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Dillon SampsonChem 111Section 104April 3, 2012Determining the Hardness of Water Using Multiple TechniquesGroup Members:Dillon SampsonKyle SamSydney ShermanMike ShawTA: Nick McCoolIntroduction:Water Hardness refers to the concentration of Mg2+ and Ca2+ along with other polyvalent cations in a water sample. Water increases in hardness as it picks up magnesium and calcium cations from the earth (1). Due to its polarity water has an overall negative charge. This charge is what attracts (or picks up) these positively charged magnesium and calcium cations and their compounds. In theory, water hardness increases as the amount of contact with soil and rock increases because more of these cations are taken up (dissolved). The United States Geological Survey (USGS) on water quality gives a scale on the hardness of water. Usually water hardness is measured in terms of the levels of calcium carbonate. A water sample with a greater amount of calcium carbonate (by concentration) would be considered harder, and softer with less concentration. The USGS scale puts the water into categories of soft, moderately hard, hard, and very hard based on the calcium carbonate concentration (2,3). While it has little affect on health, the hardness of natural waters is relevant when dealing with washing processes, plumbing, and in industrial boilers and evaporators. Calcite deposits which are known as scales form that can cause various problems including energy inefficiency, blockages, and expenses for removal (1,4). There are multiple ways to measure water hardness including TDS (total dissolved solids), EDTA (ethylenediaminetetraacetic acid) titration, and AA (atomic absorption spectrophotometry). The presence of cations in water is commonly measured in parts per million (12). EDTA titration uses a complexation reaction to determine quantitatively the total amount of divalent cations in a sample of water using ethylenediaminetetraacetic acid as a chelating agent. The EDTA chelating agent along with an NH3/NH4 buffer and a high pH indicator (erichrome black T, or EBT) are used in the titration. The EDTA reacts with the cations to basically remove them from the sample and soften the water. The buffer is used to set the pH to about 10 in order for the blue form to predominate. Once EBT is added it should be in its blue form. More drops of EDTA required to turn the water blue meant the water sample was harder. The harder water had a higher number of ions and therefore required more EDTA to be fully chelated. The point in the well set where the color changes from red to blue is the point at which full chelation has occurred. Using the amount of EDTA to fully chelate the sample ions, the number of ions in the sample can be determined. AA is another way to determine the concentration of ions in a sample, but is much different than EDTA titration. AA uses a light source set to the wavelength and energy of the atoms in question, Mg2+ and Ca2+ in this case, which is shined through the sample being burned inside the instrument. The light absorption level is proportional to the ion concentration of the water sample. An AA instrument is calibrated to the specific targeted ion under observation. Only one absorbance level can be determined at a time (Mg2+ value separately obtained from the Ca2+ value) (1). While EDTA provides values for concentration, AA gives values for light absorption which must be converted over to molarity. AA offers a more consistent procedure, but it is not necessarily more accurate. The biggest difference in AA and EDTA titration is the technology involved with AA. The addition of Baking Soda (NaCO3) and cation exchange resin were the two methods for softening the sample. In both cases cation exchange occurs where the divalent cations (Mg2+ and Ca2+) take the place of the softer monovalent cations (Na+ and H+) in the commercial water softener and the resin. The sample of water taking on the softer monovalent cations then becomes “softer” (5). Water samples in State College taken from the first floor of the HUB (6), the second floor of the HUB (7), the third floor of the HUB (8), and the filtration station on the second floor of the HUB (9) were analyzed to determine hardness. State College receives water from the same ground water supply. The State College water is relatively hard with calcium concentration between 180-220 parts per million (10). The only variability in samples taken across State College should be from filtration setups and aged piping. Since our samples were taken from the same building the piping should play no factor in the hardness of