OptiMelt Automated Melting Point System - Stanford Research ...

78 Pharmacopeia vs. Thermodynamic Melting Points Appendix ATable 6 is an example of the dependence of the clear point determination on ramping ratefor phenacetin samples (MP: 135 ºC) analyzed with an OptiMelt system. As expected, theclear point increases with increasing ramping rates. The need to specify ramping ratesalong with the results of the melt is demonstrated by this data.Ramp Rate, r [°C/min]Clear Point [°C]0.1 134.20.2 134.40.5 134.91 135.42 136.25 137.9Table 6. Clear Point of Phenacetin at various ramp rates.The heating stands used by commercial melting point apparatuses are divided into twocategories:Liquid BathThe capillaries are immersed in a liquid bath (typically silicone oil) that is continuouslybeing heated up.Metal BlockA small, dry thermal block (metal oven) has proven to be a good alternative to a liquidbath.The melting point procedures described in early pharmacopeias were designed for “liquidbath” heating stands; however, in recent years most monographs have been updated andsupplemented to include the more prevalent metal block setups.TipA slight drift in melting point results is expected when the temperatures obtained with aliquid bath oven are compared to those obtained with a metal block stand. This differenceis to be expected and generally ignored since it most generally falls within the intrinsicuncertainty of the measurement.OptiMelt Automated Melting Point System