Standardization:
There are two methods available to do the standardization based on the amount of water present in the sample to be tested.
Procedure for standardization
There are two methods available to do the standardization based on the amount of water present in the sample to be tested.
By DST[ for water less than 1%, it is
preferable to use reagent with water equivalency factor Not more than 2.0]
Weigh
accurately about 75 to 125 mg of
disodium tartarate[DST], add into titration vessel with sufficient quantity of
methanol. Titrate with KF reagent until end point.
[USP : 20 to 125 mg]
Water
Equivalency factor F = W/v(36.04/230.08)
36.04
is two times the molecular weight of water and 230.08 is the molecular weight
of Sodium tartarate dihydrate and W is weight in mg of Sodium tartarate dihydrate.
Calculate
the Water Content = Sample in mg of tartarate x 0.1566 / Volume of KF consumed
0.1566
is the water reagent factor for KF reagent.[ 36.04/230.08]
By Purified Water[Precise determination
of Significant amount of water 1% or more use purified water as reference
substance]
Quickly
add 25 to 250 mg of water accurately weighed by difference, from weighing
pipette or from pre-calibrated syringe or micropipette , the amount taken being
governed by the reagent strength and the burette size. Titrate to end point.
Calculate the water equivalency factor f in mg per ml by the formula
F=
W/v
W
is the weight in mg and V is the volume in ml reagent required.
Decide
the Which equivalency factor to be used based on the reagent equivalency
factor, recommended titration volume, burette size and amount of standard.
Consider
a setup in which the reagent equivalency factor is 5mg per ml and the burette
volume is 5ml and an instrumental end point. Standard amounts equivalent to
between 2.5 mg and 22.5mg of water [10-90%of burette capacity] could be used
based on the burette and equivalency factor. The upper end of this range would
involve excessive amount of sodium tartarate dehydrate. If purified water or a
standard is weighed an analytical balance appropriate to the amount weighed is
required.
Testing samples
WATER
CONTENT BY KFR: Transfer about 50 mL Methanol in to the titration vessel then
switch on the instrument. Adjust the speed and proper rotation of magnetic
needle. Neutralize the Methanol by adding Karl Fischer reagent. When end point
reached remove the cork and add about 0.4
g of sample being tested
into the titration vessel and replace the cork and fill the burette with KFR. Start the titration, after
getting the end point note down the titer value. Calculate the water content as
follows.
Volume
of KFR consumed X KFR factor[mg/mL]
Water content by KFR =
------------------------------------------------------------------x 100 = %
Weigh of the sample[g] X 1000
Water
content in Liquid
Transfer
about 50 mL of fresh methanol into the titration vessel then switch on the
instrument. Adjust the speed and proper rotation of magnetic needle. Neutralize
the methanol by adding Karl Fischer reagent. When end-point reached, add 2.0 ml
of sample being tested, into the titration vessel and replace the cork and fill
the burette. Start the titration, after getting the end point note down the
titre value. Calculate the water content as follows.
Volume of
KFR consumed X KFR factor [mg/mL]
Water content by KFR =
--------------------------------------------------------------------- X 100 = %
Volume
of the sample X 1000
How to decide the sample quantity for water determination:
Unless otherwise specified in the individual monograph, use an accurately weighed or measured amount of the specimen under test estimated to contain 2 to 250 mg of water.
The amount of water depends on the water equivalency factor of the Reagent and on the method of endpoint determination. In most cases, the minimum amount of specimen, in mg, can be estimated using the formula:
FCV / KF
in which F is the water equivalency factor of the Reagent, in mg per mL;
C is the used volume, in percent, of the capacity of the burette;
V is the burette volume, in mL; and
KF is the limit or reasonable expected water content in the sample, in percent.
[Note: C is between 30% and 100% for manual titration, and between 10% and 100% for the instrumental method endpoint determination]