each sample. It is predicted that the samples taken from the three floors will have equal hardness values (divalent ion concentrations) because they are from the same source and flow through the same piping system. The water sample taken from the filtration station will be softer because the system removes impurities.Procedure:In order to test differences in water from State College the four samples had to be taken. Three of the samples were taken from taps located in the bathrooms of the designated floors (1st,2nd, and 3rd). The fourth sample was taken from a filtered water tap on the second floor. The PSU Chemtrek Lab Manual provided the procedure for going about testing the hardness of the samples. Using the AA spectroscopes provided in lab, atomic absorbance values were determined for Mg2+ and Ca2+. The concentration values in absorbance were recorded along with the AA standards and check standards for each cation’s concentration (in parts per million) (1,8). Plotting the values (Figure 1 and 2) provided a linear trend line that could be used to determine the concentration values for both cations in parts per million. By using a tin foil piece and a hot plate TDS (total dissolved solids) could be found. By using the “good transfer technique” the samples, distilled water, and Ca2+ solution were placed on the tin foil until fully evaporated. Samples were compared with distilled water and a Ca2+ (1.0x10^-3M) solution and the results were recorded. The residue left behind determined the hardness of the water (the more residue, the harder the water). In some cases sample had to be diluted to proceed with EDTA titration. EDTA titration involved placing different volumes of EDTA in each of a twelve well set in order to examine color, and then determine concentration of the sample (as stated above). Determining the concentration utilized the equation (1,8): Equation 1: MEDTA x VEDTA = MCa2+ x VCa2+This equation can be used to determine the concentration of Ca2+ in a sample using the number of drops of EDTA taken to produce a color change in the well set. A similar equation was used for Mg2+. Once the concentration was obtained, it was then converted to parts per million (ppm) and grains per gallon (gpg) using the equations (1,8):Equation 2: (M/V)*(100g/1 mole)*(1000mg/1g)=ppmEquation 3: ppm*(1 grain per gallon/17.1 ppm)=grains per gallonOnce the color changed to blue in the well set, the determined the volume (measured in drops by a previously calibrated microburet) of EDTA required to make the color change was plugged into the equation 1 to determine concentration of the sample. EDTA titration was repeated to note the change in hardness after the application of a commercial softening agent (baking soda) and divalent cation exchange resin (1,8).Results: (6,7,8,9)The results of TDS for the four samples and control samples (distilled water and CaCl2) are listed below. A description of each is provided in comparison to a reference sample.Table 1.Total Dissolved Solids (TDS) Observations of Different SamplesWater SampleObservationCaCl2 Sample (Reference)A feint white residue circle was left behind.Distilled WaterLittle to no white residue was left, less than the reference.Water taken from a tap on first floor of the HUB (6)A white ring of residue was left that was darker than the reference with a defined outer ring.Water taken from a tap on second floor of the HUB (7)A white ring of residue was left that was darker than the reference with a defined outer ring.Water taken from a tap on the third floor of the HUB (8)A white ring of residue was left that was darker than the reference with a defined outer ring.Water taken from a filtration station on the second floor of the HUB (9)A white ring of residue was left that was slightly darker than the reference, no defined outer ring.The calibration graphs of AA for Ca2+ (figure 1) and Mg2+ (figure 2) are shown below (8). The graphs show absorbance values for calcium and magnesium (respectively) vs. concentration in ppm. A trend line is shown with an equation for it, along with an R2 value to show point deviation.Figure 1. Figure 2.Below is a table of absorbance values obtained through AA and the corresponding calculated concentrations.Table 2.AA and Hardness Values Calculated for all SamplesSample/IonsAA Absorbance Concentration (ppm)Hardness (ppm CaCO3)First Floor HUB Tap (6)Total: 230.8155Mg2+0.499631.1871128.3420Ca2+0.393240.9894102.4735Second Floor HUB Tap (7)Total: 229.2492Mg2+0.493130.7677126.6162Ca2+0.393841.0532102.6330Third Floor HUB Tap (8)Total: 241.6594Mg2+0.514932.1742132.4041Ca2+0.418743.7021109.2553Second Floor Filtration Station (9)Total: 