Chem126 Lab – Instrumental Analysis
EXPERIMENT 3 Analysis of Permanganate – Dichromate Mixture Vanessa lga !" Dagon#on an# $en M" Mene% Department of Chemistry, Chemistry, College of Arts and Sciences, ni!ersity of the "hilippines – #isayas, $iag%ao Iloilo
A&'TRA(T Spectrophotometry can used to determine the concentration of the components of a binary system. The sample that was analyzed was a mixture of KMnO 4 (permanganate and K !"r!O# (dic (dichr hrom omat ate e solu soluti tion ons. s. $oth $oth of thes these e comp compon onen ents ts abs absorb orbs at di%erent wa&elengths. Therefore ore' a sim simple spectrophotometric method and calculations cannot be used to dete deterrmine mine the the conc concen entr trat atio ion n of thes these e comp compon onen ents ts.. n this this exper xperim imen ent' t' two two sets sets of stan standa darrds wer were prep prepar ared ed)) KMnO KMnO4 (per (perma mang ngan anat ate e and and K !"r!O# (dic (dichr hrom omate ate stan standar dards ds** two two wa&e wa&ele leng ngth ths s wer were sele select cted ed at maxi maximu mum m abso absorb rbanc ance* e* four four calibratio calibration n cur&es cur&es were were plotted* plotted* and' four molar +bsorpti&iti +bsorpti&ities es were calculated to simultaneously determine the concentration of the permanganate and dichromate ion. The concentration of the dichr dichroma omate te and perman permangan ganate ate ion was was deter determin mined ed to be !!#.,4 !!#.,4 ppm and -!., ppm' respect respecti&el i&ely y. erc ercentage entage errors errors of /.-01 and -.2/1 -.2/1 resulted resulted from from the analysis. analysis. This implies good accu accura racy cy in the the anal analys ysis is and and ther theref efor ore e yiel yields ds no sign signi3 i3ca cant nt di%e di%errence ence betw betwee een n the the sp spec ectr trop ophot hotom omet etri ric c and and theo theorretic etical al measurements. INTRD)(TIN Se&eral methods ha&e been de&eloped o&er the years to determine determine the indi& di&idual compositions of mixtures under a binary system. The concentrations of the isolated substances are usually also measured as to obta obtain in comp comple lete te refer eferen ence ce of the the su subs bsttance ances. s. This This exper xperim imen entt us used ed spectr spectroph ophoto otomet metry ry to separa separate te the two &ery &ery di%er di%erent ent compone components nts of a binary system by their concentration and absorbance. Typically' Typically' as with this experiment' experiment' the substances of (per (perma mang ngan anat ate e and and K !"r!O# (di (dichrom hromat ate e were chos chose en as age * of *+
KMnO4 bina binarry
Chem126 Lab – Instrumental Analysis components for ha&ing di%erent properties' most especially on maximum absorbance di%erence. Since the absorbances are considered additi&e and must ne&er react with each other in any way as shown in this formula) A λ1= λ1 A 1+ λ 1 A2
(-
A λ2= λ2 A 1+ λ2 A 2
(!
where + the measured absorbance and 5 wa&elength The maximum absorbances of each component should correspond to a uni6ue wa&elength to pre&ent each resulting absorpti&ity cur&e from o&erlapping and creating constant and identical results. The permanganate solution absorbs at a maximum wa&elength around ,2/ nanometers (nm' while dichromate absorbs at a maximum wa&elength of 72/ nm. The molar absorpti&ity coe8cient' denoted by
ε ' which is the only
constant &alue between measurements' is calculated from the linear regression function obtained from the graph of concentration &ersus the absorbance of the substances. Substituting this formula to (- and (! will pro&ide the concentration of each component) A = ε C b
(7
where " is the concentration of the sample and b is the path length of the cu&ette. The molar absorpti&ity coe8cient that was ta9en from the standardization process of the permanganate and dichromate solutions is used to determine the indi&idual absorbances of each component of the mixture or binary system. The formula is) 350
A total= ε 350
A total= ε
350
350
b Mn [ Mn ] + ε
b M n [ Mn ] + ε
350
350
b Cr [ Cr ]
(4
b Cr [ Cr ]
(2
where :Mn; and :"r; are the concentrations of each component at the speci3c wa&elength
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Chem126 Lab – Instrumental Analysis The determination of the concentrations of each component at di%erent wa&elengths should show a signi3cant di%erence in the two components' as long their maximum and minimum absorbance refrain from o&erlapping.
