Experiment #10

 

The Chemistry of Natural Waters

 

Lab Report

 

Ali Rowell

 

Group: Nate Rotunda, Matt Sabo, Dave Rumbaugh

 

11/16/06

 

Chem 014 Section 105

 

Introduction:

Living creatures need fresh water in order to survive, whether it be plant or animal. Thus, it is important to understand to chemistry of natural water. Water is considered hard when there are high concentrations of the divalent cations Magnesium and Calcium. It is considered soft when there is a low concentration of the two [8]. Hardness can also be divided into carbonate hardness (calcium and magnesium hydrocarbonates) and non-carbonates (calcium and magnesium salts of strong acids) [3]. Carbonates, when boiled leave a precipitate. Water hardness depends on which geological strata it had contact with, limestone and chalk areas have hard water. Soft water tends to come from areas with granite and other impermeable rocks [2]. The water does not become absorbed and percolated through the rock and thus does not pick up divalent cations such as magnesium and calcium. Other minor cations associated with hardness are Strontium, Iron, and Manganese. The major natural sources of Calcium and Magnesium are “soils and rocks containing limestone, dolomite & gypsum (calcium sulfate), and small amounts from igneous and metamorphic rocks” [4].

Water hardness is very important. Scale formation is when large deposits of Calcium carbonate form in pipes, jets, tubes in industrial boilers and evaporators. Pipe volume becomes smaller and heat transfer is made less efficient by these mineral deposits [8]. When it comes to cleaning with very hard water, a scum will form and the object will not become clean until a water-softening agent is used. If the water has a hardness and concentration of calcium carbonate of greater than 500 mg/liter, it will have a very unpleasant taste [2]. However, Calcium and Magnesium do not promote health problems in humans. Water is considered soft 0-75mg/L, at 76-150mg/L is moderately hard, at 151-300mg/L is hard, and anything over 300mg/L is considered very hard [2].

EDTA is ethylenediaminetetraacetic acid is used to analyze the hardness of water. In a titration method, the EDTA complexes with Magnesium and Calcium and the indicator eriochrome black T (EBT). The bluer the water, the softer it is [8]. This method is a rather inexpensive way to calculate water hardness. Atomic Absorption Spectrophotometry (AA) is an expensive, but accurate method of determination. The water sample is aspirated into a flame in the device. A light beam is directed at the flame onto a detector which measures the amount of light absorbed by the aspirated water. The source lamp is made of the element, because each element has it owns wavelength, one of the reasons this method is very expensive. “The amount of energy at the characteristic wavelength absorbed by the flame is proportional to the concentration of the element in the sample over a limited concentration range”[1]. Both methods were used to determine the hardness of the water samples.

            Samples for this lab were taken from Long Island Sound, CT, private well water in Berwyn, PA (southeastern), a water fountain on the 7th floor of Snyder Hall at PSU, and the 4th sample came from East campus as well.

Procedure:

            The full procedure can be found in PSU Chemtrek: Small-Scale Chemistry for General Chemistry. Written by Stephen Thompson, edited by Joseph T. Keiser for August 2006-July 2007, published by Hayden McNeil, and located on pages 10-1 to 10-22.

Section A involved the Determination of Water Hardness by Atomic Absorption Spectroscopy. The water sample was aspirated into the flame for the calcium device and the magnesium device. Hardness was determined with a calibration graph discussed later in report. In Section B a water sample is placed on aluminum foil on a hot plate in order to evaporate the sample sequentially to determine Total Dissolved Solids.  Section C involves the Divalent Cation Analysis by EDTA titration.  Serial titrations are then utilized: add one drop of the 0.001M calcium solution to each of 12 wells and then add one drop of EBT to each well. Then add on drop of the NH3/NH4Cl/MgEDTA buffer to each well. To titrate add one drop of .0002M EDTA to first well, 2 drops in 2nd well, 3 drops in 3rd well, etc. The first blue well is considered the “endpoint” and is how you calculate hardness. Section D determines the hardness of the water sample using the same serial titration method as Section C, except you add one drop of the water into each of the 12 wells. Hardness, again, is determined by the endpoint. In Section E, the water sample is treated with a water-softening agent, washing soda. Twenty milligrams of the agent is placed in a vial with the water sample and dissolved. The serial titration method used in Section D is used again with the now softened water. Section F involves adding resin to the water sample in order to remove divalent cations and replace them with ions. The resin and water are placed in a vial, shaken and then allowed to settle. A microburet is used to collect the liquid in the vial and use the same serial titration method in Section D. Section G explains how to calculate and report the hardness of water in parts per millions and grains per gallon.

 

 

Results:

Section A (AA):

Sample Name

Calcium Absorption (nm)

Magnesium Absorption(nm)

Well Water[9]

0.1550

0.2559

Long Island Sound Water [10]

-

1.0366

Synder Hall Drinking Water [11]

0.4925

0.5228

East Campus Water [12]

0.4130

0.4617

These are the values obtained using the AA device. The absorption value is taken at 202.5 nm. 

