Cyanide interference

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... 

Cyanides interfere because of the formation of cyanogen iodide they are therefore removed before the test either by heating with sodium hydrogen carbonate solution or by acidifying and heating ... 

Cyanide interferes with respiration by forming stable compounds with respiratory enzymes notably the oxidized hemoproteins. 

Carbon monoxide and methemoglobin-forming agents interfere with oxygen transport, resulting in cellular hypoxia. Cyanide interferes with oxygen use and therefore causes an apparent cellular hypoxia. 

Apart from cyanide, interferences in the determination of copper are also caused by thiosulphate and species which reduce Cu(II) to Cu(I) or oxidize DDTC. 

Asphyxiant A vapor or gas that can cause unconsciousness or death by suffocation (lack of oxygen). Most simple asphyxiants are harmful to the body when they become so concentrated that they reduce (displace) the available oxygen in air (normally about 21%) to dangerous levels (18% or lower). Chemical asphyxiants, like carbon monoxide (CO), reduce the blood s ability to carry oxygen or, like cyanide, interfere with the body s utilization of oxygen. 

Cyanogen chloride (CAS 506-77-4) Vapors extremely irritating to eyes and respiratory tract pulmonary edema may result. Cyanide interferes with cellular respiration (see p 177). 

The presence of halide is determined by first acidifying a portion of the FAQS with dilute nitric acid and boiling the solution in the hood to remove any sulfide or cyanide ions as hydrogen sulfide or hydrogen cyanide, respectively. This is necessary because sulfide and cyanide interfere with the test for halogens. Silver nitrate solution is then added, and the formation of a precipitate of silver halide signals the presence of halide in the FAQS (Eq. 25.1). 

Cyanides interfere with the reaction because they form stable complex alkali cuprocyanide. Under these circumstances, the procedure must include fuming with sulfuric acid, or ignition, to remove or destroy the cyanide. Tartrates, citrates, oxalates, and phosphates do not interfere since they are far weaker complexing agents for copper. ... 

Calcium and magnesium can be titrated readily with disodium ethylenediaminetetraacetate, with Eriochrome Black T as the indicator. The solution is buffered at pH 10.0. Certain metal ions interfere with this procedure by causing fading or indistinct end points. Cyanide, sulfide, or hydroxjiamine can be used to eliminate or minimise the interferences. 

Cyanide compounds are classified as either simple or complex. It is usually necessary to decompose complex cyanides by an acid reflux. The cyanide is then distilled into sodium hydroxide to remove compounds that would interfere in analysis. Extreme care should be taken during the distillation as toxic hydrogen cyanide is generated. The cyanide in the alkaline distillate can then be measured potentiometricaHy with an ion-selective electrode. Alternatively, the cyanide can be determined colorimetricaHy. It is converted to cyanogen chloride by reaction with chloramine-T at pH <8. The CNCl then reacts with a pyridine barbituric acid reagent to form a red-blue dye. 

Both of these haUdes can also be determined potentiometricaHy with an appropriate ion-selective electrode. Sulfide and cyanide both interfere with the electrode response. 

ASPHYXIA The result of a diminished supply of oxygen to the blood and tissues and interference with the respiratory function. Simple anoxia may be caused by inert gases , e.g. nitrogen, and some flammable gases, e.g. methane. Toxic anoxia may be caused by certain substances, e.g. carbon monoxide and hydrogen cyanide, which interfere with the body s ability to transfer or utilize oxygen in the tissues. Rapid unconsciousness and death can occur in either case. 

As a group, the cyanides are among the most toxic and fast-acting poisons. (This is due to the cyanide ion which interferes with cellular oxidation.)... 

The poisoning effect of molecules such as CO and PF3 (p. 495) arises simply from their ability to bond reversibly to haem in the same manner as O2, but much more strongly, so that oxygen transport is prevented. The cyanide ion CN can also displace O2 from oxyhaemoglobin but its very much greater toxicity at small concentrations stems not from this but from its interference with the action of cytochrome a. 

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. 

Determination of silver as chloride Discussion. The theory of the process is given under Chloride (Section 11.57). Lead, copper(I), palladium)II), mercury)I), and thallium)I) ions interfere, as do cyanides and thiosulphates. If a mercury(I) [or copper(I) or thallium(I)] salt is present, it must be oxidised with concentrated nitric acid before the precipitation of silver this process also destroys cyanides and thiosulphates. If lead is present, the solution must be diluted so that it contains not more than 0.25 g of the substance in 200 mL, and the hydrochloric acid must be added very slowly. Compounds of bismuth and antimony that hydrolyse in the dilute acid medium used for the complete precipitation of silver must be absent. For possible errors in the weight of silver chloride due to the action of light, see Section 11.57. 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

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. 

Blood/pulmonary agents, which interfere with metabolic functions such as hydrogen cyanide (AC) or phosgene (CG). 

Dissolve the sample in nitric acid. Separate Pb by extraction with CHC solution of sodium diethyl dithiocarbonate using alkaline cyanide solution to mask interferences. Titrate Pb in an ammonia-ammonium chloride medium at pH =10 using eriochrome black T. 

By careful control of these parameters aluminium may be separated, and few interferences are observed if the precipitation is carried out from an ammoniacal-cyanide-EDTA solution. When large amounts of Ca or Mg are present the homogeneous precipitation procedure (Table 5.18) is usefully employed at pH = 5. The precipitate is readily filtered and may be weighed after drying at 150°C as the anhydrous compound. 

The sample is extracted with a mixture of hexane, acetone and water. After separation, the hexane phase is reduced in volume and divided into two aliquots, one of which is first shaken with 7% fuming sulphuric acid to remove lipids, and then with cyanide to eliminate interference by elemental sulphur. The other aliquot is evaporated to dryness and heated with ethanolic potassium hydroxide. The two aliquots are injected into a gas chromatograph fitted with a glass capillary column and an electron capture detector. Hexabromobenzene is used as an internal standard. Polychlorinated biphenyls are determined quantitatively by comparing the peaks of the sample with those of Clophen A... 

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