Complexometric Determination of Water Hardness

The water hardness for unknown water sample number 40 was determined. From the results of the two complexometric titrations, the water hardness of the unknown sample was calculated to be 250.9 ppm CaCO3, which agrees with the ranges of acceptable water hardness in the city of Phoenix and Tempe Arizona. Introduction

When rainfall picks up impurities from the soil, ions of sodium, magnesium, calcium, iron, and other metals are dissolved into the water. These impurities are what cause residues left on glassware from the reaction with the soap used for cleaning. Water hardness is from the metal ions with a +2 charge or higher being dissolved into the rainfall. When reporting water hardness, it is reported in units of mg CaCo3/L of solution, or in ppm due to one mg of solute having one millionth of the mass of a liter of water or dilute aqueous solution. It is reported using only CaCo3 because typically, Calcium is the largest contributor to the water hardness. The purpose of this experiment was to determine the concentration of metal ion impurities in an unknown sample of hard water by performing a complexometric titration with EDTA. The results were then compared to the expected range for municipal water hardness from multiple city websites. Methods

All procedures from the CHM 152 Lab, Complexometric Determination of Water Hardness (1) were followed precisely. Unknown water sample #40 was mixed with approximately 20 ml of DI water, 3.0 ml of Ammonia/Ammonium Chloride (NH(aq) Buffer pH=10), and 4 drops of Eriochrome Black T indicator solution. It was then titrated with .004197 M EDTA, which was also made in the lab from 0.7582 g of , added to 500 mL of DI water, that was then standardized with a stock Calcium Ion Solution (CaCO(aq) 1.00g/1.00L) by titration. A total of 3 titration trials were completed to find the average molarity of the EDTA, and the average water hardness of unknown water sample #40. All
mass measurements were taken from a AND Balance, serial number 12321601 and all titrations were done using a KIMAX buret, #173. Pipet measurements were taken from a 25mL Bel-Art, and 15mL Bel-Art pipet. The numerical techniques used in this experiment were, stoichiometry with mole ratio for calculating the molarity of the EDTA solution and water hardness and relative average absolute deviation in ppt to show how closely the results from the Standardized EDTA titration of each trial agree with each other. Data Analysis

Based on the experiment and calculations, the average calculated concentration of metal ion impurities (water hardness) in unknown sample number 40 from this experiment was 250.9 ppm. From table 1, the average molarity of the titrated was calculated to be 0.004197 M. Table 1

The molarity of the EDTA solution for each trial n was calculated as follows: .0.01000 L CaCO3 SolnTotal EDTA Delivered L×1.000 g CaCO31 L CaCO3 Soln×1 mol CaCO3100.1 g CaCO3×1 mol Na2EDTA1 mol CaCO3=Molarity of Trial n The Average Molarity of the EDTA solution was calculated as follows: Average Molarity of EDTA Solution =Calculated molarity from each trialNumber of titration trials To determine how closely the results of each trial from the Standard Disodium EDTA titration (Table 1) agreed with each other, a relative average absolute deviation calculation was done. It was calculated as follows:

Absolute deviation for trial n= Na2EDTA avg molarity-Total Na2EDTA Trial n
absolute deviations3Na2EDTA avg molarity×1000=Estimated Prescision (ppt) The results of the above calculation came out to be 14.45 ppt. With the expected range of precision being at most five parts per thousand, it has been observed that the molarity of the standard Disodium EDTA solution includes some obvious error. It seems that the observation of color change when Na2EDTA was titrated to the mixture of CaCO3, DI water, Eriochrome Black T, and ammonia/ammonium chloride was the most error prone. Not knowing when to stop titrating the EDTA solution, would have accounted for a misreading of measurement for the total volume of EDTA solution added to complete the titration, making the final molarity for that trial less than
what was expected.

Table 2

Based on the molarity results from table 1 and the data from table 2, the average calculated water hardness of unknown water sample 40 was calculated to be 250.9 ppm CaCO3. The water hardness of unknown water sample 40 for each individual trial was calculated as follows:

=Total EDTA soln Delivered L0.02500 L CaCO3 soln×0.004197 mol EDTA1 L EDTA soln×1 mol CaCO31 mol EDTA×100.1 g CaCO31 mol CaCO3×1000mg1.0 g Average Water Hardness (ppm)≡mg CaCO31 L CaCO3
Interpretation of Results
Based on the data, the calculated water hardness of the unknown sample number 40 (250.9 ppm), is within range of the city of Phoenix’s calculated range of 164-291 ppm (2) and the city of Tempe’s calculate range of 150-400 ppm (3). References

1. Complexometric Determination of Water Hardness, Procedures, Mesa Community College CHM152LL website,, accessed 9/7/2013. 2. City of Phoenix website., accessed 9/8/2013. Copyright 2013 3. City of Tempe Az website., accessed 9/8/2013. Copyright