A thick-film graphite-containing electrode modified with formazan for determining manganese in natural and drinking waters by stripping voltammetry

Abstract

Manganese is an essential element capable of activating many enzymes involved in the metabolism of humans, animals, and plants. Manganese enters living organisms from the environment, in which it is present mainly as hydrated Mn 2+ . The latter can be oxidized under aerobic conditions to give Mn 3+ or Mn 4+ , which form soluble and insoluble compounds. The concentration of manganese in natural water varies from 0.0001 to 1 mg/L and may attain 50 mg/L in contaminated water [1]. The maximum permissible concentration (MPCs) for manganese is 0.01 mg/L in fresh water and 0.05 mg/L in sea water [2].Spectrophotometry and atomic-absorption spectrometry are used for determining trace manganese(II) in environmental samples. These methods require the preconcentration of manganese, which complicates and prolongs analysis. Stripping voltammetry (SVA) is the most acceptable method for determining trace amounts of metals in natural waters, especially in on-site and online modes. Two versions of this method, one with an anodic sweep of potential (ASVA) [3][4][5], the other with an cathodic sweep of potential (CSVA) [5][6][7][8][9][10][11][12] are used for determining manganese(II). In the first version of SVA, stationary mercury and mercury-film electrodes are used, whereas the second version of SVA employs both mercury and carbon-containing electrodes (impregnated graphite, glassy-carbon, composite carbon-containing, or modified with polymeric film). The determination of manganese(II) by ASVA is associated with a number of problems that restrict its wide use. Among these are the high negative accumulation potential of manganese (-1.7 V), which lies in the region of hydrogen liberation; the low solubility of manganese in mercury; and the formation of intermetallic compounds at mercury electrodes. On the whole, these problems may result in random and systematic errors and deteriorate the sensitivity of analysis. CSVA offers greater promise, because it can be performed without mercury and eliminates the effect of dissolved oxygen and elements forming intermetallic compounds with manganese. Ligands forming complexes with manganese(II) are introduced into the supporting electrolyte or into the indicator electrode to reduce the detection limit for manganese and enhance the reversibility of its Red/Ox system [13,14]. Diphenylcarbazone has been found to be very promising. The detection limit for manganese(II) found with the use of this electrode modified with diphenylcarbazone was 5 × 10 − 9 M at an accumulation time of 600 s [14].In our opinion, continuation of the work devoted to the further investigation and use of a wider spectrum of formazans in the mercury-free voltammetric determination of metals is of particular scientific and practical interest. This work is dedicated to the development of a procedure for determining manganese(II) by CSVA using new mercury-free electrodes modified with formazans with the aim of lowering the detection limit for manganese and enhancing the rapidity of analysis. EXPERIMENTALThe reagents used were...

Formazans and their metal complexes

“…50 The determination limit of manganese(II) with this electrode was 0.04 mg litre 71 ; the preconcentration time was 60 s. The dependence of the analytical signal of Mn(II) on its concentration in solution was linear in the range from 0.1 to 30 mg litre 71 . This procedure was used for the analysis of natural and drinking waters for the detection of manganese(II) ions.…”

Voltammetric Determination of Manganese on Organo – Modified Electrodes

“…Mercury-free thick-film graphite electrode modified with formazan was proposed to determine Mn 2+ by cathodic stripping voltammetry. The detection limit of Mn 2+ with that electrode was 0.04 g/l 27 . On the basis of formazan derivative a mercury-free voltammetric sensor, used for the analysis of drinking and natural waters, was created 28 .…”

Preconcentration Versions and New Sensors in Stripping Voltammetry

“…In determining Cu 2+ , Pb 2+ , Cd 2+ , and Zn 2+ , 1-(o-chlorophenyl)-3-phenyl-5-(6-methyl-4-hydroxy-pyrimidinyl-2)formazan [19,20] was used as a modifier of TGEs. TGEs modified with 2,3-diaminonaphthalene [21], nitroso R salt [22], pyrocatechol [23], 1,10-phenanthroline [24], and 1-(o-tolyl)-3-methyl-5-(1-benzylbenzimidazolyl-2)formazan [25] were used for determining Se 4+ , Co 2+ , Fe 3+ , Fe 2+ , and Mn 2+ , respectively. The A-Ch-E sequence of electrode reactions was the most typical of the preliminarily modified TGEs, and the Ch-A-E sequence, of the in situ modified electrodes.…”