Wet oxidation procedures

The essence of a successful perchloric acid based wet oxidation is to arrange for the redox potential of the solution to rise from a level where only the most labile materials are oxidised to the final fuming perchloric acid state. This is easily arranged by diluting the perchloric acid with a less powerful oxidiser, usually nitric acid, and then raising the temperature. Provided that the sample dissolves in the hot acid mixture, and that the amount of nitric acid and the rate of temperature rise are such that the more easily oxidised materials are destroyed during the evaporation, the reaction will proceed in a smooth and controlled manner to completion. 

A common mistake is to use too little perchloric acid even with acid soluble materials posing no particular difficulty, there is considerable energy given off during the actual perchloric oxidation, and the acid is both consumed as oxidant and vigorously boiled off. If the total amount of acid available is insufficient, the mixture will go to dryness before the oxidation is complete and the dry residues will catch fire. True explosions on the 1—2 g sample scale are rare at this stage, but the beaker may shatter because of the thermal shock, and in any case the analysis will be ruined. 

Wet oxidation of dried herbage, blood, meat, custard powder or other cellulose/protein low fat materials 

Cover with a pyrex glass cover (to keep Pb, A1 and Zn blanks low), and heat to boiling. There may be some effervescence but it will usually rapidly dissipate as the oxidation proceeds. Copious brown fumes are often evolved at this stage, and are an indication that oxidation of labile materials is proceeding satisfactorily. As the nitric acid evaporates, the brown fumes often clear, indicating the end of the destruction of the easily oxidised entities, followed by a further evolution of brown fumes as the solution temperature rises to about 150° C and the more resistant substances begin to be attacked. Observe the solution at this stage, and if an oily film separates, swirl the beaker to disperse the oily layer into the acid. 

As the last nitric acid evaporates, the nuclear boiling of the liquid may change to effervescence again, which may become vigorous. If the reaction rate appears to be increasing too quickly for comfort, remove the beaker from the hotplate. If the reaction does not abate in 30 to 60 s, add 1—2 ml of nitric acid to lower the temperature and hence the redox potential, and then allow the reaction to proceed again it may be necessary to heat the solution if the reaction has subsided too far. 

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Widening interest in the quaHty of the environment has led to increased demand for information on a wide range of trace-metal contents of foodstuffs. Trace metals in foodstuffs are normally determined by spectroscopic techniques after complete destruction of the organic matrix. Destruction is achieved either by wet oxidation or by dry ashing additional treatment is normally required in order to obtain the metals of interest in a form suitable for analysis. Both methods of destruction are time consuming and tedious this is particularly true of the wet-oxidation procedure, which has the additional disadvantage of being potentially hazardous the methods require considerable analytical skill and experience. Both methods are prone to produce erroneous results either by the loss of an element of interest or by adventitious contamination from the component parts... 

Specific wet oxidation procedures should be chosen for each type of compound as a function of the ease or difficulty of complete combustion. The method applied after oxidation for the determination of the inorganic phosphorus should also be taken into account. 

Having stressed some of the precautions to be taken, it is perhaps necessary to list the advantages of using perchloric acid, with particular reference to its use in wet oxidation procedure. 

The stable isotope of carbon, C , is obtained in the form of methane, cyanide or a carbonate. This isotope is available in larger quantities than are the radioactive carbons although highly enriched material is difficult to obtain. Measurements are readily and accurately made with a mass spectrometer. The usual procedure involves combustion of an organic substance containing by the wet oxidation procedure of Van Slyke and Folch or by the usual combustion methods followed by purification of the resulting CO2. The sample of CO2 is then analyzed by means of the mass spectrometer. It should be emphasized at this point that analyses of necessity must be carried out on analytically pure substances since most contaminants contain carbon. This is particularly true of biological substances which have to be isolated from natural sources. 

A wet oxidation procedure which utilizes sulfuric, nitric, and perchloric acid was established as a satisfactory method for destroying the organic matrix of petroleum samples with quantitative retention of the arsenic. Table 5.1 compares the amount of arsenic added to three petroleum samples with the amount of arsenic found on analysis by the method subsequently adopted. In each case the amount found by analysis agrees with the amount added within the precision of the method and indicates that the arsenic is quantitatively retained by the sample preparation procedure. Quantitative retention of the arsenic was further substantiated by neutron activation analysis of a sample which was spiked with a known amount of triphenylarsine. The arsenic concentration was determined at each step of the sample preparation procedure (Table 5., II). The results were in general agreement with the amount added and confirmed the earlier conclusion that arsenic is quantitatively retained during sample preparation. 

