|MadSci Network: Chemistry|
The water is not a very conductive substance itself. The fact that we know we should not mix water and electricity is that water can dissolve many substances. The substances that dissolve better in water are ionic compounds. These are substances composed by ions, positive (cathions) and negatively (anions) charged particles. These ions are separated when dissolved in the water. This “ionic solution” is what we call an electrolyte and because it contains positive and negatively charged particles it can now conduct electricity. And it is an obvious idea to relate the behaviour of the electrolyte to an electrical current with the concentration of these particles in the solution. First of all we have to model a circuit, where the electrolyte can be pictured as a resistance. By measuring that resistance of the electrolyte we can therefore calculate the concentration of ions in the solution. But when we study the resistance given by the electrolyte we can see it does not work as just a simple resistor but if we want to take accurate mesures we have to take into account at least another element, a capacitor, connected in series (the simplest model). The problem is that a capacitor stops the current so we cannot use a dc current to make the measurements; we need to use alternating current, ac, and the resistance is called impedance instead (so we do not confuse with the simple one) In order to measure the impedance or its inverse, conductivity, we need a EnC (Electrical Conductivity) meter (http://en.wikipedia.org/wiki/EC_meter). “The common laboratory conductivity meters employ a potentiometric method and four electrodes. The electrodes are usually cylindrical and arranged concentrically. The electrodes are usually made of platinum metal. An alternating current is applied to the outer pair of the electrodes. The potential between the inner pair is measured. Conductivity could in principle be determined using the distance between the electrodes and their surface area using the Ohm's law but generally, for accuracy, a calibration is employed using electrolytes of well-known conductivity.”
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