Selective Precipitation
AgCl and Ag2CrO4
Chemicals and Equipment Needed
 d-H2O
 0.1 M NaCl – H2
 0.1 M Na2CrO4 – H2
 0.1 M AgNO3 – H2
 2 black backgrounds – A1
 1 white background – A1






1-250 mL graduated cylinder – Q4
3-100 mL graduated cylinder – Q3
4-25 mL graduated cylinder – Q3
2-10 mL graduated cylinder – Q3
3 glass stir rods (one long) – U1
Rubber stoppers for all cylinders – U3
Preparation
 Make 0.005 M Na2CrO4:
o First make 0.01 M stock solution – measure 10 mL 0.1 M Na2CrO4 into one of the 100 mL
graduated cylinders, and dilute with d-H2O to 100 mL. Stopper and label.
o Measure 25 mL 0.01 M Na2CrO4 into a 100 mL cylinder, dilute with d-H2O to 50 mL.
Stopper and label “Na2CrO4”
 Make 0.05 M NaCl:
o Measure 25 mL 0.1 M NaCl into a 100 mL cylinder, dilute with d-H2O to 50 mL.
Stopper and label “NaCl”
 Make combined cylinder:
o Measure 60 mL 0.01 M Na2CrO4 stock solution into the 250 mL graduated cylinder. Add 60 mL
0.01 M NaCl to the same 250 mL graduated cylinder and mix the two solutions well.
Stopper and label “NaCl + Na2CrO4”
 Label the bases of the 10 and 25 mL graduated cylinders “0.1 M AgNO3” Right before delivery, add
20 mL 0.1 M AgNO3 to the 25mL cylinders and stopper (4 total). Add 5 mL 0.1 M AgNO3 to the 10 mL
cylinders and stopper (2 total).
 On delivery, place the “NaCl” and “NaCl + Na2CrO4” cylinders in front of the black backgrounds, and the
“Na2CrO4” cylinder in front of the white background. Place a 10 mL cylinder next to each of the smaller
cylinders, and all 4-25 mL cylinders near the “NaCl + Na2CrO4” cylinder.
Presentation
Revised Spring 2015
AMM



“NaCl” cylinder:
o Add one 10 mL graduated cylinder of 0.1 M AgNO3 and stir.
o White cloudy precipitate, AgCl.
“Na2CrO4” cylinder:
o Add the other 10 mL graduated cylinder of 0.1 M AgNO3 and stir.
o Orange-red precipitate, Ag2CrO4.
“NaCl + Na2CrO4” combined cylinder:
o Add the first 25 mL graduated cylinder of 0.1 M AgNO3 and stir.
o
▪ Cloudy light yellow precipitate, AgCl (tinted with CrO42-)
Add another 25 mL graduated cylinder of 0.1 M AgNO3 and stir.
o
▪ Cloudy light yellow precipitate, AgCl, some orange precipitate, Ag2CrO4
Add the third 25 mL graduated cylinder of 0.1 M AgNO3 and stir.
o
▪ Some orange precipitate, Ag2CrO4.
Add the fourth 25 mL graduated cylinder (if desired) and stir.
▪
Orange-red precipitate, Ag2CrO4.
Discussion
 AgCl
o Ksp = 1.8x10-10 = [Ag+][Cl-]
Ag2CrO4

o Ksp = 1.1x10-12 = [Ag+]2 [CrO42-]
 If 0.1 M AgNO3 is gradually added to a solution of 0.05 M NaCl and 0.05 M Na2CrO4, what will happen?
o Precipitation of AgCl: M Ag+ required
o Precipitation of Ag2CrO4: = [Ag+]2; M Ag+ required
 AgCl precipitates first and keeps precipitating until [Ag+] > M.
o How much [Cl-] is left?
Clean-Up
 Dispose of excess Na2CrO4 stock solution in the white waste container.
 All solutions should be disposed of in the white waste container right after class. If the silver
compounds cannot be scrubbed out of the cylinders, use some 6 M HNO3 to clean them.
Revised Spring 2015
AMM
Lecturer Instructions
 “NaCl” cylinder:
o Add one 10 mL graduated cylinder of 0.1 M AgNO3 and stir.
o White cloudy precipitate, AgCl.
 “Na2CrO4” cylinder:
o Add the other 10 mL graduated cylinder of 0.1 M AgNO3 and stir.
o Orange-red precipitate, Ag2CrO4.
 “NaCl + Na2CrO4” combined cylinder:
o Add the first 25 mL graduated cylinder of 0.1 M AgNO3 and stir.
▪ Cloudy light yellow precipitate, AgCl (tinted with CrO42-)
o Add another 25 mL graduated cylinder of 0.1 M AgNO3 and stir.
▪ Cloudy light yellow precipitate, AgCl, some orange
precipitate, Ag2CrO4
o Add the third 25 mL graduated cylinder of 0.1 M AgNO3 and stir.
▪ Some orange precipitate, Ag2CrO4.
o Add the fourth 25 mL graduated cylinder (if desired) and stir.
▪ Orange-red precipitate, Ag2CrO4.
Revised Spring 2015
AMM