Experiment 14
Simultaneous Determination of Chromium and
Manganese in a Mixture
Name
Student ID
：A.G. Odeesha Kumari
：FGS/MSc/IEC/2021/66
[2022.06.17]
1
Objectives;
1. Determinations of molar absorption coefficient of dichromate
and permanganate ions
2. Established the additivity of absorbance values of dichromate
and permanganate ions.
3. Determination of the concentration of dichromate
and
permanganate ions in the mixture.
Introduction
In this experiment was determined the amount of chromium and manganese in a given
mixture. These were presented as dichromate and manganate ions respectively. In fact, this
determination mimics the determination of the amount of chromium and manganese in alloy steels
wherein these are oxidized to dichromate and permanganate ions respectively. The spectra of these
ions overlap to certain extent. On dissolution of alloy steel containing chromium and manganese,
get these elements as their ions; Cr3+ and Mn2+ ions respectively. In the determination, these are
oxidised respectively to dichromate and permanganate ions using potassium persulphate and
potassium periodate, respectively. The orange red coloured dichromate shows maximum absorption
(max) at 440 nm while for the pink coloured permanganate the max is at 545 nm. However,
permanganate also absorbs at 440 nm to a smaller extent. Similarly, dichromate ions also have small
absorption at 545 nm. In simple words, both the species absorb at the wavelengths of maximum
absorptions mentioned above.
2
Thus, the absorption of the solution containing a mixture of dichromate and permanganate
ions at these wavelengths would be the sum of the absorbances of the two species. It can be shown
that the absorbances of these two ions obey Beer-Lambert’s law when measured individually and
also in a mixture in presence of 0.5 M sulphuric acid.
Under these conditions following expressions can be mentioned assuming a unit path length.
Here, the meaning of the terms used is self-evident. For example,
Cr, 545 refers to the molar
absorption coefficient at 545 nm for chromium as dichromate ion; A440 refers to the absorbance of
the mixture at 440 nm and [Cr2O72-] represents the concentration of dichromate ions and so on. This
kind of equations are needed to solve these simultaneous equations to determine both the species
without their separation. Solving Equation 1 and 2 can be get the following expressions for the
concentrations of dichromate and permanganate ions.
3
Thus, the concentrations of the two ions was obtained from the absorbance measurements at
440 and 545 nm, molar absorption coefficients for both the ions wavelengths was given. The values
of the molar absorption coefficients are obtained from absorbance measurements of pure solutions
of the two substances at the designated wavelengths. Fe3+, Ni2+, Co2+ and V2+ also absorb in these
regions, therefore these should be absent from the analyte solution.
Apparatus
Required
Chemicals
amount
Spectrophotometer/
Filter
1
photometer 1 Matched cuvette
Volumetric flasks (1 litre)
Potassium
dichromate
2
Potassium
permanganate
Volumetric flasks (250 cm3)
1
Graduated pipette (10 cm3)
2
Beakers (100 cm3)
1
Weighing bottle
10
Burettes
1
Spectrophotometer/
Filter
Sulphuric acid
2
photometer 1 Matched cuvette
4
Procedure
1 M sulphuric acid (Reference blank):
It was prepared by adding (with constant swirling) 53.3 cm3 of analytical grade concentrated
sulphuric acid in to a one litre flask containing about 500 cm3 of distilled water, allowing the flask to
cool and making up the volume using distilled water.
Potassium dichromate (0.002 M) in 1M sulphuric acid:
It was accurately weighted 0.1471 g of analytical grade potassium dichromate and quantitatively
transferred to a 250 cm3 volumetric flask and adding sufficient amount of 1 M sulphuric acid to
dissolve it. Once the compound dissolves the volume is made upto the mark with 1M sulphuric acid.
Oxalic acid (0.005 M):
It was prepared by accurately weighted 0.063 g of oxalic acid and quantitatively transferred to a 100
cm3 volumetric flask and added sufficient amount of water to dissolve it. Once dissolved the volume
was made upto the mark with distilled water.
Potassium permanganate (0.002 M) in 1 M sulphuric acid:
It was prepared as follows.
➢ A.R. potassium permanganate was weighed about 1.8 g and dissolved in 50 cm3 of distilled
water taken in a 100 cm3 beaker.
➢ The solution was heated to boil and gently continued for about 15 minutes.
➢ The solution was c ooled and filtered through a plug of glass wool in the funnel.
➢ 10 cm3 of the filtrate w a s t aken in a 100 cm3 beaker and diluted to 100 cm3 with 1M
sulphuric acid.