Industry Values (table given on 10/26/06) [9]

Calcium Concentration(ppm)/

Magnesium Conc. (ppm)

Absorbance Value (nm)

Calcium/Magnesium

Check Standard (ppm)

Calcium/Magnesium

0

          -

        -

1.000

0.02408/  0.3800

0.82/  1.15

5.00

0.11075/  0.16408

4.66/  4.89

10.00

0.21627/  0.31223

10.97/  10.12

25.0

0.40533/  0.70112

25.55/  25.68

50

0.65849/  0.81996

50.28/  30.20

 

The chart above and the graphs below are used to calculate a calibration curve. The equations for the Calcium and Magnesium graphs are used to calculate the hardness ppm for each respectively.

 

 

Calibration Graphs [9]

y=0.013x + 0.0385 Calcium Absorbance

y=0.0169x + 0.0828 Magnesium Absorbance

     

 

Sample

Calcium Conc. (ppm)

Magnesium Conc. (ppm)

Total Hardness (ppm/grains per gallon)

Well Water[9]

10.75

68.7

79.5/  4.65

Long Island Sound [10]

1164

3853.5

5017/  293.4

Snyder Hall [11]

90

83

173/  10.11

East Campus [12]

71.2

66.75

137.95/  8.07

These values are obtained using the calibration equations respectively. Using the equation to calculate calcium carbonate hardness, the total hardness is calculated for the water samples.

Section B (evaporation & TSD):  The water sample left a light white ring with some white in it (TSD present). The distilled water left nothing and the calcium solution (TSD present) left a very faint white ring.

Section D (Hardness Determination using EDTA titration):

Sample

Molarity of Calcium and Magnesium (mol/L)

Total hardness (ppm/grains per gallon)

Well Water [9]

0.001

100/  5.8

Long Island Sound [10]

0.0576

5760/  337

Snyder Hall [11]

0.0018

180/  10.5

East Campus [12]

0.0012

120/  7.01

Using the serial EDTA titration method, the hardness is calculated by identifying the endpoint and thus calculating the molarity. The molarity is then used with the calcium carbonate equation to determine the total hardness of the water sample.

Section E (Water-softening agent)

Sample

Hardness (ppm/grains per gallon)

Well Water [9]

80/  4.7

Long Island Sound [10]

4500/  263

Snyder Hall [11]

160/  9.4

East Campus [12]

120/  7.01

Washing soda is added to the water sample in order to soften it. The titration method is used and the same calcium carbonate conversion. Washing Soda’s active ingredient is Sodium Carbonate.

Section F (cation/ion exchange with resin)

Sample

Hardness (ppm/grains per gallon)

Well Water [9]

0/  0 (all wells were blue, no endpoint)

Long Island Sound [10]

3000/  175

Snyder Hall [11]

40/  2.3

East Campus [12]

28/  1.63

Resin beads are added to the water sample and the water is softened again. The well water is softened to the point where all wells are blue and hardness is almost undetectable.

Section G (Calculations) [9]

0.002M of Calcium solution equals 250ppm hardness and 14.6 grains/gallon

Part B is utilized in the charts for AA.

Discussion:

            The well water results for the AA and EDTA were very similar with values of 80ppm and 100ppm respectively. Values for Long Island Sound were both in the 5000ppm range. Both samples of the campus water were within the 10-20ppm range concerning the AA and EDTA difference. Of course, the AA results should be more accurate because there is less chance for human error, this is a good advantage. It is also a good technique to determine metals dissolved or suspended in the solution, just not Magnesium and Calcium. This benefit provides for a setback. There must be a separate AA device for each metal tested, because the lamp must be made of the element, not to mention the expense of the actual device minus the lamp. To determine hardness and other factors, AA is a pricey device and illogical for anyone besides university and large company labs [8]. EDTA titration is a more practical, and inexpensive approach if the person knows how to conduct the procedure. Besides obvious chances for human errors, EDTA has one fault. If the water sample does not have a sufficient amount of Magnesium it will be extremely hard to titrate the sample to distinguish the endpoint. As evident with the results, there does not seem to be a huge difference in values of hardness for EDTA and AA.

The two samples taken from campus had similar hardness values. This makes sense because one would assume campus water, especially when coming from the East side of campus, comes from the same water source. Differences in the two could occur because of where the water was taken from (water fountain or faucet?) and also human error in lab procedure. According to the State College Borough Water Authority, its water has a hardness value of 180-240ppm [6]. This is considered hard water. The two campus samples fall between moderately hard and hard levels. State College is visibly surrounded by mountains and sediments so the water hardness makes sense. The water from Long Island Sound can be considered a mixture of fresh water and seawater and thus the high level of hardness is explained. In seawater Magnesium is found at a concentration of 1,350 mg/L and Calcium at 410mg/L [8]. The well water from Berwyn (Philadelphia suburb) fell more on the soft to moderately hard scale. As a reference, Bryn Mawr and the Philadelphia suburbs municipal water’s raw hardness value was 109mg/L [4]. A map of Chester County and my specific town showed deposits of azoic slates, trap, and hydromica schists. All of which are medium-grade metamorphic rocks [5]. I would say my hypothesis of expected hardness and its sources are correct as demonstrated by the observations made above.

