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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
ANALYSIS OF COBALT
COMPLEXES
I. Determining The % Co in a Cobalt Complex Sample
Using Atomic Absorption Spectroscopy
(AAS). II. Analysis of Colored Complexes by UV-Visible
Spectroscopy. III. Determining the % NH3 in Ammine
Cobalt Complexes by Thermal Decomposition and
Volumetric Titration. IV. Determining the Total % Cl in
a Complex by the Gravimetric Chloride Method. Determining Ionic Chloride by
Mercury (II) Titration
Co
INTRODUCTION:
In the Cobalt Marathon you made three complexes of Cobalt (III): Orange crystalline
hexamminecobalt(III)chloride, purple crystalline pentamminecobalt(III)chloride, and green crystalline trans-dichlorobis
cobalt(III) chloride. To analyse the products, you will use the Atomic Absorption Spectrometer to determine the %
Co in each of your three complexes. You will decompose hexamminecobalt(III)chloride and
pentamminecobalt(III)chloride in sodium hydroxide, capture the ammonia gas in HCl and determine the % ammonia
in the complexes. Since each complex is colored (thus absorbing visible light at characteristic wavelengths) you will
obtain a UV-visible spectrum of each complex. You may also analyse each complex for Cl by two methods. CHECK
WITH YOUR INSTRUCTOR FOR THE ANALYTICAL METHODS ON OFFER THIS YEAR.
I. COBALT ANALYSIS BY AAS:
Metals are conveniently analysed by Atomic Absorption spectroscopy. Very briefly, the AA spectrometer consists of:
 a sipper to get sample into the instrument;
 a flame to get the sample into the gas phase; an element-specific energy source to excite the electrons of the
element you are analyzing;
 and a detector, to measure how much light is absorbed by the sample.
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
FIGURE 1: SCHEMATIC DIAGRAM OF AN ATOMIC ABSORBTION SPECTROPOTOMETER
Digital readout
Amplifier
Ligh
Detector
Burner Flame
Monochromator
Light Source
Sample
Fuel Tanks (O2 and HCCH)
Light Source – An Element-specific hollow cathode lamp.
Incoming light beam
Exiting light beam
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
In direct-aspiration atomic absorption spectroscopy, a sample is aspirated and atomized in a flame. A light
beam from a hollow cathode lamp or an electrodeless discharge lamp is directed through the flame into a
monochromator, and onto a detector that measures the amount of absorbed light. Absorption depends upon the
presence of free unexcited ground-state atoms in the flame. Because the wavelength of the light beam is
characteristic of only the metal being determined, the light energy absorbed by the flame is a measure of the
concentration of that metal in the sample. This principle is the basis of atomic absorption spectroscopy.
EXPERIMENT:
The determination of cobalt in your cobalt complexes by AAS requires two things: 1) that you have solutions
of known mass of complex in a known volume of solution; 2) that you calibrate the AA by making a standard curve of
[Co] vs Absorbance, using standard solutions with known [Co].
YOU WILL NEED THE FOLLOWING:
In this experiment you will:
1.
2.
3.
4.
5.
Make up solutions containing known amounts of each cobalt complex ;
Run all samples thought the Atomic Absorbtion Spectrometer, using the cobalt –specific lamp at 240.7 nm.
Construct a standard curve of absorbance vs. [Co] using a set of known Co standards (In the interests of time, the standard
solutions will be made by the lab staff;
Determine the % Co in each of your complexes;
Compare your experimentally obtained data with the known % Co in each complex.
You will need the following glassware and equipment:
Volumetric Flask, 100.00 mL/stopper
3
You will need the following reagents:
5 Cobalt Stock Solutions of various
known concentrations . Record the
EXACT concentrations on the stock
bottles IN YOUR NOTEBOOKS
when you get to lab
2 mg/L, 4 mg/L, 6
mg/L, 10 mg/L
15 mg/L
PRELAB:
To be done by TA:
Make a stock solution of Co2+, “THE COBALT STOCK SOLUTION”
according to the procedure on page 4. Record the exact concentration on the container. Make 4 quantitative
dilutions according to the procedure on page 4. Record the exact concentration on each container.
To be done by the student
: RECORD the exact concentration of each known
cobalt solution in your notebook. WRITE IT DOWN!
