Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
Redox titration of iron using methylene blue as indicator and its application in ore analysis
Erik Prasetyo, Fika R. Mufakhir Indonesian Institute of Sciences Jl. Ir. Sutami km. 15, Tanjung Bintang Bandar Lampung, Indonesia
[email protected]
Abstract
-
Dichromatometry
titration
procedure to determine iron content using
Keywords - methylene blue, dichromate titration, iron ore analysis.
methylene blue as redox indicator was investigated.
The
effectiveness
of
this
I. INTRODUCTION
procedure was then compared with standard
Iron is one of the most routinely analyzed
dichromatometry titration in which barium
elements in laboratory. The analysis methods
diphenylamine sulfonate was applied as
chosen generally depend on sensitivity or iron
indicators. The result showed that the titration
content in sample, sensitivity, and accuracy
procedure using methylene blue could be
required. Titrimetric analysis is usually chosen
served as alternative procedure in terms of
for range of iron content in percent – permil.
reproducibility and accuracy. This procedure
This method measures the volume of a
even owned advantage compared to existing
solution
standard methods, since it did not incorporate
concentration (titrant) which is required to
Zimmermann Reinhardt reagent as in standard
react quantitatively with a measured volume of
permanganometry and phosphoric acid as in
a solution of the substance to be determined
standard dichromatometry titration in iron
(analyte).
determination.
determined based on volume of titrant,
Nov 2011
of
substance
Analyte
ATBAS-60118057©Asian-Transactions
with
concentration
known
is
then
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Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
chemical reaction occurred and molecular
diphenylamine-4-sulfonic acid as barium or
mass of substance involved in reaction [1].
sodium salt [4], oxidation of iron(II) by
Technically the end of quantitative reaction is
potassium permanganate (permanganometry)
marked by physical change including but not
in presence of sulfuric acid and phosphoric
limited
and
acid [5] and manganese(II) [1]. Iron(III) is
precipitation. Color change in the end of
generally determined as iron(II) after reduction
reaction could be imparted by the excess of
with tin(II) chloride. Complexation based
colored titrant or addition of a substance
procedure uses EDTA as titrant to form
known as indicator. Indicators are usually
complex with iron(III) with variamine blue as
organic
indicator [6].
to:
color,
potential,
substances
which
pH
change
color
according to the condition of analyte: pH,
Permanganometry own its advantage
potential, concentration and species of ions
since it does not required additive substance as
contained. So far, after more than a century the
indicator. The permanganate itself serves as
application of indicators is still considered
internal indicator since in the end of titration a
important in analytical chemistry and has yet
little excess titrant could change the color of
to be restricted by application of modern
analyte from colorless to very pale pink.
instrumental technique in detecting end of
However this procedure has disadvantages in
reaction [2].
terms of potassium permanganate instability
Several titrimetry procedures have
and its characteristic as very strong oxidizing
been proposed to determine iron content.
agent.
Generally these procedures are based on redox
decomposed and difficult to obtain in pure
and complex forming reaction. Redox based
form) does not meet the criteria as primary
procedure
and
standard, so it must be frequently standardized
considered as standard includes the oxidation
with oxalic acid before use. As strong
of
dichromate
oxidizing agent permanganate tends to react
(dichromatometry) in 1 – 2 M sulfuric acid
with chloride ion present in analyte. This
solution and in the presence of phosphoric acid
chloride generally comes from the aqua regia
with diphenylamine as indicator [3], with
used in sample destruction. So that to lower
iron(II)
Nov 2011
which
by
is
widely
potassium
used
Permanganate
ATBAS-60118057©Asian-Transactions
instability
(easily
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Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
the tendency of permanganate reacts with
staining agents, medicine, pharmaceutical and
chloride and increase its tendency to react with
aquaculture [7].
iron(II), manganese(II) sulfate and phosphoric acid
are
added
(Zimmerman
Reinhardt
reagent). Potassium
dichromate
solves
the
problem posed by permanganometry in terms of stability and its characteristics as a weaker oxidizing agent compared to permanganate. So that in titration dichromate does not react with chloride to some extent (max. 2 M). However, the lack of detectable of color change in the end of titration makes this procedure require diphenylamine-4-sulfonate (DAS) as indicator.
Figure 1. Methylene blue structures.
