Question: During boiling point determination, our glycerin start bubbling at 255 oC but temperature does not be constant and continuously vary, at 270 oC bubbling stopped but temperature vary is continue till 280 oC.
If you want to accurately determine the saturation temperature at one atmosphere and not simply the "boiling point" which depends on a lot of things other than the fluid properties alone, this isn't the best way to go about it. Think about a triple point tube, which you can buy online (jarrett-isotech). When you have water as a solid, liquid, and vapor in the same place you know exactly what the temperature and pressure is. A precision pressure-controlled experimental apparatus with RTDs is the most accurate way of measuring this for a fluid. There may be someone at the University who already has this equipment. When I was doing boiling experiments at University, I used equipment from several other departments, which they didn't even know could be used for that purpose.
How well stirred was the glycerine? What was the heat source? How did you measure the temperature?
It's possible, if the heat was applied locally, that you were boiling the glycerine at a fair distance from the point of measurement. Stirring the liquid vigorously, and using a multitude of thermometers and heaters might reveal a more reliable measure.
Glycerol is hygroscopic; it could possibly contain some water. Consider to dry it over activated alumina followed by filtering. You may further dry/purify it by fractional distillation under vacuum and over fresh activated alumina.
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