[ramsuthar2001]

Dear friends,

   I wish to know some details about specific optical rotation. I’m from INDIA.

I’ve been asked to check the specific optical rotation of a substance at 20 degree Celsius, for which our standard testing procedure says, we should weigh the substance in a standard flask which was marked 50ml at 27 degree Celsius as per Indian condition and the substance should be dissolved in and made upto the mark with methanol. Then, the solution must be cooled to 20 degree Celsius and the specific optical rotation should be measured.

I wish to know whether this procedure is correct or not. More over I would like to know, if the substance is dissolved in methanol cooled to 20 degree Celsius and made up to volume with the same and specific optical rotation is measured at 20 degree Celsius, would be the correct procedure.

Kindly reply.

With regards,
RAM KUMAR.S

[sjb]

I'd be tempted to say that the latter procedure is more usual, if only because 20° is a more common temperature to take measurements at.

As alpha-d is measured as a concentration in terms of mass (or moles) per volume, anything that changes that volume will affect the measured concentration. Changes in termperature will change the volume of solution.

So if you make up a standard solution at 27°, then cool it to 20°, then the volume will usually decrease and the concentration increase, giving you an abnormally high alpha-d.

S

[ramsuthar2001]

I would like to know if, the sample is made up with methanol at 27C and specific optical rotation is measured at 20C using polarimeter with auto cooler, is correct or not ,to get the specific optical rotation result at 20C.

I post this question because in pharmacopoeias it is mentioned that the 20C is the temperature of measurement and it doesn't say anything about the temperature at which the solution should be prepared except for the formula to calculate the concentration of the solution for which different solvents are used.

[kevins]

I am not the expert in this area. I perfer to cool the solution near 20C first because I think the function of cooler is to maintain the working condition at around 20C and not to cool the solution as well as to bring the solution to 20C in short time.

[sjb]

As far as I understand what you're saying, you have a volumetric flask (or similar) that is graduated to 50ml at 27°C. If you were to make a solution up to this mark, then cool it to 20°C to take your alpha-d measurement, then (usually) the volume would change (as indeed would the volume of glass, but they would not cancel out), meaning that you no longer have an accurately known volume so you cannot calculate a standard alpha-d-20.

However, there is nothing wrong in principle in measuring alpha-d at 27°C, but there are less data points to check (in my experience). If your pharmacopeia uses 20°C as its standard it may be better to find glassware that is calibrated at that temperature and work as far as possible at that temperature.

Bear in mind increasing temperature may change conformer populations and the like, so if you for instance want to measure the alpha-d of N-(1-methyl)propyl formamide (the amide from 2-aminobutane and formic acid, in case I've misnamed it) this may not be sufficiently precise to be sure of what you're actually measuring...

S