In fact the procedure requires phosphoric acid addition to change redox potential system of
MB as a redox indicator has a
iron(II) – dichromate from +0.56 V to +0.78
potential transition which depends on the pH
V, in order to get near to the transition
of the medium [2] (+0.532 V at pH 0). This
potential of DAS (+0.76 V) so that the color
value is close to potential redox system Fe (II)
change (end of titration) occurs in the end of
- dikromate (+0.56 V), so it is assumed that
reaction [4].
methylene blue could be used to detect the end
The use of phosphoric acid could be
point in titration. This paper will review the
avoided if an appropriate indicator is used. An
possibility of methylene blue as indicator for
indicator
redox titration of Fe (II) with dichromate
which
might
be
proposed
is
methylene blue (MB). MB (Figure 1) is a
including
substance easily obtained in the market due to
concentration in analyte as well as its
its broad application which is not limited only
applicability in ore analysis.
variables
of
pH
and
MB
to chemical applications but also to biology as Nov 2011
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Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
II. EXPERIMENT
deionized water and homogenized before
In this experiment the reagents used include ammonium
iron(II)
sulfate
potassium
dichromate,
titration.
hexahidrate,
tin(II)
chloride
III. RESULTS AND DISCUSSIONS
dihidrate, mercury(II) chloride, sulfuric acid
The result of alternative dichromatometry
and methylene blue chloride obtained from
procedure is summarized in Table 1. In this
Merck
For
titration the initial color of analyte was blue.
comparison, titration of iron(II) for the same
Grey tinge would be marked the end of
samples would also be carried out using
titration
standard dichromatometry procedure in which
addition of 2-3 more drops of titrant would
barium diphenylamine-4-sulfonate (BDS) acts
change the color to emerald green.
all
with
analytical
grade.
which
rapidly
disappeared
and
as indiactor. The standard procedure refers to
In Table 1, Fe added stands for
Vogel’s Textbook of Quantitative Chemical
iron(II) added in the form of ammonium
Analysis [1].
iron(II) sulfate while Fe found stands for
The
alternative
dichromatometry
amount of iron(II) determined by alternative
procedure is carried out using potassium
dichromatometry procedure. The difference
dichromate 0.005 M as titrant. The solution
between Fe added and Fe found is summarized
was prepared by weighing the crystals
in column deviation. SA and MB denote the
followed by dilution with deionized water into
concentration of 25 ml sulfuric acid and
appropriate volume in volumetric flask. As
volume of 6% MB solution added into analyte
analyte ammonium iron(II) sulfate hexahidrate
respectively.
was weighed (varied between 0.3 – 0.9 gram)
Based on results in Table 1 it is shown
then transferred into 500 ml Erlenmeyer flask
that the least deviation is obtained in the
and
to
titration with addition of 25 ml of 7 M sulfuric
approximately 50 ml. Into this solution 25 ml
acid and 5 ml of 6% MB solution into analyte.
sulfuric acid (1 M, 4 M or 7 M) and 6% MB
This case is different from the assumption
solution (1 ml, 3 ml or 5 ml) were added then
established before that the optimum condition
the solution was marked to 100 ml by
would be obtained if the proton concentration
diluted
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using
deionized
water
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Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
reached 2 M in analyte (addition of 25 ml of 4
However
the
titration experiment
M sulfuric acid). The value 2 M was obtained
showed that the ideal result obtained in
by solving the Equation 1 where E value was
extremely acid condition (proton concentration
set as 0.56 (redox potential system of iron(II) –
close to 3.5 M) with the addition of 25 ml of 7
dichromate).
M sulfuric acid. This is the case due to the assumption that some of hydrogen ion would
Table 1. Result of iron(II) determination using
be
alternative dichromatometry titrimetry with
dichromate (Reaction 1) so that during
MB served as indicator.
titration hydrogen ion concentration would
Fe added
Fe found
Deviation
No
consumed
in
iron(II)
oxidation
by
MB
decrease and approach the theoretically ideal
(ml)
concentration in the end of titration. Several
SA (gr)
(gr)
(gr)
1
0.0776
0.0837
0.0061
1M
1
2
0.0772
0.0837
0.0066
4M
1
3
0.1091
0.1138
0.0047
7M
1
4
0.0904
0.0954
0.0050
1M
3
extremely acid condition i.e. tin(II) by iron(III)
5
0.0943
0.1004
0.0061
4M
3
in 4 – 6
6
0.0844
0.0887
0.0043
7M
3
molybdenum(VI) by iron(II) in 11.5 – 13 M
7
0.1178
0.1306
0.0128
1M
5
8
0.0625
0.0736
0.0111
1M
5
9
0.0621
0.0637
0.0016
4M
5
in 10.5 M
10
0.0735
0.0770
0.0036
4M
5
uranium(VI) by iron(II) in 11.6 M phosphoric
11
0.1098
0.1122
0.0024
7M
5
acid [11].
