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7.3 DETERMINATION OF VOLATILE OIL

7.3 DETERMINATION OF VOLATILE OIL

         Volatile oils are characterized by their odour, oil-like appearance and ability to volatilize at room temperature. Chemically, they are usually composed of mixtures of, for example, monoterpenes, sesquiterpenes and their oxygenated derivatives. Aromatic compounds predominate in certain volatile oils. Because they are considered to be the “essence” of the vegetable drugs, and are often biologically active, they are also known as “essential oils”. The term “volatile oil” is preferred because it is more specific and describes the physical properties.

         The determination of volatile oil in vegetable drugs is carried out by steam distillation in a special apparatus in the conditions described below. The distillate is collected in its graduated tube using xylene to take up the volatile oil; the aqueous phase is automatically recirculated into the distillation flask.

Apparatus

         The apparatus (see Fig. 1) is constructed of resistant glass of low coefficient of expansion and has the following dimensions:

         (a) a round-bottomed flask (A) of suitable capacity with a short, ground-glass neck having an internal diameter of about 29 mm at the wide end;

         (b) a condenser assembly that closely fits the flask and consists of the following parts fused into one piece:

---- a vertical tube (BD), 215 to 265 mm long and 14 to 16 mm in internal diameter,

---- a bent tube (DEF) in which the distances DE and EF are each 145 to 155 mm long and 9 to 10 mm in internal diameter,

---- a bulb-condenser (GH), 145 to 155 mm long and 9 to 10 mm in diameter at the restrictions,

---- a vented stopper (M) and a tube (N) with an orifice of diameter about 1 mm that coincides with the vent. The wide end of the tube (N) is of ground-glass, having an internal diameter of 10 mm,

---- a tube (HJ), 75 to 85 mm long and 9 to 10 mm in internal diameter, making a 30º to 40º angle (HJN) with the tube (JN),

---- a graduated tube (LP), graduated over 105 to 115 mm to give 1 ml subdivided in 0.01 ml. Above the graduation are two circular marks (K and L),

---- a pear-shaped swelling (S), about 3 ml in capacity,

---- a bulb-shaped swelling (P), about 2 ml in capacity,

---- a three-way tap (Q), and

---- a connecting tube (CQ), 7 to 8 mm in internal diameter, fitted in the middle with a filling funnel (R). The junction (C) is at a level 20 mm higher than uppermost graduation;

(c) a suitable heating device, allowing a fine control; and

(d) a vertical support with horizontal ring covered with insulating material.

         Before use, clean the apparatus by successive washings with acetone, water and chromic acid cleansing mixture inverting several times, and rinse with water. Drain the apparatus and mount it in a place protected from a draught.

Procedure

         Carry out the assay according to the nature of the drug to be examined.

         Place the prescribed volume of distillation liquid in the flask, add a few pieces of porous porcelain and attach the condenser assembly. Introduce water through the filling funnel (R) until it is at the level (C). Remove the stopper (M) and introduce the prescribed quantity of xylene, using a pipette with its tip at the bottom of the tube (N). Replace the stopper (M) and ensure that the orifice is coincidental with the vent. Heat the liquid in the flask to boiling and adjust the distillation rate to 2 to 3 ml per minute, unless otherwise prescribed.

         To determine the rate of distillation, lower during distillation the level of the water by means of the threeway tap (Q) until the meniscus is in level with the lower mark (L) (see Fig. 2). Close the tap and record the time taken for the meniscus to reach the upper mark (K). Open the tap and continue the distillation, modifying the heat to regulate the distillation rate. Distil for the time prescribed in the monograph. Stop the heating, and after at least 10 minutes read off the volume of xylene in the graduated tube.

         Introduce into the flask, the prescribed quantity of the drug and continue the distillation as described above for the time and at the rate prescribed. After a further 10 minutes, read the volume of xylene previously noted. The difference represents the quantity of volatile oil in the weight of the drug taken. Calculate the result as millilitres per 100 g of drug.

         When the volatile oil is to be used for other analytical purposes, the water-free mixture of xylene and volatile oil may be recovered as follows. Remove the stopper (M) and introduce 0.1 ml of a 0.1 per cent w/v solution of sodium fluoresceinate and 0.5 ml of water. Lower the level of xylene and volatile oil mixture into the bulb-shaped swelling (P) by means of the three-way tap, allow to stand for 5 minutes and lower the mixture layer slowly until it just reaches the tap (Q). Turn the tap clockwise so that the water flows out of the connecting tube (CQ). Wash the tube with acetone and with a little toluene introduced through the filling funnel (R).

Turn the tap clockwise in order to recover the mixture of xylene and volatile oil in an appropriate flask.

APPENDICES • 7.3 DETERMINATION OF VOLATILE OIL
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หมายเหตุ / Note : TP II 2011 PAGE 516-518