Page 122 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

funnels and flasks, centrifuge tubes, conical flasks, filter crucibles, filter funnels,
measuring cylinders, scoops, spatulas, stoppers, tubing, weighing bottles, etc.;
such products are often cheaper than their glass counterparts, and are frequently
less fragile. Although inert towards many chemicals, there are some limitations
on the use of plastic apparatus, not the least of which is the generally
rather low maximum temperature to which it may be exposed: salient properties
of the commonly used plastic materials are summarised in Table 3.4.
Attention is drawn to the extremely inert character of Teflon, which is so
lacking in reactivity that it is used as the liner in pressure digestion vessels in
which substances are decomposed by heating with hydrofluoric acid, or with
concentrated nitric acid (see Section 3.31).

3.20 METAL APPARATUS
Crucibles and basins required for special purposes are often fabricated from
various metals, amongst which platinum holds pride of place by virtue of its
general resistance to chemical at tack.
Platinum. Platinum is used mainly for crucibles, dishes and electrodes; it has
a very high melting point (1773 OC), but the pure metal is too soft for general
use, and is therefore always hardened with small quantities of rhodium, iridium,
or gold. These alloys are slightly volatile at temperatures above 1100 OC, but
retain most of the advantageous properties of pure platinum, such as resistance
to most chemical reagents, including molten alkali carbonates and hydrofluoric
acid (the exceptions are dealt with below), excellent conductivity of heat, and
extremely small adsorption of water vapour. A 25 mL platinum crucible has
an area of 80-100 cm2 and, in consequence, the error due to volatility may be
appreciable if the crucible is made of an alloy of high iridium content. The
magnitude of this loss will be evident from Table 3.5, which gives the approximate
loss in weight of crucibles expressed in mg/100cm2/hour at the temperature
indicated. An alloy consisting of 95 per cent platinum and 5 per cent gold is
referred to as a 'non-wetting' alloy and fusion samples are readily removed
from crucibles composed of this alloy; removal is assisted by keeping the crucible
tilted while the melt is solidifying. Crucibles made of this alloy are used in
preparing samples for investigation by X-ray fluorescence.
A recent development is the introduction of ZGS (Zirconia Grain Stabilised)
platinum. This is produced by the addition of a small amount of zirconia
(zirconium(1V) oxide) to molten platinum, which leads to modification of the
microstructure of the solid material with increased hot strength and greater
resistance to chemical attack. Whereas the recommended operating temperature
for pure platinum is 1400°C, the ZGS material can be used up to 1650°C.

Table 3.5 Weight loss of platinurn crucibles