Chapter 35 Manganese

Chapter 35

Manganese

Manganese (Mn, atomic weight 54.94, melting point 1244°C, d = 7.3 g cm -3 ) is a white-grey, hard, brittle metal. It is fairly abundant in the earth's crust (ca 0.1%) where it occurs primarily as pyrolusite (MnO2), in Fe ores and in deep sea nodules. The metal dissolves in dilute mineral acids with formation of Mn(II). Oxidation states of Mn from II to VII are known, the most popular being II, IV, VI and VII. The only common cation is Mn2 + (pale pink). Sodium hydroxide precipitates Mn(OH) 2 which is insoluble in excess of the reagent and is rapidly oxidized to the brown-black MnO 2aq. Manganese(II) forms many labile complexes, e.g. with EDTA, tartrate, cyanide. Strong oxidants (e.g. IO 4) oxidize Mn(II) in acidic solution to the violet permanganate, MnO4, that is a strong oxidant. Manganese in oxidation states higher than II can be reduced to Mn(II) by boiling with concentrated HCl. Manganate (MnO4-) is stable in strongly alkaline solution; otherwise it disproportionates into Mn(VII) and Mn(IV). The most widely used organomanganese compounds include ethylenebisdithiocarbamate(used as fungicide) and methylcyclopentadienyl tricarbonyl (used as antiknock agent). Manganese is a biologically essential trace element and a constituent of many enzymes. In higher concentrations it is toxic. The environmental occurrence of Mn can be either natural (erosion of rocks) or anthropogenic, mostly as a result of mining and metallurgy [1]. 35.1 SEPARATION AND PRECONCENTRATION Extraction

Manganese can be extracted at pH 6-8 as the dithiocarbamate into a variety of solvents (e.g. CHC13, CC14 or Freon TF) and then stripped 509

into HNO 3 [2]. Extraction of Mn salicylate with liquid anion [3,4] or chelating exchangers [5] into non polar solvents followed by stripping into dilute acids was reported. The Mn-TTA complex could be fairly selectively extracted from a complex matrix (Cu, Fe, Pb, Zn, Al) as the ion-pair with TBA (MIBK) in the presence of thiosulphate and sulphosalicylate [6] or as the ion-pair with DB-18-crown-6 (o-dichlorobenzene) (pH 3) [7]. Extraction of Mn with bis-2-ethylhexylphosphoric acid (kerosene) has been reported [8]. Other techniques Coprecipitation of Mn(II) with APDC (with Co(II) as carrier) or with 8-quinolinol (with Mg as carrier) has been used [9]. Colloidal Mn(III)/ Mn(IV) oxides coprecipitate with the same yield as Mn(II) [9]. On-line electrodeposition of Mn (both anodic and cathodic) was found to be successful [10]. On-line sorption on the immobilized 8-hydroxyquinoline has been reported [11].

35.2 DETERMINATION TECHNIQUES Spectrophotometry Spectrophotometric determination of Mn has been reviewed [12]. 2Oxidation of Mn(II) to MnO4 by powerful oxidants (S2O , Ce(IV), + BiOs, IO4) in acidic solutions, often in the presence of Ag or Co2 + as catalysts, forms the basis of a poorly sensitive ( = 2.4x103 at 528 nm) but very selective method, especially if derivative spectra processing is used [13]. A higher sensitivity ( = 1.1x10 4 at 455 nm) is offered by the formaldoxime method [14,15] which gives appreciable selectivity in the presence of cyanide. The catalytic effect of Mn on the oxidation of some basic dyes [16-18], succimide dioxime [19] or 7,7,8,8-tetracyanoquinodimethane [11] (chemiluminescence) has been employed for the determination of Mn in various matrices, and also in FI mode. Flame atomic absorptionspectrometry Flame AAS offers a sensitivity of ca 0.05 Rg ml-l in the recommended air-C 2H 2, oxidizing (lean, blue) flame at the most sensitive 279.5 nm line [3,5,8]. The Mn signal is depressed by Si which can be overcome by the addition of CaCl 2. Large excess of Fe (>10 g 1-1) increases the Mn absorption. Water-organic emulsions diluted with kerosene can be analyzed [8]. 510

