Percent_Potassium_and_Iron_Determination by nuhman10


                              OXALATO SALT BY ION EXCHANGE
                         (NSF Summer Project in Chemistry, Hope College)
                    Read this entire laboratory procedure before starting this lab.
     This experiment involves determining both the percent potassium and iron in a single
titration after passing a solution containing a known mass of the complex salt through an ion
exchange column.
Ion Exchange:
     Certain materials called ion exchange resins consist of rather large molecules, which contain
ions that can be displaced. The resins are solids, insoluble in water, usually granular in texture,
which when added to water swell to form a slurry.
     In our specific case, an aqueous solution containing K+ ions is poured into the ion exchange
resin filled column. This solution displaces an equal volume of liquid surrounding the resin,
which elutes from the bottom of the column. In the process, as the solution containing K+ ions
passes down the column, the K+ ions will exchange with H3O+ ions. The K+ ions will bind to the
resin and release H3O+ ions. The solution coming out of the column will contain a quantity of
H3O+ ions equal to the number of K+ ions that were added to the column. Anions are unaffected
by this ‘cation exchange resin’ and pass through.
The Titration
     When the solution eluted from the ion exchange column, called the eluate, is titrated with
standard NaOH using a pH meter to follow the course of the reaction, a titration curve looking
something like that in Figure 1 is obtained.

                                              Figure 1
   Two titrimetric equivalence points are obtained; the first, after the addition of V1 mL of
NaOH and the second after V2 mL of NaOH have been added. The first equivalence point
represents the completion of the neutralization of H3O+ ions that were released by an equal
number of K+ ions.

                            H3O+ + OH–  2 H2O                                                   (1)

Thus for each mole of K+ added to the column one mole of H3O+ elutes from the column. If the
eluted solution is titrated with standard NaOH, the moles of H3O+ in the solution eluted from the
column (hence moles of K+ added to the column) can be determined.
    mol K+(aq) added = mol H3O+(aq) eluted = mol NaOH required in titration = VL x MOH (2)
                             VL = volume in liters of standard NaOH
                      MOH = the molarity in mol/L of the standard NaOH
Thus V1, converted to liters, is the value of VL in Equations 3a and 3b. (The atomic mass of K is
39.10 g/mol.)
                  mass of K+ in the sample = VL x MOH x 39.10 g/mol                          (3a)

                   % Potassium in sample = (VL x MOH x 39.10 g/mol) x 100                         (3b)
                                                 mass of sample

After all of the acid is neutralized in the titration, further addition of NaOH results in the reaction
represented by Equation 4. The Fe(OH)3 precipitates from the solution as a reddish-brown
gelatinous precipitate.

                    Fe(C2O4)yx– + 3 OH–  Fe(OH)3(s) + y C2O42–                                     (4)

Equation 4 indicates that 3 moles of OH– are required to react with each mole of Fe in the salt.
Or, stated in a different way, one mole of Fe is chemically equivalent to three moles OH– in the
reaction. Thus if it requires VL liters of standard NaOH solution of concentration MOH mol/L to
completely precipitate all of the iron in the weighed sample, it follows that the number of moles
of iron in the sample is given by equation (5). V2–V1, converted to liters, is the value of VL in
Eqn. 5, 6 and 7. Thus the second equivalence point represents the completion of the precipitation
of Fe(OH)3.
                                                                       1 mol Fe
                                mol Fe  (VL x M OH ) mol OH _ x                                   (5)
                                                                      3 mol OH _

The mass of Fe present is given by Equation 6. (The atomic mass of Fe is 55.85 g/mol.)

                            (VL x M OH- ) mol Fe x 55.85 g/ mol Fe VL x M OH- x 55.85 g
             mass of Fe                                                                          (6)
                                               3                            3
Finally, the % Fe in the salt is given by Eqn. 7.

                                           VL x M OH- x 55.85 x 100
                                  % Fe                                                           (7)
                                             3 x mass of sample

   Thus, from a single pH titration curve of a weighed sample of KxFe(C2O4)y zH2O that has
been passed through a cation exchange resin, it is possible to determine both the percent K and
percent Fe in the compound.

1. To learn the principles and practice of using ion exchange columns.
2. To determine the percent potassium (K) and percent iron (Fe) in KxFe(C2O4)y zH2O using an
   ion exchange column and titration techniques.

Ion exchange column packed with cation exchange resin
10-mL graduated cylinder
pHydrion paper
0.001 g electronic balance
50-mL beaker
two 150-mL beakers
short stem funnel
pH meter
buret clamps
magnetic stirrer motor
stir bar.

Standardized NaOH (~0.1000 M)
buffer solutions for pH meter standardization
1 M HCl
student prepared KxFe(C2O4)y zH2O

Safety, Environmental, and Economic Concerns:
1. Be sure to wear safety goggles.
2. All solutions used in this experiment may be safely discarded down the drain.
3. A pH meter is an expensive and delicate instrument. If solutions spill on it wipe up the spill
   immediately and inform your teacher.
3. NaOH is very corrosive and destructive to body tissue, especially to the eyes. It is extremely
   important to wear safety glasses or goggles while working with it.

