Page 234 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE

of the liquid does not fa11 below the top of the resin bed, so that the latter is
always wholly immersed in the liquid. The ratio of the height of the column to
the diameter is not very critical but is usually 10 or 20: 1. Another form of
column is depicted in Fig. 7.4(b) (not drawn to scale): a convenient size is 30 cm
long, the lower portion of about 10 mm and the upper portion of about 25 mm
interna1 diameter. A commercially available column, fitted with ground-glass
joints is illustrated in Fig. 7.4(c).
The ion exchange resin should be of small particle size, so as to provide a
large surface of contact; it should, however, not be so fine as to produce a very
slow flow rate. For much laboratory work 50-100 mesh or 100-200 mesh
materials are satisfactory. In al1 cases the diameter of the resin bead should be
less than one-tenth of that of the column. Resins of medium and high
cross-linking rarely show any further changes in volume, and only if subjected
to large changes of ionic strength will any appreciable volume change occur.
Resins of low cross-linking may change in volume appreciably even with small
variations of ionic strength, and this may result in channelling and possible
blocking of the column; these effects limit the use of these materials. To obtain
satisfactory separations, it is essential that the solutions should pass through
the column in a uniform manner. The resin particles should be packed uniformly
in the column: the resin bed should be free from air bubbles so that there is no
channelling.
To prepare a well-packed column, a supply of exchange resin of narrow size
range is desirable. An ion exchange resin swells if the dry solid is immersed in
water; no attempt should therefore be made to set up a column by pouring the dry
resin into a tube and then adding water, since the expansion will probably
shatter the tube. The resin should be stirred with water in an open beaker for
several minutes, any fine particles removed by decantation, and the resin slurry
transferred portionwise to the tube previously filled with water. The tube may
be tapped gently to prevent the formation of air bubbles. To ensure the removal
of entrained air bubbles, or any remaining fine particles, and also to ensure an
even distribution of resin granules, it is advisable to 'backwash' the resin column
before use, i.e. a Stream of good-quality distilled water or of de-ionised water
is run up through the bed from the bottom at a sufficient flow rate to loosen
and suspend the exchanger granules. The enlarged upper portion of the exchange
tube shown in Fig. 7.4(b) or (c) will hold the resin suspension during washing.
If a tube of uniform bore is used the volume of resin employed must be suitably
adjusted or else a tube attached by a rubber bung to the top of the column;
the tube dips into an open filter flask, the side arm of which acts as the overflow
and is connected by rubber tubing to waste. When the wash water is clear, the
flow of water is stopped and the resin is allowed to settle in the tube. The excess
of water is drained off; the water level must never fa11 below the surface of the
resin, or else channelling will occur, with consequent incomplete contact between
the resin and solutions used in subsequent operations. The apparatus with a
side-arm outlet (Fig. 7.4~) has an advantage in this respect in that the resin
will not run dry even if left unattended, since the outlet is above the surface of
the resin.
Ion exchange resins (standard grades) as received from the manufacturers
may contain unwanted ionic impurities and sometimes traces of water-soluble
intermediates or incompletely polymerised material; these must be washed out
before use. This is best done by passing 2M hydrochloric acid and 2M sodium