Arsenic aqueous solutions

Figure 6. Adsorption isotherms for arsenate aqueous solutions with Ag- and Pb-clinoptilolites and chromate aqueous solutions with Ag- and Pb-clinoptilolites, Tconst.= 23°C (downwards from the right)...

In aqueous solution arsenic(lll) oxide is a reducing agent being oxidised to arsenate(V) by halogens, chlorate(I), nitric acid and even iron(III) chloride. 

Both chloramine-T and dichloramine-T have marked antiseptic properties, chloramine-T being most frequently used because of its solubility in water. Aqueous solutions of chloramine-T can be used either for external application, or for internal application to the mouth, throat, etc, as chloramine-T in moderate quantities is non-toxic its aqueous solution can also be effectively used when the skin has come in contact with many of the vesicant liquid poison-gases, as the latter are frequently organic sulphur or arsenic derivatives which combine with or are oxidised by chloramine-T and are thus rendered harmless. 

In metallic form, barium is very reactive, reacting readily with water to release hydrogen. In aqueous solution it is present as an ion with a +2 charge. Barium acetate, chloride, hydroxide, and nitrate are water-soluble, whereas barium arsenate, chromate, duoride, oxalate, and sulfate are not. Most water-insoluble barium salts dissolve in dilute acids barium sulfate, however, requkes strong sulfuric acid. 

Brasses are susceptible to dezincification in aqueous solutions when they contain >15 wt% zinc. Stress corrosion cracking susceptibiUty is also significant above 15 wt % zinc. Over the years, other elements have been added to the Cu—Zn base alloys to improve corrosion resistance. For example, a small addition of arsenic or phosphoms helps prevent dezincification to make brasses more usefiil in tubing appHcations. 

A flow-injection system with electrochemical hydride generation and atomic absorption detection for the determination of arsenic is described. This technique has been developed in order to avoid the use sodium tetrahydroborate, which is capable of introducing contamination. The sodium tetrahydroborate (NaBH ) - acid reduction technique has been widely used for hydride generation (HG) in atomic spectrometric analyses. However, this technique has certain disadvantages. The NaBH is capable of introducing contamination, is expensive and the aqueous solution is unstable and has to be prepared freshly each working day. In addition, the process is sensitive to interferences from coexisting ions. 

Compounds of Tl have many similarities to those of the alkali metals TIOH is very soluble and is a strong base TI2CO3 is also soluble and resembles the corresponding Na and K compounds Tl forms colourless, well-crystallized salts of many oxoacids, and these tend to be anhydrous like those of the similarly sized Rb and Cs Tl salts of weak acids have a basic reaction in aqueous solution as a result of hydrolysis Tl forms polysulfldes (e.g. TI2S3) and polyiodides, etc. In other respects Tl resembles the more highly polarizing ion Ag+, e.g. in the colour and insolubility of its chromate, sulfide, arsenate and halides (except F), though it does not form ammine complexes in aqueous solution and its azide is not explosive. 

Discussion. Arsenates in solution are precipitated as silver arsenate, Ag3 As04, by the addition of neutral silver nitrate solution the solution must be neutral, or if slightly acid, an excess of sodium acetate must be present to reduce the acidity if strongly acid, most of the acid should be neutralised by aqueous sodium hydroxide. The silver arsenate is dissolved in dilute nitric acid, and the silver titrated with standard thiocyanate solution. The silver arsenate has nearly six times the weight of the arsenic, hence quite small amounts of arsenic may be determined by this procedure. 

As an alternative, a reversible indicator may be employed, either (a) 1-naphthoflavone (0.5% solution in ethanol, which gives an orange-coloured solution at the end-point), or (b) p-ethoxychrysoidine (0.1% aqueous solution, colour change pink to pale yellow). Under these conditions, the measured 25 mL portion of the arsenic solution is treated with 10 mL of 10 per cent potassium bromide solution, 6 mL of concentrated hydrochloric acid, 10 mL of water and either 0.5 mL of indicator (a) or two drops of indicator (b). 

Reactions in aqueous solution can have complicated kinetics. An example Is the reaction between arsenic acid and iodide ions ... 

The most general method for the simultaneous analysis of oxyanions by gas chromatography is the formation of trimethylsilyl derivatives. Trimethylsilyl derivatives of silicate, carbonate, oxalate, borate, phosphite, phosphate, orthophosphate, arsenite, arsenate, sulfate and vanadate, usually as their ammonium salts, are readily prepared by reaction with BSTFA-TMCS (99 1). Fluoride can be derivatized in aqueous solution with triethylchlorosilane and the triethylfluorosilane formed extracted into an immiscible organic solvent for analysis by gas chromatography [685). 