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
To be done by TA:
Make a stock solution of Co by accurately weighing 1 g of cobalt metal to
the nearest 0.0001g. Dissolve the cobalt in approximately 10 mL conc. HNO 3. Dilute to 1.0000L with
deionized water. RECORD THE EXACT CONCENTRATION on the bottle.
To be done by TA:
Making calibration standards:
1. Obtain a clean Erlenmeyer flask (any size) and six 500.00 mL volumetric
flasks.
2. Label the Erlenmeyer flask "0" and the volumetric flasks "1" through "6".
3. Fill Erlenmeyer flask "0" with deionized water.
4. To flask "1" carefully pipet in 1.00 mL of the stock Co solution using a volumetric pipet.
Dilute to the mark on the neck with deionized water.
You want to follow the same convention as you did when reading a buret: The BOTTOM of the
meniscus should touch the mark on the neck. Cap the flask and mix it thoroughly!
5. Following the chart, add 2.00 mL of the stock Co to flask 2, 3 mL of the stock Co3+ to flask 3, and
so on, and dilute to the mark with deionized water. Remember to thoroughly mix each flask.
To be done by TA: FILL IN THE FOLLOWING CHART We will supply copies to the students
Flask Number
[stock Co3+]:
mL stock Co3+ used:
Diluted to a final volume of (mL):
Final [Co3+]
(g/L)
0
1.0000 g/l
0
500 mL
0
1
1. 00
0.002
2
2. 00
0.004
3
3. 00
0.006
4
5.000
0.010
5
10.00
0.015
For best results, calibration standards should be prepared fresh each time a batch of samples is analyzed.
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
SAMPLE PREPARATION
Weigh out 0.10 g of each cobalt complex. Record the mass to +/- 0.0001 g. Place each weighed sample in a
labeled 100.00 mL volumetric flask. Dilute to the mark with distilled water. Cap and invert a minimum of 30x until
the solid dissolves.
ATOMIC ABSORPTION METHODS
PROCEDURE
Direct aspiration (flame) procedure
Your instructor will show you how to operate the AA instrument. Run your series of
standards of the element under analysis. Beginning with the blank and working toward the highest
standard, aspirate the solutions and record the readings. Repeat the operation with both the calibration
standards and the samples a sufficient number of times to secure a reliable average reading for each
solution.
Construct a calibration curve by plotting the concentrations of the standards against absorbances.
Aspirate the samples and determine the concentrations from the calibration curve. Standards must be
run each time a sample or series of sample is run.
SAVE YOUR COBALT COMPLEX SOLUTIONS AND GET A UV-VISIBLE SPECTRUM
OF EACH.
REFERENCES
1.
Methods for Chemical Analysis of Water and Wastes; U.S. Environmental Protection Agency. Office of Research and
Development. Environmental Monitoring and Support Laboratory. ORD Publication Offices of Center for Environmental
Research Information. Cincinnati, OH, 1983; EPA-600/4-79-020.
Calculations:
 Obtain the g/L Co in each of your standards. Make sure they are recored in your notebook,
and on your spreadsheets.
 Generate a standard curve of absorption vs. concentration for your standards. Indicate what
wavelength you used. Calculate the best fit straight line for your data.
 Using your standard curve best fit line, calculate the g Co/L in each of your 3 complexes.
 Calculate gCo/gComplex for each of your complexes.
 Calculate a % Co for each complex and a theoretical % Co. Report each and the % absolute
error.
WASH & RETURN ALL GLASSWARE!
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
ALL WASTE FROM THIS EXPERIMENT GOES IN THE JUG LABELLED “COBALT WASTE” IN THE HOOD.
CALCULATIONS:
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
II. UV-visible Spectra:
When a compound absorbs visible light it appears colored; The color you observe is the visible light NOT
absorbed by the compound.
For example, consider Chlorophyll A):
Chlorophyll A absorbs blue light (400-450 nm, λmax = 427 nm) and red light (650-700 nm, λmax = 642 nm).
Green and yellow light are not absorbed, so chlorophyll a is green.
