CHEM 222 Harry S Ion Exchange Separation and Spectrophotometric Determination Questions

ADH 10/28/16
CHEM 222
KH 5-1: Part I – Determination of Iodide in Salt (Ion-Selective Electrode)
Name: ______________________________________________Date: ___________________
TA’s Name: __________________________________________UNKNOWN #: ____________
CALIBRATION CURVE
mass KI for ∼ 0.01 M stock, g
standards I− conc, M
log[I−]
voltage, mV
Prepare and include a graph of voltage vs. log[I−] in the standard solutions. Do a linear
regresssion on the data and report the regression parameters and their errors:
m
sm
b
sb
sy
r2
UNKNOWN ANALYSIS
trial 1
trial 2
trial 3
∆E, mV
slope m
A
S, M
X, M
µg I− per g salt
average µg I− per g salt
std deviation
KH 5-1: Part II – Testing Table Salt for Its Iodide Content by Titration
g sodium thiosulfate in 250 mL stock solution
M 250 mL sodium thiosulfate stock solution
regular salt
mass of salt, g
iodized salt
volume of thiosulfate, mL
moles of thiosulfate
moles of I− in sample
mass of I− in sample
% I− in salt
FOR GRADER’S USE ONLY (do not write below)
real µg I− per g salt
µg I− per g salt
experimental error
lab report grade
/100
ADH 11/09
CHEM 222 DU Exp. 10.5: Ion-Exchange Separation and Spectrophotometric Determination
of Ni and Co
Name: ______________________________________________ Date: ___________________
TA´s Name: _________________________________________
CALIBRATION CURVE
VOLUME STD SOLUTIONS
0.50 mL
1.00 mL
1.50 mL
2.00 mL
2.50 mL
conc Ni standard, mg/mL
absorbance of Ni standard
conc Co standard, mg/mL
absorbance of Co standard
Prepare and include two graphs of absorbance vs. mg/mL for the standard Ni and Co solutions.
Do a linear regression on the data and report the regression parameters and their errors:
m
sm
b
Ni
Co
UNKNOWN #: ___________
UNKNOWN ANALYSIS
absorbance of Ni unknown
concentration of Ni in original unknown solution
uncertainty in Ni concentration (use Eq. 4-27)
relative uncertainty (%)
absorbance of Co unknown
concentration of Co in original unknown solution
uncertainty in Co concentration (use Eq. 4-27)
relative uncertainty (%)
sb
sy
r2
Post-lab Question 1. Use only one page max.
Post-lab Question 2. Use only one page max.
ADH 10/28/16
CHEM 222
KH 5-1: Part I – Determination of Iodide in Salt (Ion-Selective Electrode)
Name: ______________________________________________Date: ___________________
TA’s Name: __________________________________________UNKNOWN #: ____________
CALIBRATION CURVE
mass KI for ∼ 0.01 M stock, g
standards I− conc, M
log[I−]
voltage, mV
Prepare and include a graph of voltage vs. log[I−] in the standard solutions. Do a linear
regresssion on the data and report the regression parameters and their errors:
m
sm
b
sb
sy
r2
UNKNOWN ANALYSIS
trial 1
trial 2
trial 3
∆E, mV
slope m
A
S, M
X, M
µg I− per g salt
average µg I− per g salt
std deviation
KH 5-1: Part II – Testing Table Salt for Its Iodide Content by Titration
g sodium thiosulfate in 250 mL stock solution
M 250 mL sodium thiosulfate stock solution
regular salt
mass of salt, g
iodized salt
volume of thiosulfate, mL
moles of thiosulfate
moles of I− in sample
mass of I− in sample
% I− in salt
FOR GRADER’S USE ONLY (do not write below)
real µg I− per g salt
µg I− per g salt
experimental error
lab report grade
/100
ADH 11/3/17
Kennedy Handout 5-1
Determination of I- in Salt – Part 1(Ion-Selective Electrode)
Principles
According to Equation 5-1 (see Quantitative Chemical Analysis 9th ed., D.C. Harris, pp. 345-347
and 354-364), the measured cell potential for an electrochemical cell containing an ion-selective electrode
is related to the activity of the selected ion
$%
𝐸!”## = 𝐾 + &’ ln 𝒜
(equation 1)
where K is a constant, n is the charge on the ion, and 𝒜is the activity. While 𝒜 = 𝛾I-[I-], you will be
working with sufficiently dilute solutions so that one can assume that the activity coefficient for iodide is
1. For an iodide ion-selective electrode, n = −1, and, therefore, the potential should decrease by 59 mV
for a tenfold increase in concentration. A calibration curve can be prepared by plotting the measured
potential versus the log of the concentration of known standard solutions. The calibration curve may then
be used to determine the concentration of an unknown sample. Although this analytical method is not of
high accuracy (± 3 to 10%) because the logarithmic relationship tends to amplify errors, it is fast and
convenient. For these reasons, it finds many uses.
