Sulphide Ions, Removal

Active Site Structure of Rubredoxin There are several non-heme iron-sulphur proteins that are involved in electron transfer. They contain distinct iron-sulphur clusters composed of iron atoms, sulphydryl groups from cysteine residues and inorganic or labile sulphur atoms or sulphide ions. The labile sulphur is readily removed by washing with acid. The cysteine moieties are incorporated within the protein chain and are thus not labile. The simplest type of cluster is bacteria rubredoxin, (Cys-S)4 Fe (often abbreviated FelSO where S stands for inorganic sulphur), and contains only non labile sulphur. It is a bacterial protein of uncertain function with a molecular weight of 6000. The single iron atom is at the centre of a tetrahedron of four cysteine ligands (Fig.). 

It should be kept in mind that many of these decomposition reactions are equilibria. The decomposition of thiourea in the absence of a metal ion will normally be much slower than in the presence of such an ion. The metal ion removes sulphide as metal sulphide—the less soluble the sulphide, the more effective the removal at very low sulphide concentrations. This continuous removal of sulphide shifts the equilibrium to the direction of more sulphide production. The same principle holds for many other anion precursors. 

Sulphur, as sulphide ion, may be detected by predpitation as black lead sulphide with lead acetate solution and acetic add or by the purple colour produced on addition of disodium pentacyanonitrosoferrate(m). Halogens are detected as the characteristic silver halides by the addition of dilute nitric acid and silver nitrate solution. Cyanide and sulphide ions both interfere with this test for halide by forming silver cyanide and silver sulphide precipitates. If nitrogen or sulphur has been detected, therefore, the interfering ions must be removed by boiling the acidified fusion solution as detailed later, before the silver nitrate solution is added to detect the halogen. 

Nitrogen and/or sulphur present. To remove cyanide and sulphide ions, make 2-3 ml of the fusion solution just acidic with dilute nitric acid, and evaporate to half of the original volume in order to expel hydrogen cyanide and/or hydrogen sulphide which may be present (CAUTION). Dilute with an equal volume of water and proceed as in test (1), (2) and (3) above. 

The use of the diagram is illustrated as follows. Assess the feasibility of removing copper from a solution containing 30gL Zn and 2gL Cu at pHO by precipitation with gaseous H2S. If the solution is saturated with H2S at atmospheric pressure, the sulphide ion activity log S at pH 0 will be —21,... 

Figure 4.6 summarizes the total process. Normally the sludge removal is by filtration of the electrolyte although in some cases (e.g. Pb) the sludge collects on the anode which periodically must be removed from the cell and scraped. Before recycle, the electrolyte must be purified and the concentration of metal ion, electrolytes and addition agents adjusted to the correct values. The purification process normally involves cementation or precipitation (by addition of hydroxide or sulphide ion or by change of oxidation state, e.g. Sn(II) -> Sn(IV)4). The exact procedure will depend on the level and type of impurities in the electrolyte. 

Procedure. Dilute 1-2 ml. of gasoline 20 to 50 times with ethanol. Add sulphuric acid so as to make the final concentration 0-025 N, and add 0-05 ml. of a 3 per cent solution of cadmium nitrate (to remove the sulphide ions present). Then dilute the solution to a certain volume with water and ethanol so as to make the final concentration of ethanol 85-90 per cent. Finally, measure the anodic wave at —0-02 V. 

The total sulphur (elemental, xanthogenate, thiocarbonate and sulphidic) can be determined ) by heating the sample at 95-lOO C with about 1 N sodium hydroxide for 30 min in the absence of oxygen. By this treatment all the sulphur present is transformed into sulphides, and the anodic wave of sulphide ions is recorded. Elemental sulphur can be determined if the bound sulphur is removed by treating the fibre at 70°C with 10 per cent sulphuric acid for 30 min. 

The influence of sulphide ion depends on the redox conditions if anaerobic HgS (low solubility) will remove much of the Hg avoiding it from being... 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

Reference 92 describes not a normal CD process, but one closer to the SILAR technique described in Sec. 2.11.1. However, while the SILAR method involves dipping the substrate in a solution of one ion (e.g., sulphide), rinsing to remove all but (ideally) a monolayer of adsorbed ions and then dipping in a solution of the other ion (e.g., Ag ), the present technique omits the intermediate rinsing step. This means that a relatively large amount of solution can remain on the substrate between dips, and layer formation proceeds much more rapidly than for SILAR, albeit with less control. A typical rate was 4 nm/dip cycle. In this case, a visible layer of Ag2S formed after several dips. Since interference colors were ob-... 

Pass the solution through the column at a rate of 5 ml/min, and then wash the column with 50 ml of hot water to remove the mineral acid. Check the completeness of absorption of tbe copper and cadmium ions. For tbis purpose, pour 3-5 ml of the washing water into a test tube and add a solution of ammonium sulphide or hydrogen-sulphide water to it. The formation of a precipitate indicates incomplete absorption, which occurs only if there is an error in performing the operations described above. In this case, the experiment has to be repeated. To do tbis, wash the column with a solution of hydrochloric acid (1 3), next with water until the washing solution has a neutral reaction, and then pass the solution through the column again. 

This type of reaction is particularly interesting when the salt of the weak acid is sparingly soluble. Ferrous sulphide is a salt of the weak acid H2S. It is very insoluble in water, but it passes completely into solution in dilute HC1. The formation of the un-ionized H2S removes S ions and thus allows the ionization of the ferrous sulphide to continue unchecked... 

Zinc sulphide does not precipitate from an acidified solution because the S ion concentration is repressed by the H+ ions of the strong acid, H2S 2H+ + S, and the solubility product of zinc sulphide cannot be reached. Acetate ions, however, remove H+ ions, and, the hydrogen sulphide thus being allowed to ionize to a greater extent, the solubility product of zinc sulphide is exceeded and the white precipitate appears. (See Solubility Product, page 131, and Experiment 22, page 175.)... 

Reducing agents (sulphides, thiosulphates, etc.) interfere since they yield molybdenum blue hexacyanoferrate(II) ions give a red colouration. Arsenates (warming is usually required), arsenites, chromates, oxalates, tartrates, and silicates give a similar reaction with some variation in the colour of the precipitate. All should be removed before applying the test. 

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