Talk:Specific rotation

Units: part 1

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:

• = 66.47 deg (10 cm)^-1 ml g^-1
• = 66.47 deg (10 cm)^-1 cm³ g^-1
• = 6.647 deg cm² g^-1

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)

The definition is indeed in term of a 10 cm path. Note that the units for the 6.647 value are in terms of "(cm)^-1" not "(10 cm)^-1". You've changed the units by a factor of 10, so the value hasalso changes by a factor of 10. DMacks 01:07, 28 February 2006 (UTC)

Shouldn't the units should just follow from the equation?:

$[\alpha ]_{\lambda }^{T}={\frac {\alpha }{l*c}}={\frac {66.47{\mbox{ deg}}}{(10{\mbox{ cm}})*(1{\mbox{ gm}}/{\mbox{ml}})}}=66.47{\frac {0.1{\mbox{ deg}}{\mbox{ cm}}^{2}}{\mbox{gm}}}$

It looks to me like the "formal unit for specific rotation" is really "0.1 deg cm² g^-1". Spiel496 20:36, 6 March 2006 (UTC)

I agree that the "formal unit" description seems inconsistent with the math (which is consistent with the definition, i.e., 10 cm pathlength). Putting a scalar in the "units" seems weird, even though it might be technically correct. The key point that needs to be made in the text is that "degrees" is the common literature notation. Maybe the "formal unit" should use dm as the unit for the pathlength and not reduce (cm³ dm^-1) into pure cm, so the scalar really does become part of the units? Would be good to get input from those who wrote the original here... DMacks 20:53, 6 March 2006 (UTC)

Units Nightmare

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)

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)

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)

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)

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 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)

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)

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)

Units Nightmare Clarified

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)

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 (), 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

What, Why or How is 'Specific rotation'?

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)

I couldn't remember the template name, so that was all I could come up with. Ah yes, Template:Lead missing and too Template:technical. I was hoping some expert would fix it rather than revert it, but it was not to be.

The polarized light bit is clear, the need for a fluid is clear, but the "observed angle"..? Observed how? With what? What's the difference between an angle of zero and the max angle, whatever it is?

A picture of the test setup would be good. [light source (type?)] -> [polarization filter (type? circular?)] -> [fluid (purpose? In a chamber? Do the chamber walls affect the measurement?)] -> [measurement system?]

DMahalko (talk) 17:17, 27 October 2010 (UTC)

Can we approach the units in a more standard way?

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):

${\mathcal {V}}={\frac {\beta }{Bd}}$

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)

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)

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)

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)

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)

Does 'specific rotation' deserve an article on Wikipedia at all?

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)

It's a commonly-used technique, enough scientists care about it that companies sell expensive equipment to do it. Sounds like you're saying this article lacks context (and there already are tags about missing intro/lede and poor accessibility for lay readers). But this article really does need context. It does explain the use in identifying enantiomers and determining ee, but extensive discussion of why anyone cares about that is off-topic here (has its own article). Is the optical rotation page a good background for the topic? Seems like these two articles are two facets of the same idea: one about the concept/importance/theory and the other about the technique of measuring it. What about merging this one into that one? DMacks (talk) 15:54, 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)

Grumpy Andy: To answer your question, specific rotation is an attribute of all chiral compounds -- which itself is probably 80% or more of chemistry. To place it in a context for which you may be more familiar, it would be the equivalent of "litres" - not everything have a volume, but many important ones do, and all that does have an "litre" attribute.

As to its handling in Wikipedia, I agree with DMacks. My thought (as a chemist) is that the optical rotation article should be expanded substantially with an diagram like that in absorption spectroscopy done up, and eventually merge specific rotation into that. At the same time, I do not object to specific rotation being as it is. The link to optical rotation as an prerequisite seems clear to me, and there are clear warning signs.

PS, I intend to fill in diagrams for chemistry/physics articles, and to try doing so via a modular kit. Original illustrations are thankless and time-consuming, so it'll take some time before any substantial headways are made. Jon C (talk) 09:07, 23 January 2011 (UTC)

Links to other Wikipedia articles

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)

Lead Section

Optical rotation (optical activity) experiment. The LASER light passes inside a sucrose solution box, then passes by a analyzing polaroid.

