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The units for specific rotation are handled in a sloppy way -- not just in this article, but in the CRC Handbook as well. Because the definition specifies a path length of 10 cm, I assume one could correctly write that the rotation of a sucrose solution is:
However, the statement "The formal unit for specific rotation values is deg cm2 g-1 " implies that Sucrose has a specific rotation of +66.47 deg cm2 g-1 . I genuinely do not know which is correct. Spiel496 19:51, 27 February 2006 (UTC)
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
What is a novice reader supposed to make of the following statement?:
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)
I was initially confused about this as well. Orgchemprof is right about them being inequivalent chemically, though he doesn't give a reason why this would be significant. Since this is a phenomenon not entirely unlike that described with Beer's Law, it may be that the optical rotation values don't scale linearly with concentration when at high concentrations. In such an instance, the exact concentration used to generate the specific rotation would become important. -- Uberhobo 17:20, 23 February 2007 (UTC)
Actually, upon asking around my department, the organic chemists use the "wrong" form of the equation, and my CRC handbook of chemistry and physics lists the g/mL version of the equation, as well. I'd be interested to see where Orgchemprof acquired his version. -- Uberhobo 17:32, 23 February 2007 (UTC)
The equation does imply linear scalability, and it holds for concentration ranges that would normally be used. Lots of linear relationships for solutions break down at high concentrations. But the equation is pretty essential here. I tried finding the IUPAC definition, since that would be as official as you get, but I couldn't find it on their website anywhere. The definition you propose above would be too small by 2 orders of magnitude. It does seem like a minor quibble, though. -- Uberhobo 23:23, 23 February 2007 (UTC)
I just wasted the first half of my day trying to figure out why the specific rotation for one of my compounds was off from the literature value by two orders of magnitude. The reason, I soon learned, was that the equation for solutions listed in this article was simply incorrect: The equation without the "correction factor" of 100 was shown, but the units for concentration were listed as g/100mL. This is inconsistent with either of the proposed equations discussed here on the talk page. Since I'm of the opinion that the g/100mL unit is a dumb historical artifact, I've changed the article to use the g/mL unit for solution concentration. If you do change the units back to g/100mL, please be sure to correct the equation as well. Ohnodoctor ( talk) 13:34, 27 June 2014 (UTC)
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.
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 ( [1]), see the “Guidelines for Authors”. User:Orgchemprof|Orgchemprof]] ( talk) 19:58, 12 December 2008 (UTC))
I decided to clarify the confusing units related rant on the main page. It was entirely unnecessary, and largely untrue. -
94.192.232.74 (
talk)
23:42, 24 November 2009 (UTC)
I wonder if one piece of valuable information, that by convention the path length is one decimeter unless noted, should have been preserved. Is it somewhere else? -- AJim ( talk) 20:35, 25 November 2009 (UTC)
References
A rapidly-reverted edit to this article here added this at the start:
"In stereochemistry, specific rotation is a method of doing something that is important for some reason or another, and should be explained a lot better by the brainiacs responsible for creating this article, so that casual readers can understand why this is important".
While the edit was perhaps mischievous, I'm inclined to agree with the editor. An article like this needs to indicate not only how whatever 'specific rotation' is is done/performed/measured/revolved/'turned upside-down so the makers label can be inspected', but also why. Can a brainiac please help out? AndyTheGrump ( talk) 16:58, 27 October 2010 (UTC)
Imagine if you were to encounter the following while trying to learn about "velocity"...
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):
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 . 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)
Well, does it? This article (sort of) tells you what it is, but doesn't tell you why anyone should be interested in it as a subject. It tells you nothing about its purpose, or anything else much at all. Who invented/discovered it? Are there other methods to get the same results? Why would anyone who didn't already know how to do it be remotely interested in the article?
