3500-Mn MANGANESE*
3500-Mn A. Introduction
1.
Occurrence and Significance
Manganese (Mn) is the first element in Group VIIB in the
periodic table; it has an atomic number of 25, an atomic weight
of 54.94, and common valences of 2, 4, and 7 (and more rarely,
valences of 1, 3, 5, and 6). The average abundance of Mn in the
earth’s crust is 1060 ppm; in soils it is 61 to 1010 ppm; in
streams it is 7
g/L, and in groundwaters it is ⬍0.1 mg/L.
Manganese is associated with iron minerals, and occurs in nod-
ules in ocean, fresh waters, and soils. The common ores are
pyrolusite (MnO
2
) and psilomelane. Manganese is used in steel
alloys, batteries, and food additives.
The common aqueous species are the reduced Mn
2⫹
and the
oxidized Mn
4⫹
. The aqueous chemistry of manganese is similar
to that of iron. Since groundwater is often anoxic, any soluble
manganese in groundwater is usually in the reduced state
(Mn
2⫹
). Upon exposure to air or other oxidants, groundwater
containing manganese usually will precipitate black MnO
2
. El-
evated manganese levels therefore can cause stains in plumbing/
laundry, and cooking utensils. It is considered an essential trace
element for plants and animals. The United Nations Food and
Agriculture Organization recommended maximum level for
manganese in irrigation waters is 0.2 mg/L. The U.S. EPA
secondary drinking water standard MCL is 50
g/L.
2.
Selection of Method
The atomic absorption spectrometric methods (3111B and C),
the electrothermal atomic absorption method (3113B), and the
inductively coupled plasma methods (3120 and 3125) permit
direct determination with acceptable sensitivity and are the
methods of choice. Of the various colorimetric methods, the
persulfate method (B) is preferred because the use of mercuric
ion can control interference from a limited chloride ion concen-
tration.
3.
Sampling and Storage
Manganese may exist in a soluble form in a neutral water
when first collected, but it oxidizes to a higher oxidation state
and precipitates or becomes adsorbed on the container walls.
Determine manganese very soon after sample collection. When
delay is unavoidable, total manganese can be determined if the
sample is acidified at the time of collection with HNO
3
to pH
⬍2. See Section 3010B.
3500-Mn B. Persulfate Method
1.
General Discussion
a. Principle: Persulfate oxidation of soluble manganous com-
pounds to form permanganate is carried out in the presence of
silver nitrate. The resulting color is stable for at least 24 h if
excess persulfate is present and organic matter is absent.
b. Interference: As much as 0.1 g chloride (Cl
⫺
) in a 50-mL
sample can be prevented from interfering by adding 1 g mercuric
sulfate (HgSO
4
) to form slightly dissociated complexes. Bro-
mide and iodide still will interfere and only trace amounts may
be present. The persulfate procedure can be used for potable
water with trace to small amounts of organic matter if the period
of heating is increased after more persulfate has been added.
For wastewaters containing organic matter, use preliminary
digestion with nitric and sulfuric acids (HNO
3
and H
2
SO
4
) (see
Section 3030G). If large amounts of Cl
⫺
also are present, boiling
with HNO
3
helps remove it. Interfering traces of Cl
⫺
are elim-
inated by HgSO
4
in the special reagent.
Colored solutions from other inorganic ions are compensated
for in the final colorimetric step.
Samples that have been exposed to air may give low results
due to precipitation of manganese dioxide (MnO
2
). Add 1 drop
30% hydrogen peroxide (H
2
O
2
) to the sample, after adding the
special reagent, to redissolve precipitated manganese.
c. Minimum detectable concentration: The molar absorptivity
of permanganate ion is about 2300 L g
⫺1
cm
⫺1
. This corre-
sponds to a minimum detectable concentration (98% transmit-
tance) of 210
g Mn/L when a 1-cm cell is used or 42
g Mn/L
when a 5-cm cell is used.
d. Quality control (QC): The QC practices considered to be
an integral part of each method can be found in Section 3020.
2.
Apparatus
Colorimetric equipment: One of the following is required:
a. Spectrophotometer, for use at 525 nm, providing a light
path of 1 cm or longer.
b. Filter photometer, providing a light path of 1 cm or longer
and equipped with a green filter having maximum transmittance
near 525 nm.
c. Nessler tubes, matched, 100-mL, tall form.
* Approved by Standard Methods Committee, 1999. Editorial revisions, 2011.
Joint Task Group: 20th Edition—See 3500-Al.
1
3.
Reagents
a. Special reagent: Dissolve 75 g HgSO
4
in 400 mL conc
HNO
3
and 200 mL distilled water. Add 200 mL 85% phosphoric
acid (H
3
PO
4
), and 35 mg silver nitrate (AgNO
3
). Dilute the
cooled solution to 1 L.
b. Ammonium persulfate, (NH
4
)
2
S
2
O
8
, solid.
c. Standard manganese solution: Prepare a 0.1Npotassium
permanganate (KMnO
4
) solution by dissolving 3.2 g KMnO
4
in
distilled water and making up to 1 L. Age for several weeks in
sunlight or heat for several hours near the boiling point, then
filter through a fine fritted-glass filter crucible and standardize
against sodium oxalate as follows:
Weigh several 100- to 200-mg samples of Na
2
C
2
O
4
to 0.1 mg
and transfer to 400-mL beakers. To each beaker, add 100 mL
distilled water and stir to dissolve. Add 10 mL 1 ⫹1H
2
SO
4
and
heat rapidly to 90 to 95°C. Titrate rapidly with the KMnO
4
solution to be standardized, while stirring, to a slight pink
end-point color that persists for at least 1 min. Do not let
temperature fall below 85°C. If necessary, warm beaker contents
during titration; 100 mg Na
2
C
2
O
4
will consume about 15 mL
permanganate solution. Run a blank on distilled water and
H
2
SO
4
.
