EXPERIMENT 5 - KINETIC METHOD OF TRACE ANALYSIS
OBJECTIVE
To determine the concentration of a trace amount of copper by a
kinetic method.
THEORY
A wide variety of kinetic methods of analysis have been developed, and many are in
common use. For the determination of a single component, the kinetic method can be fast
and simple.
In this experiment, the catalytic effect of Cu2+ on the following redox reaction is used to
determine [Cu2+] in unknown solutions.
catalyst

 2 SO42- (aq) + I2 (aq)
S2O82- (aq) + 2 I- (aq) Cu
2+
rxn (5-1)
The rate for the Cu2+ catalyzed reaction is found to be first order with respect to [Cu2+].
2-
rate = 
d [S 2 O 8 ]
2= k [S 2 O 8 ] [I - ][Cu 2 + ]
dt
eqn (5-1)
The rate equation for the reaction when the initial concentrations of S2O82-and I- are
kept constant is
rate = k' [Cu2+]
eqn (5-2)
where k' = k [S2O82-][I-], is a constant.
By adding solutions of known Cu2+ concentration, a standard addition method, to a
given quantity of S2O82- and I-, it is possible to construct a calibration curve of rate of
reaction versus [Cu2+] or mmoles of Cu2+. Addition of unknown concentration of Cu2+
to the reaction mixture will yield a reaction rate which can be related to concentration by
the calibration curve for the experimental conditions which are used. Care must be taken
to ensure that the unknown solution's Cu2+ concentration falls within the mid-range
region of the calibration curve.
The rate is inversely proportional to the time required to form enough iodine to turn the
solution blue. Initially, the iodine formed is consumed according to the following
reaction.
5-1
I2 (aq) + S2O32- (aq)  2 I- (aq) + S4O62- (aq)
rxn (5-2)
After the S2O32- ions are exhausted, the formation of any more I2 will react with the
starch to turn the solution blue.
5-2
PROCEDURE [NB - All glassware has to be CLEAN and DRY. The drying process involves
rinsing with acetone and blasting the inside surface with compressed air.]
1.
Obtain a 500 mL Erlenmeyer flask, label this flask 'A' and prepare solution 'A'
as follows:
(a) Using graduated cylinders measure the volumes of the following
solutions into the Erlenmeyer flask.
(i)
(ii)
(iii)
(iv)
50 mL of 0.200 M KI
65 mL of 0.0100 M Na2S2O3
35 mL of 0.2 % starch
100 mL of distilled water.
(b) Mix and stopper the solution.
2.
Obtain another 500 mL Erlenmeyer flask, label this flask 'B' and prepare
solution 'B' as follows:
(a) Using a graduated cylinder measure the volumes of the following
solutions into the Erlenmeyer flask.
(i)
(ii)
50 mL of 0.200 M (NH4)2S2O8
200 mL of distilled water.
(b) Mix and stopper the solution.
3.
Obtain sixteen 125 mL Erlenmeyer flasks. Clean, dry and label them
A1, A2, A3, A4, A5, A6, A7 and A8.
B1, B2, B3, B4, B5, B6, B7 and B8.
4.
Pipet 20.00 mL of solution A into each of the 8 Erlenmeyer flasks labeled A1
through A8.
5.
Pipet 20.00 mL of solution B into each of the 8 Erlenmeyer flasks labeled B1
through B8.
6.
Set up two burets containing the following two solutions:
(i)
(ii)
0.00100 M Cu2+
distilled water.
and make the following additions to the 'A' Erlenmeyer flasks:
5-3
Solution
Volume of 0.00100 M
Cu2+ added (mL)
Volume of distilled
H2O added (mL)
A1
9.00
1.00
A2
7.00
3.00
A3
5.00
5.00
A4
3.00
7.00
A5
2.00
8.00
A6
1.00
9.00
*A7 (Unknown ___)
10.00 mL unknown
Cu2+ solution
0.00
*A8 (Unknown ___ )
10.00 mL unknown
Cu2+ solution
0.00
* Your instructor will assign you with an unknown solution. Write down the
ID of this unknown solution on your datasheets.
[Note: Do not clean up and dispose of the solutions until you are sure that you are
satisfied with all your data.]
7.
Use lead donuts to stabilize the sixteen Erlenmeyer flasks in the 25oC water
bath. Allow the flasks to come to thermal equilibrium by leaving them in the bath
for at least 5 minutes.
In the following steps you will be mixing the 'A' solutions with the 'B' solutions.
Mix: (i)
A1 with B1
(ii)
A2 with B2
.
.
.
(viii) A8 with B8
8.
Pour the content of solution A1 rapidly into B1 while swirling. Start the timer
immediately and record the time (in sec) for the appearance of the blue colour.
For the duration of the reaction, keep swirling the flask containing the combined
solutions and keep the flask immersed in the 25oC water bath.
9.
Repeat step 8 for the remaining 7 (A2/B2 to A8/B8) pairs of solutions.
5-4
10.
Inspect the data for the unknown solution. If the measured times are not within
the mid-range of your calibration curve, dilute the unknown solution and repeat
making the unknown solutions as needed.
11.
Tell your instructor your plan of action and ask your instructor for more
Erlenmeyer flasks if needed.
5-5
DATA SHEET
Temperature of reaction = ___________________
(a) Datasheet for the Standard Solutions
Solution
mmoles of
0.00100 M
Cu2+
added
time, t
(sec)
"rate"
1000 / t
(sec-1)
A1/B1
A2/B2
A3/B3
A4/B4
A5/B5
A6/B6
Show sample calculation of mmoles of Cu2+ added using solution A4/B4.
5-6
(b) Datasheet for the Unknown Samples
Solution
time, t
(sec)
"rate"
1000 / t
(sec-1)
Dilution of the Unknown Sample
(if necessary)
Volume of
Unknown
(mL)
Volume of
Distilled Water
(mL)
A7/B7
(Unknown __)
A8/B8
(Unknown __)
Unknown ___
(Trial 2)
Unknown ___
(Trial 3)
Unknown ___
(Trial 4)
Which trials are going to be used to determine the mmoles of Cu2+ for the unknown
sample that requires dilution? Explain your reasoning.
5-7
SUMMARY OF RESULTS
Unknown _______
Unknown _______
mmoles of Cu2+
in unknown solution
concentration of Cu2+
in the unknown (moles/L)
Show the calculations of the [Cu2+] for your unknowns.
5-8
TREATMENT OF DATA
1000
versus mmoles of Cu2+". This is your calibration curve.
t
1.
Plot a graph of "
2.
Determine the concentration of the Cu2+ in the unknown solutions from the
calibration curve.
3.
Enter your results, the mmoles the Cu2+ and the concentration Cu2+ in the
unknown solutions, in the Summary of Results table.
QUESTION
1.
Examine the calibration curves showing "rate versus mmoles of Cu2+".
Comment on the validity of the rate equation.
5-9