How do I calculate impedance in a four wire test.

How do I calculate impedance in a four wire test. i.e. I am passing a 1 amp (A.C.) current through 2 x 10 ohm resistors. One of the resistors is connected in paralel with a switch. When the switch is closed it shorts the resistor causing a voltage drop accros this resistor. The switch can be coated with certain contaminants and it is the impedance of these contaminats I am trying to measure. I measure the voltage accross the two resistors and the current phaze difference between them. How can I use this data to calculate the impedance of the contaminant?