Synthesize Potassium Dichromate

Make Potassium Dichromate

Potassium dichromate is an orange colored inorganic salt of chromium in its 6+ oxidation state. It can be used in to increase chlorate production efficiency in a chlorate cell. Below I will be producing potassium dichromate from the characteristic green oxide that is well know for being nearly impossible dissolve and produce useful salts out of. 

   2KNO₃  + Cr₂O₃  --------> K₂Cr₂O₇ + 2NO 

Calculations:

1 mol of potassium nitrate is 101.10 grams

1 mol of chromium oxide is 151.99 grams

I will run the reaction on a 0.25 molar scale

50.55grams of potassium nitrate will react with 38.00grams of chromium oxide to make a theoretical of 73.55 grams of potassium dichromate.

Materials:

•  2 250ml beakers

•  50.55g potassium nitrate

•  38.00g chromium oxide

•  100ml fused quartz beaker

•  Blowtorch

•  Electronic mass balance

•  Funnel

•  Filter paper
•  Hotplate 

Procedure:

1.   Weigh out 50.55g of potassium nitrate and place into the fused quartz beaker. 

2.   Weigh out 38.00g of chromium oxide and mix with the potassium nitrate in the quartz beaker.

3.   Clamp using a metal clamp either in a fume hood or outdoors.

4.   Heat strongly with the blowtorch until the potassium nitrate has melted. 

5.   Continue heating and brown nitrogen dioxide vapors should start to be evolved. Keep even heating until no more nitrogen dioxide vapors are produced. 

6.   Once cool, invert the beaker and tap on a piece of paper, a slug of potassium dichromate and unreacted reagents should fall out. 

7.   Heat 150ml of water to boiling and add the slug, stir until most of the potassium dichromate has dissolved. 

8.   Filter the resulting solution while hot to remove unreacted chromium oxide. 

9.   Evaporate the filtrate until dry, be prepared for creep, where by capillary action the crystals crawls up and out the sides of the beaker. 

Conclusion:

The end result is about 30g of reasonably pure potassium dichromate. However, as the solubility curve of potassium nitrate and that of potassium dichromate are rather similar there may still be some potassium nitrate contamination. Also, I think I got a poor yield because I did not heat the beaker long enough. 

Closing Remarks:

1. Remember that this experiment requires the use of a QUARTZ beaker, or at least a ceramic crucible. Using a normal borosillicate glass beaker will result in breakage. 
2. Heat for as long as you can see brown fumes. I didn't heat mine enough which is why I had a poor yield. 
3. Potassium dichromate is TOXIC and is CARCINOGENIC. Wear gloves when handling and also limit exposure to dust. 
4. Hexavalent chromium needs to be reduced with a suitable reducing agent for clean up. I use sodium sulfite. 
5. Chromium is a heavy metal and waste should not be poured down the drain. Instead, evaporate it down and place it into a container for later disposal at a waste treatment facility.