(http://www.ch.ic.ac.uk/local/projects/steer/chloro.htm)
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
π* (antibonding)
π  π*
n  π*
n (nonbonding)
π (antibonding)
σ (bonding)
The visible region of the spectrum has energies (E= hν = hc/λ)
between 170 kJ/mole photons (red light) to 300 kJ/mol (blue
light) to 600 kJ/mol in the UV. These energies are just right
to promote (excite) a molecular electron to a higher energy
orbital. In general, energetically favored electron promotion
will be from the highest occupied molecular orbital (HOMO)
to the lowest unoccupied molecular orbital (LUMO). When
an electron in a molecule is in a high energy π* level, the
molecule is said to be in an excited state. When the electron
falls back down to the lower energy level, it has returned to
the ground state.
When molecules are hit with light having an energy that matches an electronic transition within the molecule,
some of the light energy will be absorbed as the electron is promoted to a higher energy orbital. A UV-visible
spectrophotometer measures the wavelengths at which absorption occurs, together with the degree of absorption at
each wavelength. The resulting spectrum is a graph of absorbance versus wavelength, such as the one for
chlorophyll shown on page 7. You will obtain spectra for each of your 3 complexes, and report λmax, the
wavelength at which absorbance of light is MAXIMUM.
EXPERIMENTAL:
SOLUTIONS NEEDED
1 g/L Co(NO3)2
1 g/L CoCl2
0.5 g/L [Co(NH3)5Cl]Cl2
Use your solutions of each cobalt complex you made for the AA.
Obtain a spectrum from 260 – 800 nm for each complex that you made.
Obtain a spectrum from 260 – 800 nm for [Co(NH3)5Cl]Cl2 obtained from Sigma-Aldrich for comparison purposes
with your [Co(NH3)5Cl]Cl2 complex that you made.
Obtain a spectrum from 260 – 800 nm for each starting cobalt salt.
Report λmax for each complex and salt. Organize this data neatly in a table.
Compare the spectrum of YOUR [Co(NH3)5Cl]Cl2 with [Co(NH3)5Cl]Cl2 from Sigma-Aldrich. How are they
alike? How are they different? Is YOUR complex identical to Sigma’s as far as you can tell by UV-vis?
Compare the spectrum of starting material to the spectrum of the product (eg CoCl2 with [Co(NH3)6]Cl3). How
are they alike? How are they different? Overlay the 2 spectra ans answer the question “If your product still had
some unreacted starting material in it, could you detect it by UV-vis?”
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
III. AMMONIA ANALYSIS:
You may determine the number of moles of ammonia per mole of hexamminecobalt(III)chloride (Orange
complex I) and pentamminechlorocobalt(III)chloride (purple complex II) by decomposing a known mass ofyour
cobalt complex in sodium hydroxide. This liberates NH3 gas, which you can trap as ammonium ion (NH4+) in a
known volume of HCl; By determinining how much HCl reacted, by titration with standard NaOH, you can
determine how many moles of NaOH were liberated form the complex.
heat
Co(NH3)nClm + NaOH
Co2O3(s, black) + nNH3(g )
Std HCl
xs
NH4+(aq) + left over HCl
Std NaOH to a bromocresol
green endpoint
H2O
You will need:
Equipment:
Glass funnel
Rubber tubing
200 mm test tube
1 hole-stopper with
long bent glass
delivery tube and
short piece of
rubber tubing
600 mL beaker
250 mL ehlenmeyer
Ring stands
Test tube clamps
Bunsen burner and
striker
10.00 volumetric
pipet and blue bulb
2005: 20.00 mL
pipet
10 mL Grad.
Cylinder
50.00 mL buret
Stir bar and plate
1
1
1
Reagents:
1.000 M HCl,
standardized.
3
1-3
2
2
1
1
1
1
1
For 2005: 0.5000 M
HCl, Standardized.
RECORD EXACT
CONCENTRATION
0.300 M NaOH,
Standardized.
RECORD EXACT
CONCENTRATION
3.0 M NaOH
1% Bromocresol
green indicator
50 mL
250 mL
15 mL
In dropper
bottles on the
riser
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
Protocol:
Set up the apparatus as shown in the photo.
For each ammonia determination you will
need to weigh out 0.25 g of complex (record
the mass to the nearest 0.0001 g) and put it in
the large test tube.
Write down in your notebook the exact
concentration of standardized 1.0 M HCl.
(FOR 2005: Use 20.00 mL of 0.5000 M HCl.