MET-D./0 + stoc9 solution of about /./- M solution of KMnO 4 was prepared and standardized* while a solution of -.2 liters of /.!2 M < !SO4 was also prepared. Separate &olumes of /.2' -./' 7.//' and 4.// milliliter (m= portions were then pipetted from the standard KMnO 4 solution and placed into separate -// m= &olumetric >as9s. +nother empty >as9 was added to the set and each >as9 was labeled respecti&ely from - to 2. ?ach solution was then diluted to mar9 with the /.!2 M solution of < !SO4. +bout /.22 to /.,/ grams of dried K !"r!O# reagent was weighed on an analytical balance. This dry reagent was then transferred into a -// m= &olumetric >as9 and diluted to the mar9 with distilled water. @sing a pipette' !.//' 4.//' 0.//' and -/.// m= portions of the diluted K !"r!O# were placed separately into -// m= &olumetric >as9s. +n empty >as9 was added to the set and the >as9s were numbered from - to 2' respecti&ely. ?ach >as9 was then diluted to the mar9 with the /.!2 M < !SO4. The middle permanganate solution' the third one with intermediate concentration' had its absorbance measured against a blan9 of pure /.!2
Chem126 Lab – Instrumental Analysis was calculated for each ion. The path length was calculated from the b A &alues for each wa&elength. + 9nown amount of KMnO4 and K !"r!O# solution was prepared in a -// m= &olumetric >as9. The absorbance was measured for each of the two pre&iously chosen wa&elengths. The concentration of each of the primary ions present in the solution was then calculated from the absorbances. The calculated concentrations were then compared with the standard concentrations obtained from the pre&ious processes.
RE').T' AND DI'()''IN $eer A =ambertBs law is only &alid for a binary system in a set of conditions. One of these conditions includes the independence of the components of the sample analyzed from each other such that they are nonC interfering* are homogenous* and' the incident radiation must be monochromatic (D. SanEee&' !/-7. Thus' an instrument cannot di%erentiate two species present in the sample absorbing speci3c wa&elengths. t can only determine the total absorbance as shown in e6uations - and !. n this experiment' an Fun9nownG mixture of K !"r!O# and MnO4 are analyzed by measuring the absorbances of the solution at two determined wa&elengths. Hour calibration cur&es were constructed each of which contains the ions present in the sample. The wa&elengths which correspond to the highest absorbances of each standard solution' K !"r!O# and KMnO4 were determined by the +bsorption spectrum shown in Higure -. The highest absorbance in the KMnO4 standard solution corresponds to a wa&elength of 47#.2 nm. The K!"r!O# standard corresponds to a wa&elength of 2!, nm. These were used to record the absorbances of the series of standards containing both of the components of the sample.
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Chem126 Lab – Instrumental Analysis
/.72 /.7 /.!2 /.!
Ia&elength (nm /.-2 otassium permanganate
otassium dichromate
/././2 / 4//
42/
2//
22/
,//
,2/
#//
#2/
+bsorbance
&i gure 1' Absorption Spectrum( )a!elength !s' Absorbance for ma*imum absorbance determination Higure ! and 7 shows the calibration cur&es obtained from measuring the absorbances at 47#.2 nm. Higure ! is the plot of the absorbance against the concentration of the permanganate ion. Higure 7 is the plot of the absorbance against the concentration of the dichromate ion. "onse6uently' Higure 4 and 2 are the calibration cur&es obtained from measuring the absorbances at the 2!, nm of the permanganate and dichromate ion' respecti&ely. The information needed from these calibration cur&es is the molar absorpti&ity which is the slope of the calibration cur&es. +ccording to e6uation 7' if the path length is - cm (such that in this experiment' the absorbance is linearly related to the concentration with the molar absorpti&ity as its slope.
