Determination of water hardness
Water that has not been purified is what is known as "hard water." Hard water can contain substances like Ca2+, Mg2+, and Fe2+. These "hard ions" are not always unhealthy necessarily, but there are several good reasons that we remove them. First, they can combine with other compounds to form soap scum. Second, it can lead to the buildup of scale in pipes which may require costly repairs. Finally, the scale and soap scum will lead to more expensive energy bills and more repairs needed in the long run. (Dean, Reck, Stone, & Robinson, 2009)
The amount of hard ions in water can be determined by the process of titration. In this case, the titration is done by adding the chemical EDTA, ethylenediaminetetraacetic acid, to the water (which has a few drops of the indicator Eriochrome Black T) until the indicator changes from red to blue. The point at which the color changes is called the endpoint. (Dean, Reck, Stone, & Robinson, 2009)
For EDTA to be able to bind to the hard ions, it is important that it remains deprotonated. To do this, a buffer solution is added to keep the pH at a high level. This allows EDTA to bind freely with hard ions like Ca2+ as shown in the following reaction:
EDTA4- + Ca2+ à Ca(EDTA)2-
In this experiment, the presence of Ca2+ in a water sample will be found. The type of titration using EDTA and Eriochrome Black T explained above will be used to determine the level of calcium ions in parts per million. This will then be compared to a scale of water hardness to see how "hard" the water actually is. (Dean, Reck, Stone, & Robinson, 2009)
Taking the average of the differences gives a value of 30.79 mL. Using this in conjunction with the reaction listed above, you can determine the molarity of the EDTA solution.
Since the average volume of EDTA used was used, this value is the average concentration of EDTA. Using Excel to calculate the standard deviation, a value is obtained of 0.008120 ± 0.000003 M EDTA. This has an rsd value of 0.0004%.
Using the values from trials 2 and 3 (trial 1 was not within the precision needed), the concentration of Ca2+ in parts per million (ppm) can be calculated. The average from trials 2 and 3 is 15.12 mL.
Since the average volume of EDTA used was used, this value is the average concentration of EDTA. Using Excel to calculate the standard deviation, a value is obtained of 98.4±0.2 ppm Ca2+. This has an rsd value of 0.2%.
Results / Discussion
Using the scale in the lab manual, 98.42ppm corresponds to moderately soft water. The water sample used was obtained from the "Jordan River" (of Bloomington, not Israel), so it was expected that it might fall under the hard or very hard categories. This was because Bloomington has many limestone deposits that might transfer hard ions to the water.
There are a few places in this experiment where errors could have occurred. A graduated cylinder had to be used instead of a volumetric pipette, because the correct pump wasn't available, and this is a less accurate instrument. Also, at some points, it was difficult to tell when the solution had turned from purple to blue, but the blank allowed for the shade of blue required for the titration to be "finished" to be static across the whole experiment.
Standardization is important when doing a titration experiment since the end result of the titration depends upon the accuracy of the concentration of the titrant. Standardization allows for the concentration of the titrant to be confirmed and thus, increases the accuracy of the final result of the titration.
Hard water has both advantages and disadvantages. Hard water can make washing clothing less effective. It also causes soap scum to build up on the body when bathing. Buildups in pipes lead to inefficiency in appliances that use water, which can increase costs. ("Hard Water Problems," n.d.) Studies have shown that hard water doesn't seem to have any negative effects on humans. It is even possible that it could help defend the body against disease. Some studies have shown that people that drink hard water are less likely to have heart disease, but the evidence isn't complete enough to make a sure connection. ("Hardness in Drinking-water," 2003)
This experiment shows a practical application of how chemistry can be used to help deal with real world issues. Testing water with chemical methods to determine hardness can prove useful in preventing the previously mentioned issues of inefficiency that hard water can cause. Along with the chemical processes used to purify water, these concepts can help to lower costs and enhance efficiency in everyday life.
- Dean, Norman, Cathrine Reck, Todd Stone, and Jill Robinson. Chemistry C117: Principles of Chemistry and Biochemistry: Laboratory Manual. 8th ed. Plymouth: Hayden-McNeil, 2009. Print.
- Hard Water Problems - Limescale Deposits, Reduced Heating Efficiency, Skin Irritation. (n.d.). Retrieved October 07, 2009, from http://www.hardwater.org/hard_water_problems.html
- Hardness in Drinking-water. (2003). Retrieved October 7, 2009, from http://www.who.int/water_sanitation_health/dwq/chemicals/en/hardness.pdf
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