COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS LAB

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UNKNOWN SAMPLE #97

COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS
DR. BUDRUK
CHM 152 LL
SEAN MARKIS
4 FEBRUARY 2015

Introduction
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



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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