Precipitation of Lead Chromate

Curriculum Notes 

This demo is usually used when solubility rules are being introduced. It can also be used to illustrate a double replacement reaction. The use of lead chromate as "chrome yellow," a commercial pigment, means that this demo can be used in a consumer chemistry unit also. Allow about 5 minutes for this demo.

Lead Time 
One day of lead time is required for this project.
Discussion 

Almost all alkali metal compounds and nitrates are soluble, but most lead compounds are insoluble (except for acetates, perchlorates, chlorates, and nitrates). Therefore, when the soluble salts lead(II) nitrate and potassium chromate are mixed, insoluble lead(II) chromate forms and precipitates out (Ksp = 2.8 x 10-13).1Pb(NO3)2(aq) + K2CrO4(aq) ==> PbCrO4(s) + 2 KNO3(aq) or Pb2+(aq) + CrO42-(aq) ==> PbCrO4(s) Lead(II) chromate is also known as "chrome yellow" and has been used as a yellow pigment since the 1800's. It is, however, fairly toxic. It occurs naturally in the rare mineral "crocoite".2

1 Solubility Product Constants near 25 Degrees Celsius 2 Pigments Through the Ages

Materials 
  • about 20mL concentrated lead(II) nitrate solution
  • dropper bottle of potassium chromate solution
  • large test tube
  • ring stand
  • ring stand test tube clamp
  • small funnel
  • background box
Procedure 

Place the ring stand holding the test tube in front of the background box to provide better visibility. The blue background works best for this demo. Using the funnel, fill the test tube about halfway with lead(II) nitrate solution. Add a dropperful of potassium nitrate solution to the test tube. Immediately a bright yellow precipitate forms.

Safety Precautions 

Both lead(II) nitrate and potassium chromate are toxic and strong oxidizers. Wear goggles. Avoid getting either chemical on your skin. If your skin comes into contact with either chemical, immediately wash thoroughly with soap and water. If you get either chemical in your eyes, flush with water for 15 minutes and seek medical help.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group