Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

LANTHANUM BLUB. 45

and the contents will differ only in the amount of iodine. After

24 hours, during which time the bottles should be frequently

shaken, take 10 c.c. of the clear solution from each and determine

the iodine content by titration with 0.1-normal sodium thio-

sulphate solution. The difference between the original weight

present and the amount of iodine thus determined gives the quan-

tity adsorbed by the charcoal. Arrange the results in a table and

plot them on coordinate paper with the adsorbed amounts as or-

dinates and the corresponding amounts left in solution as abscissas.

It is found that from dilute solutions relatively more iodine is

adsorbed than from concentrated ones. This relation is quite

general for adsorption equilibria, and can frequently be represented

by the equation — = k, in which ci is the concentration of the

adsorbed iodine, c 2 that of the unadsorbed iodine, and n and k are

constants.

1

Since in these experiments the same volume and the

same amount of charcoal are taken each time, the quantities of

iodine determined may be used directly in place of cx and c 2 in the

equation. Find by trial what value of n makes the quotient

remain most nearly constant; n is greater than 1.

23. Lanthanum Blue.
    The blue color which an iodine solution gives with starch depends on the
    formation of an adsorption compound between the two substances. This
    can be proved by a quantitative investigation of the adsorption curve. The
    same conclusion can be reached from the fact that another substance, the
    hydrogel of basic lanthanum acetate, which has no property in common with
    starch except the colloidal condition, gives precisely the same color reaction.
    The formation of lanthanum blue is made use of as a test for lanthanum.
    (Cf. No. 171.)

To a dilute solution of lanthanum acetate, or of lanthanum

nitrate acidified with acetic acid, add a solution of iodine in potas-

sium iodide, and then introduce ammonia cautiously, so as to not

quite cause the yellowish-brown color of the iodine to disappear;

warm very gently, and a dark-blue precipitate gradually forms.

1

This equation was proposed by von Bemmelen and later used by Kiister

and Biltz as well as by others. Freundlich has made it known more widely.

Yet the equation itself is often over-emphasized; some reactions to which the

equation appears to apply are not really adsorption reactions at all.