (UTC)
thread_title:  Can we approach the units in a more standard way? 

message: Users and editors of the article on this page should be aware that there are closely related articles on polarimeter and polarimetry. Unfortunately both are at present of fairly poor quality, but they do go a long way to answering questions like "Why is anyone interested in specific rotation?" and "Why is this page needed?"
Damocles2 (talk) 23:53, 21 October 2011 (UTC)
thread_title: Links to other Wikipedia articles

message: I changed the 'lead missing' template to 'lead rewrite' reflecting a recent reorganisation I made to separate the lead from the rest of the article. The lead does, however, still need plenty of work. I'm considering moving some of the more specific information relating to the measurement of [α] down the article, and bringing in an illustrative diagram or two from here or using the image on the right. Tomásdearg92 (talk) 13:06, 10 January 2013 (UTC)
thread_title:  Lead Section 

message: I've expanded, reorganized and revised this article over the last week or so. The issues noted in the message box at the top of the page have been, in my opinion, largely addressed. The article had two citations before, and now it has twelve, some of which are used more than once. There are still some unsourced claims that deserve sources, but those are noted in the text. The lead section was rewritten, and should be (I hope) accessible to a lay audience now. As for being too technical ... well, I think that, at least, the lead section should be understandable. Perhaps I'll leave the "citations" message box up there until I can track down some final sources, but I think the other two issues have been largely resolved. Any objections to removing the "lead rewrite" and "technical" messages? Ohnodoctor (talk) 20:58, 30 June 2014 (UTC)
replies: text: Personally, I support removing refimprove and lead rewrite: I was thinking of removing those myself after seeing your edits. I still find the treatment of the "separate" equations for pure liquids and solutions odd and overly technical-sounding (in other words, obscure). Perhaps that is just me. Tomásdearg92 (talk) 21:13, 30 June 2014 (UTC)
replies: text: Oh, yes. Looking at it again, I think you're right about the technical tone. I like the version in your sandbox better. Ohnodoctor (talk) 14:26, 1 July 2014 (UTC)
replies: text: Thank you Ohnodoctor, that was very nice of you. I took a step back from the page to see if anyone else was going to comment and the measurement section changed entirely! I still think it could benefit from a similar reorganisation/rewrite referred to above (and since then, below). Now that nothing has progressed on the page for two months I'm thinking of being a bit more WP:BOLD. It appears now that specific rotation is used in solid samples (see below), and I just checked today (this) thin films and suspensions too. Tomásdearg92 (talk) 23:38, 29 September 2014 (UTC)
thread_title:  Article revision 

message: We should consider adding a simple diagram to this webpage. It would help describe the theory and it would help remove the complexity comment on the top of this page. Visualizing that a stereospecific molecule rotates plane polarized light is different then what someone might interperate when they read the theory. Can anyone find theory that describes predicting the specific rotation? I searched my college library and I can't even find specific rotation mentioned in books other then the CRC. I wish the page include the most basic theory of predicting specific rotation. In my opinion, the page doesn't deserve the complexity marker at the top of the page. Specific rotation is a physical property of stereospecific molecues and it just has complicated units. 
:Hi TerpeneOtto! As far as I'm aware, there is no theoretical framework for predicting the specific rotations of individual substances. There are some images floating around on Wikimedia Commons, and I meant to add one to the Lead two years ago. Maybe I'll get around to that now! Tomásdearg92 (talk) 23:45, 21 November 2016 (UTC)
replies: text: It's only been a fortnight already... Tomásdearg92 (talk) 01:02, 7 December 2016 (UTC)
thread_title: Picture request