256.4138Mg2+0.528833.0710136.0945Ca2+0.460348.1277120.3193To find the concentrations of Mg2+ and Ca2+ in parts per million the absorbance values were plugged in as the y value in the equation on the graph that corresponds to the ion (8).Equation 4: y=mx+bThird Floor HUB sample calculations:Mg2+ 0.5149=0.0155x+0.0162 x=32.174ppmCa2+ 0.4187=0.0094x+0.0079 x=43.7021ppmTo find the overall hardness of the sample in terms of divalent cations the ppm values for Ca2+ and Mg2+ must be converted into ppm of CaCO3 through the equations shown below. The values are then added together to get the overall hardness in terms of ppm of CaCO3. The calculations for the third floor tap of the HUB are shown below.Equation 5: (ppm Ca2+)*[(100g CaCO3/1 mole)/(40.0g Ca2+/1 mole)](43.7021 ppm Ca2+)*[(100g CaCO3/ 1 mole)/(40.0g Ca2+/1 mole)]=109.2553 ppm CaCO3Equation 6: (ppm Mg2+)*[(100g CaCO3/1 mole)/(24.3g Mg2+/1 mole)](32.1742 ppm Mg2+)*[(100g CaCO3/1 mole)/(24.3g Mg2+/1 mole)]=132.4041 ppm CaCO3Equation 7: (ppm value 1)+(ppm value 2)=total hardness(109.2553 ppm CaCO3)+(132.4041 ppm CaCO3)= 241.6594 ppm hardnessTable 3.EDTA Results for Water SamplesSampleDilution Factor (parts sample: parts distilled water)Drops Needed for Color ChangeHardness (ppm)First Floor HUB Tap (6)1:35400Second Floor HUB Tap (7)1:17280Third Floor HUB Tap (8)1:34320Second Floor Filtration Station (9)1:19360To calculate the hardness CaCO3 from drops of EDTA the following equation below (Equation 8) was used. The calculations for the Third Floor of the HUB tap are shown (8).Equation 8: MEDTA * VEDTA = MSample * VSample(2x10^-4M)*(4 drops EDTA)=MSample*(1 drop sample) Msample=(8.0x10^-4 M)*4 (dilution factor)=3.2x10^-3M(.0032 moles CaCO3/1 L solution)*(100.0g CaCO3/ mol CaCO3)*(1000 mg/g) =320ppmTable 4.The Hardness Values For Water Samples After Softening TechniquesSampleCommercial Softening Agent (ppm)Divalent Cation Exchange Resin (ppm)First Floor Tap (6)480<96Second Floor Tap (7)576192Third Floor Tap (8)28880Filtration Station (9)384200The two water softening techniques were applied to all samples. The concentrations were determined through the same EDTA titration equations above.Discussion:The results are slightly varied from technique to technique. The TDS test showed that the taps of the three floors of the HUB had about the same residue. They all left a white circle with a defined outer ring that was darker than the reference (CaCl2), meaning more impurities (cations) were in the water samples. The second floor filtration station left a circle darker than the reference, but not quite as dark as the other three samples. It makes sense that the filtered water would leave behind less residue. In the AA tests however, the filtration sample was the hardest sample at approximately 256 ppm. The second hardest was the third floor tap sample at approximately 242 ppm. The first and second floor taps showed extremely close results, both near 230 ppm. Based on the AA tests, the filtration station seemed to produce water that was surprisingly harder than the taps of the three floors in the HUB. The EDTA results followed the same pattern as the AA analysis with the exception of the first floor tap. The first floor tap came out much harder than the rest at 400 ppm. The next three were the filtration at 360 ppm, followed by the third floor tap at 320 ppm, and finally the second floor tap at 280 ppm. While these results show a steady decline of 40 ppm, the arrangement is quite random.The hypothesis that the tap water samples from the three floors are equal in hardness was incorrect. Also, it was incorrect in saying that the second floor filtration station would give a softer sample of water. Based on the AA results in Table 2, the filtration station provided water that was harder than the other three floors. With the exception of the first floor, the filtration station was harder than the taps in EDTA as well. In the EDTA titration there may have been sources of error that skewed the results of the first floor tap sample. The third floor appeared to be harder than the other two taps. There may have been scale buildup in the upper floor’s piping that lead to a harder water sample. If the pipes had been more recently replaced on the lower floors the samples would be softer, as seen in the AA results. While it was assumed that the filtration station would remove these divalent cations from the water, it proved false. The filtration station may only remove large debris and particles and play no role on the concentration of Mg2+ and Ca2+ along with other polyvalent cations in the water. Based on the results it seems that the station increased the hardness of the