Quantitative Determination of Total Hardness in Drinking Water by Complexometric Titration

Pages: 2 (1077 words) Published: November 1, 2014

Quantitative Determination of Total Hardness in Drinking Water by Complexometric EDTA Titration G.S.O. Banzuelo
Department of Chemical Engineering, College of Engineering
University of the Philippines, Diliman, Quezon City, Philippines Date Performed: October 01, 2014
Instructor’s name: Ericka Itang
2349510731400
Answers to Questions
The concept of complexometric titration and water hardness (why is it expressed as ppm CaCO3): Complexometric titration is the process wherein the ligand is considered as the titrant (Lewis base), the metal ion is the analyte (Lewis acid) and the endpoint is determined with the use of a metallochromic indicator. It can be used to determine water hardness. Additionally, water hardness is the measure of the calcium, magnesium and in lesser amounts, iron in water. Most hardness is attributed to the carbonate mineral deposits so water hardness is measured by getting the sum of the concentrations of calcium and magnesium and converting them into a concentration equivalent to CaCO3 in mg/L of water. The use of EDTA as complexing agent and titrant:

EDTA or ethylene-diamine-tetra-acetic acid is known to be a versatile chelating agent. A chelating agent is a substance whose molecules are capable of forming several bonds to a single metal ion. EDTA is a hexadentate and tetraprotic ligand and forms a stable 1:1 complex with many metals. By complexing, the metals that soften the water were captured. When used as a titrant, the endpoint is reached when all of the cations have reacted. The use of EBT as indicator; significance of adding MgCl2•6H2O crystals to the titrant: EBT or Eriochrome Black T is an organic dye with acid-base indicator properties. It undergoes a color change when a proton is added to and from a dye. EBT forms a very stable wine-red complex, MgIn-, with the magnesium ion. MgCl2•6H2O crystals were added because the indicator requires a trace of Mg2+. Adding the crystals makes the determination of calcium possible and...

References: [3] http://homepages.ius.edu/DSPURLOC/c121/week13.htm
[4]http://www.env.gov.bc.ca/wsd/plan_protect_sustain/groundwater/library/ground_fact_sheets/pdfs/hardness(020715)_fin2.pdf
M std solution= (g CaCO3)(%purity)(FW CaCO3)(L std soln) = (1.2487)(0.995)(100.09)(0.25) = 0.0496536
Molarity of working standard solution = (M standard solution)(V standard solution)V working standard solution
= (0.0496536)(0.01.05L= 0.004965 M
Molarity of EDTA= (M working std)(V working std)V EDTA = (0.004965)(0.01)(0.0118) = 0.00420793M
Titer= (M EDTA)(FW CaCO3) = (0.00420793)(100.09) = 0.421172 g/L
ppm CaCO3 = (Titer)(ml EDTA)V sample = (0.421172)(17.3mL)0.05 L = 145.7254064
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