Iodized salt contains a small amount of KI, which is readily determined with an iodide ionselective electrode. Although standards prepared in NaCl solutions could be used to determine K in
Equation 5-1 (using a calibration curve) it is better to use the standard addition method to compensate for
any matrix effects. Also, measurements of sample and sample plus standards can be made so close
together that problems with electrode drift are minimized.
An expression for the analyte with a 10% volume increase upon addition of the standard is
𝑋 = 𝑆+
(.(*(*
+,(.*(*
,
equation 2
In Equation 2, X is the analyte concentration in the sample, S is the analyte concentration in the added
standard solution, and A is antilog (∆E/m), where ∆E is the change in potential observed and m is the
actual slope. Theoretically, m = −59 mV for an iodide electrode. To derive Equation 2, use Equation 1
above and the standard addition equations in 5-3 in the Harris text, correcting for the volume change.
In this procedure the slope m is measured with standard iodide solutions and the potential (EX) of
a salt solution is measured. Then a standard addition of concentration S is made and the new potential
(EX+S) is measured. From the change in potential, ∆E = EX+S − EX, and the values of S and m the
concentration of iodide, X, can be calculated from Equation 5-2.
Directions
Warm up the pH meter and connect the iodide ion-selective electrode and the reference electrode.
Prepare a stock standard containing 10-2 M I- in water using reagent grade KI. Prepare working standards
containing 10-3, 10-4, and 10-5 M I- by sequential dilution (10 to 100 mL), starting with the stock standard.
Measure their potentials, using a 3-min equilibration time for the electrode (Note 1). Plot the potential
versus log of the concentration of the solution for each working standard and determine the slope, m.
Keep these standard solutions.
You will be doing three trials. Weigh out 3 g (± 10 mg) of the salt sample (Note 2), and dissolve
in 40 mL of water (Note 3). Then dilute to 50 mL in a volumetric flask. Transfer the entire 50 mL (do
not rinse) to a dry beaker, and measure the potential of the salt solution, using the expanded scale (Note
4). Allow a 5-min equilibration time (Note 1). Select a standard solution with at least ten times the
anticipated concentration of the sample (Note 5). With a volumetric pipet, add 5 mL of the selected
standard solution, stir, and measure the new potential after a 5-min equilibration time. Subtract the initial
electrode potential EX from the final potential EX+S to determine ∆E. Calculate A and then X. From X,
calculate and report the percentage of I- in the salt sample.
Notes
1. Some ion-selective electrodes are slow to equilibrate, especially in dilute solutions. In some cases,
1 or 2 min may be an ample equilibration time. Note how quickly the reading becomes stable to
see whether a shorter equilibration time may be acceptable. Drifting observed after 5 min usually
indicates incomplete mixing of the solution, a faulty electrode, or a faulty meter, the instructor
should be consulted.
2. No drying is necessary because this technique has an uncertainty > 1%.
3. Some cloudiness will frequently be observed, which presents no problem. This is due to a filler
material added to prevent caking in humid weather (“when it rains, it pours”).
