UNKNOWN SAMPLE #97
COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS
CHM 152 LL
4 FEBRUARY 2015
Using a Lewis base neutral molecule to donate electron pairs (ligands) to a Lewis acid metal ion center to form a single cluster (complex) ion. When the complex ions forms with a metal ion (chelation) the ligand used is called the (chelating agent). EDTA acts as a great chelating agent due to the Nitrogen and Oxygen donating an electron pair to the metal ion center to form an octahedral complex. The metal ions especially with a +2 charge or higher are the reason for water hardness to form on various objects known as “scum”. Calcium ions are typically the most common contributing factor for water hardness so this experiment uses CaCO3 (Calcium Carbonate) to analyze the hardness of an unknown sample. A scale of water hardness identifies “soft” water with a value less than 60 ppm (parts per million) and “hard” water with a value more than 200 ppm. 3 mL of ammonia/ammonium chloride buffer (pH 10) is added to the mixture prior to the titration to capture the calcium metal ions so the indicator can work properly. The experiment adds 4 drops of Eriochrome Black T as the indicator to visually see the color change as complexes are formed and the solution undergoes chelation of metal impurities. The color change from indicator starts as pink and changes to a violet then light blue color to signify the chemical phase changes throughout the reaction until the endpoint. 3 titrations are experimentally conducted to calculate the mean average of the Na2 EDTA for experimental accuracy. The EDTA mean average is then used to calculate the water hardness of an unknown water sample (#97) using 3 more titrations to calculate a mean average of the unknown water sample. An absolute deviation is calculated for each titration experiment to calculate the experimental estimated precision. The final experimental result is then compared to the city of Tempe standard for water hardness and acceptable standards.
“A complex ion is an ion containing a central metal cation bonded to one or more molecules or ions” (Chang, 2013). Just like complex ions, a ligand is a molecule or ion that is bonded to the metal ion in a complex ion (Chang, 2013). A chelating agent is a substance that forms complex ions with metal ions in a solution (Chang, 2013). The process of the chelating agent forming is called chelation. E.D.T.A. (ethylenediaminetetraacetic acid) is a common chelating agent that will be used in this experiment to chelate the metal ions. Tetraamminecopper (II) [Cu(NH3)4]2+ will be the complex ion in this lab experiment. In chapter 11.1-3 the “Kinetic Molecular Theory” is being tested in this experiment (Chang, 2013). Solids are denser than liquids and allow very little empty space to exist between molecules limiting the freedom of motion. The liquids are less dense than solids, held closely together with little space between molecules (less than solids), however, the molecules in liquid do not break away from the attractive forces allowing them to move past each other freely. Gases are the least dense and have the largest amount of distance between molecules allowing them to move around more freely. According to the theory, the experimental Carbon, Hydrogen, Nitrogen and Oxygen ions are able to quickly attract and find the metal ion in the liquid by donating their electron pair to the metal ion center creating the complex. These complex ions in the experiment use intermolecular and intramolecular forces to break and hold chemical bonds thru the experimental process of chelation to identify the hardness of the unknown sample. After the reaction is complete, when evaporation and or vaporization of the liquid and gas in the molecules is separated the remaining metal impurities known as “scum” are left. In...
References: Chang, G. (2013). Chemistry Mesa Community College. McGraw-Hill.
Complexometric Determination of Water Hardness Lab. (n.d.). Retrieved from CHM 152 labs: http://www.physci.mc.maricopa.edu/Chemistry/CHM152/page2/page23/files/5-edta-titration.pdf
Department, C. o. (2013). City of Tempe Water Quality Report. Retrieved from City of Tempe Water Department: http://tempe.gov/home/showdocument?id=23826
Spurlock, D. (2014, March 31). Retrieved from Southeast Indiana University: http://homepages.ius.edu/DSPURLOC/c121/week13.htm
Water filteration systems. (n.d.). Retrieved from https://images.search.yahoo.com/images/view;_ylt=AwrTcX4xqttUeskAX_mJzbkF;_ylu=X3oDMTIzZnJvb2NuBHNlYwNzcgRzbGsDaW1nBG9pZANiMThhN2IzN2I1MjM3Yjc2Njg3MjlhNTE3YTM4MGQyMQRncG9zAzM0BGl0A2Jpbmc-?.origin=&back=https%3A%2F%2Fimages.search.yahoo.com%2Fyhs%2Fsearch%
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