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Chem126 Lab – Instrumental Analysis
Asorance 4s" 55m Mn1 at 6137"2 /./4 /./4
f(x /x J /./D -
/./7 /./7 Asorance
/./! /./! /.//.// /
2
-/
-2
!/
!2
7/
72
4/
42
55m Mn1
&igure 2' Calibration Cur!e( Absorbance !s' ppm $n+ at -./'0
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2/
Chem126 Lab – Instrumental Analysis
Asorance 4s" 55m (r,7 at 6137"2
Asorance
/.L /.0 /.# /., /.2 /.4 /.7 /.! /./
f(x /x J / D -
/
-//
!//
7//
4//
2//
,//
55m (r,7
&igure .' Calibration Cur!e( Absorbance !s' ppm Cr 2+/ at -./'0
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Chem126 Lab – Instrumental Analysis
Asorance 4s" 55m Mn1 at 62,+ -.! f(x /./!x J /./D -
/.0 Asorance
/., /.4 /.! / /
2
-/
-2
!/
!2
7/
72
4/
42
2/
55m Mn1
&igure 2' Calibration Cur!e( Absorbance !s' ppm $n+ at -026
Asorance 4s" 55m (r,7 at 62,+ /./, /./2
f(x /x J /./D /.LL
/./4 Asorance
/./7 /./! /.// /
-//
!//
7//
4//
2//
,//
55m (r,7
&igure 0' Calibration Cur!e( Absorbance !s' ppm Cr 2+/ at -026
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Chem126 Lab – Instrumental Analysis Table - summarizes the molar +bsorpti&ities in each calibration cur&es. These were obtained by using the least s6uare methods in determining the slope of a regression line (Seep +ppendix) "alculations. Molar +bsorpti&ities (=mgC- cmC- −¿
MnO 4 ¿437.5 0.00068112 7
ℇ ¿
¿
−¿
Mn O4
¿526 0.02013437 8
ℇ ¿ ¿ 2−¿
Cr 2 O 7
¿437.5 0.00145308 9
ℇ ¿ ¿ 2−¿
Cr 2 O7 ¿526
−5 7.56909 × 10
ℇ ¿
¿
Table -. Summary of the Molar +bsorpti&ities obtained in each calibration cur&e. These molar +bsorpti&ities are used to determine the concentration of each ion present in the solution. +s mentioned' only the total absorbance in each speci3c wa&elength can be detected in the instrument. ncorporating this concept to e6uation 7 and by measuring the absorbances of the sample at the two selected wa&elengths' we may be able to get the concentration of each ion in the sample as shown in e6uations 4 and 2. Manipulating this system of e6uation' the concentration of the dichromate ion and permanganate ion can be calculated as follows)
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Chem126 Lab – Instrumental Analysis 526 −¿ ℇ Mn O ¿
¿
4
2−¿
Cr 2 O7 ¿ 437.5 ¿ ℇ ¿
−¿
MnO4 ¿ 437.5 −¿ ℇ ¿
2−¿
Cr2 O7 ¿526 ¿ ℇ ¿
(,
¿ b¿
−¿ ℇ Mn O ¿ 4
526
¿
−¿
MnO4 ¿437.5− A 440 ¿ ℇ ¿
A 545 ¿ [ C r 2 O7 ]=¿
−¿ ¿
MnO4 ¿ 2 −¿ ¿
Cr 2 O7 ¿ ℇ MnO
−¿ 4
(#
¿545
¿ 2 −¿
Cr 2 O7
¿545 ¿
ℇ ¿
A545 −¿ ¿
The sample that was analyzed in the experiment consists of 4ml K !"r!O# and -ml KMnO 4. Table ! shows the summary of the concentrations obtained experimentally and theoretically. The percentage errors in each determination were calculated. Minimal error resulted in the analysis. This suggests an accurate laboratory handling and experimentation and also' a successful experiment.
