Manganese , in oxidation

Figure 3. The hioavailability of zinc (Zurinm/Zuscdiment) to Macoma balthica as related to the ratio of hijdroxylamine—soluble iron/ammonium acetate-soluble manganese in oxidized sediments of San Francisco Bay (r = 0.77).

For an element exhibiting several different oxidation states in aqueous solution, we must consider a number of different half-reactions in order to obtain a clear picture of its solution chemistry. Consider manganese as an example aqueous solution species may contain manganese in oxidation states ranging from Mn(II) to Mn(VII), and equations 7.42-7.46 give half-reactions for which standard reduction potentials can be determined experimentally. 

Mn " " also activates several oxidases. The role of manganese in oxidation-reduction processes of plants is its most important function, and is related to the valency change between Mn " " and Mn (Kabata-Pendias and Pendias 2001). Because of its high redox potential, manganese is the main metallic activator of enzymes in the citrate cycle and controls oxidation, reduction, and carboxylation reactions in carbohy-... 

A very sensitive test for nitrogen consists of heating the sample in admixture with manganese dioxide or other oxidants such as lead dioxide, red lead oxide (Pbg04), cobalt(m) oxide, nickel(m) oxide, or manganese (in) oxide. Nitrous acid is evolved and can be detected by the Griess reaction . 

Uncombined elements are all given zero oxidation state. Consider (a) manganese in the permanganate ion, MnO there are four... 

In the laboratory, bromine is prepared by oxidation of bromide ion the oxidation is carried out by mixing solid potassium bromide with manganese(IV) oxide and distilling with concentrated sulphuric acid ... 

Iodine is rarely prepared in the laboratory the method used is the oxidation of an iodide by manganese(IV) oxide and sulphuric acid, for example with sodium iodide ... 

We may note (a) the common occurrence of oxidation state +2 where the 4s electrons have been formally lost, (b) the increase in the number of oxidation states from scandium to manganese in the latter element, the oxidation state + 7 corresponds to the formal loss of the and 3d electrons, (c) the sharp decrease in the number of oxidation states after manganese—suggesting that removal of the paired id electrons is less easy (d) the oxidation state 0, occurring for many of the later elements in the series. ... 

Manganese is the third most abundant transition metal, and is widely distributed in the earth s crust. The most important ore is pyrolusite, manganese(IV) oxide. Reduction of this ore by heating with aluminium gives an explosive reaction, and the oxide Mn304 must be used to obtain the metal. The latter is purified by distillation in vacuo just above its melting point (1517 K) the pure metal can also he obtained by electrolysis of aqueous manganese(II) sulphate. 

Manganese(IV) oxide is the only familiar example of this oxidation state. It occurs naturally as pyrolusite, but can be prepared in an anhydrous form by strong heating of manganese(II) nitrate ... 

Manganese(IV) oxide is a dark-brown solid, insoluble in water and dilute acids. Its catalytic decomposition of potassium chlor-ate(V) and hydrogen peroxide has already been mentioned. It dissolves slowly in alkalis to form manganates(lW), but the constitution of these is uncertain. It dissolves in ice-cold concentrated hydrochloric acid forming the complex octahedral hexachloromangan-ate(IV) ion ... 

Potassium permanganate (KMn04) will also oxidize pri mary alcohols to carboxylic acids What is the oxidation state of manganese in KMn04 ... 

Ground-state electronic configuration is ls 2s 2p 3s 3p 3i 4s. Manganese compounds are known to exist in oxidation states ranging from —3 to +7 (Table 2). Both the lower and higher oxidation states are stabilized by complex formation. In its lower valence, manganese resembles its first row neighbors chromium and especially iron ia the Periodic Table. Commercially the most important valances are Mn, Mn ", or Mn ". ... 

The Mn ion is so unstable that it scarcely exists in aqueous solution. In acidic aqueous solution, manganic compounds readily disproportionate to form Mn ions and hydrated manganese(IV) oxide, Mn02 2H20 in basic solution these compounds hydroly2e to hydrous manganese(III) oxide, MnO(OH). Sulfuric acid concentrations of about 400 450 g/L are required to stabilize the noncomplexed Mn ion in aqueous solutions. 

Manganese(III) Oxides. The sesquioxide, Mn202, exists in dimorphic forms. The a-Mri202 exists in nature as the mineral bixbyite. 

Aqueous potassium permanganate solutions are not perfectiy thermodynamically stable at 25°C, because Mn02, not MnO is the thermodynamically stable form of manganese in water. Thus permanganate tends to oxidize water with the evolution of oxygen and the deposition of manganese dioxide, which acts to further catalyze the reaction. 

R. G. Bums and V. M. Bums, Structural Relationships Between Manganese(IW) Oxides in The Manganese Dioxide Symposium, Vol. 1,1. C. Sample Office, 1975. 

---