COLLOIDAL GOLD SOLUTIONS. 47tion, drop by drop, with a colloidal solution of zirconium oxide.
1After enough of the former has been added, a molybdenum-blue-
zircon lake is precipitated. Regarding the mutual precipitation
of colloids, see p. 42.- Colloidal Gold Solutions; Precipitating Colloids and Protective
Colloids.
During recent years, gold solutions have been used with great success,
particularly by Zsigmondy, in making clear the nature of the various prop-
erties of colloidal suspensions. Red colloidal solutions of gold can be pre-
pared by electrical disintegration of the metal or by reduction of gold salts
in various ways. The red color of gold-ruby glass is likewise due to colloidal
gold. The adsorption compound of colloidal gold and tin oxide hydrogel,
called Purple of Cassius, serves for the qualitative detection of gold. (Cf.
No. 9.)
- Colloidal Gold according to Zsigmondy.
2
Mix together in a
round-bottomed, 300-400 c.c. flask of Jena glass, 120 c.c. of pure,
distilled water
3
and 2.5 c.c. of chlorauric acid solution (prepared
by dissolving 0.6 g. of HAuCU • 3 H 2 O crystals in 100 c.c. of dis-
tilled water) and add 3.0 to 3.5 c.c. of 0.18 normal, pure potassium
carbonate solution. After this, add about 0.5 c.c. of an ethereal
phosphorus solution, prepared by saturating ether with phos-
phorus and finally diluting to five times the original volume with
ether. If the mixture is allowed to stand several hours, it will
gradually turn brown, often showing a blue or black color as well,
and finally turn red. After about 24 hours at room temperature,
a fairly homogeneous colloidal solution of gold will be obtained
with the particles of gold so small that they cannot be seen with
the ultra-microscope (amicrons). The reduction of the gold from
the tervalent state, in which it exists in chlorauric acid, takes place
more rapidly if the liquid is heated to boiling after the introduction
of the ethereal solution of phosphorus. Then, after the red colora-
(^1) Pseudo-solutions of zirconium oxide can be prepared by dialyzing a solu-
tion (about 16 per cent) of zircon nitrate for about 5 days with frequent
change of water.
(^2) R. Zsigmondy, Z. Anal. Chem. 40, 697 (1901). The most beautiful gold
solutions can be obtained by this classic but very difficult procedure.
(^3) This preparation is a difficult one. It is best to use distilled water which
has been redistilled, after adding some potassium permanganate, using a
silver condenser and a receiver of Jena glass.