➢ The solution was standardized by titrating with a standard solution of oxalic acid in
acidic medium.
➢ The volume w a s c alculated by solution required to prepare 250 cm3 of 0.002 M solution.
➢ The requited volume was taken by standardized solution of potassium permanganate in a
250 cm3 volumetric flask. The volume w a s m ade up with 1M sulphuric acid.
Sample solution:
5
The 0.002 M solutions of potassium dichromate and potassium permanganate was Prepared by suitably
mixing to make 10 cm3 of the mixture and added 10 cm3 of 1M sulphuric acid to it.
a) Determination of molar absorption coefficients for dichromate and permanganate ions
1) The 0.002 M stock solution of potassium dichromate w a s p ipetted out 20 and 40 cm3
respectively of into two 100 cm3 standard flasks and made up the volume with 1M
sulphuric acid. 0.0004 M and 0.0008 M solutions were given respectively of potassium
dichromate. Mark the flasks accordingly.
2) Pipette out 20 and 40 cm3 respectively of the 0.002 M stock solution of potassium
permanganate into two 100 cm3 standard flasks and make up the volume with 1M sulphuric
acid. This would give 0.0004 M and 0.0008 M solutions respectively of potassium
permanganate. Mark the flasks accordingly.
3) The stock solutions (0.002 M) of potassium dichromate and potassium permanganate was
measured the absorbance values at 440 nm and 545 nm for and the dilutions prepared in step
1 and 2. The cuvettes of 1 cm path length was measured and used 1M sulphuric acid as the reference.
Record the values in Observation Table 1.
4) The molar absorption coefficients for potassium dichromate and potassium permanganate at
440 nm and 545 nm were calculated as per the expressions given under column 4 and 6 and
recorded appropriately in the Observation Table 1.
b) Establishing the additivity of absorbance values of dichromate and permanganate ions at
440 and 545 nm
1) 0.0008 M potassium dichromate and 0.0004 M potassium permanganate were taken in to
two clean burettes and fill them respectively.
2) 0.0008 M potassium dichromate, 0.0004 M potassium permanganate and 1M sulphuric acid
were taken into 7 beakers of 100 cm3 capacity labelled from 1 to 7 and transferred to solutions
of as per the details given in column 2, 3 and 4 of the Observation Table 2.
3) The absorbance values of the resulting solutions using 1M sulphuric acid was measured as
the reference and recorded observations in the column 5 and 7 of Observation Table 2.
cuvettes were used having a path length of 1 cm.
4) The absorbance values for the mixtures of potassium dichromate and potassium
permanganate at 440 nm and 545 nm were calculated by using Eq. 1 and Eq. 2 respectively
and recorded them in the column 6 and 8 of Observation Table 2.
5) The observed and calculated absorbance values were compared for the validation of
additivity. Observations were recorded in provided space after Observation Table 2.
6)
6
c) Determination of the concentrations of dichromate and permanganate ions in the mixture.
The absorbance values of the given sample solution taken in a cuvette of path length equal to 1 cm
were measured. 1M sulphuric acid was used as the reference and recorded observations in the column
1 and 2 of Observation Table 3.
1) The concentration of potassium dichromate and potassium permanganate in the mixture
w e r e c alculated by using Eq.3 and Eq. 4 respectively and recorded the results in the column
3 and 4 of Observation Table 3.
7
Result and Discussions:
Table 1: Determination of molar absorption coefficients for dichromate and permanganate ions
A. Potassium dichromate
1
2
3
4
5
6
Conc. of
Absorbance
Absorbance
545nm
K2Cr2O7
at 440 nm
440nm
(CK2Cr2O7)
(A440nm)
1
0.002
0.71
355
2
0.0004
0.12
300
0.06
150
3
0.0008
0.25
312.5
0.09
112.5
322.5
Average
134.17
Sample #
(A440nm/CK2Cr2O7)
Average
at 545 nm
(A545nm/CK2Cr2O7)
(A545nm)
0.28
140
B. Potassium permanganate
Sample #
Conc. of
Absorbance
KMnO4
at
(CKMnO4)
(A440nm)
1
0.002
2
0.0004
3
0.0008
Average
440
0.74
0.12
0.28
nm
440nm
Absorbance
(A440nm/CKMnO4)
(A545nm)
at
545
545nm
nm
(A545nm/CKMnO4)
370
0.25
125
300
0.06
150
350
0.09
112.5
340
Average
129.17
No of expression in valid only if the absorbances means is made in cuvette of path length 1cm.