Ex5erimental
Theoretical
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Percentage Error
Chem126 Lab – Instrumental Analysis −¿ ¿
Mn O4
−¿
2
¿
Cr 2 O 7
12.06 ppm
11.88 p
pm
1.50
227.64 ppm
227.24 ppm
0.18
Table !. Summary of the analysis of the un9nown sample. (N(.)'IN Simultaneous determination of the concentration of a sample composed of two or more components can be done using spectroscopic measurements. $y using a series of standards of solution present in the sample' four calibration cur&es were obtained. The slope of these calibration cur&es were calculated to the molar absorpti&ity. These &alues were used to calculate for the concentration of the two ions present in the sample. The sample composed of 4ml K !"r!O# and -ml KMnO4 was analyzed. The spectroscopic measurements determined the concentration of the dichromate and permanganate ion to be !!#.,4 ppm and -!., ppm' respecti&ely. The theoretical &alues for these concentrations were calculated in order to compare the accuracy of the results of the analysis. + percentage error of -.2/1 and /.-01 resulted from the determination of the permanganate and dichromate concentrations. These minimal &alues indicate accuracy in the analysis and therefore yield no signi3cant di%erence between the experimental and theoretical &alues.
.ITERAT)RE (ITED
SM@=T+R?O@S N?T?DMR+TOR OH "OMOSTOR OH $R+D MT@D?S (S?"TDO
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Chem126 Lab – Instrumental Analysis chemistry.niser.ac.inPlabhandoutsP"!4-C?xp0.pdf. March -L' !/-,.
Nate
+ccessed)
?xperiment 0) Simultaneous Spectrophotometric Netermination. Nepartment of "hemistry' "ollege of Science' Sultan Uaboos @ni&ersity' Sulatanat of Oman. +pril 7/' !/--. Detrie&ed from) www.eoman.almdares.netPupP772#,P-7/,4/,0-,.doc . Nate +ccessed) March -L' !/-,.
APPENDI(E' T+$=?S
m. $ ,(r,7 stan#ar# a##e#
55m (r,7;
Asorance at 6137"2
Asorance at 62,+
,
--7.,!
/.-,-
/./-L
1
!!#.!4
/.770
/./!L
8
424.40
/.,#-
/./4#
*:
2,0.-
/.0!
/./27
able .' Summary of Data using the "otassium Dichromate Standards
m. $Mn1 stan#ar# a##e#
55m Mn1;
Asorance at 6137"2
Asorance at 62,+
:"2
2.L7L0,-247
/.//L
/.-4
*
--.0#L#!7/L
/./-4
/.!47
3
72.,7L-,L!,
/./!L
/.#7!
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Chem126 Lab – Instrumental Analysis
1
4#.2-00L!74
/./70
/.L#
able ' Summary of Data using the "otassium $anganese Standards =m.> =m.> * 7#.#L /./ 7#.#L
,
7-.-L
-./
7/.-L
3
4!.//
--.L/
7/.-
able 0' Data for the standardi3ation of "otassium $anganese stoc4 solution
"+="@=+TORS Least S5uares $ethod o determine molar absorpti!ity7 +bsorbance &s. ppm MnO4 at 5 47#.2 (Hig. ! 2
S xx = Σ x −
S xy = Σxy −
−¿
MnO4
( Σx )
2
=3704.60529 −
n
( ΣxΣy )❑ n
( 100.9776462 )
2