4. Potential changes observed with ion-selective electrodes are normally a few millivolts, which
cannot be measured accurately with the normal pH meter scale (10 mV/division). Readings to ±
1 mV are esential, and readings to ± 0.3 mV are preferred, requiring an expanded scale of 1
mV/division.
5. A good estimate can be made from the measured electrode potential and comparing with the
calibration line determined from the standard solutions.
Pre-Lab (Your TA will check and grade your notebook when you arrive in lab to verify the prelab is done properly.)
1. Determine how many grams of KI are needed to prepare 0.01 M KI. Show your calculations.
2. Write a brief procedure in your notebook for how you will perform a serial dilution to make
standards containing 10-3, 10-4, and 10-5 M I-.
3. Prepare a table in your notebook for collecting standardization data in a neat and logical way. Be
sure to include: mass of KI used for stock, and for each standard, its standard concentration and
measured voltage. Be sure to include units for all measures! You may also want to include the
log[I-] value to facilitate analysis.
4. Prepare a table in your notebook for collecting your lab data. Be sure to include the weight of
your salt sample, the measured potential, the standard added, and the measured potential upon
standard addition. Be sure to include units for all measures!
Exp. 12: Ion-Exchange Separation and Spectrophotometric Determination of Ni and Co Page 1 of 5
Ion-Exchange Separation and Spectrophotometric Determination of Nickel and
Cobalt
The ion-exchange method of separation is discussed in sections 23-2 and 26-1 of Quantitative
Chemical Analysis 7th ed. D.C. Harris and there is a short discussion below. The experiment
illustrates this method by the separation of nickel and cobalt with a strong-base anion exchanger. In
solutions containing a high concentration of hydrochloric acid, many metals are converted into
complex anions (chlorides) and are adsorbed by an anion resin. Other metals that are not readily
converted into complex anions are not adsorbed by such a resin. Cobalt forms the deep blue complex,
CoC142- and this is strongly adsorbed by 9 M hydrochloric acid. Nickel is not adsorbed from such a
solution, and a separation of nickel from cobalt is easily effected. The cobalt is readily removed by
washing the resin with hydrochloric acid less concentrated than about 4 M.
The two metals can be determined by standard spectrophotometric procedures. Nickel is oxidized
with bromine in ammoniacal solution and then treated with dimethylglyoxime. A winered or brown
complex is formed:
[Ni(NH3)6]2+(aq) + 2(CH3CNOH)2(alc) Ni[ONC(CH3)C(CH3)NOH]2(s)+2NH4+(aq) + 4NH3 (aq)
Cobalt is determined by conversion of the metal to the blue complex cobalt thiocyanate, Co(CNS)4
An acetone (or ethanol) solution is used because the complex is somewhat dissociated in water unless
a large excess of thiocyanate is added. Both procedures are based on those given by Sandell in
Colorimetric Determination of Traces of Metals (Interscience Publishers, Inc., 1959).
Procedure
1. Prepare Solutions: (will be prepared and available for use on the day of the lab.)
Standard nickel solution. Prepared by dissolving 0.405 g of uneffloresced crystals of NiCl2x6H20
in 100 mL of 0.1 M hydrochloric acid. Then 10-mL of this solution is diluted in another 100-mL
volumetric with 0.1 M hydrochloric acid. The standard solution contains 0.100 mg/mL of nickel.
Standard cobalt solution. Prepared by dissolving 0.404 g of CoC12x6H20 in 100 mL of 0.1 M
hydrochloric acid in a volumetric flask. This solution contains 1.00 mg/mL of cobalt.
Ammonium thiocyanate. Prepared by dissolving 50 g of the salt in 100 mL of aqueous solution.
Dimethylglyoxime. Prepared by dissolving 2 g of the solid in 200 mL of ethyl alcohol.