Ashokkumar et al. [128] have reported the sonochemical conversion of As(III) to As(V) in an aqueous solution as a process for the removal of arsenic in contaminated water. A very interesting pH independent sonochemical conversion of arsenic (III) to arsenic (V), besides its precipitation as sulphide in aqueous solutions, in the pH range 5 to 9, and subsequent adsorption on coagulants such as Fe(OH)3 and Al(OH)3 has also been reported [129] from this laboratory. 

Farquhar ML, Chamock JM, Livens FR, Vaughun DJ (2002) Mechanisms of arsenic uptake from aqueous solution by interaction with goethite, lepidocrocite, mackinawite and pyrite as X-ray absorption spectroscopy study. Environ Sci Tecnol 36 1757-1762... 

A 0.5-gram mass of either the organo-treated or inorganic cation exchanged zeolite and 50 mL of 10 mM/L arsenate or chromate aqueous solutions were placed into Erlenmeyer flasks and mechanically shaken in reciprocating mode to attain equilibrium. Different equilibrium periods for individual zeolite modifications and both aqueous oxyanions species have been established. The adsorption isotherm experiments were conducted using above mass/ volume ratio of samples with an initial metal concentrations ranged from 0.5 to 100 mM/L at laboratory temperature. The... 

X-ray diffraction measurements indicated that the zeolite rock consisted primarily of clinoptilolite (60-70%), volcanic glass (10%), feldspar (10%) and minor quantities of cristobalite, quartz and plagioclase (20%). Fig. 1 represents the XRD pattern of ODA-Clinoptilolite-rich tuff used for arsenate or chromate removal from aqueous solutions. 

Figure 5. Adsorption isotherms for arsenate (upper line) and chromate (lower line) aqueous solutions with ODA-clinoptilolite, Tconst.= 23°C...

Some differences in arsenate and chromate adsorption on ODA-clinoptilolite and Pb-(Ag-linoptilolites) as well were recorded (Figs. 5 and 6). ODA-clinoptilolite exhibited more efficient arsenate and chromate removal from aqueous solutions than the inorganically exchanged modifications. However, silver exchanged clinoptilolite revealed higher capacity values for both oxyanions uptake than lead exchanged clinoptilolite did. This phenomenon supports preferred silver treated clinoptilolite utilization for specific water purification process even on the base of environmental acceptability. 

By batch description trials Organo- and inorganically- modified zealot was subjected up to 24 hr in distilled water, tap water and 2% Nalco aqueous solutions in laboratory shaken machine to demonstrate how strongly the examined oxyanions are bound on the modified zeolite. While only slightly chromate desorption in the maximum extent about 20 mg/L was observed, approximately one order higher arsenate desorption was found, corresponding to increased ionic strength in waters. However, in both cases ODA-clinoptilolite exhibited the lowest desorption characteristics. Here, the... 

Figure 8. Regeneration of ODA-clinoptilolite column loaded with arsenate by means of 2% NaCl aqueous solution and breakthrough curves for ODA-clinoptilolite in arsenate solution of co = 25 mg/L repeated cycle after regeneration, first cycle breakthrough curve on Pb-clinoptilolite (from the left).

Inorganic arsenates are found to be extremely toxic in nature and hence careful control is maintained by the addition of magnesium ammonio-sulphate solution to an aqueous solution of the sample, thereby producing an instant white precipitate. 

G. J. Stridh. On the Slow Oxidation of Arsenic(llT) Oxide to ArsenidV) Oxide in Aqueous Solution by Elemental Oxygen and its Effect on the Precise Determination of Energies of Combustion of Organic Chloro- and Bromo-compounds. J. Chem. Thermodynamics 1975, 7, 703-705. 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

To obtain MSMA, the DSMA solution is partially acidified with sulfuric acid and the resulting solution concentrated by evaporation. As the aqueous solution is being concentrated, a mixture of sodium sulfate and sodium chloride precipitates out (about 0.5 kg per 100 kg of active ingredient). These salts are a troublesome disposal problem because they are contaminated with arsenic. The salts are removed by centrifugation, washed in a multistage, countercurrent washing cycle, and then disposed of in an approved landfill. 

Cleavage reactions are best carried out in aqueous solution. In aprotic solvents, electrogenerated bases lead to the conversion of onium salts to the ylids which are not reducible [49]. The sequence of reactions shown in Scheme 5.2 shows that the bond cleavage process for phosphonium salts proceeds with retention of configuration around the phosphorus atom [50]. Retention of configuration at arsenic is also observed [51]. This electrochemical process is a route to asymmetric trisub-stituted phosphorus and arsenic centres. 

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