Normality of KMnO
4
⫽gNa
2
C
2
O
4
(A⫺B)⫻0.067 01
where:
A⫽mL titrant for sample and
B⫽mL titrant for blank.
Average results of several titrations. Calculate volume of this
solution necessary to prepare1Lofsolution so that 1.00 mL ⫽
50.0
g Mn, as follows:
mL KMnO
4
⫽4.55
normality KMnO
4
To this volume add 2 to 3 mL conc H
2
SO
4
and NaHSO
3
solution dropwise, with stirring, until the permanganate color
disappears. Boil to remove excess SO
2
, cool, and dilute to 1000
mL with distilled water. Dilute this solution further to measure
small amounts of manganese.
d. Standard manganese solution (alternate): Dissolve 1.000 g
manganese metal (99.8% min.) in 10 mL redistilled HNO
3
.
Dilute to 1000 mL with 1% (v/v) HCl; 1 mL ⫽1.000 mg Mn.
Dilute 10 mL to 200 mL with distilled water; 1 mL ⫽0.05 mg
Mn. Prepare dilute solution daily.
e. Hydrogen peroxide,H
2
O
2
, 30%.
f. Nitric acid, HNO
3
, conc.
g. Sulfuric acid, H
2
SO
4
, conc.
h. Sodium nitrite solution: Dissolve 5.0 g NaNO
2
in 95 mL
distilled water.
i. Sodium oxalate, Na
2
C
2
O
4
, primary standard.
j. Sodium bisulfite: Dissolve 10 g NaHSO
3
in 100 mL distilled
water.
4.
Procedure
a. Treatment of sample: If a digested sample has been prepared
according to directions for reducing organic matter and/or excessive
chlorides in Section 3030G, pipet a portion containing 0.05 to 2.0 mg
Mn into a 250-mL conical flask. Add distilled water, if necessary, to 90
mL and proceed as in ¶ b.
b. To a suitable sample portion add 5 mL special reagent
and 1 drop H
2
O
2.
Concentrate to 90 mL by boiling or dilute to
90 mL. Add 1 g (NH
4
)
2
S
2
O
8
, bring to a boil, and boil for 1
min. Do not heat on a water bath. Remove from heat source,
let stand 1 min, then cool under the tap. (Boiling too long
results in decomposition of excess persulfate and subsequent
loss of permanganate color; cooling too slowly has the same
effect.) Dilute to 100 mL with distilled water free from
reducing substances and mix. Prepare standards containing 0,
5.00,...1500
g Mn by treating various amounts of standard
Mn solution in the same way.
c. Nessler tube comparison: Use standards prepared as in ¶ 4b
and containing 5 to 100
g Mn/100 mL final volume. Compare
samples and standards visually.
d. Photometric determination: Use a series of standards from
0 to 1500
g Mn/100 mL final volume. Make photometric
measurements against a distilled water blank. The following
table shows light path length appropriate for various amounts of
manganese in 100 mL final volume:
Mn Range
g
Light Path
cm
5–200 15
20–400 5
50–1000 2
100–1500 1
Prepare a calibration curve of manganese concentration vs.
absorbance from the standards and determine Mn in the samples
from the curve. If turbidity or interfering color is present, make
corrections as in ¶ 4e.
e. Correction for turbidity or interfering color: Avoid filtra-
tion because of possible retention of some permanganate on the
filter paper. If visual comparison is used, the effect of turbidity
only can be estimated and no correction can be made for inter-
fering colored ions. When photometric measurements are made,
use the following “bleaching” method, which also corrects for
interfering color: As soon as the photometer reading has been
made, add 0.05 mL H
2
O
2
solution directly to the sample in the
optical cell. Mix and, as soon as the permanganate color has
faded completely and no bubbles remain, read again. Deduct
absorbance of bleached solution from initial absorbance to ob-
tain absorbance due to Mn.
5.
Calculation
a. When all of the original sample is taken for analysis:
mg Mn/L ⫽
g Mn/100 mL
mL sample
⫻100
mL portion
b. When a portion of the digested sample (100 mL final
volume) is taken for analysis:
mg Mn/L ⫽
g Mn (in 100 mL final volume)
mL sample
MANGANESE (3500-Mn)/Persulfate Method
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MANGANESE (3500-Mn)/Persulfate Method
6.
Precision and Bias
A synthetic sample containing 120
g Mn/L, 500
g Al/L, 50
g Cd/L, 110
g Cr/L, 470
g Cu/L, 300
g Fe/L, 70
g Pb/L,
150
g Ag/L, and 650
g Zn/L in distilled water was analyzed
in 33 laboratories by the persulfate method, with a relative
standard deviation of 26.3% and a relative error of 0%.
A second synthetic sample, similar in all respects except for 50
g Mn/L and 1000
g Cu/L, was analyzed in 17 laboratories by
the persulfate method, with a relative standard deviation of
50.3% and a relative error of 7.2%.
7. Bibliography
RICHARDS, M.D. 1930. Colorimetric determination of manganese in
biological material. Analyst 55:554.
NYDAHL, F. 1949. Determination of manganese by the persulfate method.
Anal. Chem. Acta. 3:144.
MILLS, S.M. 1950. Elusive manganese. Water Sewage Works 97:92.
SANDELL, E.B. 1959. Colorimetric Determination of Traces of Metals,
3rd ed. Interscience Publishers, New York, N.Y., Chapter 26.
DELFINO, J.J. & G.F. LEE. 1969. Colorimetric determination of manga-
nese in lake waters. Environ. Sci. Technol. 3:761.
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