Thanks to James Stauffer for this suggestion)
Carefully pipet 10.00 mL (Starting 2005:
20.00 mL) of this HCl into the 600 mL beaker,
using a VOLUMETRIC PIPET and BULB.
Add 2-3 drops of bromcresol green indicator
to the HCl. Record the color. Carefully place
the funnel in the beaker, clamped up so a seal
is just formed between the rim of the funnel
and the HCl. This HCl will trap the ammonia given off by your complex as ammonium ion.
Now add 3 mL of 3 M NaOH to the complex in the test tube, and put in the stopper.
CAUTION: DO NOT PIPET BY MOUTH! HCl is corrosive…it will corrode you!
NaOH is caustic…wear safety glasses!
When your apparatus is charged and sealed, GENTLY GENTLY GENTLY (and I am yelling, here)
start heating the test tube with a LOW LOW LOW flame. Let the solution come to a gentle rolling boil.
Gently pass the flame up & down the test tube from time to time. The NaOH will decompose your complex,
liberating NH3(g) which will pass through the gas delivery tube and be trapped in the HCl under the funnel as
soluble NH4+
The complex will turn black (Co2O3). Gently gently gently heat the complex to dryness. Think “gentle rolling
boil” here. Record in your notebook color changes of the complex, and of the bromocresol green indicator.
When the complex is decomposed completely, unstopper the test tube and rinse the glass delivery tube into the
beaker with a small amount of distilled water.
CAUTION: If you heat your complex too fast, your analysis will be DRECK. Ammonia will
escape, you will get spattering, your lab mates will make you wait outside, and you will have wasted
your time. This analysis is tricky…inevitably, some NH3 will be lost, Don’t make it more difficult
by heating too fast!
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
You can now clean out the test tube, but SAVE THE 600 mL beaker and contents!!!! Quantitatively transfer the
contents of the 600 mL beaker to a clean 250 mL ehrlenmeyer flask. That is, carefully pour the solution into the
ehrlenmeyer (don’t spill!) and rinse the beaker 3x with 5 mL of distilled water. Add the rinsings to the
ehrlenmeyer. Set the ehlenmeyer flask aside to titrate (next paragraph) and set up another ammonia digestion.
Now, TITRATE THE HCl with standardized 0.3 M NaOH (RECORD THE EXACT CONCENTRATION!) to a
bromcresol green end point. As usual, obtain a buret, rinse it with distilled water, and then rinse it with small
amounts of previously standardized 0.3 M NaOH. Record the exact [NaOH] in your notebook! Fill the buret with
0.3 M NaOH, record the volume, titrate the left-over HCl in your 600 mL beaker until the bromcresol green
changes color. Record the volume of NaOH.
Repeat the ammonia analysis 2 more times with hexamminecobalt(III)chloride and three times with
pentamminechlorocobalt(III)chloride.
CALCULATIONS:
1. Calculate the number of moles of 1 M HCl (use exact concentration) you STARTED with in your 600 mL
beaker.
2. Write the balanced equation for the reaction between HCl(aq) and NH3(g). This will use up some of your
HCl.
3. Write the balanced equation for the reaction between HCl(aq) and NaOH(aq). This is how you will determine
how much HCl is left over.
4. Calculate the number of moles of 0.3 M NaOH (use exact concentration) you added to neutralize the leftover HCl in your beaker.
5. Calculate how many moles of HCL got USED UP by ammonia (HClstart – HClleft over)
6. Calculate how many moles of ammonia were liberated by your complex. (See 2 above)
7. Determine the molecular weight of ammonia.
8. Calculate (use results from step 6) the number of grams of ammonia liberated by your complex.
9. Using the mass of complex you started with, calculate the % ammonia in the complex (g NH3/g complex) x
100.) Calculate average, Standard deviation and % error.
10. Using the chemical formula and the molecular weight of your complex, calculate the theoretical %
ammonia in the complex.
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
11. Calculate the absolute error between your data and the theoretical data.
WASH ALL GLASSWARE WITH DISTILLED WATER AND RETURN THEM TO BENCH AREA.
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
IV ANALYSIS OF Cl
A. Total Cl:
The gravimetric method with silver can be used in a quantitative determination of the total percent chloride (ionic
and covalent) in your products.
