Inorganic Synthesis

Potassium dichromate crystals

potassium dichromate

Theory:

Potassium dichromate is a salt of chromic acid with nicely reddish colour and plenty of interesting properties. It is a strong oxidizing-agent in acidic medium:

Cr2O7 2- + 14H+ + 6 electron -> 2Cr3+ + 7H2O

This property is mainly used in organic chemistry  to oxidize alcohols. It converts primary alcohols into aldehydes and, under more forcing conditions, into carboxylic acids. Potassium dichromate  is also used as an analytic reagent to determinate presence of ethanol, sulfur dioxide and also for purity tests of various metals. In nature potassium dichromate occurs as mineral Lopezite which is however extremly rare. Although potassium dichromate has a lot of interesting properties, the hexavalent chromium anion is extremly toxic and carcinogenic so  maximum carefulness must be taken when handling it.

Experimental:

100 mL of distilled water was poured into the beaker and then potassium dichromate powder (30 g, 102 mmol)  was added. The solution was  heated and constantly stirred until all K2Cr2O7 has dissolved. A nylon thread was immersed into the solution. After that, the beaker was placed into a dewar flask in order to slow down the cooling as much as possible. As the water cooled down, small seed crystals have grown on the thread.  The thread was removed from the solution and the solution was heated once again to approximately 40 °C. The K2Cr2O7 powder was added till the solution was saturated. The thread with seed crystals was immersed again and the beaker was placed into the dewar flask. This procedure was  repeated several times.