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Unformatted text preview: METHODS AND MANIPULATION 67 be about 10 cm. long and 2 mm. in diameter. It is advisable to use solvents in which the solute is sparingly soluble, so that larger volumes of liquid can be employed. Light petroleum and cyclohexane are often satisfactory for this purpose. Choice of Solvent. In general, a higher boiling solvent is preferable to a low-boiling solvent, as evaporation is then considerably less. When the correct solvent for recrystallisation is not known a pro- cedure similar to that given on pp. 15-16 should be followed, but on the semi-micro scale not more than io mg. of the solid should be placed in the tapered-end test-tube (Fig. 29(5)) and about o-i ml. of the solvent should be added from the calibrated dropping-pipette (Fig. 30(8)). If the compound dissolves readily in the cold, the solvent is unsuitable, but the solution should not be discarded. [In this case recourse should be had to the use of mixed solvents (p. 18). For example if the substance is very soluble in ethanol, water should be added from a...
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- Fall '06
- Organic chemistry