Graphitefurnace atomic absorptionspectrometry

Graphite furnace AAS offers a DL of ca 0.01 ng ml-l (characteristic mass 2 pg) at the 279.5 nm line using a pyrocoated tube and platform atomization with Mg(NO 3) 2 as matrix modifier. The less sensitive 403.1 nm line was used in the case of samples with elevated Mn contents [20,21]. Loss of Mn prior to atomization is the basic problem. Vaporization of Mn in a GF has been studied by a radiotracer (56Mn) method [22]. Detailed studies of interferences of inorganic acids and salts [23] with particular emphasis on chloride has been reported [24]. The addition of EDTA helps to eliminate negative interference by Ca and Mg [25]. Without a modifier a pyrolysis temperature of 1100 C can be used for Mn. To increase this temperature to 1400-1500 C various modifiers including HNO 3 [20,26,27], Ca2+-H3PO 4-HNO 3 [23], Mg(NO3 ) 2 (NH 4)2 HPO4 [28,29], NaOH [30], ascorbic acid [24], thiourea [31], Pt [32] and Pd-Mg [33] have been proposed. The Ni(NO 3) 2 modifier was recommended to alleviate the interference from the NaNO3 -H 3BO3 medium on fusion [34]. Slurry GF AAS analysis of single cell proteins [21] and other biological materials [20,29] has been developed. Atomic emission spectrometry

ICP AES offers a DL of ca 1 ng ml-' at the most sensitive 257.61 nm, 259.37 nm and 260.57 nm lines. The two latter lines are interfered with by high Fe concentrations. Mineral acids suppress the Mn signal and need to be compensated for, e.g. by real-time internal standardization with Sc [35]. Other sources for AES included a magnetically altered thin-film plasma [36] and a dual cathode discharge lamp [37]. Other techniques

Manganese is readily determined by XRF, usually in a multielement array [9]. Laser-excited ETA AFS offers an ADL of 0.1 pg which is contamination controlled to the 1 pg level [29]. Neutron activation of Mn yields

56

Mn (t1 2 = 2.56 h, E = 0.845 MeV) of which y-counting forms the

basis of INAA. Iron interferes. Manganese has only one stable isotope 55Mn which can be determined by ICP MS, usually in a multielement array. 35.3 ANALYSIS OF REAL SAMPLES Manganese is often determined in many samples in a multielement array (cf. Part II). The concentrations of Mn in geological samples are 511

easily accessed by FAAS, especially after extraction of an Mn complex into MIBK [6] or spectrophotometry [13]. Environmentaland geological samples

Occupational exposure for Mn in air was estimated by near-real-time electrostatic precipitation GF AAS with a DL of 0.05 ng/m 3 [38]. The concentration of Mn in environmental waters is estimated to be 10-100 ng 1-1 [1]. Shipboard analyses by an FI chemiluminescence system [11] or using a photometric submersible analyzer for in-situ analysis have been reported [39]. Albeit direct GF AAS of sea [30,32] and lake [26] water is apparently feasible, preconcentration of Mn commonly precedes water analyses (cf Table 35.1). Sediment cores should be handled in an inert atmosphere to avoid oxidation, e.g. Fe(II) present may oxidize to Fe(III) and precipitate as Fe(OH)3 carrying trace Mn by coprecipitation [9]. Clinical materials

