2.2.14. MELTING POINT - CAPILLARY METHOD

1 MELTING POINT - CAPILLARY methodEUROPEAN PHARMACOPOEIA Apparatus for the determination of waterby distillationDimensions in millimetresMethod. Clean the receiving tube and the condenser of theapparatus, thoroughly rinse with water, and 200 ml oftoluene Rand about 2 ml ofwater Rinto the dry flask. Distil for 2 h, then allow to cool for about30 min and read the water volume to the nearest in the flask a quantity of the substance, weighed withan accuracy of 1 per cent, expected to give about 2 ml to 3 mlof water. If the substance has a pasty consistency, weigh it ina boat of metal foil.

2 Add a few pieces of porous material andheat the flask gently for 15 min. When the toluene beginsto boil, distil at the rate of about two drops per second untilmost of the water has distilled over, then increase the rateof distillation to about four drops per second. When thewater has all distilled over, rinse the inside of the condensertube withtoluene R. Continue the distillation for 5 min,remove the heat, allow the receiving tube to cool to roomtemperature and dislodge any droplets of water whichadhere to the walls of the receiving tube. When the waterand toluene have completely separated, read the volume ofwater and calculate the content present in the substance asmillilitre per kilogram, using the formula.

3 M=themassingramsofthesubstancetobeexamin ed,n1=thenumberofmillilitresofwaterobtai nedinthefirst distillation,n2=the total number of millilitres of water obtainedin the 2 MELTING POINT - CAPILLARYMETHODThe MELTING POINT determined by the CAPILLARY METHOD is thetemperature at which the last solid particle of a compactcolumn of a substance in a tube passes into the liquid prescribed in the monograph, the same apparatus andmethod are used for the determination of other factors, suchas meniscus formation or MELTING range, that characterisethe MELTING behaviour of a The apparatus consists of.

4 A suitable glass vessel containing a liquid bath (forexample, water, liquid paraffin or silicone oil) and fittedwith a suitable means of heating, a suitable means of stirring, ensuring uniformity oftemperature within the bath, a suitable thermometer with graduation at not more range of the thermometer is not more than 100 C, alkali-free hard-glass CAPILLARY tubes of internal mm to mm with a wall mm to mm Unless otherwise prescribed, dry the finelypowdered substancein vacuoand overanhydrous silicagel Rfor 24 h. Introduce a sufficient quantity into a capillarytube to give a compact column 4 mm to 6 mm in Cbelowthepresumed MELTING POINT and then adjust the rate of heatingto about 1 C/min.

5 When the temperature is 5 C below thepresumed MELTING POINT , correctly introduce the capillarytube into the instrument. For the apparatus described above,immersethecapillarytubesothatthecl osedendisnearthecentre of the bulb of the thermometer, the immersion markof which is at the level of the surface of the liquid. Recordthe temperature at which the last particle passes into theliquid of the apparatus. The apparatus may becalibrated using MELTING POINT reference substances such asthose of the World Health Organisation or other MELTING POINT - OPENCAPILLARY METHODFor certain substances, the following METHOD is used todetermine the MELTING POINT (also referred to as slip pointand rising MELTING POINT when determined by this METHOD ).

6 Use glass CAPILLARY tubes open at both ends, about 80 mmlong, having an external diameter of mm to mm andan internal diameter of mm to into each of 5 CAPILLARY tubes a sufficient amountof the substance, previously treated as described, to formin each tube a column about 10 mm high and allow thetubes to stand for the appropriate time and at the otherwise prescribed, substances with a waxyconsistency are carefully and completely melted on awater-bath before introduction into the CAPILLARY the information section on general monographs (cover pages)