Subject: Hard Water Lab Activity
Several newer high school textbooks have laboratory activities pertaining to hard water. These lab activities usually have the students attempt to quantify the level of hardness in several water samples using one of two methods. The first is to use a set number of drops of dish detergent and, after shaking, measure the "sudsiness" (height of suds above water sample) of the water. A second, better method suggested is to precipitate the calcium and magnesium ions with a sodium oxalate solution. The sodium oxalate precipitates the calcium and magnesium ions as calcium oxalate and magnesium oxalate. While this method is better at achieving quantitative results, it requires the user to observe the amount of precipitate formed against a black background. Again, this can still be rather subjective.
I was interested in using the idea of quantifying the levels of hardness in water as a lab activity. It would be relevant in a chapter on the elements, ions, water chemistry, or environmental chemistry. In addition it could be used in conjunction with a lab activity on water softening. Having worked in a former career as a chemical engineer at a nuclear processing facility, I had the opportunity to perform some water quality testing including water hardness. One of the companies whose equipment I used was Hach Company. I visited their website and looked up the procedure I had used to test for water hardness. Upon finding it, I modified it for use by high school students in a high school lab setting. Provided below is a list of the chemicals required to perform this lab activity.
The method is a simple titration from pink to blue with EDTA. The lab activity works very well and provides quantitative results in mg/L as CaCO3. There are only two chemicals needed, besides the water samples: EDTA for a 0.01M solution, which can be purchased from most chemical supply companies such as Flinn Scientific, Inc. [E0044, 25g, $8.25], and Hach UniVer® 3 Hardness Reagent which must be purchased from Hach Company. In addition to the chemicals, Hach does sell a 5.83 mL plastic measuring tube for collecting the correct sample size, at a cost of $1.40 each (their chemicals can be ordered premeasured and prepackaged for their set sample sizes). The total cost of performing this lab activity with 80 students working in pairs, each testing two samples of water is under $40.
|UniVer 3 Hardness Reagent, pk/100 Powder Pillows||96299||$11.05|
|Tube, measuring plastic 5.83mL||43800||$1.40|
|EDTA, disodium salt, dihydrate reagent powder, 25g||E0044||$8.25|
I hope this activity proves useful in helping you reach a few more students, and that you enjoy using it as much as I do. If you have any comments or suggestions, please feel free to contact me.
* Phillips, John; Strozak, Victor; and Wistrom, Cheryl, Chemistry: Concepts and Applications. New York: Glencoe/McGraw-Hill, 1997, pg. 452.
* Dingrando, Laurel; Gregg, Kathleen V.; Hainen, Nicholas; and Wistrom, Cheryl; Chemistry: Matter and Change. New York: Glencoe/McGraw-Hill, 2002, pg. 202-203.
* Water Quality Association. "What Makes Water Hard & How Hard Water Can Be Improved." [online] 26 January 1999. <http://www.wqa.org>
Deer Park High School, Cincinnati, OH