Oxygen Flask Combustion Methods

Oxygen flask combustion methods have been used to determine traces of chlorine in PVC [15] and in polyolefins and chlorobutyl rubber [16]. 

Traces of chlorine have been determined in polyolefins [17] at levels between 0 and 500 ppm. The Schoniger oxygen flask combustion technique requires a 0.1 g sample and the use of a 1 litre conical flask. Chlorine-free PE foil is used to wrap the sample, which is then supported on a platinum wire attached to the flask stopper. Water is used as the absorbent. Combustion takes place at atmospheric pressure in oxygen. The chloride formed is potentiometrically titrated in nitric acid/acetone medium with 0.01 M silver nitrate solution. 

A method for the determination of fluorine in fluorinated polymers such as polytetrafluoroethylene (PTFE) is based on decomposition of the sample by oxygen flask combustion followed by spectrophotometric determination of the fluoride produced by a procedure involving the reaction of the cerium(III) complex of alizarin complexan (1,2-dihydroxy-anthraquinone-3-ylmethylamine N,N-diacetic acid). The blue colour of the fluoride-containing complex (maximum absorption, 565 nm) is completely distinguishable from either the yellow of the free dye (maximum absorption, 423 nm) or the red of its cerium(III) chelate (maximum absorption, 495 nm). 

A method has been described [17] for the determination of chlorine in polymers containing chlorine, fluorine, phosphorus and sulfur, which involves oxygen flask combustion over water, addition of ethanol, and titration to the diphenylcarbazide indicator end point with 0.005 M mercuric nitrate  

Using this method Johnson and Leonard [18] obtained from PTFE, 75.8% of fluorine using a silica or boron-free glass combustion flask against a theoretical value of 76%. Using a borosilicate glass combustion flask they obtained a low fluorine recovery of 72.1%. 

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While in these laboratories the oxygen flask combustion method has been used without any difficulty for the determination of fluorine in triamcinolone and other fluorinated steroids use of the Pregl-Roth distillation method has also been recommended . 

Prepare an analar solution of 12 g ammonium ferric sulphate in water and add 40 ml of analar nitric acid. Dilute to 100 ml. Prepare a solution containing 0.4 g of mercuric thiocyanate crystals in 100 ml of absolute alcohol. Mix 5 ml of the test solution from the oxygen flask combustion method (described above) with the ammonium ferric sulphate solution. Add 1.5 ml of mercuric thiocyanate solution. When chlorine is present an orange to red colour will develop in the test sample. If a semi-quantitative estimation of chlorine is required, set the solution aside for 10 min, then measure the optical density of this coloured solution against the blank solution at 460 nm in 20 mm cells. Typical calibration figures for the examination of plastic coatings such as PVC are as follows ... 

Despite the recent development of commercial automated oxygen combustion equipment, the oxygen flask combustion method remains a method for choice for elemental analysis in many laboratories due to its simple, rapid, and economic procedure using readily available apparatus. From the record of the past 5 years, three directions of development of the oxygen flask method can be seen. First, it shows a trend toward microdetermination, in particular for samples of less than 1 mg. Second, new procedures have been developed for multielement determination after oxygen flask combustion, mostly based on 1C separation of the anions produced. Third, extensive validation of the procedure developed has been carried out for real sample analysis with a parallel method comparison with two to six different procedures. 

An oxygen flask combustion method for the determination of between 2 and 80% of chlorine, bromine and iodine in polymers is described in Methods 2.2 and 2.3 at the end of this chapter. 

This qualitative oxygen flask combustion method [29] enables nitrogen, fluorine, chlorine, bromine, iodine, sulfur and phosphorus to be identified in polymers in amounts down to 0.01%. 

Because of the volatility and high chlorine content of this material, the oxygen-flask combustion method is not applicable. 

Sulphobromophthalein sodium, C2oHgOioBr4S2Na2, Mol, Wt. 838-1 can be assayed for bromine and sulphur by the oxygen-flask combustion method (Appendix IV). 

Another method, which is satisfactory and rapid, is the oxygen-flask combustion method (Johnson and Vickers ). The method is described in detail in Appendix IV and for this determination 0 05 g of the sample should be used and the titration should be carried out with 0 01 N sodium thiosulphate. 1 ml O-OIN = 0 0002115 g I. 

The iodine may also be determined by the oxygen-flask combustion method, using 8 mg and titrating with 0 01 N thiosulphate. 1 ml O-OIN = 0-000423 g I. 

However, if the oxygen-flask combustion method is to be used the tablet coating must first be removed and a 20-mg portion of the powdered cores used because, since these tablets are enteric coated, the small amount of sample required for this method might not be representative if the usual practice of powdering the coated tablets is followed. 

Application of the oxygen-flask combustion method to the determination of fluorine in organic combination has now been made by a number of workers. The fluoride in solution after combustion may be determined by thorium nitrate titration or colorimetrically as the alizarin complexan chelate or with a chloranilate (see Halogen Acids and Salts, p. 302). A method based upon the alizarin complexan chelate is given in Appendix IV. 

The oxygen-flask combustion method (see Appendix IV) has been applied to a number of sulphur-containing materials of pharmaceutical and horticultural interest. The method is suitable for the determination of sulphur in organic compounds except those containing a high proportion of nitrogen. 

The oxygen-flask combustion method can be employed for this preparation using 50 mg. The absorbing liquid is 25 ml of water and 2 ml of 20 volume hydrogen peroxide, neutralised to methyl red with 0-02N sodium hydroxide. After rinsing, a further 100 ml of water is added, shaken for one minute and then boiled until the volume is reduced to about 50 ml. After cooling rapidly it is titrated with 0-02N sodium hydroxide using methyl red as indicator. 1 ml 0-02N = 0-0003206 g S. 

Thyroxine sodium, Ci5Hio04Nl4Na,5H20, Mol. Wt. 889-0, is the sodium salt of L-thyroxine. It may be assayed by the oxygen-flask combustion method as described in Appendix IV using about 13 mg and titrating with 0-02N sodium thiosulphate. 1 ml 0-02N = 0-0006657 g anhydrous. 

Ammonium limit test (31) Oxygen flask combustion method... 

One such method is the oxygen flask combustion method, which can be used on PVC products to determine the amount of PVC in the formulation by quantifying the chlorine present. 

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