Friend, M.T., Smith, C.A. and Wishart, D. (1977) Ashing and wet oxidation procedures for the determination of some volatile trace metals in foodstuffs and biological materials by AAS. Atom. Abs. Newsl., 16, 46-49. 

Pujo-Pay M. and Raimbault P. 1994. Improvement of the wet-oxidation procedure for simultaneous determination of particulate organic nitrogen and phosphorus collected on filters. Mar. Ecol. Prog. Ser. 105 203-207. 

Procedure. The arsenic must be in the arsenic (III) state this may be secured by first distilling in an all-glass apparatus with concentrated hydrochloric acid and hydrazinium sulphate, preferably in a stream of carbon dioxide or nitrogen. Another method consists in reducing the arsenate (obtained by the wet oxidation of a sample) with potassium iodide and tin(II) chloride the acid concentration of the solution after dilution to 100 mL must not exceed 0.2-0.5M 1 mL of 50 per cent potassium iodide solution and 1 mL of a 40 per cent solution of tin(II) chloride in concentrated hydrochloric acid are added, and the mixture heated to boiling. 

The following procedure has been recommended by the Analytical Methods Committee of the Society for Analytical Chemistry for the determination of small amounts of arsenic in organic matter.20 Organic matter is destroyed by wet oxidation, and the arsenic, after extraction with diethylammonium diethyldithiocarbamate in chloroform, is converted into the arsenomolybdate complex the latter is reduced by means of hydrazinium sulphate to a molybdenum blue complex and determined spectrophotometrically at 840 nm and referred to a calibration graph in the usual manner. 

The use of microwave digestion-wet oxidation overcomes sample preparation problems for many polymer-based materials. However, this will result in a reduction in sensitivity compared with an ashing procedure, because of dilution. Use of an aqueous phase is not... 

Wet oxidation of samples of biological origin with a mixture of nitric acid and perchloric acid is a common procedure that may be problematic in certain cases. An alternative procedure for sample preparation is irradiation with a strong UV source. Acidified samples of hair (0.1 g) containing about 2 pg Pb and other trace elements, were irradiated for 3 h with a 500 W source a buffer (pH 5.5) and a small amount of catechol violet (9) were added, and the complex of the dye with the trace elements was determined polarographically RSD ca 5% for Pb95. 

Ventilation of sewers may not only reduce the hydrogen sulfide concentration in the sewer atmosphere but also the moisture that is a fundamental requirement for establishment of microbial activity on the sewer walls. It is important that the ventilation be well controlled otherwise, odorous problems in the vicinity of the sewer network may occur. In some cases, operational procedures like treatment of the vented air, e.g., by wet oxidation, by chemical scrubbing or by passing a biofilter, may need to be considered. 

Phosphorus is a common component of additives and appears most commonly as a zinc dialkyl dithiophosphate or triaryl phosphate ester, but other forms also occur. Two wet chemical methods are available, one of which (ASTM D1091) describes an oxidation procedure that converts phosphorus to aqueous orthophosphate anion. This is then determined by mass as magnesium pyrophosphate or photochemically as molybdivanadophosphoric acid. In an alternative test (ASTM D4047), samples are oxidized to phosphate with zinc oxide, dissolved in acid, precipitated as quinoline phosphomolybdate, treated with excess standard alkali, and back-titrated with standard acid. Both of these methods are used primarily for referee samples. Phosphorus is most commonly determined using x-ray fluorescence (ASTM D4927) or ICP (ASTM D4951). 

At the Laboratory of the Government Chemist, increased analytical demands and the expense of suitable staff resulted in a requirement for the complete, or partial mechanization or automation of this kind of work. A review of the hterature, together with an examination of the procedures then used in the laboratory, suggested that a wet-oxidation system would be the most suitable for automation. This is described in detail here because it provides a valuable lesson in systems design. The specification of the automatic system was that it should be modular in construction in order to facifitate modification and maintenance, and that it should have a multi element capabihty. 

The phosphorus content can be determined by wet oxidation of the starch with sulfuric (or nitric) acid and hydrogen peroxide followed by colorimetric estimation of the phosphomolybdate complex. A detailed procedure for gravimetric determination can be found in AACC method 40-57 (AACC, 2000). However, large sample sizes (2-5 g) are required in this method. 

The low level of aluminium in some samples may require concentration by chelation—solvent extraction prior to introduction into the flame. A procedure described for wines [36] involves digestion with HN03—H2S04, extraction with 8-hydroxyquinoline in MIBK and determination with a nitrous oxide—acetylene flame. Aluminium has also been extracted with 2,4-pentanedione [185], EAAS has been applied to measuring aluminium in beer [186]. Gorsuch [139] proposes wet oxidation to be preferred and mentions that adverse comments reported in respect of sample destruction have usually applied to dry ashing. 

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