water. An explanation for why the samples gave these results is that the samples were obtained on different days of the week. Though the water comes from the same source, the water may have been harder on the day the filtration station sample was taken compared to the other three. It may have also been harder on the day the third floor tap water was taken in comparison to the first and second floor taps. All of the samples had calcium hardness lower than the 2008 State College study, where the hardness was between 180 and 220 ppm (10). Perhaps in the past few years measures have been taken to soften the water in State College. The samples are still considered “hard” based on the 120-180 ppm scale (11).EDTA can provide us with an analysis of total divalent cation concentration while AA analyzes specific ions. AA seems to be more precise and accurate. EDTA has more room for error because there is more human involvement and steps taken. Some possible sources of error include the wells not being completely clean or dried. The wells could dilute the solutions further where it may not be appropriate if they are not dried fully. They could also provide unwanted contaminations if not properly cleaned. Another source of error is in the microburets, considering not all drops come out the exact same size. Small pressure differences can produce different sized drops, along with the speed at which the drops come out. Observational error could play in when judging the color change from red to blue in the wells. People may observe a well as blue when it is actually in an intermediate phase of red and blue. Also, if the appropriate amount of EDTA to fully chelate the ions lies between two drops, the results are either too high or too low. A source of error in the AA analysis is in the check standards. The AA values, although close, were not exactly what they were supposed to be and so the absorbance values may have been slightly inaccurate. The two softening techniques worked in opposite ways. The commercial softening agent actually increased the hardness of all four samples. There was perhaps error involved that produces inaccurate results. The agent may have been applied incorrectly, or it may have been a faulty batch of product. The more effective of the two techniques was clearly the exchange resin. The resin worked extremely well, softening the water by more than 100 ppm in two of the samples (third and first floor taps), and more than 50 ppm in the other two (filtration station and second floor tap). The resin, which is made primarily for substance exchange, worked much better as it replaced divalent cations from the water with monovalent (softer) cations.Conclusion:Our hypothesis was almost entirely incorrect since the filtration proved to be the hardest sample and the values for the taps of the three floors were not all the same. The only accuracy was that the first and second floor taps had hardness values that were within 2 ppm of eachother. The second floor filtration station had the hardest water. The third floor tap had the second hardest water sample. The first floor was the third hardest by AA analysis, although the EDTA titration showed it as being the hardest which is likely attributed to error. The second floor tap, although very close to the first floor tap, was the softest sample. The results show that State College water is relatively hard. The softening results showed that certain softening agents (in this case cation exchange resin) can work remarkably to soften a water sample.References:(1)PSU Chemtrek by Stephen Thompson, Prentice Hall, Englewood Cliffs, NJ 2011-2012 version 10.0-10.22(2)Briggs, J.C., and Ficke, J.F., 1977, Quality of Rivers of the United States, 1975 Water Year -- Based on the National Stream Quality Accounting Network (NASQAN): U.S. Geological Survey Open-File Report 78-200, page 436 (3) Water Hardness and Alkalinity . (accessed March 26, 2012).(4)Hard Water Problems (accessed March 26, 2012)(5) Brown, Theodore. Chemistry: The Central Science, 11th Edition; Prentice Hall. Upper Saddle River, 2009, pg 786(6) Sam, Kyle, Chem 111 Laboratory Notebook, pp. 24-26(7) Shaw, Mike, Chem 111 Laboratory Notebook, pp. 26-31(8) Sampson, Dillon, Chem 111 Laboratory Notebook, pp. 28-32(9) Sherman, Sydney, Chem 111 Laboratory Notebook, pp. 27-30(10) State College Water may Cause Kidney Stones. . (July 7, 2008)(11) Water Resource Characterization Dss. (n.d.). Hardness. (accessed March 27, 2012) water.ncsu.edu/watershedss/info/hardness.html(12) Facts About Your Water.?City of Philadelphia. (accessed March 29, 2012) ................
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