The molar absorption coefficient for the Cr2O7-2 and MnO4 -1 ions are found to be
440nm CK Cr O = 322.5
2
2
7
545nm CK Cr O = 134.17
2
2
7
440nm CKMnO = 340
4
545nm CKMnO = 129.17
4
8
Table 2: Establishing the additivity of absorbance values of dichromate and permanganate ions at 440 and 545 nm
The observed and calculated absorbances values for the mixture of K2Cr2O7 and KMnO4
1
2
Sample
Volume
#
0.0008
K2Cr2O7
3
4
5
of Volume of
Volume
Absorbance
M 0.0004 M
of 1 M
440 nm
545 nm
H2SO4
(A440nm)
(A545nm)
(cm3)
Obsd.
KMnO4
(cm3)
(cm3)
6
7
at
8
Absorbance
at
Calc.
Obsd.
Calc.
1
50.0
00.0
1.0
0.25
0.129
0.05
0.054
2
40.0
10.0
1.0
0.24
0.117
0.14
0.048
3
30.0
20.0
1.0
0.22
0.105
0.28
0.042
4
25.0
25.0
1.0
0.20
0.099
0.33
0.04
5
20.0
30.0
1.0
0.21
0.092
0.40
0.037
6
10.0
40.0
1.0
0.19
0.08
0.61
0.031
7
00.0
50.0
1.0
0.16
0.068
0.68
0.026
K2Cr2O7 Calculations for sample No. 01 to 07;
K2Cr2O7 concentration after mixing with KMnO4 = 0.0008 X 10-3 molL-1 X (50ml/100ml)
K2Cr2O7 concentration in sample No. 01
= 4 X 10-4 molL-1
K2Cr2O7 concentration in sample No. 02
= 3.2 X 10-4 molL-1
K2Cr2O7 concentration in sample No. 03
= 2.4 X 10-4 molL-1
K2Cr2O7 concentration in sample No. 04
= 2.0 X 10-4 molL-1
K2Cr2O7 concentration in sample No. 05
= 1.6 X 10-3 molL-1
K2Cr2O7 concentration in sample No. 06
= 0.8 X 10-4 molL-1
K2Cr2O7 concentration in sample No. 07
= 0 molL-1
9
KMnO4 Calculations for sample No. 01 to 07;
KMnO4 concentration after mixing with K2Cr2O7 = 0.0004 X 10-3 molL-1 X (10ml/100ml)
KMnO4 concentration in sample No. 01
= 0 molL-1
KMnO4 concentration in sample No. 02
= 0.4 X 10-4 molL-1
KMnO4 concentration in sample No. 03
= 0.8 X 10-4 molL-1
KMnO4 concentration in sample No. 04
= 1.0 X 10-4 molL-1
KMnO4 concentration in sample No. 05
= 1.2 X 10-4 molL-1
KMnO4 concentration in sample No. 06
= 1.6 X 10-4 molL-1
KMnO4 concentration in sample No. 07
= 2.0 X 10-4 molL-1
For K2Cr2O7 , and KMnO4 b y applying to equation 1 ;
1. K2Cr2O7 + KMnO4 Mixture A440 = (322.5 X 4 X 10-4 +340X 0) molL-1
= 0.129
2. K2Cr2O7 + KMnO4 Mixture A440 = (322.5 X 3.2 X 10-4 +340X 0.4 X 10-4) molL-1 = 0.117
3. K2Cr2O7 + KMnO4 Mixture A440 = (322.5 X 2.4 X 10-4 +340X 0.8 X 10-4) molL-1 = 0.105
4. K2Cr2O7 + KMnO4 Mixture A440 = (322.5 X 2.0 X 10-4 +340X 1.0 X 10-4) molL-1 = 0.099
5. K2Cr2O7 + KMnO4 Mixture A440 = (322.5 X 1.6 X 10-3 +340X 1.2 X 10-4) molL-1 = 0.092
6. K2Cr2O7 + KMnO4 Mixture A440 = (322.5 X 0.8 X 10-4 +340X 1.6 X 10-4) molL-1 = 0.080
7. K2Cr2O7 + KMnO4 Mixture A440 = (322.5 X 0.0+ 340X 2.0 X 10-4) molL-1
= 0.068
For KMnO4 and K2Cr2O7 , and by applying to equation 2 ;
1. KMnO4 + K2Cr2O7 Mixture A545 = (134.17X 4 X 10-4 +129.17X 0) molL-1
= 0.054
2. KMnO4 + K2Cr2O7 Mixture A545 = (134.17X 3.2 X 10-4 +129.17X 0.4 X 10-4) molL-1 = 0.048
3. KMnO4 + K2Cr2O7 Mixture A545 = (134.17X 2.4 X 10-4 +129.17X 0.8 X 10-4) molL-1 = 0.042
4. KMnO4 + K2Cr2O7 Mixture A545 = (134.17X 2.0 X 10-4 +129.17X 1.0 X 10-4) molL-1 = 0.04
5. KMnO4 + K2Cr2O7 Mixture A545 = (134.17X 1.6 X 10-3 +129.17X 1.2 X 10-4) molL-1 = 0.037
6. KMnO4 + K2Cr2O7 Mixture A545 = (134.17X 0.8 X 10-4 +129.17X 1.6 X 10-4) molL-1 = 0.031
7. KMnO4 + K2Cr2O7 Mixture A545 = (134.17X 0.0 +129.17X 2.0 X 10-4) molL-1
10
= 0.026
Absorbance at 440 nm
Volume of (8 M+ 4
M)X 10-4 K2Cr2O7
+ KMnO4
Sample No
1
2
3
4
5
6
7
50+10
40+10
30+20
25+25
20+30
10+40
0+50
Obsd.
