Interference sulphide

In electrochemical cells sample oxidation produces an electric current proportional to the concentration of test substance. Sometimes interferences by other contaminants can be problematic and in general the method is poorer than IR. Portable and static instruments based on this method are available for specific chemicals, e.g. carbon monoxide, chlorine, hydrogen sulphide. 

Traces of many metals interfere in the determination of calcium and magnesium using solochrome black indicator, e.g. Co, Ni, Cu, Zn, Hg, and Mn. Their interference can be overcome by the addition of a little hydroxylammonium chloride (which reduces some of the metals to their lower oxidation states), or also of sodium cyanide or potassium cyanide which form very stable cyanide complexes ( masking ). Iron may be rendered harmless by the addition of a little sodium sulphide. 

Discussion. The theory of the titration of cyanides with silver nitrate solution has been given in Section 10.44. All silver salts except the sulphide are readily soluble in excess of a solution of an alkali cyanide, hence chloride, bromide, and iodide do not interfere. The only difficulty in obtaining a sharp end point lies in the fact that silver cyanide is often precipitated in a curdy form which does not readily re-dissolve, and, moreover, the end point is not easy to detect with accuracy. 

Discussion. Minute amounts of beryllium may be readily determined spectrophotometrically by reaction under alkaline conditions with 4-nitrobenzeneazo-orcinol. The reagent is yellow in a basic medium in the presence of beryllium the colour changes to reddish-brown. The zone of optimum alkalinity is rather critical and narrow buffering with boric acid increases the reproducibility. Aluminium, up to about 240 mg per 25 mL, has little influence provided an excess of 1 mole of sodium hydroxide is added for each mole of aluminium present. Other elements which might interfere are removed by preliminary treatment with sodium hydroxide solution, but the possible co-precipitation of beryllium must be considered. Zinc interferes very slightly but can be removed by precipitation as sulphide. Copper interferes seriously, even in such small amounts as are soluble in sodium hydroxide solution. The interference of small amounts of copper, nickel, iron and calcium can be prevented by complexing with EDTA and triethanolamine. 

Further, recommendations by suppliers for avoiding interferences must be followed, e.g., high concentrations of non-alkali metal ions, sulphide and com-plexing agents such as cyanide may be harmful. 

Good results are obtained for electrodes with sulphides of Pb, Cd and Cu(II), but with certain other sulphides the response time is unsatisfactory. Interference occurs in highly acidic solutions (H2S formation) and in alkaline solutions (at pH > 11) other metal ions sometimes disturb determinations with the metal ISE also, anions may cause difficulties, e.g., in a Cu(H) determination at a Cu(H) ISE if Cu2+ and Cl" are simultaneously present in the... 

It is interesting to note that exactly the same interference occurred for both organic and inorganic mercury standards, since methyl mercuric chloride does not directly react with sodium sulphide to form mercuric sulphide. Therefore the interference could not be the result of incomplete digestion of mercuric sulphide or CH3Hg+. 

Sulphur, ozone and hydrogen sulphide were investigated as possible causes for the interference. No interference was observed. When the automated method was used, the interferences which were observed for the standards and blank spiked with lOOmg sulphide L-1, occurred when an excess of dichromate did not exist in the solutions. 

A method [62] has been described for the determination of down to 2.5ppb alkylmercury compounds and inorganic mercury in river sediments. This method uses steam distillation to separate methylmercury in the distillate and inorganic mercury in the residue. The methylmercury is then determined by flameless atomic absorption spectrophotometry and the inorganic mercury by the same technique after wet digestion with nitric acid and potassium permanganate [63]. These workers considered the possible interference effects of clay, humic acids, and sulphides, all possible components of river sediment samples on the determination of alkylmercury compounds and inorganic mercury. 

An ideal ISE would exhibit a specific response to a certain ion J and the effect of interferents would be excluded. Except for the silver sulphide electrode, which is specific for sulphide or silver ions, no ion-selective electrode has this property. The others exhibit selectivity only for a particular ion with respect to the others. The selective behaviour of an ISE follows from (3.1.7). If the activity of the interferent is sufficiently low, i.e. if... 

The solution pH affects the function of all ISEs, either through interference of hydroxonium or hydroxide ions in the membrane reaction (for example the interference of OH" with the function of fluoride ISEs at pH values greater than about 5.5), or through chemical interference in solution (for example, formation of poorly dissociated HF and HF in acid solutions, which are not sensed by a fluoride ISE or formation of HS and H2S with decreasing pH, which are not sensed by a sulphide ISE), or both. Moreover, the pH value can affect the equiUbria of interferents in the solution. The pH must thus be adjusted with all these effects in mind. Fortunately, it is usually sufficient to maintain the pH within a certain region rather than at a single precise value. 

The properties of this electrode approach those of an ideal specific electrode. It has an anionic response almost exclusively for sulphide ions in a broad activity interval (up to 10 molar concentration in hydrogen sulphide solutions). Only the cyanide ion interferes here at high concentrations. This electrode has a cation response for silver ions in a wide activity range (in Ag(I) complexes up to 10 m Ag [325]), where only Hg interferes [417]. 

