Interference copper

The presence of tin and of considerable quantities of iron and copper interfere with the determinations. 

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. 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

A drop reaction for tervalent arsenic consists in treating a drop of the sample on filter paper with hydrochloric acid and a 0-5 per cent, aqueous solution of kairin (N-ethyl-8-hydroxytetrahydroquinoline hydrochloride) on adding a drop of aqueous ferric chloride and warming the test paper, a reddish-brown colour appears.3 The test is sensitive to 6 x 10 1° g. Mercury, lead and copper interfere. 

R.O. Kadara and I.E. Tothill, Resolving the copper interference effect on the stripping chronopotentiometric response of lead (II) obtained at bismuth film screen-printed electrode, Talanta, 66 (2005) 1089-1093. 

The presence of up to 30 pg in the portion analysed does not interfere in this procedure, but 100 mg copper depresses the selenium signal by 9%. Therefore, one should be aware of copper interference in the case of plant material which is contaminated with copper spray residues. 

The ions of bismuth, cerium, manganese, thallium, cobalt, and nickel give a similar reaction iron and large quantities of copper interfere. 

Thiourea test, CS(NH2)2. Solid or dissolved thiourea precipitates selenium as a red powder from cold dilute solutions of selenites. Tellurium and bismuth give yellow precipitates, whilst large amounts of nitrite and of copper interfere. 

Both iron and copper interfere. If, however, potassium iodide solution is added, they are reduced to the non-reactive iron(II) and copper(I) ions the liberated iodine may be decolourized with sodium thiosulphate solution. Alternatively, the reduction may be carried out with sodium thiosulphate solution alone on a spot plate, the copper acting as a catalyst for the reduction of the iron ... 

Copper interferes in the determination of iron(II) with bathophenanthroline and should, therefore, be separated, e.g., as CuSCN, or masked with thiourea [42]. Higher concentrations of Co, Ni, Zn, and Cd interfere slightly. 

J.A. Gomes-Neto, A.P. Oliveira, G.P.G. Freshi, C.S. Dakuzaku, M. Moraes, Minimization of lead and copper interferences on spectrophotometric determination of cadmium using electrolytic deposition and ion-exchange in multi-commutation flow system, Talanta 53 (2000) 497. 

Bingley, J. B. (1963). Molybdenum in plants and animals. Determination of molybdenum in biological materials with dithiol-control of copper interference. 7. Agric. Food Chem. 11 130-1. 

Anodic stripping voltammetry was in use already in 1972 to determine Bi in plasma [120]. Bismuth is one of the elements most easily determined with this method. It can be deposited on electrodes at potentials at which most other elements ate in solution. Glassy carbon electrodes coated with films of mercury [100,121-124], but also of gold [101,125], are used. As reference served calomel or silver/silver chloride electrodes [121]. Platinum foils or wires were used as counterelectrodes. At -0.7 V carbon electrodes are precoated with mercury from acid Hg(II) or gold from acid Au(III) solutions. To deposit Bi on the electrode -0.2 to -0.3 V is a sufficient potential for preelectrolysis, but mostly higher voltages are chosen to determine other metals as well. Bi is stripped fiom the electrode at potentials of about -0.10 to -0.16 V. Only some elements interfere with Bi, such as arsenic, mercury, copper, and antimony. Copper interferes at high concentrations and is separated by extraction [101]. Arsenic or antimony in urine are oxidized to their pentavalent states [125]. 

Tyszczuk K, Korolczuk M, Grabarczyk M (2007) Application of gallium film electrode for elimination of copper interferences in anodic stripping voltammetry of zinc. Talanta 71 2098-2101... 

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