Calc.
0.25
0.24
0.22
0.2
0.21
0.19
0.16
0.129
0.117
0.105
0.099
0.092
0.08
0.068
Comparison of observed Absorptions & calculated Absorptions at
A-440nm
0.3
0.25
0.24
0.25
0.22
0.21
0.2
0.19
Absorbance at 440 nm
0.2
0.16
0.15
0.129
0.117
0.105
0.1
0.099
0.092
0.08
0.068
0.05
0
Sample No
Obsd.
Calc.
11
Absorbance at 545 nm
Volume of (8 M+ 4
M)X 10-4 K2Cr2O7
+ KMnO4
Sample No
1
2
3
4
5
6
7
50+10
40+10
30+20
25+25
20+30
10+40
0+50
Obsd.
Calc.
0.05
0.14
0.28
0.33
0.4
0.61
0.68
0.054
0.048
0.042
0.04
0.037
0.031
0.026
Comparison of observed Absorptions & calculated
Absorptions at A-545nm
0.8
0.68
0.7
Absorbance at 545 nm
0.61
0.6
0.5
0.4
0.4
0.33
0.28
0.3
0.2
0.14
0.1
0.05
0.054
0.048
0.042
0.04
0.037
0.031
0.026
2
3
4
5
6
7
0
0
1
8
Sample Number
Obsd.
Calc.
12
Table 3: Determination of the concentrations of dichromate and permanganate ions in the mixture.
Absorbance Absorbance (C K2Cr2O7 ) =
at 440 nm
at 545 nm
(A440nm)
(A545nm)
0.54
0.82
(CKMnO4) =
0.0196 M
0.00058 M
Mixture was prepared by adding 0.002 M Potassium di chromate and 5ml 0.002M Potassium Per
Manganate - 5ml.
1.0 Calculations;
Concentration of K2Cr2O7 = (0.82 X 129.17) – (0.54 X 340)
(322.5 X129.5) – (340 X 134.17)
Concentration of K2Cr2O7 = 0.0196 M
2.0 Calculations;
Concentration of KMnO4 = 0. 82 – (134.17 X 0. 0196)
129.17
Concentration of KMnO4 = 0.0140 M
•
The concentration of Potassium di chromate given mixture is found to be = 0.0196 M
•
The concentration of Potassium Per Manganate given mixture is found to be = 0.0140 M
13
Discussion;
All the reagents were used as in AR grade. All the glassware used as in cleaned conditions.
Absorptions were taken in cuvettes of 1cm path length, if not so need to be modified molar
absorption coefficient and concentrations would need to be modified.
Concentration of given mixture were calculated by applying factors to eq. 3 and 4. Concentrations.
Equation 4 is given the negative concentration. That may be the reason of higher absorption at 545
nm conditions and dilutions gaps in samples were taken for finding of absorptions coefficient. When
considering the comparison graphs, which were generated base on table 02; Observable absorption
values higher than calculated values in both wavelengths. Standard deviation of 440nm data sets
were taken from both as observed and Calculated given as 0.063 and consequently 545 nm as 0.226.
By comparing both data sets can be come to decision as 545 nm data set indicates values are spread
out over a wider range. It may due to the residuals contains in cuvettes, due to electrostatic
interactions between molecules in close proximity, and scattering of light due to particulates in the
sample.
14