The Ag2 S ISE has Nemstian response dE/d log a( = 0.0296 V in the sulphide concentration range 10" to 10" M and silver ions from 10 to 10 M if the solutions are prepared from pure salts, as a further concentration decrease is prevented by adsorption on the glass (see p. 76 and [87, 163]). After prolonged use, the limit of the Nemstian behaviour shifts to about 10" m [130] as a result of formation of mixed potentials on accumulation of metallic silver in the membrane surface. An analogous deterioration in the membrane function in the presence of iodine results from surface oxidation [23]. Cyanide interferes only at large concentrations the equilibrium constant of the reaction... 

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-... 

The various kinds of alkali, cf commerce usually contain a. greater or less proportion of chlorides and sulphates. The presence of these dees not interfere In the slightest degree with the above methods of estimation. In some cases, however, sulphides, sulphites, and hyposulphites aro also present, and these, neutralizing a certain quantity of the tost acjd, Tender the determinations more or less inaccurate. The first of these salts evolves sulphide of hydrogen, the second sulphurous acid, and the last hyposulphurous acid, which is immediately decomposed into sulphurous ecitl... 

A sensitive colour test for sulphite ions consists in adding, drop by drop, a 0-01 per cent, solution of Fast Blue R crystals, shaking after each addition, until the violet coloration disappears and a yellow solution is produced the test is sensitive to one part of sulphurous acid in about 175,000. Thiosulphates and polythionates do not interfere, but sulphides and hydroxides must be absent.1... 

Evolution of nitrogen from sodium azide-iodine mixture is brought about by traces of thiocyanate, and the latter may readily be detected by this means in the presence of most inorganic oxy-acids and the common organic acids.2 Sulphides and thiosulphates interfere (see pp. 65, 205) and must previously be removed by means of mercuric "hloride. 

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. 

Mix together 5 drops of a 1.5 per cent solution of p-nitrobenzaldehyde in 2-methoxyethanol, 5 drops of a 1.7 per cent solution of o-dinitrobenzene in 2-methoxyethanol and 2 drops of a 2.0 per cent aqueous solution of sodium hydroxide. To this mixture add 1 drop of fusion solution. A deep purple coloration is positive for cyanide ions, a yellow or tan colour is negative. Neither halide ions nor sulphide ions interfere. 

The main disadvantage of mercury sensors based on bare gold layers is their poor selectivity. This is illustrated in Fig. 12.6 an incubation at 100% humidity (Fig. 12.6a), with saturated vapour of sulphuric acid (Fig. 12.6), volatile sulphides or thiols (10 pg/1 of 1-butanethiol vapour, Fig. 12.6c), or halogens (10 pg/1 of iodine vapour, Fig. 12.6d), results in conductivity changes of the same magnitude as an incubation with 10-20 ng/1 of mercury vapour. This interference with widely spread substances is a serious problem in applications of such sensors for real probes and makes necessary a pre-treatment of probes. 

Direct binding of the ion-selective component to the electrode has also been studied. For example, graphite combined with an antimony compound has been screen printed and the resultant electrodes shown to give selective responses to sulphide ion in simulated wastewater samples (0.01-0.7 mM sulphide) with high stability to repeated testing and low interference from other compounds [31]. 

Lui et al. [109] have described an automated system for determination of total and labile cyanide in water samples. The stable metal-cyanide complexes such as Fe(CN)63 are photo-dissociated in an acidic medium with an on-line Pyrex glass reaction coil irradiated by an intense mercury lamp. The released cyanide is separated from most interferences in the sample matrix and is collected in a dilute sodium hydroxide solution by gas diffusion using a hydrophobic porous membrane separator. The cyanide ion is then separated from remaining interferences such as sulphide by ion exchange chromatography and is detected by an amperometric detector. The characteristics of the automated system were studied with solutions of free cyanide and metal-cyanide complexes. The results of cyanide determination for a number of wastewater samples obtained with this method were compared with those obtained with the standard method. The sample throughput of the system is eight samples per hour and the detection limit for total cyanide is 0.1 pg L 1. 

Funazo et al. [812] have described a method for the determination of cyanide in water in which the cyanide ion is converted into benzonitrile by reaction with aniline, sodium nitrite and cupric sulphate. The benzonitrile is extracted into chloroform and determined by gas chromatography with a flame ionisation detector. The detection limit for potassium cyanide is 3 mg L 1. Lead, zinc and sulphide ion interfere at lOOmg L 1 but not at lOmgL-1. 

The second sample contains ozone and nitrogen peroxide and the difference between the nitrite contents of the two bottles is equivalent to the ozone present. The presence in the air of ammonia, sulphur dioxide, and hydrogen sulphide does not interfere with the estimation of ozone and nitrogen peroxide by this method as all three gases are completely absorbed during passage through the chromic anhydride tube. 

Notes. The method removes the sulphide interference from the cold-vapour mercury signal. Concentrations of sulphide as high as 20mgr1 S2-(as Na2S) do not interfere with the recovery of inorganic mercury added to distilled water. However, the oxidation technique suffers from chloride interference. If chloride is present in the sample it utilises oxidant and is oxidised to chlorine which interferes with the cold-vapour detection by absorbing radiation at the same wavelength as mercury. 

Acid dissolution is a particularly favourable approach for carbonates and sulphides, where the matrix anion will be removed during the evolution of carbon dioxide or hydrogen sulphide, and for salts of organic acids, where the anion seldom causes interference problems. Conversely, sulphates can cause problems during flame atomisation and chlorides during furnace atomisation ways of dealing with such problems are discussed below. 

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