How to Calculate Specific Rotation of a Sample

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Solutions of chemical compounds rotate light rays differently. Chemists identify substances observing the direction and extent to which they rotate plane-polarized rays. Sucrose, for example, rotates light more than lactose. To accurately compare different substances, you must observe them under standard conditions. Otherwise, calculate their specific rotation by converting their rotation to a standard unit: "degrees milliliters per gram per decimeter."

  • Convert density to grams per milliliter. For instance, if your sample contains 750 grams per liter:

    750 divided by 1000 equals 0.75 g/ml.

  • Multiply your answer by the length of the cell path in decimeters. For instance, if you're looking down a 1.5 decimeter polarimeter:

    0.75 * 1.5

    = 1.125

  • Divide the observed rotation by this answer. For instance, if you observe a rotation of 102 degrees:

    102 / 1.125

    = 90.7 degrees

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  • Photo Credit solution and powder image by Radu Razvan from Fotolia.com
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