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Melting Range or Temperature (Apparatus and Deternination)


Learn how to determine the Melting Range or Temperature by different methods in Pharmaceuticals.
In this Pharmacopoeia, melting range or temperature of a substance is defined as those points of temperature within which, or the point at which, the substance begins to coalesce and is completely melted except as defined otherwise for certain substances. The following procedures are suitable for the various substances described in the Pharmacopoeia. Any other apparatus or method capable of the same accuracy may also be used. The accuracy should be checked frequently by using certified reference substances of declared melting point, such as those of the World Health Organization or other suitable substances, the reference substance selected being one that melts nearest to the melting range of the substance to be examined.
Unless otherwise specified in the individual monograph, use Method I.

Method I

Apparatus

(a) A glass heating vessel of suitable construction and capacity containing one of the following or any other suitable bath liquid, to a height of not less than 14 cm.
(i) Water for temperatures upto 60°C.
(ii) Glycerin for temperatures upto 150°C.
(iii) Liquid paraffin of sufficiently high boiling range for temperatures upto 250°C.
(iv) Sesame oil or a suitable grade of silicone oil for temperatures upto 300°C.
(b) A suitable stirring device capable of rapidly mixing the liquids.
(c) An accurately standardised thermometer suitable for the substance under examination. The thermometer must be positioned in the bath liquid to its specified immersion depth and yet leave the bulb about 2 cm above the bottom of the bath.
(d) Thin-walled capillary glass tubes of hard glass, closed at one end, about 12 cm long, with a thickness of 0.2 to 0.3 mm and an internal diameter of 0.8 to 1.1 mm. The tubes should preferably be kept sealed at both ends and cut as required. A suitable magnifying glass may be used for observation of melting in the capillary tube.
(e) A source of heat (open flame or electric heater).

Procedure

Reduce the substance to a very fine powder and, unless otherwise directed, dry it at a temperature considerably below its melting temperature or at a pressure of 1.5 to 2.5 kPa over self-indicating silica gel for 24 hours. Introduce into a capillary glass tube, a sufficient quantity of the dry powder to form a compact column 4 to 6 mm high. Heat the bath until the temperature is about 10°C below the expected melting temperature. Remove the thermometer and quickly attach the capillary tube to the thermometer by wetting both with a drop of the liquid of the bath or otherwise and adjust its height so that the closed end of the capillary is near the middle of the thermometer bulb. Replace the thermometer and continue the heating, with constant stirring, sufficiently to cause the temperature to rise at a rate of about 10°C per minute. Continue the heating and note the temperature at which the column of the sample collapses definitely against the side of the tube at any point, when melting may be considered to have begun and note also the temperature at which the sample becomes liquid throughout as seen by the formation of a definite meniscus. The two temperatures fall within the limits of the melting range.

Method II

Apparatus

Use the apparatus described under Method I except that the glass capillary tube is open at both ends and has an internal diameter of 1.1 to 1.3 mm, an external diameter of 1.4 to 1.7mm and length of 50 to 60 mm.

Procedure

Rapidly melt the substance under examination, at a temperature not more than 10° above the point of complete fusion. Draw it into a capillary tube to a depth of about 10 mm. Cool the charged tube at 10° or lower for 24 hours, or keep in contact with ice for at least 2 hours. Attach the tube to the thermometer and adjust it so that the column of the substance under examination is in level with the thermometer bulb. Suspend the thermometer in the heating vessel containing water at 15°C so that the lower end of the column of the substance is 30 mm below the surface of the water and heat the water with constant stirring so that the temperature rises at the rate of 1°C per minute. The temperature at which the partly melted substance is observed to rise in the capillary tube is the melting temperature.

Method III

Apparatus

(a) A glass boiling-tube of overall length 110 mm and internal diameter 25 mm.
(b) A cork about 25 mm long to fit into the boiling-tube, bored with a central hole to fit a standard thermometer and with a groove cut in the side.
(c) A glass beaker of such a size that when the apparatus is assembled the boiling-tube can be immersed vertically to two-third of its length in the water present in the beaker with its lower end about 2.5 cm above the bottom of the beaker.
(d) A stirrer or any other device which will ensure uniformity of the temperature throughout the water in the beaker.
(e) An accurately standardised thermometer suitable for the substance under examination .
(f) Suitable means for heating the water in the beaker.

