Page 119 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

WASH BOTTLES 3.18

adapted as a method of purifying tap water. The tap water, at a pressure of
3-5 atmospheres, is passed through a tube containing the semi-permeable
membrane. The permeate which is collected usually still contains traces of
inorganic material and is therefore not suitable for operations requiring very
pure water, but it will serve for many laboratory purposes, and is very suitable
for further purification by ion exchange treatment. The water produced by
reverse osmosis is passed first through a bed of activated charcoal which removes
organic contaminants, and is then passed through a mixed-bed ion exchange
column and the resultant effluent is finally filtered through a sub-micron filter
membrane to remove any last traces of colloidal organic particles.
The high-purity water thus produced typically has a conductance of about
0.5 x O-' cm-' (0.5pScm-') and is suitable for use under the most
stringent requirements. It will meet the purity required for trace-element
determinations and for operations such as ion chromatography. It must however
be borne in mind that such water can readily become contaminated from the
vessels in which it is stored, and also by exposure to the atmosphere. For the
determination of organic compounds the water should be stored in containers
made of resistant glass (e.g. Pyrex), or ideally of fused silica, whereas for
inorganic determinations the water is best stored in containers made from
polythene or from polypropylene.

3.18 WASH BOTTLES
A wash bottle is a flat-bottomed flask fitted up to deliver a fine stream of distilled
water or other liquid for use in the transfer and washing of precipitates. A
convenient size is a 500-750mL flask of Pyrex or other resistant glass; it
should be fitted up as shown in Fig. 3.4. A rubber bung is used, and the jet
should deliver a fine stream of water; a suitable diameter of the orifice is 1 mm.
Thick string, foam rubber, or other insulating material, held in place by copper
wire, should be wrapped round the neck of the flask in order to protect the
hand when hot water is used. In order to protect the mouth from scalding by
the back rush of steam through the mouth-piece when the blowing is stopped,
it is convenient to use a three-holed rubber stopper; a short piece of glass tubing
open at both ends is inserted in the third hole. The thumb is kept over this tube
whilst the water is being blown out, and is removed immediately before the
mouth pressure is released. All-glass wash bottles, fitted with ground-glass joints,
can be purchased. They should be used with organic solvents that attack rubber.

Fig. 3.4