Chemistry 111 Experiment 10 The Chemistry of Natural Waters

Transcription

1 Chemistry 111 Experiment 10 The Chemistry of Natural Waters Greg Wiltsie November 11, 2009 Chemistry 111 Experimental Chemistry Section 105 Group Members Greg Wiltsie, Gene Williams, Madhu Yennawar, Kyle Witmer, Dave Zatezalo, and Binyang Zhang. TA Bill Chartte 1

2 Introduction Water hardness is described by Dr. William Wurts at Kentucky State University as a measure of the quantity of divalent ions such as calcium, magnesium and/or iron in water. There are many different divalent ions that are used in measuring the hardness of water, but the two most common ions are magnesium and calcium. The hardness of water is important in that more soap and detergents for laundry and washing is needed in hard water, and scaling is increased in boilers and in other industrial equipment (USGS). According to Virginia Tech, hard water in households can cause problems by decreasing the life of household plumbing and other appliances that use water, the decreased efficiency of water heaters, and hard water stains and deposits on pots, pans, and in sinks. It was reported by the 1997 National Water Quality Survey that one out of five Americans were dissatisfied with the quality of their water due to hard water. (Virginia Tech) The Water Quality Association lists some problems that are associated with hard water. Some of these problems include pipe clogging up, appliances that use water clogging up, mineral deposits on coffee pots, or reduced water flow. Other problems include soaps and detergents to not have the suds that they normally have and a film covering showers, tubs, or even people. (WQA) Although none of these problems are massive, they are an inconvenience to anyone that goes through it. Having water that is too soft may also be a problem, so a good level of calcium carbonate must be maintained in a water source for good health. An example of a health condition caused by water that was too soft was found by Anderson, et al. in the Canadian Medical Association Journal in 1975, which stated that the high death rates that were found in the areas of research were due to a suboptimal intake of magnesium, and that water-borne magnesium exerts a protective effect on the residents of hard-water areas. (Anderson) This proves that any attempt to soften water from a hard state should be monitored to ensure that all amounts of magnesium in calcium carbonate are not removed from the water. Water hardness is measured by the amount of milligrams per liter of calcium carbonate in water. The guidelines for hardness from the USGS can be found in table 1. 2

3 Table 1 General Guidelines for Classification of Hardness Amount of Calcium Carbonate in Water (mg per L) Hardness Classification 0-60 Soft Moderately Hard Hard Greater than 180 Very Hard Data from U.S. Geological Survey Table 1 shows data from the USGS that determines the hardness classification of a water sample based on the amount of calcium carbonate (in milligrams) found in that water (per liter). Water hardness can be tested by a variety of methods, either for home use or by laboratories, with labs doing two main tests: EDTA and AA. In any test that is done to determine water hardness, the amount of calcium and magnesium in a water sample, either listed in grains per gallon, milligrams per liter or parts per million (ppm). One grain equals about 17 milligrams per liter and equals one part per million. (Wilkes) Another test to determine the hardness of water is with the total dissolved solids (TDS) test, in which water is evaporated and the remaining residue is examined. (PSU Chemtrek) One of the methods to determine water hardness is ethylenediaminetetracetic acid (EDTA) titration. In this method of water hardness determination, an amount of the sample water is taken and the ph of that water is adjusted to 10 using a NH 3 /NH 4 buffer. After adjusting the ph of the water, a dye called eriochrome black T (EBT) is added to this solution. The dye turns the solution a blue color. After adding the EBT, the solution will turn a red color if magnesium is present in the water. EDTA solution, of which is a known concentration, is added to this solution. The EDTA will react with calcium in the solution to form a colorless chelate. After the EDTA reacts with all the calcium, it begins to react with the magnesium, returning the indicator to its blue color. At the end of the titration, the EDTA will react with all the calcium and magnesium in the water sample and the color will range from a red color to a blue color. There needs to be some amount of magnesium in the sample for this method to work correctly. (PSU Chemtrek) Another method used to determine water hardness is with atomic absorption spectrophotometry (AA). In this method, monochromatic light is passed through the water sample that is to be analyzed. Some of the atoms in the sample will absorb this light. The amount of light absorbed 3