Procedure

Melt a quantity of the substance under examination slowly, while stirring, until it reaches a temperature of about 90°C. Cool and allow the temperature of molten substance to drop to a temperature 8°C to 10°C above the expected melting temperature.
Chill the bulb of the thermometer to 5°C, wipe it dry and while it is still cold dip it in the molten substance so that the lower half of the bulb is submerged. Withdraw it immediately, and hold it vertically away from the heat until the wax surface dulls, then dip it for 5 minutes into a water bath at a temperature not higher than 15°C.
Fit the thermometer through the bored cork into the boiling tube so that the lower part is 15 mm above the bottom of the tube. Suspend the tube in the beaker filled with water adjusted to about 15° and raise the temperature of the bath to 30° at the rate of 2° per minute, adjust the rate to 1° per minute and note the temperature at which the first drop of melted substance leaves the thermometer. Repeat the determination twice on a freshly melted portion of the substance. If the three readings differ by less than 1°, take the average of the three as the melting temperature. If they differ by more than 1°, make two additional determinations and take the average of the five readings.
Melting Range Apparatus

Method IV

Apparatus

The apparatus (see Fig.) consists of the following components:
(a) An accurately standardised thermometer calibrated for 100-mm immersion, covering the range -5°C to + 105°C, and conforming to Indian Standard 4825:1968 but with the following modifications:
Bulb diameter 3.35 to 3.65 mm
Bulb length not greater than 5 mm
Stem diameter, immediately above the bulb, approximately equal but not less than the diameter of the bulb for a length of at least 26 mm measured from the bottom of the bulb.
b) A metal cup made from chromium-plated brass or from other suitable metal which is not susceptible to corrosion by the substance under examination and conforming to the following dimensions:
Apparatus DimensionsInternal depth of wide part of the cup: The lower part of the wide portion of the cup is approximately part of a hemisphere. When a steel ball, 7.00 mm in diameter is placed in the cup, the top of the ball is 12.05 to 12.35 mm above the bottom of the tube forming the orifice. The top of the cup and the bottom of the tube forming the orifice are smooth, parallel to each other and at right angles to the axis of the cup. The bottom edge of the orifice is not chamfered or radiused.
c) To the lower end of the thermometer is securely and coaxially cemented with a cylindrical metal sleeve onto which a metal case is screwed. This metal case has two small apertures, one in front and one behind, to act as air vents. It also has a rim stop, guide lugs to aid the coaxial insertion of the cup into the case and spring clips to enable the cup to be retained firmly with its top edge against the rim stop. The metal sleeve is fixed to the thermometer in such a position that, when the metal case is screwed on to the sleeve, the bottom of the thermometer bulb is 7.9 to 8.2 mm below the rim stop.
d) Auxiliary apparatus assembled as shown in Fig and consisting of the following parts.
(i) A glass tube, overall length 110 mm, internal diameter 25mm.
(ii) A cork about 25 mm long, bored with a central hole to fit the standard thermometer and with a groove cut in the side.
(iii) A glass beaker, of such size that when the apparatus is assembled the boiling tube can be immersed vertically to two third of its length in the liquid in the beaker with its lower end about 25 mm above the bottom of the beaker. The liquid used in the beaker should be water for melting points (flow and drop points) below 80°C and liquid paraffin or glycerin for melting points above 80°C.
(iv) A stirrer or any other convenient device that will ensure uniformity of temperature throughout the liquid in the beaker.
(v) An auxiliary thermometer.
(vi) A stand with clamps for holding the boiling tube and auxiliary thermometer in position in the beaker and for holding the beaker above the source of heat.
(vii) A suitable means of heating the water in the beaker.

Procedure

Heat the sample, with stirring, to 118°C to 122°C, to ensure uniformity, and then cool to 103°C to 107°C. Warm the metal cup to 103°C to 107°C in an oven, remove it from the oven, place on a clean plate or ceramic tile and pour sufficient of the melted sample into the cup to fill it completely. Allow the filled cup to cool for 30 minutes on the tile or plate and then place it in a water-bath at 24°C to 26°C for a further 30 to 40 minutes. Level the surface of the sample with a single stroke of a knife or razor blade, avoiding any stirring of the sample. Push the cup, without lateral movement, into the metal case as far as the rim stop and wipe away the excess of the substance that is squeezed out of the bottom of the tube, ensuring that the air vents are not blocked. Fit the thermometer, with the cup attached, through the bored cork to the boiling tube such that the bottom of the cup is 24 to 26 mm above the bottom of the boiling tube. Fix the boiling tube vertically within the beaker so that at least two thirds of its length is immersed in the liquid contained in the beaker. Adjust the temperature of the outer bath so that the temperature of the substance rises at the rate of 1°C per minute. The temperature at which the first drop of melted liquid falls from the metal cup is regarded as the melting point (drop point) of the substance. Note the temperature at the fall of the first drop. Carry out at least three determinations, each time with a fresh sample of the substance under examination. The difference between the readings must not exceed 3°C. The mean of three readings is taken as the melting point of the substance.
Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
Email: .moc.enilediugamrahp@ofni Need Help: Ask Question


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