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

46 CHANGES OF CONDITION.

If a very dilute solution of lanthanum acetate is used, a blue

colloidal solution is produced instead.

Prepare also a little iodo-starch from a very dilute starch

solution and a few drops of iodine solution, and compare the

colors.

24. Molybdenum Blue, Mo 3 Os.
    Blue molybdenum oxide, Mo 3 O 8 , is soluble as a reversible colloid in water
    and behaves in this respect, as well as in its behavior towards vegetable and
    animal fibers, like many of the organic dyestuffs.

Dissolve 15 g. of commercial ammonium molybdate,

5 (NH 4 ) 2 MoO 4 • 7 MoO 3 • 7 H 2 O, in 250 c.c. of water, add 35 to 40 c.c.

of 2-normal sulphuric acid, heat the solution to boiling, and keep
boiling gently for 30 minutes to an hour, meanwhile introducing a

stream of hydrogen sulphide. Reduction takes place, causing the

appearance in a few moments of a dark-blue color. With a smaller

amount of sulphuric acid a precipitation of molybdenum sulphide

takes place; with more acid the yield becomes poorer. Filter from
precipitated sulphur and subject the solution to dialysis for from

4 to 6 days (cf. No. 21), until the outside water is free from sul-

phuric acid, and is only faintly blue. Evaporate the contents of

the dialyzing tube in a porcelain dish, at first over a free flame and
finally on the water bath, until, after frequent stirring, the resinous,

deep-blue residue has become a dry powder. Yield, 5 to 7 g.

The preparation is soluble without residue in water. Use the

solution for the following experiments:

1. Boil a portion with a piece of undyed silk. The silk is col-

ored blue. This dyeing experiment can also be carried out with
the undialyzed solution.

2. Repeat Experiment 1 with the addition of a considerable
   amount of sodium sulphate. More of the coloring matter is taken

up by the silk. This is due to the salting-out action of the elec-

trolyte.

3. Shake a little of the solution with some freshly-precipitated,
   washed aluminium hydroxide. A mixture of molybdenum blue
   and hydrated aluminium oxide is precipitated (a lake) and the
   solution becomes lighter colored. Hydrogels can thus be dyed
   !ike fabrics.


4. Mix a few cubic centimeters of dilute molybdenum blue solu-