## 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 nodules 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 Mn4 . 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 . Elevated 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 mercuricion can control interference from a limited chloride ion concentration.

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 compounds 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. Bromide 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 eliminated 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 2O 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 corresponds to a minimum detectable concentration (98% transmittance) 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.

13. 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, (NH4) 2 S 2 O 8 , solid. c. Standard manganese solution: Prepare a 0.1 N potassium 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 Na2C 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 1 H 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 Na2C 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  g Na2 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 prepare 1 L of solution 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 2O 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, Na2C 2O 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 ¶ 4 e

.

e. Correction for turbidity or interfering color: Avoid filtration because of possible retention of some permanganate on the filter paper. If visual comparison is used, the effect of turbidityonly can be estimated and no correction can be made for interfering 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. Deductabsorbance of bleached solution from initial absorbance to obtain 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

26. 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. M ILLS , S.M. 1950. Elusive manganese. Water Sewage Works 97:92. S ANDELL , E.B. 1959. Colorimetric Determination of Traces of Metals, 3rd ed. Interscience Publishers, New York, N.Y., Chapter 26. D ELFINO , J.J. & G.F. LEE . 1969. Colorimetric determination of manganese in lake waters. Environ. Sci. Technol. 3:761. MANGANESE (3500-Mn)/Persulfate Method

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