GREEN CRYSTALLINE COMPOUND PART III: DETERMINATION OF
PERCENT POTASSIUM AND PERCENT IRON
PURPOSE: In this third of four experiments involving the synthesis and analysis of a green crystalline compound with the formula, K x
Fe y
(C
2
O
4
) z
· a H
2
O, you will determine both the percent potassium and percent iron. The % K and % Fe will be obtained by carrying out a pH titration after passing a solution containing a known mass of the compound through an ion exchange column to prepare for analysis.
The ion exchange column contains a resin. Resins are solids which are insoluble in water.
They usually have a granular texture and when they are added to water, they form a slurry. Ion exchange resins are made up of large molecules containing ionizable groups.
The ionizable portion of the resin molecule to be used in this experiment is a sulfonic acid group (-SO3-H). This group ionizes in the presence of water.
R-SO
3
-H ---------------> R-SO
3
-1 + H +1
R symbolizes the rest of the resin molecule. H
3
O
+
is the hydronium ion which is bound to the sulfonate ion on the resin. This is a cation resin, so called because other cations can be substituted for H
3
O
+
(the acid form of the resin). When a slurry of the resin and water is poured into a glass tube, plastic column or buret, with a cotton plug at the bottom to hold the resin in and let excess water flow out, a workable ion exchange column is obtained. When a massed sample of the green crystalline compound is dissolved in water, the salt dissociates by the equation:
K x
Fe(C
2
O
4
) y
· zH
2
O
-----------------> xK + (aq) + Fe(C2O4) y x(aq) + zH
2
O
If this solution is passed through the cation exchange resin in the acid form, each of the
K
+
ions will become attached to the resin in place of an H
3
O
+
ion. The equation is: xR-SO
3
- H
3
O + + xK + + Fe(C
2
O
4
) y x---------------> xR-SO3 K + + xH
3
O + + Fe(C
2
O
4
) y x-
The number of moles of K + in the solution of the green crystalline compound is equal to the number of moles of H
3
O
+
in the solution (the eluate) that comes off the resin column.
The number of moles of H3O+ and thus the number of moles of K+ can be determined by doing an acid-base titration using a standardized solution of NaOH: mol K + = mol H
3
O + = mol NaOH used in titration = V
NaOH
(in liters) · M
NaOH
The mass of K+ can be obtained from the number of moles: mass K + in sample = mol K + · 39.1 g/mol
The percent potassium can then be calculated:
%K = [(mass of K + in sample)/(mass of sample of crystal)] x 100

Notice that iron comes through the ion exchange column in the form of Fe(C
2
O
4
) y x-
.
When the solution from the ion exchange column is titrated with standardized NaOH solution, the reaction that takes place first is given by the equation:
H
3
O + + OH ----------->
2H
2
O
Once all of the acid (H
3
O + ) has reacted, further addition of NaOH results in the reaction of Fe(C
2
O
4
) y x-
by the following equation:
Fe(C
2
O
4
) y x(aq) + 3OH (aq)
---------------->
Fe(OH)
3
(s) + yC
2
O
4
2(aq)
The Fe(OH)
3
precipitates from solution as a rust colored precipitate. The equation shows that three moles of OH
-
from the NaOH are needed to react with one mole of Fe in
Fe(C
2
O
4
) y x-
. mol Fe = V
NaOH
(in liters) · M
NaOH
· 1 mol Fe
3 mol NaOH
The mass of Fe can be calculated from the number of moles. mass Fe = mol Fe · 55.8 g/mol
Since the mass of the green crystalline compound is known, the percent iron can be determined.
%Fe = [(mass Fe)/(mass of crystalline compound)]x100
The solution obtained from the ion exchange column is to be titrated with a standard solution of NaOH. A pH meter is used to follow the reaction. A titration curve of pH versus number of mL of NaOH solution added will be graphed. The curve will have two equivalence points as shown in the figure below:

