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Rotation of PPL by opticaly active compounds (mathematical feeling)

  1. May 6, 2014 #1
    i read formulae in textbook :

    [α]λt°C = Θ/L*C ........(i)

    where Θ is rotation in light
    and L is length of tube
    and C is concentration in gm/litre

    if we consider tube of lenth L and cross section area 'A'
    then C= M/A*L
    M is mass of sample

    so (i) becomes

    [α]λt°C = Θ/[M/A]

    so are scientist so blind to observe or i am doing something wrong.
  2. jcsd
  3. May 6, 2014 #2


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    This appears to be a variation on Verdet's equation for measuring the concentration of an optically active substance, allowing for temperature variations.

    You have neatly removed the variable for concentration.
  4. May 6, 2014 #3


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    To blind to observe what?
    Your formula is correct, however, I doubt that it is more practical. The point is that you would have to fill the sample of the given mass in a tube which has precisely the cross section you sprecified. Have you tried to fill a tube with precisely the amount of substance you want and to put glass windows on it without neither liquid spilling out or gas bubbles remaining inside?
  5. May 7, 2014 #4
    the older equation has a feeling that if length of tube is increased one of factor will have to change and my equation marks that rotation(θ) and [α] are both independent of length of tube. I am just a bit confused for example help out with this

    Q) if length of tube is doubled what will be the effect on Θ?
  6. May 7, 2014 #5


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    Depends on what you hold constant. If M is constant, nothing, if c is constant, theta will double.
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