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)

Article revision

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)

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)

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)

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)

What about solids?

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)

Pure liquids an solids seem to be essentially the same, though why the density should be used is beyond me. Unlike a solution, one cannot change the density of the optically active compound, and a simple measure of degrees per decimetre seems appropriate, and might this go by another name. It does seem like these cases should either be linked to in their own article or covered together here. However, I'm unfamiliar with this discipline, and cannot make a properly informed comment though. —Quondum 03:19, 13 July 2014 (UTC)

How about we remove any explictit reference to "in solution" or "for liquids" in the lead? Why add constraints to the definition that needn't apply—unless they are in the source(s)?

Additionally, the density of a pure liquid is equal to its "concentration"

For example, water has a density of 1 g cm^-3. The amount of water found in 1 ml of water is 1 g, so the concentration must also be 1 g cm^-3, or 1 g/ml. Similarly, ice has a density of 0.9 g cm^-3 at 0 °C, meaning the concentration of pure ice is also 0.9 g/ml.

and the "density" term is largely superfluous. How is the density of any pure liquid expected to change without changes in temperature (ignoring changes in applied pressure)? The two "different" expressions given in the article are equivalent and so an extension of this expression to solids would take the same form as that for pure liquids. The inclusion of the density term serves to allow numerical comparison between solutions and pure liquids/solids. Tomásdearg92 (talk) 11:33, 14 July 2014 (UTC)

It's simple enough. If specific the rotation is how the rotation in solids and pure liquids is generally expressed (i.e. in terms of density in notable sources), the article should cover it. There is nothing wrong with the concept, only WP should not invent a way of making the concept apply where some alternative is generally used. It actually does make sense if expansion (thermal and pressure-related) is significant. Phases changes (e.g. freezing as in this example) might orient molecules, potentially creating much larger effects, though. If you feel you have a reasonable source, put it in. But if there is an alternative dominant convention for solids/liquids, be sure to mention that too. —Quondum 13:53, 14 July 2014 (UTC)

OK, I left this a while in case anyone had anything to add, maybe too long :) Several sources refer to specific rotation measurements in solutions, pure liquids (the phrase neat being used in place of the concentration remark, it seems),[2] solids such as quartz plates (including some standard plates used for saccharimetry[3]),[4] suspensions of ground and sieved solids (using a rather curious measurement cell),[5] thin films (for which the thickness is measured separately)[6]‡ and I think I even came across a mention of gases. Some of them refer to the specific rotation explicitly, some refer simply to "optical rotation" but then present [α]_D or [α]^T values. Otto Vogl is quite prolific in this area, or so it seems.[7] A review by him could be ideal. The International Pharmacopoeia tends to muddy the waters with the introduction of several variables that appear to cancel down to concentration, albeit with correction factors and unit changes thrown in to the bargain.[8] Secondary sources on the former dispense with much of this.[9] Here is a page notable only for it's use of a scanned portion of a pharmacy textbook in which the form for solids is almost identical to that currently given in the article (with correction factor of 100).[10] And then, a phycics book comes along and dispenses with the standards used in chemistry and pharmacy (and it is likely other physics books do the same).[11]

‡Quote from above source: "The optical activity of suspensions was measured in a rectangular quartz cell with a path length of 10mm with stirring. The speed of the rotation of the magnetic stirring bar was 700 & 50rpm. The results were recorded using a Linear 1200 recorder. The procedure for accurate measurements was described in detail earlier. [Bartus, J., & Vogl, O., Polymer Bull., 28 (1992) 203.]

The optical activity of polymer solutions was measured in a glass cell with a path length of 10cm. The polymer solutions in the appropriate solvents were prepared in 10 ml volumetric flasks.

Polymer films were cast on a glass plate from concentrated solutions. The thickness of the films was measured with a micrometer and the optical activity was recorded after mounting of films in a cell compartment of the polarimeter."

So, where do we go from here? Tomásdearg92 (talk) 00:17, 30 September 2014 (UTC)

Picture request

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)

 Done It's only been a fortnight already... Tomásdearg92 (talk) 01:02, 7 December 2016 (UTC)

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)