Telluric iron

Against a white surface this colour reaction may be observed even with a 0-00001 per cent, concentration of selenious acid. The reaction is more sensitive with sulphuric acid above 75 per cent, concentration than with dilute acid. The test is not applicable if the sulphuric acid contains iron. Tellurous acid retards the reaction.1... 

Tellurium oxide, Te02, is found in rhombic crystals as the mineral tellurite, whilst ferrotellurite is probably an iron tellurate, emmonsite and dividensite being similar compounds. Other naturally occurring tellurates are montanite, a basic bismuth tellurate, and magmlite, a... 

Tellurium usually occurs in nature as metallic tellurides. This toxic element is used in the production of iron, steel, and other alloys. Tellurium can have oxidation states of -2, +2, +4, and +6. Tellurites tend to be more toxic than tellurates [12]. 

Metallic potassium and sodium are explosively converted into the hydroxides when brought into contact with concentrated solutions of hydrogen peroxide many of the heavier metals such as zinc and iron, and especially aluminium,3 are readily changed into their respective hydroxides, whilst chromium, arsenic, and molybdenum are oxidised respectively to chromic, arsenic, and molybdic acids. Colloidal tellurium yields telluric acid with very dilute solutions of peroxide4 the crystalline modification reacts slowly with 60 per cent, peroxide at 100° C. 

Selenious acid oxidizes pyrrole to a blue dyestuff of unknown composition ( pyrrole blue ). Iron salts accelerate the reaction when it is carried out in phosphoric acid solution. Selenic, tellurous and telluric acids do not react under the conditions given below the test therefore provides a method of distinguishing selenites and selenates. 

Selenites and selenates are quantitatively reduced to elementary selenium when their acid solutions are warmed with ferrous sulfate. In contrast, tellurite and tellurate solutions remain unaltered because the redox potential of iron is not sufhcient to accomplish the reaction Fe i + Teiv(vi) Fe -f Te°. However, the reducing power of Fe+ ions can be raised considerably if the Fe+ ions are removed as soon as they are produced. Phosphoric acid is excellent for this purpose it immediately converts the Fe+ ions into [Fe(P04)2] " ions. Fluorides act analogously through the formation of [FeF,]- ions. Consequently, when tellurites or tellurates are warmed with a ferrous sulfate-phosphoric acid mixture there is complete reduction to free tellurium. 

Concentration. Inhibition of iron corrosion in distilled water occurs only when the anion concentration exceeds a critical value. At concentrations below the critical value, inhibitive anions may act aggressively and stimulate breakdown of the oxide films. Effective inhibitive anions have low critical concentrations for inhibition. A number of anions have been classified in order of their inhibitive power toward steel, judged from their critical inhibitive concentrations. The order of decreasing inhibitive efficiency is azide, ferri-cyanide, nitrite, chromate, benzoate, ferrocyanide, phosphate, tellurate, hydroxide, carbonate, chlorate, o-chlorbenzoate, bicarbonates fluoride, nitrate, and formate. 

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