Crystal Growth


This exercise involves a mixture containing ammonia, salt, water, and an old-time laundry whitener called BLUING. (Laundry Bluing, i.e. Mrs. Stewart's, is used to whiten fabrics which have turned yellow or gray. It is composed of a colloidal solution of Prussian Blue which is used as a blue pigment that makes fabrics appear white. Prussian Blue is made by reacting potassium ferrocyanide with iron (III) sulfate.) When poured over charcoal and left for hours, a crystaline growth is produced.

The reaction and activity is called "Crystal Tree," "Magic Tree," or "Crystal Garden" which is a miniature artificial tree that grows forming ornamental "buds" in as little as 15 minutes, and "magically" growing into a delicate tree in about 2 hours. Magic Tree is manufactured by New Tomorrow, Inc. The Bluing of choice for this activity is Mrs. Stewart's Liquid Bluing. Other hints are to use Kosher salt or pickling salt for the NaCl (do not use iodized salt) and use the regular household ammonia (not sudsy) for the ammonia solution. You may also use food coloring if you wish.

The chemistry of the reaction is pretty complex. If you omit the salt, no crystals will form. If you omit the ammonia, hard, plate-like crystals resembling salt crystals will form. Without the Bluing, only a white crust will develop without any crystals.

This is the chemistry of the crystal growth:

3K4Fe(CN)6 + 2Fe2(SO4)3 + xH2O ---> Fe4[Fe(CN)6]3 *xH2O + 6K2SO4

note x = 14-16

Prussian blue is formed as a precipitate (it is not soluble in water). This reaction, however, is the ideal case when the reaction occurs very slowly. If formed quickly, as is usually the case, the Prussian blue is more likely to have the formula

K[Fe+2Fe+3(CN)6]xH2O. It contains iron in both the +2 and +3 oxidation states, as denoted by the Roman numerals. The intense color is due to charge-transfer from Fe+2 to Fe+3 (There is no color change if both iron atoms are in the same oxidation state).

Addition of ammonia, a base, and salt does not change the color of the bluing solution, but the crystals that grow are white. It is suspected that during the evaporation of solvent and resulting crystallization there is probably a reduction of the Prussian blue forming white K2Fe+2Fe+2(CN)6 where additional K+ ions would fill the crystal lattice due to the openings left by reduction of Fe+3 to Fe+2. Since there are a few additional potassium ions in solution, the lattice is probably filled by the Na+ ions from the salt forming KNaFe+2Fe+2(CN)6 which should also be white in color. This potassium compound is known as Everitt's salt (Thomas Everitt, 1835).


See:

Cotton, F. Albert, and Wilkinson, Geoffery, Advanced Inorganic Chemistry, 5th Ed., Wiley-Interscience, New York, 1988, page 721.

Greenwood, N.N., and Earnshaw, A., Chemistry of the Elements, Pergamon Press, Elmsford, N.Y., 1984, page 1271.