The Karl Fischer Jacks of a pH Meter

Some pH meters contain a pair of sockets at the back labeled K-F or Karl Fischer. When the manufacturer s instructions are followed, a constant current (usually about 10 pA) is applied across these terminals. To perform a bipotentiometric titration, Pt electrodes are connected to the K-F sockets. The meter is set to the millivolt scale, which displays the voltage needed to maintain the constant current between the electrodes. 

The figure shows the results of a bipotentiometric titration of ascorbic acid with If. Ascorbic acid (146 mg) was dissolved in 200 mL of water in a 400-mL beaker. Two Pt electrodes were attached to the K-F outlets of the pH meter and spaced about 4 cm apart in the magnetically stirred solution. The solution was titrated with 0.04 M If (prepared by dissolving 2.4 g of K1 plus 1.2 g of I2 in 100 mL of water), and the voltage was recorded after each addition. Prior to the equivalence point, all the If is reduced to I- by the excess ascorbic acid. Reaction B can occur, but Reaction A cannot. A voltage of about 300 mV is required to support a constant current of 10 pA. (The ascorbate dehydroascorbate couple does not react at a Pt electrode and cannot carry current.) After the equivalence point, excess If is present, so Reactions A and B both occur, and the voltage drops precipitously. 

End points in Karl Fischer titrations tend to drift because of slow reactions and leakage of water into the cell from the air. Some instruments measure the rate at which I2 must be generated to maintain the end point and compare this rate with that measured before sample was added. Other instruments allow you to set a persistence of end point time, typically 5 to 60 s, during which the detector voltage must be stable to define the end point. 

A round robin study of accuracy and precision of the coulometric procedure identified sources of systematic error.33 In some labs, either the instruments were inaccurate or workers did not measure the quantity of standards accurately. In other cases, the solvent was not appropriate. Commercial reagents are designed for Karl Fischer analysis. Reagents recommended by the instrument manufacturer should be used with each instrument. 

In electrolysis, a chemical reaction is forced to occur by the flow of electricity through a cell. The moles of electrons flowing through the cell are It/F, where / is current, t is time, and F is the Faraday constant. The magnitude of the voltage that must be applied to an electrolysis cell is E = (cathode) — E(anode) — IR — overpotentials. 

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