It is a long time since I did any chemistry, and it was never my favourite subject, but at least when I did it I usually had a vague idea about what I was trying to achieve. This article seems to be describing something or other you can do to some substance or other, which results in... well a result. A number. Now this number may well be worth millions of dollars in the fertiliser industry or whatever, or it may just be something you note down on the back of an envelope and forget about. I suspect reality is somewhere in between, but it would help if this article was a little more, well, specific. Please tell us ignoramuses who does it, why, and when, and what they do with the result. If you can't do this, then I'm inclined to think that this is one more (unsourced) article that nobody will miss if they can't find it on Wikipedia. Prove me wrong. AndyTheGrump ( talk) 12:27, 22 January 2011 (UTC)
Well, I'm no chemist, but if it makes sense to do this from a technical viewpoint, the optical rotation article certainly provides context. I'd say this sounds entirely sensible: The merely curious can see the why, and those who need to know can see the how. Perhaps a few of the merely curious might even find some of the how interesting too. AndyTheGrump ( talk) 16:08, 22 January 2011 (UTC)
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)
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)
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)
Optically active solids rotate light too, but this article only discusses solutions and pure-liquids. The CRC definition does not mention that the material be in solution at all, let alone be liquid, but the lede now explicitly specifies "solutions". The standard analysis in this state appears to be values based on a 1 mm thickness. [1] In Journal of the Chemical Society, London (volume 34, page 462) is an early discussion of specific rotation of insoluble solids. And seems to be a discussion in the preceding pages about how concentration is not a linear effect (although the additional terms in the rotation formulas might be only small values). DMacks ( talk) 02:18, 13 July 2014 (UTC)
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.
One thing that doesn't appear clear is that not even quantum chemists know why specific optical rotation happens. It is such a high level chemical and physical effect and so advanced that one cannot ask to make it simpler. Gotta study chemistry to even get a grasp of it. Wikipedia is a reference, not a tool for a person that has no knowledge whatsoever to get to a BSc level. It isn't an instrument to be used when doing research at university level either, but it can help those skilled in the art to get quick reference to something. There are specific reactions in Wiki, there are frontier particle physics theories as well and in none of those cases it implies that a person without the rudimentary knowledge can be capable of understanding ANY bit of it. And it's perfectly fine the way it is. — Preceding unsigned comment added by 194.72.164.66 ( talk) 09:28, 23 May 2017 (UTC)
I just looked at the picture, which is quite nice, but I wonder about one detail. I am not sure about actual practice in specific instruments, but I believe that the general rule is that it is much more accurate to detect the angle of the extinction null, when the analyzer is crossed, than to detect the broad peak, where the intensity changes very slightly with small angle changes. AJim ( talk) 01:43, 31 July 2019 (UTC)
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Reporting errors |
The units for specific rotation are handled in a sloppy way -- not just in this article, but in the CRC Handbook as well. Because the definition specifies a path length of 10 cm, I assume one could correctly write that the rotation of a sucrose solution is:
However, the statement "The formal unit for specific rotation values is deg cm2 g-1 " implies that Sucrose has a specific rotation of +66.47 deg cm2 g-1 . I genuinely do not know which is correct. Spiel496 19:51, 27 February 2006 (UTC)
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
What is a novice reader supposed to make of the following statement?:
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)
I was initially confused about this as well. Orgchemprof is right about them being inequivalent chemically, though he doesn't give a reason why this would be significant. Since this is a phenomenon not entirely unlike that described with Beer's Law, it may be that the optical rotation values don't scale linearly with concentration when at high concentrations. In such an instance, the exact concentration used to generate the specific rotation would become important. -- Uberhobo 17:20, 23 February 2007 (UTC)
Actually, upon asking around my department, the organic chemists use the "wrong" form of the equation, and my CRC handbook of chemistry and physics lists the g/mL version of the equation, as well. I'd be interested to see where Orgchemprof acquired his version. -- Uberhobo 17:32, 23 February 2007 (UTC)
The equation does imply linear scalability, and it holds for concentration ranges that would normally be used. Lots of linear relationships for solutions break down at high concentrations. But the equation is pretty essential here. I tried finding the IUPAC definition, since that would be as official as you get, but I couldn't find it on their website anywhere. The definition you propose above would be too small by 2 orders of magnitude. It does seem like a minor quibble, though. -- Uberhobo 23:23, 23 February 2007 (UTC)
I just wasted the first half of my day trying to figure out why the specific rotation for one of my compounds was off from the literature value by two orders of magnitude. The reason, I soon learned, was that the equation for solutions listed in this article was simply incorrect: The equation without the "correction factor" of 100 was shown, but the units for concentration were listed as g/100mL. This is inconsistent with either of the proposed equations discussed here on the talk page. Since I'm of the opinion that the g/100mL unit is a dumb historical artifact, I've changed the article to use the g/mL unit for solution concentration. If you do change the units back to g/100mL, please be sure to correct the equation as well. Ohnodoctor ( talk) 13:34, 27 June 2014 (UTC)
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.