2. Acquire a chromatography column from the stockroom. Rinse the column with deionized
water and drain. Place about 1 g of a strong-base anion in a 50 mL beaker and add about 10 mL of
deionized water. Stir the resin to form a slurry and then add the slurry to the column. Allow the resin
to settle and the water to flow through the column and then add more slurry until the entire 1 g is
added to the column. Be sure that there are no air pockets in the column. Now add the small portions
of 9 M hydrochloric acid to the resin and allow this to flow over the resin until 4 to 5 mL of the acid
has been used. The resin will shrink somewhat and darken slightly on treatment with the
concentrated acid. Make two small marks, 1 cm apart, on the side of the column using a marker
above the resin and adjust the screw clamp until the time require for the solution to flow this distance
is I to 2 min.
Exp. 12: Ion-Exchange Separation and Spectrophotometric Determination of Ni and Co Page 2 of 5
3. Secure from the stockroom an unknown sample of a solution containing nickel and cobalt in
9 M hydrochloric acid. The instructor will have available 250 microliter pipettors for you to use to
dispense 0.20 mL of the solution to the resin column. Place the tip into the column as close to the
resin bed as possible and add the solution as slowly as possible to the column. Place a clean 250 mL
volumetric flask below the column to receive the solution. Allow the solution to seep into the resin
and note the blue band (cobalt) at the top of the column. Then add slowly, about 1 mL at a time, 4
mL of 9 M hydrochloric acid, allowing each portion to settle into the resin before addition of the
next portion. Note that the cobalt band spreads somewhat during the washing, but does not come off
the column. Near the end of the last wash, catch a drop of the solution on a spot plate (or in a small
beaker), add a drop of concentrated ammonia, and then add a drop of dimethylglyoxime. No red
color should be formed, showing that the nickel has been completely removed.
4. Set aside the flask containing the nickel and replace it with a 50 mL volumetric flask to
receive the cobalt. Add to the column about 4 to 5 mL of 1 M hydrochloric acid in successive 1 mL
portions. Note that the cobalt band quickly begins to move down the column and observe the color
of the drops as the cobalt is removed. (Why is the color first blue, then pink?)
5. Into five 50 mL volumetric flasks pipet 0.5, 1.0, 1.5, 2.0 and 2.5 mL portions of the standard
nickel solution. Use a sixth flask for the blank. Add to each flask 17.5 mL of 95% ethanol, and 10
mL of the dimethylglyoxime solution. In the hood, add 0.5 mL of saturated bromine water and 2
mL of concentrated ammonia to each flask. Dilute to the 50 mL mark with water and measure the
absorbance of each solution, using the blank as a reference. (Do not wait longer than h hour before
making the measurements.) A wavelength of 450 nm is employed with a spectrophotometer. To the
volumetric flask containing the nickel unknown add 2.5 mL of bromine water, 14 mL of
concentrated ammonia, 88 mL of 95% ethanol and 50 mL of dimethylglyoxime solution. Dilute to
the mark with water and measure the absorbance.
6. Into five 50 mL volumetric flasks pipet 0.5, 1.0, 1.5, 2.0 and 2.5 mL portion of the standard
cobalt solution. Use a sixth flask for the blank. Add to each flask 5 mL of ammonium thiocyanate
solution and 25 mL of 95% ethanol. Dilute each solution to the 50 mL mark and measure the
absorbance at 625 nm with a spectrophotometer. To the volumetric flask containing the cobalt
unknown, add concentrated stock aqueous ammonia until the solution is only slightly acidic by pH
paper and then add 5 mL of ammonium thiocyanate and 25 mL of 95% ethanol. Dilute the solution
to the mark with water and measure the absorbance.
7. Plot absorbance vs. concentration for both the nickel and cobalt standard solutions. From the
absorbances of each unknown solution calculate the concentrations in mg/mL of each metal.