4 by the sample is recorded and this amount is proportional to the concentration of the certain metal atom being tested for. A separate spectrophotometer is used for each of the metals being tested for, as the light being passed through the water sample has different energy levels for the different metals. Water having concentrations higher than certain levels will need to be diluted prior to testing with AA, since the analysis machine will only read the sample correctly is the concentrations are within certain values. The water samples for AA are placed in large microburets and the aspirator tube from the machine is placed into the sample. The machine will then analyze the sample and get a readout of the absorbance value for that sample, for either calcium or magnesium, depending on which machine was used. This value can be used with a chart of absorbance values and concentrations to determine the unknown concentration of that metal in the sample. (PSU Chemtrek) Both EDTA titration and AA will determine the hardness of the water sample. AA is more sensitive and can be used to test for any metal that is wanted. EDTA titration is more quantitative and is used to only test for calcium and magnesium. By using both methods of determining water hardness, we can compare the results to each other to determine the hardness of the water sample being tested. Hard water can be softened in a variety of methods. One method used in washing with hard water is with the fabric and dishwashing detergents, which are made with certain ingredients that are designed to complex these calcium and magnesium ions. According to the background information in the PSU Chemtrek, these ingredients in washing detergents are usually polyphosphates or citrates. For industries that use a higher volume of water, different methods of softening hard water are used. Again according to PSU Chemtrek, the addition of lime or washing soda to hard water causes the calcium and magnesium ions to precipitate out in the water. These precipitates can then be allowed to settle to the bottom and removed or are filtered out of the water, then the softened water is used in the process. Another method used to soften water is with ion exchange, in which resins of plastic beads with covalently bonded anions are added to hard water, and monovalent cations in the resin, such as sodium or hydrogen, are exchanged with divalent cations in the hard water, such as calcium. The cations in the water push off the sodium in the anions in the resins, causing the sodium to go into the water and the calcium to attach to the resin. In general, according to the Chemistry 110 textbook, for every one 4

5 calcium that is removed from the water, two sodiums are added into the water. The addition of sodium does not cause the problems that calcium and magnesium does in water, although those watching their sodium intake would need to watch the amount of water they take in that is softened in this method. (Brown, 788) This method of softening water is more expensive than other methods, so it is usually used only in instances of lower quantity applications. Filters can be attached to home water systems to soften hard water for household uses. Water softeners and water softening methods are used primarily to prevent the formation of calcite scale in pipes to prevent costly repairs and removal, and to make detergents more effective in cleaning when used in softened water. Therefore, the main reason that hard water is softened is to save time and money and to make our lives easier. My hypothesis is that my water sample from the Allegheny River will be softer than the other water samples taken, most of which come from residential drinking water sources. I think that the river water sample will be softer because the way the Allegheny River is, most of the water that enters the river comes directly from rainfall or by surface level runoff from level ground. These two methods of introducing water into the river will not introduce any magnesium or calcium into the river, as groundwater or water running over surface for a period of time would. There is some runoff from the mountains bordering the river that would introduce Mg 2+ and Ca 2+ ions into the river, adding some level of hardness to the water. Because most of the water that is in the Allegheny River does not have the chance to move through groundwater and pick up magnesium and calcium ions, the river water will have a lower level of hardness than water that has been through the groundwater. (Frank) Being that the other water samples were from either on the Penn State campus or from the surrounding area, I would take a guess that these samples will have higher hardness levels than my sample, because these waters probably ran through the groundwater before being added into the drinking water. Because of the fact that Penn State is built on and surrounded by limestone, these water samples would have higher amounts of calcium and magnesium picked up from when they were in the groundwater. Therefore, if the drinking waters spent any time in the groundwater, the level of hardness would be higher because of the higher levels of ions in the water. Procedure All of the following sections of the experiment came directly from the PSU Chemtrek book. The first step in this experiment was to test the water sample using the atomic absorption 5