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 ( [1]), see the “Guidelines for Authors”. User:Orgchemprof|Orgchemprof]] ( talk) 19:58, 12 December 2008 (UTC))
I decided to clarify the confusing units related rant on the main page. It was entirely unnecessary, and largely untrue. -
94.192.232.74 (
talk)
23:42, 24 November 2009 (UTC)
I wonder if one piece of valuable information, that by convention the path length is one decimeter unless noted, should have been preserved. Is it somewhere else? -- AJim ( talk) 20:35, 25 November 2009 (UTC)
References
A rapidly-reverted edit to this article here added this at the start:
"In stereochemistry, specific rotation is a method of doing something that is important for some reason or another, and should be explained a lot better by the brainiacs responsible for creating this article, so that casual readers can understand why this is important".
While the edit was perhaps mischievous, I'm inclined to agree with the editor. An article like this needs to indicate not only how whatever 'specific rotation' is is done/performed/measured/revolved/'turned upside-down so the makers label can be inspected', but also why. Can a brainiac please help out? AndyTheGrump ( talk) 16:58, 27 October 2010 (UTC)
Imagine if you were to encounter the following while trying to learn about "velocity"...
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):
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 . 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)
Well, does it? This article (sort of) tells you what it is, but doesn't tell you why anyone should be interested in it as a subject. It tells you nothing about its purpose, or anything else much at all. Who invented/discovered it? Are there other methods to get the same results? Why would anyone who didn't already know how to do it be remotely interested in the article?
It is a long time since I did any chemistry, and it was never my favourite subject, but at least when I did it I usually had a vague idea about what I was trying to achieve. This article seems to be describing something or other you can do to some substance or other, which results in... well a result. A number. Now this number may well be worth millions of dollars in the fertiliser industry or whatever, or it may just be something you note down on the back of an envelope and forget about. I suspect reality is somewhere in between, but it would help if this article was a little more, well, specific. Please tell us ignoramuses who does it, why, and when, and what they do with the result. If you can't do this, then I'm inclined to think that this is one more (unsourced) article that nobody will miss if they can't find it on Wikipedia. Prove me wrong. AndyTheGrump ( talk) 12:27, 22 January 2011 (UTC)
Well, I'm no chemist, but if it makes sense to do this from a technical viewpoint, the optical rotation article certainly provides context. I'd say this sounds entirely sensible: The merely curious can see the why, and those who need to know can see the how. Perhaps a few of the merely curious might even find some of the how interesting too. AndyTheGrump ( talk) 16:08, 22 January 2011 (UTC)
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)
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)
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)
Optically active solids rotate light too, but this article only discusses solutions and pure-liquids. The CRC definition does not mention that the material be in solution at all, let alone be liquid, but the lede now explicitly specifies "solutions". The standard analysis in this state appears to be values based on a 1 mm thickness. [1] In Journal of the Chemical Society, London (volume 34, page 462) is an early discussion of specific rotation of insoluble solids. And seems to be a discussion in the preceding pages about how concentration is not a linear effect (although the additional terms in the rotation formulas might be only small values). DMacks ( talk) 02:18, 13 July 2014 (UTC)
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.
One thing that doesn't appear clear is that not even quantum chemists know why specific optical rotation happens. It is such a high level chemical and physical effect and so advanced that one cannot ask to make it simpler. Gotta study chemistry to even get a grasp of it. Wikipedia is a reference, not a tool for a person that has no knowledge whatsoever to get to a BSc level. It isn't an instrument to be used when doing research at university level either, but it can help those skilled in the art to get quick reference to something. There are specific reactions in Wiki, there are frontier particle physics theories as well and in none of those cases it implies that a person without the rudimentary knowledge can be capable of understanding ANY bit of it. And it's perfectly fine the way it is. — Preceding unsigned comment added by 194.72.164.66 ( talk) 09:28, 23 May 2017 (UTC)
I just looked at the picture, which is quite nice, but I wonder about one detail. I am not sure about actual practice in specific instruments, but I believe that the general rule is that it is much more accurate to detect the angle of the extinction null, when the analyzer is crossed, than to detect the broad peak, where the intensity changes very slightly with small angle changes. AJim ( talk) 01:43, 31 July 2019 (UTC)