Iron Mask Formation

Harare bell Iron Mask Formation Rhyolite Z1M 92/29 36... 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

Colorimetric Methods have frequently been suggested,1 but of these, that originated by Skey and studied by several others 2 appears to be the most useful. It hinges on the fact that potassium thiocyanate yields a blue colour with solutions of cobalt salts, due to the formation of cobalt thiocyanate. On adding alcohol and ether to the liquid, a blue layer is produced. This is destroyed by mercuric chloride, sodium acetate, or sodium thiosulphate, and is masked by the presence of iron salts in consequence of the intense red colour of ferric thiocyanate consequently these substances should not be present when the colorimetric test is applied. 

Oxyacids, like citric or tartaric acids, and polyols, like saccharose are also used, mainly as masking agents, in qualitative analysis. The action of some specific reagents, like oc-a -bipyridyl for iron(II) and dimethylglyoxime for nickel(II), is also based on the formation of chelate complexes. In quantitative analysis the formation of chelates is frequently utilized (complexometric titrations). ... 

A solution of 0.001 Af iron(lll) is masked by the addition of 0.05 Af EDTA. (a) Calculate the masking index at pH 2, assuming formation of hydroxy complexes is negligible, (b) Calculate the masking index for aluminum(III) under the same conditions. 

Aluminum is extracted from sea water into chloroform after complexation with pyrocatechol violet and the formation of an ion associate of the aluminum complex with zephiramine (tetradecyldimethylbenzylammonium chloride). With 5 ml of chloroform as much as 94.7% of the aluminum was separated from 110 ml of sea water, i.e. a 20-fold aluminum concentration could easily be achieved. Several ions, such as manganese, iron(II), iron(III), cobalt, nickel, copper, zinc, cadmium, lead, and uranyl also react with pyrocatechol violet and to some extent are extracted together with aluminum. However, the interferences of these ions and other metal ions present in sea water could be eliminated by masking with sodium diethyl-dithiocarbamate and 8-hydroxyquinaldine. In presence of these agents all the above metal ions except aluminum were extracted into chloroform51). 

Large quantities of chloride should be avoided since they result in the formation of mixed chloride-bromide complexes, which are less intensely coloured. Iron(III) interferes in the determination of Au by forming an orange bromide complex, but it can be masked with phosphate or fluoride. In larger quantities, Cu, Ni, and Cr(III) also interfere. 

Figure 7.12 shows a chromatogram of the same sample in which EDTA is added to complex the iron(lll). The additional peak is from an iron(ll) impurity in the iron(II) solution used. Work thus far indicated that any metal ion that has an EDTA formation constant of about 10 - or higher should be masked effectively by adding EDTA to the sample. 

Eriochrome Cyanine R (C. I. Mordant Blue 3, Solochrome Cyanine R), a triphenylmethane dye, forms, in weakly acid solution, a violet lake with aluminum ions which is the basis of a spot test for the metal, One drop of the acidified sample is placed on a test plate and mixed with one drop each of Na acetate-acetic acid buffer solution (pH 5) and 1 % aqueous Eriochrome Cyanine R. The pH should be 4-6. One drop of NaHSOg solution is added. The formation of a violet color which is stable for 1 minute indicates the presence of aluminum. Iron i salts must first be reduced with ascorbic acid. Ti, Zr and Th ions form colored precipitates which must be centrifuged off. Cu must be masked with thioglycolic acid. 

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