Complexometric Determination of Water Hardness
Abstract: Two sets of acid-base neutralization titrations were conducted for experimental analysis. The first set of titrations was to standardize a solution manufactured in the lab. An approximate solution of Na2EDTA of 0.004 M was titrated against a known solution of 1.000 g CaCO3/L to deter mine to exact molarity of the Na2EDTA. Ca2+ + Na2EDTA → CaEDTA + 2Na+
The second set of titrations was to use the now standardized Na2EDTA solution to determine an unknown water sample’s hardness. The unknown water sample is # 89. Water’s hardness arises from metal ions with a charge of +2 or higher. So for this experiment, the previously stated reaction can be used as follows: (Unknown #89)2+ + Na2EDTA → (Unknown #89)EDTA + 2Na+
In conclusion, unknown sample #89 has a water hardness value of 209.5 ppm
The purpose of this experiment is to determine the hardness of a sample of water (#89). Water hardness can be evaluated by an acid-base neutralization titration. (When an acid and a base are placed together, they react to neutralize the acid and base properties, producing a salt.) From the Bronsted-Lowry acid-base reaction theory, the H+ cation of the acid combines with the OH- anion of the base to form water. In this titration experiment, there are no definitive acidic or basic agents being evaluated. This requires the use of the Lewis acid-base theory. Instead of defining acid-base reactions in terms of protons or other bonded substances, the Lewis definition defines a base (referred to as a Lewis base) to be a compound that can donate an electron pair, and an acid (a Lewis acid) to be a compound that can receive this electron pair. Similarly, in either type of acid-base neutralization experiment, an indicator is used to display when the solution is neutralized. The standardized Na2EDTA water solution is will be titrated against the unknown water sample (#89). Upon neutralization, the unknown water sample’s metal cation electrons will transfer to the Na2EDTA solution and will subsequently remove the sodium from the EDTA. (Unknown #89)2+ + Na2EDTA → (Unknown #89)EDTA + 2Na+
Water hardness is expressed in in milligrams per liter.
The first thing to be done was to manufacture the 0.004 M Na2EDTA solution. This was done by adding 0.7319 g Na2EDTA to approximately 500 mL of DI water. I obtained a 50.0mL burret, a 10.00 mL and a 25.00 mL transfer pipet, and the unknown sample (#89) from the stockroom. The 50.0 mL burret was set in the burret stand. The Na2EDTA was in the burret and CaCO3 solution was in a 250 mL Erlenmeyer flask. The CaCO3 solution was comprised of 10.00 mL of 1.000g CaCO3/mL, approximately 30mL of DI water, 2.5 – 3.5 mL of buffer (ammonia/ammonium chloride), and 4 drops of Eriochrome Black T indicator. The Na2EDTA was being titrated against the CaCO3 while simultaneously being mixed by a magnetic stir bar in the 250 mL Erlenmeyer flask. The titration was run to completion when theCaCO3 solution turned from a violet to a blue color change. This was done three times for the standardization of the Na2EDTA. The titration for the now standardized Na2EDTA solution against the unknown water sample #89 follows a similar procedure as the first titration except the contents of the 250 mL differ. The Unknown sample solution was comprised of 25.00 mL of unknown, approximately 20 mL of DI water, 2.5 – 3.5 mL of buffer (ammonia/ammonium chloride), and 4 drops of Eriochrome Black T indicator. The Na2EDTA was being titrated against the Unknown while simultaneously being mixed by a magnetic stir bar in the 250 mL Erlenmeyer flask. The titration was run to completion when the unknown solution turned from a violet to a blue color change. This was done three times to determine the hardness of water sample #89.
Experimental Data & Calculations:
1). Na2EDTA & CaCO3 titration.
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