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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.
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-In case ofiodo-compound, a mixture ofiodine &
iodate are formed.
-Oxidation is carried out withbromine.
-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.
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-The method is adapted to determine elemental
sulphur.
-Sulphur is oxidised to sulphur trioxide absorbed in
water.
-To ensure the complete oxidation, hydrogenperoxide is also added in the water.
-Then sulphur is determined by titrating withbarium perchlorate solution using indicator
alizarin red S or thoron/methylene blue as
appropriate.
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- The method is applicable in the determination
organo-mercury compounds as well.
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Apparatus:
i)Iodine flask(500ml) as per British standard
specification (BS 2735:1956).
ii)To the stopper platinum wire isfused.
iii)To the wire a platinum gauze of specified
dimension is attached.
iv)The gauze complies to dimension ofNo.36 sieve.
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Propyliodone: Determination of the percentage of
C10H11I2NO3
C10H11I2NO3
3I2+ 6NaOH 5NaIO3+ 3H2O
5[NaI+3Br2 +3H2O NaIO3 +6HBr]6[NaIO3+5KI +6HBr 3I2 +NaBr+5KBr]
I2+ 2Na2S2O3 2NaI +Na2S4O6
3X447g C10
H11
I2NO
36I NaIO
336I 36000ml M
0.7450mg C10H11I2NO3 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 gauzecup.
-A narrow strip of filter paper is inserted in thefolded paper to act as a fuse.
-Air is displaced from the flask with oxygen & theneck is moisten with water.
-The flask is swirled to saturate the solution with O2
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-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.
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-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 fortwo minutes.
-Iodide is oxidised to iodate by bromine in
potassium acetate-acetic acid buffer solution.
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-Excess bromine(reduction to HBr) is removed by
addition offormic acid.
-Potassium iodide is added & titrated with sodium
thiosulpahte(V.S.) using starch mucilage as
indicator.
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Cognate determination:
Iophendylate injection,Liothyronine sodium
Propyliodone suspension,
Proppyliodone oily suspension,Thyroxine sodium,
Nitroxynil
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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 fluorineblue & cerous nitrate in an acetate buffer.
-Standard used is sodium fluoride.
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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.
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Reference:
IP2007.A.H. Beckett/J.B.Stenlake
Practical pharmaceutical chemistry.