Experimental Procedure: This experiment should be started at the beginning of the laboratory
period in order to complete it in one period.
Ion Exchange
     Obtain an ion exchange column that is mounted on a ring stand
(Figure 2). It is important to make sure that the resin bed is filled with
liquid at all times. (Note 1)
     Using a clean 10-mL graduated cylinder rinse the column by pouring
4 mL of dH2O water into the column and collect the liquid that elutes
from the column in a small beaker.
     Using a piece of pHydrion paper test the pH of the solution that first
elutes from the column, to make sure it is distinctly acidic (pH << 7). If
it is not acidic, immediately inform your instructor.
     Allow the level of rinse liquid in the column to fall to the top of the
resin (Note 1).
Repeat the above rinse procedure two more times using 4 mL portions
of dH2O. (Note 2).                                                             Figure2

    When the level of water in the third rinse has dropped to the level of the top of the resin, test a
drop of eluate with pHydrion paper to confirm that the pH is about that of dH2O (Note 2). If the
eluate is still acidic, continue to rinse until it is about the pH of dH2O.
    There will be some waiting time accompanying the rinse procedure; make good use of this
time by looking ahead and preparing everything that will be needed for the day's work.
    Using the 0.001 g balance weigh a sample of between 0.155 and 0.165 grams of your
KxFe(C2O4)y zH2O in a 50-mL beaker. Use unheated crystals, not the sample that has lost its
water of hydration. Make sure that the sample mass does not exceed 0.165 g. Record the mass of
sample. Remember that no chemical is ever added to or removed from a container while the
container is in the balance compartment.
    Using the 10-mL graduated cylinder add 4 mL of dH2O to the sample of crystals and gently
swirl the beaker until the salt is completely dissolved.
    Place a clean 150-mL beaker under the ion exchange column and quantitatively transfer the
solution to the column. Collect the eluate in the 150-mL beaker.
    Rinse the beaker that you just emptied into the column with 4 mL of dH2O and when the level
of liquid in the column has dropped to the top of the resin, pour this rinse water into the column.
    Repeat this rinse procedure with two more 4 mL dH2O rinses (Note 3), waiting each time until
the liquid level in the column has dropped to the top of the resin level before adding more liquid
to the column.
    Before beginning the titration pour 10-mL of 1.0 M HCl into the ion exchange column and
place a beaker under the column to collect the eluate (Note 4). Take care that none of this acid
gets into the solution of crystals that has been eluted from the column.

pH Meter Setup and Standardization
    Set up a clean buret and pH
meter assembly (Figure 3). A pH
meter is an expensive, delicate
instrument and should be treated
with great care. The electrode
system is especially vulnerable.
1. Take the following precautions
with regard to electrode care.
    a. When the electrodes are not
        in use keep them immersed
        in pH 7.0 buffer.
    b. Do not keep the glass
        electrode in solutions having
        pH 10 or above for extended
        periods time.
    c. The membrane of the glass
        electrode is very thin and it
        should be protected from
        mechanical shock. Take care
        to insure that the electrodes
        clear the stirrer bar before
        starting the stirrer motor.
        Figure 4

Figure 3

   d. Electrodes must be immersed in aqueous solution to a depth that covers the plug for the
      reference part of the combination electrode. (Figure 5)

                     Figure 4                                             Figure 5
2. Remove the stirrer bar before emptying the liquid into the sink. Stirrer bars are expensive and
    care must be taken to avoid losing them down the drain.
Standardize the pH meter as follows:
    Thoroughly rinse the electrode with a stream of dH2O from a wash bottle. (Do not wipe the
glass electrode.)
    Immerse the electrode to the correct depth in the pH 7.0 buffer solution contained in the small
vial, and press the "standardize" button so that the meter gives a reading equal to the pH of the
buffer. The meter standardizes itself while you wait. Each time the meter is unplugged, it must
be standardized again.
    Set up a two-column table data sheet. Label one column, mL of titrant added, and the other,
    During the course of the titration, record a pH reading for each titrant increment beginning
with a reading at 0.00 mL of titrant added.
    Obtain about 30-mL of standard NaOH in a clean, dry 50 mL beaker and after rinsing the
buret three times with small amounts (about 5-mL portions) of the solution fill the buret with the
standard NaOH solution. In order to fill the buret it will be necessary, of course, to obtain a
second 30-mL of standard NaOH in the 50-mL beaker. Do not allow the NaOH to stand in the
beaker exposed to the air. Put it immediately into the buret where it has essentially no air contact.
    Prepare for titration by expelling air bubbles from the buret, etc.(Click here for video)
    Clamp a buret containing titrant on a ring stand and adjust the titrant level to 0.00 mL. In this
case it may be worth the extra effort required to adjust the titrant level to exactly 0.00 mL, since
so many readings will be taken and it is convenient to have the buret readings be the actual
volume of titrant added. Alternatively, the titration may be started at some initial reading near,
but greater than zero. If this is done, it is necessary to subtract the initial reading from the number
of mL at the equivalence point determined from the plot.
    Remove the drop from the buret tip with a paper towel.
    Place the 150 mL beaker containing the ion exchange solution to be titrated on the magnetic

