assays by oxygen flask combustion
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Assays by oxygen flask combustion
-Developed by Schoniger(1955).
-Rapid assay method for organically combined halogens.
-Halogen is released following the complete combustion of organic matter by oxygen.
-Halogen is absorbed by caustic soda solution.
-In case of iodo-compound, a mixture of iodine & iodate are formed.
-Oxidation is carried out with bromine.
-After acidification in the presence of iodide, six fold yield of iodine is generated.
-The iodine is titrated against sodium thiosulphate.
-The method is useful for expensive materials & product in which the halogen-containing
material is present at low dosage.
-The method is adapted to determine elemental sulphur.
-Sulphur is oxidised to sulphur trioxide absorbed in water. S→SO3
-To ensure the complete oxidation, hydrogen peroxide is also added in the water.-Then sulphur is determined by titrating with barium perchlorate solution using indicator
alizarin red S or thoron/methylene blue as appropriate.
- The method is applicable in the determination organo-mercury compounds as well.
Apparatus:
i)Iodine flask(500ml) as per British standard specification (BS 2735:1956).
ii)To the stopper platinum wire is fused.
iii)To the wire a platinum gauze of specified dimension is attached.
iv)The gauze complies to dimension of No.36 sieve.
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Propyliodone: Determination of the percentage of
C10H11I2 NO3
3I2 + 6NaOH → 5NaIO3+ 3H2O
5[NaI+3Br 2 +3H2O → NaIO3 +6HBr]6[NaIO3 +5KI +6HBr → 3I2 +NaBr+5KBr]
I2+ 2Na2S2O3 → 2NaI +Na2S4O6
3X447g C10H11I2 NO3 ≡6I ≡NaIO3 ≡36I ≡36000ml M
0.7450mg C10H11I2 NO3 ≡ 1ml 0.02M Na2S4O6
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-Accurately weighed sample is folded in an ashles- filter paper.
-The folded paper is placed in the platinum gauze cup.
-A narrow strip of filter paper is inserted in the folded paper to act as a fuse.
-Air is displaced from the flask with oxygen & the neck is moisten with water.
-The flask is swirled to saturate the solution with O2
-Water & M sodium hydroxide are placed in the flask & was filled with oxygen .
-End of the fuse is lit & the stopper is inserted immediately.
-The flask is held firmly in place.
-The flask is tilted in order to prevent incompletely burned material from falling into the liquid.
-Upon completion of the combustion, the flask is shaken vigorously for 5 minutes.
-Few ml of water is placed in the cap, the stopper is carefully withdrawn.
-The stopper & wire assembly are washed with water & excess of acetic bromine solution is
added.
-Then the solution is allow to stand for two minutes.-Iodide is oxidised to iodate by bromine in potassium acetate-acetic acid buffer solution.
-Excess bromine(reduction to HBr) is removed by addition of formic acid,
-Trace of bromine is swept out by current of air
-Potassium iodide is added & titrated with sodium thiosulpahte(V.S.) using starch mucilage as
indicator.
Cognate determination:
Iophendylate injection,
Liothyronine sodium
Propyliodone suspension,
Proppyliodone oily suspension,
Thyroxine sodium,
Nitroxynil
Fluocinolone acetonide:
-Borosilicate glass must be avoided as fluorine
reacts with boron present in the hard glass.
-Organically combined fluorine is converted to
fluorine by combustion in oxygen in a silica or soda
glass flask.
-Absorbed fluorine in water is determined
colourimetrically at 610nm using alizarin fluorine
blue & cerous nitrate in an acetate buffer.
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-Standard used is sodium fluoride.
Other uses of oxygen flask method:
-Determination of fluorine in cloxacillin sodium to supplement the assay.-To establish the presence of chlorine in diloxanide furoate & chlorthalidone.