Fusion of Titanium Dioxide and Potassium Pyrosulfate
My question is about the fusion of titanium dioxide and potassium pyrosulfate. Can I ask the proper temperature and time in the process of fusion you used in the Samples Containing Titanium portion of your Sample Preparation Guide?
Ti2O3, which has a black or blue color, is soluble in concentrated HCl orsulfuric acid. The main oxide form of titanium is TiO2. The dioxide exists in three crystalline states, namely anatase, rutile and brookite. The anatase and rutile forms are soluble in concentrated sulfuric acid, HF, and HF mineral acid combinations.
My favorite combination for dissolving anatase or rutile is 1:1 HF/HNO3 where some heat is generally required to speed dissolution. When the anatase or rutile forms are heated to temperatures at or above 800 °C, they are converted to the brookite form. My experience has been that the brookite form is not soluble in any acids and requires fusion. Fusion with sodium carbonate, pyrosulfate, alkali metal hydroxides, and sodium tetraborate are common. In this case, my favorite is the pyrosulfate fusion using a Pt crucible:
The finely powdered sample is mixed with 35 times its weight of potassium pyrosulfate and heated in a Pt crucible until the melt appears clear. It can be swirled or stirred with a Pt wire. The crucible is removed from the heat and tilted and turned slowly to distribute the melt over the walls of the crucible as it cools. The solidified melt in this way often cracks into small pieces, and falls off the walls of the crucible. Potassium pyrosulfate has a melting point of 419 °C. If heating is continued too long, the simple sulfate begins to precipitate out from the melt. Should this happen, cool and add a few drops of sulfuric acid to regenerate the flux. Note that potassium disulfate begins to decompose around 300 °C and sulfur trioxide is evolved rapidly at 500 to 600 °C.
If you wish to stay away from fusions altogether, then avoid ashing samples at temperatures above 500 °C to avoid formation of the brookite form, which may simply be referred to as the "ignited oxide."
I hope this helps and best of luck.
Serving you in chemistry,
Paul R. Gaines, Ph.D.
CEO of Inorganic Ventures & Fellow Chemist
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