MadSci Network: Chemistry
Query:

Re: How do you find the hardness of water?

Date: Thu Feb 1 09:33:40 2001
Posted By: Lon Brouse, Faculty, Chemistry, Challenge Charter School
Area of science: Chemistry
ID: 980536426.Ch
Message:

Dear Ashley,

First, let's talk about what hard and soft water mean. Water with dissolved minerals in it does not work well for cleaning clothes. The soap molecules react with the calcium and magnesium ions to prevent the soap from making suds. It is therefore, "hard" to make soap suds. This is where the term hard water came from. If you remove the dissolved calcium and magnesium through a water softener, Reverse Osmosis unit, by distillaiton, or chemical treatment, it becomes "easy" to make soap suds. Nobody calls it "easy" water, however. They chose the name "soft" as the opposite of hard.

This suggests the first way people used to measure hardness. They added small shavings of castile bar soap to a sealed 1-quart jar with 1/2 quart of the water to be tested. You count the number of small shavings as you add them one by one and shake the bottle vigorously. Keep adding and shaking until the water begins to form soap suds. Perform the test on distilled water so you can see just how little soap it takes to make suds in the absence of any dissolved minerals. Standardize the water sample size, size of the soap shavings (as much as possible), and set a standard thickness for the suds (1/2 inch or so). The greater the number of soap shavings to get the suds, the higher the concentration of calcium and magnesium in the water sample. This is a semi-quantitative test, but is easily understood by the "audience". The soap is consumed by the calcium and magnesium in making bathtub ring (calcium and magnesium stearate). You could make standards by adding a carefully measured amount of EPSOM salts (magenisum sulfate) to a carefully measured amount of distilled water and run the test with the soap shavings.

More quantitative methods use EDTA (ethylene-diamine-tetraacetic-acid) solution to titrate a sample of the water until all of the calcium and magnesium are chemically bound. A chemical indicator is added (erichrome black-T), to the water that turns from red to blue at the end point.

A simple student type test is available through PUR water testing. You just dip prepared plastic strips into the water and compare the color against a standard color bar. They have a web site at:

www.testyourwater.com

Antoher good reference site for background information on hardness and a table describing the values associated with soft through very hard water look at:

http://www.ncsa.uiuc.edu/edu/RSSE/RSEgreen/homepage.html

I hope this helps.


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