Last week we carried out Lab 1-5 (Solution Chemistry of Natural Waters) without the benefit of the conductivity meter I had in my previous job. Though I probably could have borrowed one from the chemistry department, I chose to use my digital multimeter (easily available in most electronics hobby shops and hardware stores). A physics instructor had told me that all I needed to do was to set the multimeter for measuring ohms, then simply measure the electrical resistance of the solution (which is the opposite of conductivity). Unfortunately, my multimeter is a common model with a range that is too limited for comparing solution conductivity. Consequently, I tried another strategy and got an excellent basis for comparison: I used a 9-volt battery to pass a current through the solution, then used the multimeter to measure the amps passing through the solution. I should point out that when you pass a current through an aqueous solution containing electrolytes, the solution undergoes the process of electrolysis, whereby hydrogen bubbles appear at the anode (black electrode). If you do not even have a multimeter, you can measure conductivity in terms of the amount of hydrogen collected at the anode over a given period of time. Do not measure the gas that is produced at the positive electrode because the make-up of this gas will vary depending on the electrolytes that are present (it could be either oxygen or chlorine). At any rate, it is faster to use a multimeter and it is relatively easy to set it up. Just follow the instructions for measuring electric current then incorporate the multimeter into the electrolytic circuit. An added bonus to this method is that the hydrogen bubbles produced during the electrolytic process give students a real-time visual on how much current is passing through the solution. Do not use more than 9-volts!
For the same lab, I also used an aquarium hydrometer for the specific gravity measurement. Do not use hydrometers from automotive shops that are designed for evaluating battery solution or radiator fluid because they are not sensitive enough. Hydrometers sold at aquarium shops are just right because they are specifically designed to evaluate salinity levels that are found in natural waters.
This blog was originally created in January 2013 to serve as a means for providing updates on science activities from the environmental science lab manual Ecology, Development, and Sustainability. I have now expanded its purpose to include other items of interest to science teachers.