12
0.1126
0.1155
0.0028
7M
5
13
0.1306
0.1257
-0.0048
7M
5
14
0.0887
0.0954
0.0067
7M
5
references showed that the titration using MB as indicator is generally carried out in
M hydrochloric acid [8],
phosphoric acid [9], vanadium(IV) by iron(II) phosphoric acid [10],
and
Reaction 1. Cr2O72- + 6Fe2+ + 14H+ 2Cr3+ + 6Fe3+ + 7H2O
(1) Besides, this extremely acid condition might assist to determine the end point visually since in extreme acidic solution MB is Nov 2011
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Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
known to produce the metastable free radical
transferred and diluted with deionized water in
semiquinone
of
250 ml volumetric flask. A volume of 50 ml
semiquinone was helpful in determining the
from the volumetric flask would be used for
end point since it could impart grey tinge to
each titration.
[12].
The
presence
analyte before the end point is reached before
Before the titration was carried out, all
addition of 2 – 3 more drops of titrant.
iron(III) in analyte would be reduced to
However, this grey tinge is only lasted for no
iron(II) using tin(II) chloride solution. The
more than one second as any other reversible
reduction procedure referred to [5]. For each
indicators [2]. Furthermore this explains the
sample, the titration would be executed four
excess addition of MB solution into analyte
times, twice using standard procedure as stated
since in low concentration (addition 6% MB 1
before and twice using alternative procedure.
ml or 3 ml) the grey tinge was not observed.
In the alternative procedure the optimum
This is due to decomposition rate of
condition employed was based on the result of
semiquinone which was negatively correlated
previous titration in Table 1 in which 25 ml of
with MB concentration [13 - 15].
7 M sulfuric acid and 5 ml of 6% MB solution
Iron samples used in this study were
were added. The final volume of analyte was
iron sand and iron stone. Iron sand is
maked with deionized water to 100 ml.
dominated by minerals magnetite (Fe3O4),
Titration was executed using 0.005 M
ilmenite (FeTiO3), rutile (TiO2), a little silica
potassium dichromate
(SiO2) and hematite (Fe2O3) while iron stone is
summarized in Table 2.
which
results
are
dominated by magnetite, hematite and little
The table shows that the result MB
silica. Sample preparation began with crushing
alternative was close to the results of standard
and sieving to obtain < 100 mesh fraction
BDS procedure, but slightly higher for the
which was then dried in oven at 100° C for 1
sample with lower iron content (iron sand2).
hour. As much as 0.2 gram of this dried
This problem could be solved by weighing
fraction would be destructed using aqua regia
more samples (> 0.2 gr) for destruction. Based
leaving only silica. The result was then filtered
on the low deviation in MB duplicate analysis
using Whatman 42 and the filtrate was
for each samples (Table 3) it could be deduced
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Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
that this alternative procedure has advantage in
in sample could be performed using methylene
terms of reproducibility.
blue as indicator in determining titration end point. Best results were carried out in 3.5 M
Table 2. Total iron content as results of
sulfuric acid and 0.3% methylene blue. Based
titration using ‘alternative’ methylene blue
on application in iron ore analysis it was
(MB) and ‘standard’ barium diphenylamine-4-
shown that this procedure could be served as
sulfonic acid (BDS) while deviation signify
alternative
the result differences of both procedure.
comparing with standard dichromatometry
procedure
due
to
accuracy
procedure and reproducibility. Besides, this Sample
Dev. (mg)
procedure does not require addition of
MB (mg)
BDS (mg)
Iron sand1
49.088
51.433
2.345
Zimmerman Reinhardt reagent as in standard
Iron sand2
28.230
23.622
-4.608
permanganometry and phosphoric acid as in
Iron stone1
54.449
56.543
2.094
standard dichromatometry. In order to enhance the capability (accuracy and precision) to detect
Table 3. Total iron content as results of duplicate
titration
using
‘alternative’
the
end
point
of
the
titration,
spectrophotometric titration study would be advisable.
methylene blue (MB). REFERENCES Dev. Sample
MB1 (mg)
Iron sand1
48.083
MB2 (mg) 50.093
(mg)
[1] Jeffery, G. H., Bassett, J., Mendham, J., Denney, R. C., 1989, Vogel’s Textbook
2.010
of Quantitative Chemical Analysis, 5th ed, Iron sand2
27.141
29.319
2.178
Iron stone1
51.433
57.465
6.032
Longman Scientific and Technical, Essex, England. [2] Hulanicki, A., Głab, S., 1978, Redox
IV. CONCLUSIONS Redox
titration
using
potassium
dichromate as titrant to determine iron content
Indicators:
Characteristics
and
Applications, Pure and Appl. Chem., Vol. 50, pp. 463-498.
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Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
[3] Stockdale, D., 1950, The estimation of ore by dichromate, Analyst, Vol. 75, pp. 150-155.
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Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05
[15] Granick, S., Michaelis, L., Schubert, M. P., 1939, The semiquinone radicals of methylene blue and related dyestuffs, Science, Vol. 90, pp. 422-423.
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