The Chemistry of Natural Waters

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The Chemistry of Natural Waters
Introduction:
Water will be taken from 4 different taps. The first will be Hamilton hall in the west residence halls. The second from Bigler hall in East residence halls. The third from University Towers on College Avenue. The fourth will be taken from Heritage Oaks apartment complex near Wal-Mart. My sample will be taken from Hamilton Hall in the west residence halls. I expect the water from the residence halls to be softer than the water taken from the apartments off of the Penn State University Campus. I believe this because the water on PSU campus is controlled by Penn State and I believe that they would expend more resources to make the water on campus softer for the students. I think the water off campus will be relatively hard because the apartment complexes do not have the resources to soften the water as Penn State University does, and water softening is not a very large concern. I will approach the analysis of these waters in two different ways. I will use Atomic Absorbance to measure the concentration of Mg2+ ions and Ca2+ ions with an atomic absorption spectrophotometer. I will also test the water using EDTA which measures the total solids dissolved in the water.

Procedure:

To begin this experiment I took my water sample to the Atomic Absorption Spectrophotometer. This analyzed the light absorption of two ions: calcium and magnesium. To continue this experiment I performed EDTA titration to find the hardness of the water and used this to compare with the Atomic Absorption concentration of magnesium and calcium. To perform the EDTA titration I followed section C and D of my Chemtrek lab manual. After determining the hardness I softened the water using Calcium carbonate. All of these steps are elaborated upon in the Chemtrek lab manual (1).

Results:
Graph 1
PlaceAA Ca2+ concentrationLight absorbance of Ca2+AA Mg2+ concentrationLight Absorbance of Mg2+ Hamilton Hall(7)31.17mg/L0.247021.50mg/L0.5255
Bigler Hall(8)47.06 mg/L0.373227.88 mg/L0.6691
University Towers(9)44.88mg/L0.355922.54mg/L0.5481
Heritage Oaks(10)23.25mg/L0.184116.99mg/L0.4276

These values were then compared to the calibration graphs for the Atomic Absorption Spectrophotometer for that day. These graphs are given on the next page. These graphs were used to derive a concentration of each ion given the light absorbance of each ion. The equation for each ion is given below(7): Equation 2

Equation 1
124.93(.2470)+.1633=31.02mg/L 62.094(.5255)-3.192=21.47mg/L

Chart 1

Chart 2

These concentrations of magnesium and calcium ions were then converted to parts per million so that they could be compared adequately. They were converted the following way(7): Equation 3
31.17mg/L*1l/1000g of water=31.17mg/1000g=31.17 parts per million

Table 2

PlaceCa2+ ConcentrationMg2+ Concentration
Hamilton Hall(7)31.17ppm21.50ppm
Bigler Hall(8)47.06ppm27.88 ppm
University Towers(9)44.88ppm22.54ppm
Heritage Oaks(10)23.25ppm16.99ppm

These numbers were then combined to reach a total hardness of the water. This was reached by using the equation below: Equation 4(3)

((31.17ppmCa2+)*2.5ppmCaCO3)+((21.50ppmMg2+)*4.12ppm CaCO3=166.5ppm total hardness

Table 3

PlaceTotal Hardness
Hamilton Hall(7)166.5ppm
Bigler Hall(8)232.5ppm
University Towers(9)205.1ppm
Heritage Oaks(10)128.1ppm

Results for the EDTA titration were as follows

PlaceTotal Hardness
Hamilton Hall(7)240ppm
Bigler Hall(8)480ppm
University Towers(9)310ppm
Heritage Oaks(10)200ppm

These values were reached by serial titration in the case of the Hamilton Hall water reached in this manner(1): Equation 4

Molarity of EDTA*Volume of EDTA=Molarity of Ca2+*Volume of Ca2+

2*10^-4M*12=X=.0024M

This was then converted to parts per million using...
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