The volume of NaOH solution needed to reach the first equivalence point (V
1
) along with the molarity of the solution can be used to calculate the number of moles of OH
-
needed to neutralize the H
3
O + . The volume of NaOH solution required to go from the first equivalence point to the second equivalence point (V
2
-V
1
) along with the molarity of the solution is used to calculate the number of moles of OH
-
needed to combine with iron.
PRELAB PROBLEMS:
1. Suppose 0.240 gram of the green crystalline compound is measured out for this experiment (instead of the traditional 0.160 gram). What are the mass percentages of K and Fe if the equivalence points are at V
1
= 14.10 mL and V
2
= 28.88 mL when the salt solution that has been passed through the ion exchange column is titrated with 0.100 M
NaOH?
MATERIALS:
Buret and clamp
Dowex exchange resin
.100 M NaOH
150 mL beaker
10 mL graduated cylinder Distilled water
8M nitric acid cotton ball green crystalline compound
Computer and labpro device pH probe
PROCEDURE:
1.
CONSULT YOUR INSTRUCTOR ON HOW TO PREPARE YOUR ION
EXCHANGE COLUMN.
2.
Record the precise concentration of the 0.100 M NaOH solution on your data sheet.
3.
Measure out between 0.160 and 0.165 gram of the green crystalline compound into a 150 ml beaker and record the mass.
4.
Add 4 mL of distilled water to a 10 mL graduated cylinder and transfer to the beaker containing the crystals. Swirl the beaker until the crystals are completely dissolved.
5.
Set a clean and dry 150 mL beaker under the ion exchange column and quantitatively transfer the solution containing the green crystalline compound to the resin column. Allow the solution to run through the column to just above the top of the resin and collect it in the beaker.
6.
Rinse the 50 mL beaker that originally contained the green crystalline solution with 4 mL of distilled water and pour it onto the resin column. Run this solution through the column to the top of the resin bed and collect the eluate in the 150 mL beaker.
7.
Repeat the procedure of rinsing your solution with 4 mL of distilled water and running the solution through the ion exchange resin one more time.
8.
Prepare to do a pH titration by putting a stirring bar in the green crystalline solution and setting the 150 mL beaker containing the solution on top of a magnetic stirrer. Open the program marked “Titration” on the desktop.

9.
Obtain approximately 70 mL of 0.100 M NaOH in a clean dry 100 mL beaker and after rinsing the buret three times with 5 mL portions of the solution, fill the buret to the zero mark with the 0.100 M NaOH solution. Expel air bubbles from the buret.
10.
Calibrate your pH probe as indicated by the instructor.
11.
Put the pH electrode into the 150 mL beaker containing the green solution and add enough distilled water to cover the end of the electrode in solution.
12.
Titrate the solution with 0.100 M NaOH. Add 2 mL increments of the NaOH solution until you get to 5 mL (near the first equivalence point). Take readings of the mL of NaOH solution and pH after each incremental addition.
13.
Between 5 mL and 11 mL make approximately 0.5 mL additions of the NaOH solution.
14.
Between 11 mL and 15 mL make 2 mL additions of the NaOH solution which will get you close to the second endpoint.
15.
Around the second equivalence point (between 15 mL and 21 mL) make 0.5 mL additions of the NaOH solution. End the titration with two 2 mL additions.
16.
Make a graph of pH vs. volume of 0.100 M NaOH added. Draw a smooth curve through the points. Determine the equivalence points V
1
and V
2
from the graph.
17.
Calculate the % K and % Fe in the green crystalline compound.
DATA AND CALCULATIONS:
Include the following in the data/calculations portion of your lab write up:
Concentration of NaOH solution, mass of green crystalline sample, volume in liters of
NaOH solution required for first equivalence point, moles of NaOH required for the first equivalence point, moles of K
+1
in sample, mass of K
+1
in sample, percent K
+1
in sample, volume in liters of NaOH solution required for the second equivalence point, moles of
NaOH required for the second equivalence point, moles of Fe
+3
in sample, mass of Fe
+3 in sample, percent of Fe
+3
in sample.
SHOW ALL RELEVANT CALCULATIONS THAT TOOK YOU FROM EACH
ITEM ABOVE TO THE NEXT!
CONCLUSIONS:
1.
Why is it not necessary to keep track of the amount of distilled water added to the solution of the green crystalline compound in step 9 of the procedure?
2.
Describe, using illustrations or explanation, how the ion exchange column operates.
3.
Why is the number of moles of NaOH used to titrate the sample of the green crystalline compound to the first equivalence point equal to the number of moles of potassium in the sample?