Exp. 12: Ion-Exchange Separation and Spectrophotometric Determination of Ni and Co Page 3 of 5
Ion Exchange Equilibria
Ion exchange equilibria can be treated by the law of mass action. In general for a basic ion exchange
resin in quilibrium with an aqueous ion, AX”, we can write:
(see Fig. 26-1 and Table 26-1 for properties of typical ion-exchange resins)
x{RN+ (CH3)3Cl}(resin) + Ax-(aq)
e {[RN+(CH3)3×Ax-} + xCl-(aq)
or in a more compact notation:
Cl-(aq) + [(res+)A x-]
for which
[(#$% ! )! ‘”# ][)* # (+,)]”
𝐾 = [(#$%! ))*# ]” [‘”#(+,)]
For example, when a dilute solution of CoC142- ions is brought into contact with a strongly basic ion
exchange resin (Dowex 1) [𝜙 − 𝐶𝐻- 𝑁 . (𝐶𝐻/ )/ 𝐶𝑙 0 ]the following equilibrium develops:
CoCl42- + 2 𝜙 − 𝐶𝐻- 𝑁 . (𝐶𝐻/ )/ 𝐶𝑙 0 ](resin)
2Cl-(aq) + 2 𝜙 − 𝐶𝐻- 𝑁 . (𝐶𝐻/ )/ 𝐶𝑙 0 ](resin)
The equilibrium constant (K) for this reaction is very large number at low pH and CoC14- is retained
on the resin.
If a positive ion (such as Ni2+) does not form a complex ion with chloride (to a great extent), then K
(above) for the positive ion will be very small.
If a mixture of nickel and cobalt ions is treated with concentrated HCl, the cobalt complex ions
will be retained on the ion exchange resin, while the nickel ions will not be retained on the ion
exchange resin.
Exp. 12: Ion-Exchange Separation and Spectrophotometric Determination of Ni and Co Page 4 of 5
Pre-Lab Questions:
Watch the JoVe video entitled anion exchange chromatography. (The link is fickle, to watch
the video, you may need to go to the uic library website, search for JoVe under “journals”
then enter your UIN The TA will play the video during lab. ,) Answer the following questions.
a) Why is acid required for the mobile phase in anion exchange? Draw a picture of the
anion exchange column in the presence of a neutral, positively charged, and negatively
charged analyte and explain what happens.
b) Again using a picture, indicate how salt can be used to unbind the analyte.
2. To prepare for lab, go through each step and consider how to connect what you will do
and observe with chemistry taking place. The pre-lab will be graded on completion.
The post-lab will be graded on correctness.
For each of the following steps in the procedure, answer the following questions.
Step 3: Draw the column and its contents at the beginning of this step. Draw or indicate any colors
observed and what chemical species relate to each color.
Draw the column and its contents at the end of this step. Draw or indicate any colors observed and what
chemical species relate to each color.
Step 4: Draw the column and its contents at the beginning of this step. Draw or indicate any colors
observed and what chemical species relate to each color.
Draw the column and its contents at the end of this step. Draw or indicate any colors observed and what
chemical species relate to each color.
Step 5: Write out the chemical reaction that occurs during the preparation of your unknown for
chemical analysis. Below each product and reactant, indicate it color in solution.
Step 6: Write out the chemical reaction that occurs during the preparation of your unknown for
chemical analysis. Below each product and reactant, indicate it color in solution.
Post-Lab questions:
1.
For each of the following steps in the procedure, answer the following questions.
Step 3: Draw the column and its contents at the beginning of this step. Draw or indicate any colors
observed and what chemical species relate to each color.
Draw the column and its contents at the end of this step. Draw or indicate any colors observed and what
chemical species relate to each color.
Exp. 12: Ion-Exchange Separation and Spectrophotometric Determination of Ni and Co Page 5 of 5
Step 4: Draw the column and its contents at the beginning of this step. Draw or indicate any colors
observed and what chemical species relate to each color.
Draw the column and its contents at the end of this step. Draw or indicate any colors observed and what
chemical species relate to each color.
Step 5: Write out the chemical reaction that occurs during the preparation of your unknown for
chemical analysis. Below each product and reactant, indicate it color in solution.
Step 6: Write out the chemical reaction that occurs during the preparation of your unknown for
chemical analysis. Below each product and reactant, indicate it color in solution.