Modern inorganic chemistry

276 GROUP V!

products, then the substance is heated in a current of dry nitrogen,

and the evolved water absorbed in a U tube containing, say, calcium

chloride, which is weighed before and after the experiment. (Dumas'

experiment on the composition of water made use of this method.)

A method of estimating small amounts of water in organic liquids

(and also in some inorganic salts) is that of Karl Fischer. The

substance is titrated with a mixture of iodine, sulphur dioxide and

pyridine dissolved in methyl alcohol. The essential reaction is :

H 2 O + I 2 + S0 2 + CH 3 OH -» 2HI + CH 3 HSO 4

The base pyridine removes the hydriodic acid formed. The end-

point occurs when the brown colour of free iodine is seen, i.e. when

all the water has been used up. This method is widely used.

Heavy water, deuterium oxide, D 2 O

Heavy water is obtained as a residue after prolonged electrolysis of
ordinary water. Heavy water, as its name indicates, has a higher
density than ordinary water (1.11 as against l.OOgcm"^3 ), a slightly
higher boiling point (374.6 K) and slightly different physical proper-
ties in general. Chemically, heavy water behaves like ordinary water
in the kinds of reaction which it undergoes, but the rate of reaction
is often different and the properties of the products may differ also.
Thus, deuterium oxide adds on to anhydrous salts to form deuterates
analogous to hydrates, for example the deuterate of copper(II)
sulphate, CuSO 4. 5D 2 O, which has a slightly lower vapour pressure
than the pentahydrate at the same temperature. Hydrolysis of
aluminium tricarbide to give methane is a rapid reaction; deuterium
oxide yields deuteromethane, CD 4 , only slowly. The fermentation
of glucose proceeds more slowly in heavy water than in ordinary
water.
Deuterium oxide has been used in the laboratory:

1. For exchange experiments; in these, some hydrogen-containing
   compound is mixed with deuterium oxide, and the rate and extent of
   exchange between the two are studied. It is found that compounds
   containing labile^1 hydrogen (i.e. hydrogen atoms which are rapidly
   replaceable) exchange readily; others with fixed hydrogen do not.
   Examples of labile hydrogen atoms are those in the ammonium ion,
   NH^, and in hydroxy compounds such as alcohols and sugars;
   non-labile hydrogen atoms are found in benzene, and in the phos-
   phinate ion, H 2 PO2 The non-labile atoms in the phosphinate ion