3500-Mn MANGANESE* 3500-Mn A. Introduction 1. Occurrence and Significance 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. 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 (MnO2) and psilomelane. Manganese is used in steel alloys, batteries, and food additives. The common aqueous species are the reduced Mn2⫹ 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 (Mn2⫹). Upon exposure to air or other oxidants, groundwater containing manganese usually will precipitate black MnO2. 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 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 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 HNO3 to pH ⬍2. See Section 3010B. * Approved by Standard Methods Committee, 1999. Editorial revisions, 2011. Joint Task Group: 20th Edition—See 3500-Al. 3500-Mn B. Persulfate Method 1. General Discussion Samples that have been exposed to air may give low results due to precipitation of manganese dioxide (MnO2). Add 1 drop 30% hydrogen peroxide (H2O2) 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. 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 (HgSO4) 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 (HNO3 and H2SO4) (see Section 3030G). If large amounts of Cl⫺ also are present, boiling with HNO3 helps remove it. Interfering traces of Cl⫺ are eliminated by HgSO4 in the special reagent. Colored solutions from other inorganic ions are compensated for in the final colorimetric step. 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. 1 MANGANESE (3500-Mn)/Persulfate Method 3. Reagents 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 H2O2. Concentrate to 90 mL by boiling or dilute to 90 mL. Add 1 g (NH4)2S2O8, 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: a. Special reagent: Dissolve 75 g HgSO4 in 400 mL conc HNO3 and 200 mL distilled water. Add 200 mL 85% phosphoric acid (H3PO4), and 35 mg silver nitrate (AgNO3). Dilute the cooled solution to 1 L. b. Ammonium persulfate, (NH4)2S2O8, solid. c. Standard manganese solution: Prepare a 0.1N potassium permanganate (KMnO4) solution by dissolving 3.2 g KMnO4 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 Na2C2O4 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 H2SO4 and heat rapidly to 90 to 95°C. Titrate rapidly with the KMnO4 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 Na2C2O4 will consume about 15 mL permanganate solution. Run a blank on distilled water and H2SO4. g Na2C2O4 Normality of KMnO4 ⫽ (A ⫺ B) ⫻ 0.067 01 where: A ⫽ mL titrant for sample and B ⫽ mL titrant for blank. Light Path cm 5–200 20–400 50–1000 100–1500 15 5 2 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 filtration 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 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 H2O2 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 obtain absorbance due to Mn. 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 KMnO4 ⫽ Mn Range g 4.55 normality KMnO4 To this volume add 2 to 3 mL conc H2SO4 and NaHSO3 solution dropwise, with stirring, until the permanganate color disappears. Boil to remove excess SO2, 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 HNO3. 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, H2O2, 30%. f. Nitric acid, HNO3, conc. g. Sulfuric acid, H2SO4, conc. h. Sodium nitrite solution: Dissolve 5.0 g NaNO2 in 95 mL distilled water. i. Sodium oxalate, Na2C2O4, primary standard. j. Sodium bisulfite: Dissolve 10 g NaHSO3 in 100 mL distilled water. 5. Calculation a. When all of the original sample is taken for analysis: mg Mn/L ⫽ g Mn/100 mL 100 ⫻ mL sample mL portion b. When a portion of the digested sample (100 mL final volume) is taken for analysis: 4. Procedure a. Treatment of sample: If a digested sample has been prepared according to directions for reducing organic matter and/or excessive mg Mn/L ⫽ 2 g Mn (in 100 mL final volume) mL sample MANGANESE (3500-Mn)/Persulfate Method 6. Precision and Bias 7. Bibliography 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%. 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 manganese in lake waters. Environ. Sci. Technol. 3:761. 3