Conclusion:

            Using the AA device and EDTA titration, four water samples were tested for water hardness. “Water containing a relatively high concentration of Calcium, Magnesium, and other divalent cations is called hard water” [13]. These ions are obtained by way of the water permeating through various rocks such as limestone and gypsum. In respect to AA and EDTA results: well water had 80/100ppm, water from Long Island Sound had 5017/5760ppm, Snyder Hall water had 173/180, and East Campus had 137.95/120ppm. With the exception of Long Island Sound which is EXTREMELY hard water, the water samples from Pennsylvania laid in the low end of moderately hard to the low end of hard [2].  In the lab, the water samples were also softened. Washing soda containing sodium carbonate was added to the water samples. Resin beads were also added to the water samples to perform a cation-ion exchange like with washing soda. Sodium ions are bonded with an anionic group which the calcium is attracted to. Softened water has a higher concentration of sodium ions (not a problem unless you have high blood pressure) [13]. From my research mentioned in my discussion, I can reasonably say the results of the AA/EDTA tests are legitimate. They seem to correlate with the high hardness value for seawater [8], State College value range of 180-240mg/L [6], and due to Philadelphia suburban values of 109ppm and the geological deposits in the area, the well water is reasonable [4 & 5]. We care about water hardness because of scale formations in pipes, jets, etc. in industry and the fact Calcium and Magnesium join with soap to create a precipitate (curd) and it thwarts the formation of a lather [4].

References:

[1]“Water Quality Monitoring a Practical Guide to the Design and Implementation of       Freshwater Quality Studies and Monitoring Programmes.” Chapter 8 Advanced            Instrumental Analysis. pg.1. CHEMnetBASE (PSU Library article site). 12  November 2006.            http://www.crcnetbase.com/books/1384/TF7304_CH08.pdf#xml=http://www.crcnetbase.com/search/pdfhits.asp?DocId=2                3&Index=%5c%5c172%2e16%2e1%2e41%5cSearchIndices%5cApplications%5c1049&HitCount=6&hits=68+69+6a+6      +6d+6e+

[2]“Microbiology and Chemistry for Environmental Scientists and Engineers, 2nd   Edition.”Chapter 11 Water Quality Chemistry. pg 7-9. CHEMnetBASE Article           Search.            12 November 2006.            http://www.crcnetbase.com/books/2507/TF680X_ch11.pdf#xml=http://www.crcnetbase.com/search/pdfhits.asp?DocId=4                06&Index=%5c%5c172%2e16%2e1%2e41%5cSearchIndices%5cApplications%5c1&HitCount=18&hits=81f+820+847+                48+85e+85f+88c+88d+895+896+8f8+8f9+903+904+97b+97c+9ca+9cb+

[3]“Water Quality Assessments: A Guide to the Use of Biota, Sediments, and Water in     Environmental Monitoring, 2nd Edition.” Chapter 3 Selection of Water Quality       Variables. pg. 17. 12 November 2006.

http://www.crcnetbase.com/books/2685/TF6006_Ch03.pdf#xml=http://www.crcnetbase.com/search/pdfhits.asp?DocId=4474&Index=%5c%5c172%2e16%2e1%2e41%5cSearchIndices%5cApplications%5c1&HitCount=10&hits=1544+1545+1562+1563+15ec+15ed+2b57+2b58+3129+312a+

[4]The Water Encyclopedia. 2nd Edition. Geraghty & Miller Ground Water Series. Frits      van der Leeden. Lewis Publishers 1990. pages 424, 452, and 429.

[5]Geological Map of Chester County, PA.   www.libraries.psu.edu/emsl/guides/X/Chester.jpg

[6]State College Borough Water Authority. “Water Authority Frequently Asked Questions.”

            http://www.scbwa.org/faq.htm

[8]PSU Chemtrek: Small-Scale Experiments for General Chemistry. August 2006-July      2007. Stephen Thompson. Hayden McNeil. Edited by Joseph T. Keiser.            Experiment #10 The Chemistry of Natural Water. Pg.10-1 to 10-22.

[9[Well Water Sample: Ali Rowell Lab Notebook pg. 25-27.

[10]Long Island Sound Water Sample: Dave Rumbaugh Lab Notebook pg. 49-52.

[11]Synder Hall Water Sample: Nate Rotunda Lab Notebook pg. 27-29.

[12]East Campus Water Sample: Matt Sabo Lab Notebook pg. 36-41.

[13]Chemistry: The Central Science 10th Edition. Theodore L. Brown, H. Eugene   LeMay Jr., Bruce E. Bursten. Upper Saddle River, NJ. Pearson Prentice Hall.        2006. page 790.

 

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