=1155.484031
4
❑
[ ( 100.9776462 ) ( 0.09 ) ] = =3.059028695 − 0.787031654 4
¿437.5= m=
S xy S xx
=
0.787031654 1155.484031
=0.00068112 7
ℇ ¿ ¿
+bsorbance &s. ppm MnO 4 at 52!, (Hig. 7 2
S xx = Σ x −
( Σx ) n
2
=3704.60529 −
( 100.9776462 ) 4
2
=1155.484031
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Chem126 Lab – Instrumental Analysis
S xy = Σxy −
−¿
Mn O4
( ΣxΣy )❑ n
¿526=m =
=75.89955079 −
S xy S xx
=
23.2649527 1155.484031
[ ( 100.9776462 ) ( 2.085 ) ] =23.2649527 4
= 0.02013437 8
ℇ ¿ ¿
+bsorbance &s. ppm "r !O# at 547#.2 (Hig. 4 2
S xx = Σ x −
S xy = Σxy −
2 −¿
Cr 2 O7
( Σx )
2
n
2
=593837.2024 −
( ΣxΣy )❑ n
( 1363.44 ) 4
=129095.044
❑
[ ( 1363.44 ) ( 1.99 ) ] = =865.89802 − 187.58662 4
¿437.5= m=
S xy S xx
=
187.58662 129095.044
=0.00145308 9
ℇ ¿ ¿
+bsorbance &s. ppm "r !O# at 52!, (Hig. 2
S xy = Σxy −
2−¿
Cr 2 O7
( ΣxΣy )❑ n
¿526= m =
=60.2186−
S xy S xx
=
[ ( 1363.44 ) ( 0.148 ) ]
❑
=9.77132
4
9.77132 129095.044
−5
=7.56909 × 10
ℇ ¿ ¿
$olarity of
KMnO 4
0.1264 g Na2 C 2 O 4
Trial -)
0.03779 L KMn O 4
×
1 mol Na2 C 2 O 4 133.96
g Na2 C 2 O4
×
2 mol KMn O 4 5 mol Na2 C 2 O4
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=0.00998746 M KMn O
4
Chem126 Lab – Instrumental Analysis 0.1011 g Na2 C 2 O 4
Trial !)
0.03019 L KMnO 4
0.1006 g Na2 C 2 O 4
Trial 7)
0.03010 L KMn O 4
M KMnO
4
=
×
×
1 mol Na2 C 2 O4 133.96 g Na2 C 2 O 4
1 mol Na2 C 2 O4 133.96 g Na2 C 2 O4
+
×
×
2 mol KMn O4 5 mol Na2 C 2 O 4
2 mol KMnO 4 5 mol Na2 C 2 O 4
+
0.00998746 M 0.009999376 M 0.009979674 M
( average )
3
=0.009999376 M KMn O
4
=0.009979674 M KMn O
4
=0.009988836 M
Concentration of Standards
ppm MnO 4=
ppmCr2 O7=
0.009988836 mol KMn O 4 1 L
0.5681 g K 2 Cr 2 O7 0.1 L
×
×
1 mol MnO 4 1 mol KMn O 4
1000 mg 1g
×
118.93 gMnO 4 1 mol MnO 4
×
1000 mg 1g
=5681 ppmCr 2 O7
"omposition of un9nown using the calibration cur&es 526 −¿ ℇ Mn O ¿
¿
4
2−¿
Cr 2 O7 ¿ 437.5 ¿ ℇ ¿
−¿
MnO 4 ¿ 437.5−¿ ℇ ¿
2−¿
Cr2 O7 ¿526 ¿ ℇ ¿
¿ b¿
−¿ ℇ Mn O ¿ 4
−¿
526
¿
Mn O4 ¿437.5− A 440 ¿ ℇ ¿
A 545 ¿ [ C r 2 O7 ]=¿
[C r
2
O7 ]=
(
)
(
0.260 0.00068113 −0.339 0.020134378
)
( 7.56909 × 10 ) ( 0.00068113)−( 0.00145309) ( 0.020134378) −5
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=227.64 ppm
=1187.972309 ppm≈ 118
Chem126 Lab – Instrumental Analysis
−¿
¿
MnO 4
¿ −¿
2
¿
Cr 2 O 7
¿
ℇ Mn O ¿ −¿ 4
545
=
0.260
−( 7.56909 × 10− ) ( 227.64 ppm ) 5
0.020134378
=12.06 ppm
¿
2−¿
Cr2 O 7 ¿545 ¿ ℇ¿
A 545−¿
¿
heoretical composition of un4no8n
[C r
2
−¿
O 7 ] =5681 ppmCr 2 O 7 ×
4 ml 100 ml
=227.24 pp m
¿
Mn O4
¿ ¿ "ercentage 9rror Hor
[C r
2
O7 ]
−¿ Hor
¿
MnO 4
¿ ¿
'
%Error =
%Error =
227.64 ppm− 227.24 pp m 227.24 pp m
12.06 ppm
−11.88 pp m
11.88 pp m
=0.18
=1.50
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