Contamination hazard is pertinent. Any labware as well as the environment itself are potential sources of Mn. Class-100 clean room facilities are required [2,29,42,43]. Plastic needles and cannulas cleaned by sequential leaching with HCl (1+1) and HNO3 (1+1) [2] should be used for blood sampling. The first few millilitres of sample should be discarded [44]. Attention must be paid to the possible presence of Mn in the anticoagulants used. Problems encountered during handling of serum samples have been discussed [45]. For storage at 5°C up to 7 d no significant changes were noted. For medium- and long-term storage freezing of the samples at -22°C is advised [46]. The normal range of serum Mn is 0.4-1.0 ng ml-l [47]. In blood, Mn is concentrated in erythrocytes [47-49] being bound to -globulin or transferrin [47]. Determination of Mn has been reviewed [46] with special emphasis on GF AAS [44,50,51] which is definitely the most widely used technique. Direct analysis of serum on 1+1 dilution with Triton X-100 using a pyrolytically coated tube with platform atomization and Zeeman background correction is recommended [25,43,47]. Matrix modifiers, e.g. HNO 3 [21] or Mg(NO3) 2 [48], were used for blood analysis. Deproteinization with HNO3 was used to avoid problems with the carbonaceous residues [42] but in the case of blood it may cause losses since Mn is bound to haem which coprecipitates [52]. Low temperature ashing [27] or digestion with HNO3 -H 20 2 [48] are better choices for whole blood. Calibration by standard additions is generally indispensable despite 512

TABLE 35.1 Methods for the determination of Mn in water

Water sample (amount)

Separation and/or preconcentration

Determin. technique

DL (ng/l)

Ref.

Tap (5 ml), mineral (0.25 ml)

on-line electrodeposition

GF AAS

10 -30 a

10

Sea (0.35 1)

extraction with APDC/DDTC GF AAS (Freon TF); back-extraction (HNO3 )

5

2

Sea

on-line sorption on immobilized 8-hydroxyquinoline

CL

5

11

Tap, river (0.3 1)

coprecipitation as 8-hydroxyquinoline complex with Mg carrier

GF AAS

14

40

Ground, river, lake, extraction with DDTC (DIBK) sea, tap (0.11), waste (1 ml)

LEI

Natural (10 ml)

extraction with TTA and DB-18crown-6 (o-dichlorobenzene),

0.1

41

FAAS

n.g.

7

XRF

50

9

back-extraction (H20)

Marine sediment pore (0.1 ml)

UV irradiation, coprecipitation as APDC complex with Co carrier

a Absolute detection limit, in pg.

the use of the Zeeman correction [25]. A protocol for the accuracy evaluation for the determination of Mn in blood by AAS has been developed [53]. Radiochemical NAA is an excellent method to determine

Mn in plasma and serum as after completion of activation it becomes totally insensitive to external Mn addition [45]. For the determination of Mn in urine, use of the L'vov platform and the Mg(NO3) 2 matrix modifier ensure the best performance [46]. For hair samples sequential washings with non-ionic detergent (e.g. Triton X-100), water, ethanol and again with water have been recommended [44]. Plant and animal tissues Plants may contain 1-700 ppm of Mn and can be analyzed on digestion by GF AAS [29] or on extraction preconcentration by FAAS [3,5]. Direct Mn determination in biosamples by ETA AAS using an Mo 513

tube atomizer with thiourea matrix modifier offered an ADL of 0.9 pg [31] Homogenization of tissue samples with water followed by HCl leaching was used for the determination of Mn in rat liver by FAAS [54]. Speciation Methylcyclopentadienylmanganese tricarbonyl, pentamethylcyclopentadienylmanganese tricarbonyl and cyclopentadienylmanganese tricarbonyl (cymanterene) were determined in gasoline by packed column GC with FPD with a DL of 0.6 [t g g-1 [55]. Cymantrene, methylcyclopentadienylmanganese tricarbonyl, hydroxycyclopentadiene-manganese tricarbonyl and carboxycyclopentadienemanganese tricarbonyl in rat urine were separated by reverse-phase HPLC and gradient eluted with a Tris buffer to ICP MS [56].

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