Ag+ Cl-  AgCl (s)
To analyze each of your products, place an accurately weighed 25-30 mg sample of the salt into a centrifuge tube
that has been previously dried in the oven and weighed (after it has cooled). Dissolve this sample in about 2 mL of
water and add 1 mL concentrated nitric acid. (OMIT the nitric acid for the analysis of [Co(NH3)6]Cl3 which
contains only ionic chloride.) Heat the tube in a beaker of boiling water for 10 minutes. This will serve to
decompose the sample and to liberate any covalently bound chloro- ligands into solution as chloride ions. Make
sure your sample is completely in solution, then slowly add 0.2 M silver nitrate to the solution until precipitation is
complete (about 4-5 mL). Centrifuge and test for complete precipitation with an additional drop of AgNo3; if more
precipitate forms continue adding AgNO3 dropwise until precipitation is complete. Centrifuge and carefully
decant the supernatant. Wash the precipitate with a few mL of water that has been acidified with one drop of
dilute nitric acid. Centrifuge and discard the washings. Dry the test tube(s) containing the AgCl precipitate in a
120° oven for 30 minutes. Allow the tube(s) to cool to room temperature and weigh the precipitate and tube.
Drying should be repeated to constant weight (+1 mg). Calculate the total percent chloride by weight in each of
your samples on the basis of these results. Compare the experimental % total chloride with the % expected based
on the molecular formula.
Dispose of all silver-containing mixtures in the SILVER WASTE.
B. Mercuric nitrate titration of IONIC chloride
This technique is probably more accurate and more precise than silver chloride precipitation (see Part 4.A.) with a
small sample; but it works only for ionic chlorides. IONIC CHLORIDES are Cl- counterions ionically bonded to
the cobalt complex. They are NOT the Cl’s which are bonded directly to Co3+ by coordinate covalent bonds. In
your write-up, draw the 3-D structure of each of your complexes and identify all ionic chlorides. Identify all ionic
chlorides. The ionic chlorides in complex salts can be titrated with mercuric nitrate to form mercuric chloride,
which through water soluble, is only slightly dissociated
2Cl + Hg2+  HgCl2
To detect the point at which mercuric ion is in excess of the chloride ion, an indicator, sodium nitroprusside
(sodium pentacyanonitrosylferrate(III), Na2[FeNO(CN)5] • 2H2O), is used. When all the chloride ion has been
consumed upon addition of mercuric nitrate, the excess mercuric ion forms mercuric nitroprusside, which makes
the solution turbid. This cloudiness is the end point of the titration. The reaction is:
Hg2+ + [FeNO(CN)5]2-  Hg[FeNO(CN)5](s)
Both mercuric and nitroprusside compounds are POISONOUS! Wash your hands after using them.
If you have not been provided with previously standardized Hg(NO3)2 solution, the mercury can be titrated against
potassium chloride. Accurately weigh 150 mg KCL into an Erlenmeyer flask, add about 50 mL of deionized
water, three drops of indicator, and titrate with Hg2+ solution. Do at least three trials until satisfactory precision is
achieved.
To analyze your products, weigh a sample of approximately 60 mg accurately on an analytical balance. Dissolve
the sample in approximately 50 mL of distilled water in an Erlenmeyer flask. Add about 1 mL of 10% sodium
nitroprusside and titrate with ca. 0.05 M mercuric nitrate. Calculate the % ionic chloride in the salt using the exact
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HANDOUT: CoBALT COMPLEX ANALYSIS V2004 CH107
[Hg2+] stated on the bottle. For the [Co(NH3)6]Cl3 prepared in Part 1, all of the chlorides are ionic, so your results
by the silver chloride gravimetric method should agree with the results for the mercuric nitrate titration. The other
compounds, [Co(NH3)5Cl]Cl2 (part 2) and trans-[Co(en)2Cl2]Cl (Part 3), contain both ionic and covalently bound
chloride. The percent covalent chloride can be determined by subtracting the percent ionic chloride determined in
Part 4B. from the percent total chloride determined in part 4A. For each of your samples, compare the percent
chloride based on the above determinations with the expected percent chloride calculated from the molecular
formula.
Dispose of all mercury-containing mixtures in the MERCURY
WASTE.
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