stirring motor, and carefully place a stirrer bar in the beaker. Lower the rinsed electrode into the
beaker and position it such that there is ample clearance between the bottom of the electrode and
the stirrer bar (See Figure 4). If the electrode bulb is not immersed in solution, add sufficient
dH2O to immerse it (See Figure 5). Establish a smooth stirring rate.
    Position the buret so that the stopcock is readily accessible and the tip is several millimeters
above the liquid surface.
    Make sure that the instrument is set to measure pH, and record the initial pH at 0.00 mL titrant
    Slowly add titrant until the pH changes by about 0.2 to 0.3 units.
    Record the buret reading then read and record the pH. Always record the buret reading before
reading and recording the pH. This allows the potential to equilibrate each time before you take
the pH reading. Do not waste time waiting for the reading to become perfectly constant. Read
and proceed with the next addition as quickly as you can. Always record the buret reading, then
read the pH meter, add more titrant, etc. Continue to add titrant in increments that change the pH
by 0.2 to 0.3 units. Be aware that as the equivalence point is approached a much smaller volume
of titrant will be required to change the pH than earlier in the titration.
    Near the equivalence point one drop of titrant will cause a change of more than 0.3 pH units.
In this region take a reading after the addition of each drop of titrant. (Note 5)
    After the equivalence point larger quantities of titrant will again be required to change the pH
by 0.2 units. Continue to add titrant increments until the system is 15 mL beyond the second
equivalence point (about 35 mL total). This is done in order to establish the extrapolated CD line
in Figure 2.

When the titration has been completed raise the electrode, rinse with dH2O and immerse it in the
pH 7.0 buffer.

     Carefully plot your pH data vs mL of standard NaOH added using Excel or GAX. Draw by
hand the best smooth curve through the points. Nothing is gained by expansion of the y axis
beyond the extent of the graph paper.
     Determine both equivalence points (V1 and V2) by the procedure illustrated below in Figure 6.
 You may find it helpful to make two expanded plots-one from about 3 to 17 mL NaOH added,
and the other from about 13 to 27 mL added.
     A straight-edge is used to draw line AB coincident with the linear portion of the pre-
equivalence point curve. Similarly draw line CD coincident with the linear portion of the post-
equivalence point curve. Then draw line EF coincident with the most linear portion of the steeply
rising curve in the equivalence point region. E and F are the intersections of this line with CD
and AB respectively. Determine G the midpoint of EF. Drop a perpendicular, GH, to the x-axis.
 The volume of base required to reach the equivalence point in the titration is given by H. Clearly
it is necessary to extend the titration considerably (7 to 15 mL) beyond the equivalence point in
order to construct CD accurately.
     Show all calculations. Complete the report form on the following page. Attach the pH
titration graph(s). Be sure that the numerical results have been recorded on the report form.

                                                Figure 6

Notes on Experimental Procedure:
1. Mount the ion exchange column vertically on a ring stand using a utility clamp. The column
   should always be filled with liquid up to or above the top of the resin so that air pockets
   cannot form in the resin bed.

2. When you receive the column it will contain an acid solution that has been added to insure
   that all of the resin is in its acid form. It is absolutely necessary to rinse out all of this acid.
   Never allow the liquid level to fall below the top of the resin.

3. During the time the solution of KxFe(C2O4)y zH2O and rinses are moving down the column,
   the K+ ions from the dissolved crystals are exchanging with the H3O+ of the resin (Equation

4. As the HCl solution moves down the column, the H3O+ ions of the acid exchange with the K+
   ions bound to the resin (the reverse of Equation 4). This regenerates the resin to the acid form
   so that the column will be ready for the next student.

5. Be alert! The first equivalence point may come before 10 mL of NaOH have been added and
   the second equivalence point before 20 mL of NaOH has been added.

                                 STUDENT REPORT FORM

                 Determination of the Simplest Formula of a Complex Iron Salt
                         Percent Potassium and Iron by Ion Exchange
      Mass of sample                                                              g

      Molarity of NaOH                                                           M

      V1, volume of NaOH required for first equivalence point.                  mL

      Mass of potassium in sample.                                                g

      Percent potassium in sample.                                              %K

      V2, volume of NaOH required for second equivalence point.                 mL

      V2 – V1, volume of standard NaOH that reacted with iron.                  mL

      Mass of iron in sample.                                                     g

      Percent iron in sample.                                                   %Fe



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