Water Content by KF

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.

Procedure for standardization 

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]