DETERMINATION OF WATER BY KARL FISCHER REAGENT
PRINCIPLE
For the determination of water; Karl Fischer in the year 1935, proposed a reagent prepared by the action of Sulphur dioxide, upon a solution of Iodine in a mixture of anhydrous Pyridine and anhydrous Methanol.
Water reacts with this reagent, In a two stage process as shown.
SO2
3C5H5N + I2 + SO2 + H2O = 2 C5H5NH+I- + C5H5N+ O-
OSO2OCH3
C5H5N+
---SO2 + CH3OH = C5H5N
H
Thus, each molecule of Iodine is equivalent to one
molecule of water. The end point of the reaction is conveniently determined
electrometrically, using the dead-stop end point procedure. If a small e.m.f.
is applied across two platinum electrodes immersed in the reaction mixture, a
current will flow, as long as free Iodine is present, to remove Hydrogen and
depolarise the cathode. When the last traces of Iodine have reacted, the
current will decrease to zero or very close to zero. Conversely, the technique may
be combined with a direct titration of the sample with the Karl Fischer
reagent. Here the current in the electrode circuit, suddenly increases at the
first appearance of unused Iodine in the solution.
The original Karl Fischer reagent, prepared with an
excess of Methanol, was some what unstable and required frequent
standardisation. It was found that the stability was improved by replacing
Methanol with 2-methoxy ethanol.
Present day Karl Fischer reagents have also replaced
Pyridine, with a base. Hence, these are advertised as "pyridine free
reagents".
The method is clearly confined to those cases, where
the test substance does not react either with the components of the reagent or
with the Hydrogen iodide formed during the reaction with water. Hence, this
method is unsuitable for
a) Oxidising agents such as Chromates, Dichromates,
Copper (II) and Iron (III) salts, higher oxides and peroxides.
MnO2
+ 4C5H5NH+ +
2I- = Mn++
+ 4 C5H5N + I2 +2H2O
b) Reducing agents such as, Thiosulphates, Tin(Il) salts
and sulphides.
c) Compounds which can be regarded as forming water with
the components of the Karl Fischer reagent e.g.
1. Basic oxides -
ZnO
+ 2C5H5NH+ = Zn++ + 2C5H5N
+ H2O
2. Salts of weak oxy acids -
NaHCO3 + C5H5NH+ = Na+ + C5H5N + CO2 +
H2O
Instrument
The instrument used
for determination of water by K.F. is METROHM 701 KF TITRINO, consisting of a
titrant dispenser, with a facility to draw Methanol and K.F. reagent in a closed system provided with guard
tubes containing desiccant which prohibits ingress of moisture and key board
for programming.
The titration vessel
is conditioned by neutralizing the inherent moisture present in the titration
vessel; using K.F. reagent.
The water equivalent
factor of the K.F. reagent is determined daily, by titrating known quantities
of water, in duplicate, with the reagent.
The water content of
the sample is found out by titrating known amount of the sample.
The instrument is
calibrated once a month by titrating known quantity of Disodium tartrate in
duplicate and comparing the mean result obtained, with the specification for
water content of Disodium tartrate.
Precautions
a) The
desiccants in the guard tubes must be regularly replaced.
b) The stop
cocks must be lubricated with Silicone grease and not with petroleum grease,
which reacts with the K.F. reagent.
c) The water
equivalent factor of the K.F. reagent must be determined daily and must be
within 1% R.S.D., to ensure accuracy of the water determination.
d) The water
equivalent factor of the K.F. reagent is normally around 5 to 6 mg of water per
ml of the reagent. On repeated use, this factor goes down and the reagent
having a factor of 4.5 or less, must never be used; as the volume of the reagent
required for the titration increases, reducing the sensitivity of the
determination of the water content.
e) It is
advisable to mix the sample in Methanol for at least one minute, prior to
titration with the K.F. reagent.
f) During the
determination of water, the sample and the Methanol must be thoroughly mixed.
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