6 spectroscopy (AA). Two water samples were taken, one for the AA of calcium and the second for the AA of magnesium. The spectroscope gave the absorption unit for either calcium or magnesium. Using this absorption value, we can then determine the concentration of the ions. The PSU Chemtrek listed that the water samples may need to be filtered before they are analyzed. Because my sample was clear, it did not need to be filtered. The next section of the experiment was to find the total dissolved solids in the sample. A drop of our water sample, a drop of distilled water, and a drop of 1X10^-3 M Ca 2+ was put onto a piece of aluminum foil and the foil was placed on a hot plate and allowed to evaporate all the drops. The amount of white solid that was left in each of the drops is the total dissolved solids in that sample. The three drops were compared to each other to determine the total dissolved solids in our sample compared to the distilled water and calcium. The ethylenediaminetetraacetic acid, or EDTA, titration was the next section in the experiment. In this part of the experiment, solutions of known concentrations of calcium and magnesium were used, different amounts of these solutions were added to EBT and a buffer. At a certain point with the different amounts of solution, the liquid would change color. This color change would be linked with a certain concentration of magnesium or calcium in the water. After doing two serial titrations with the known concentrations of solution, we followed with adding our water samples and putting it through the same tests to determine the concentration of cations in the water sample. We matched up the well in the tray in which the water sample changed colors, and this matched up with the serial titrations that we just did, allowing us to determine the concentration of the cations in the water. The next step in the experiment was to test two different methods of water softening. The first method that we looked at was to soften the water using a commercial water-conditioning agent, in this case baking soda. I added a portion of my water sample to a small vial and added 20 milligrams of baking soda to it and shook it up. Using the buffer solution, I put this solution through EDTA titration, again noticing in which well the solution changed colors. This information was then compared to the information for the non-softened water with EDTA. The next method of softening that we did was divalent cation removal by ion exchange. In this method, we also placed an amount of our water sample in a small vial and to this we added a 6

7 small amount of cation exchange resin to the water and shook it up. Again, this solution was put through EDTA titration and the information was compared to non-softened water. All of the above methods for this experiment were taken from the PSU Chemtrek lab manual. Results All of the results for this experiment came from my Chemistry 111 laboratory notebook. The information for the other water samples came from the Chemistry 111 laboratory notebooks from the other members of my group: Gene Williams, Kyle Witmer, Madhu Yennawar, Dave Zatezalo, and Binyang Zhang. The data gathered during this experiment from my water sample, along with the data collected from my other group members, can be found in table 2 below. In the table, the data collected for each water sample from the AA analysis, the EDTA titration, as well as the EDTA titration after water softening are included. Also listed below are the two calibration graphs that were made with the data from the AA analysis. In graph 1, the absorption values for calcium are listed along with the best fit line for known absorbance values. The second graph, graph 2, is similar to the first graph, except the data is for magnesium. Both of these graphs can be used to determine unknown concentrations of either calcium or magnesium, using the best fit line on either graph. By following the absorbance value that the AA machine gave on the readout over to the best fit line and then going down the graph, you can determine the concentration in parts per million of that cation. Using the concentration of the EDTA in the titration and the number of the well that the water sample changed colors, we can determine the hardness in molarity of the water sample: (2 X 10^-4) * (4) = 8.00 X 10^-4 M -> the hardness of the water in molarity The hardness value for the water sample did not change after the addition of baking soda, as the color still changed in the fourth well, so the hardness would be the same as above. The hardness value for the water sample after the addition of the resin did change, as the number of the well that the color changed in was different: (2 X 10^-4) * (1) = 2.00 X 10^-4 M -> the hardness of the water with resin 7

8 Taking the hardness value for the unsoftened sample, we can calculate the parts per million of CaCO3 in the water sample: 8.00 X 10^-4 M * 100,000 = 80 parts per million CaCO3 Using the ppm from the previous calculation, we can also determine the number of grains of CaCO3 found in the water sample: 80 ppm * (1 grain per gallon / 17.1 ppm) = 4.68 grains per gallon The ability to determine correctly which well number the water sample changed colors in would make a large difference in the numbers that would go into these calculations, as the hardness would be off by 2 X 10^-4 for each well that you were off. Using the best fit lines for the calcium and magnesium with AA and the absorbance values the AA machine gave for the samples, you can determine the concentration of the different cations in the water sample. Calcium -> absorbance value = > ppm Ca = Magnesium -> absorbance value = > ppm Mg = 3.79 To get the parts per million of CaCO3 from the ppm of calcium and magnesium, you simply multiply the ppm of Ca by 2.5 and multiply the ppm of Mg by 4.12 to get the parts per million of CaCO3 of these two cations. Calcium -> ppm * 2.5 = ppm CaCO3 Magnesium -> 3.79 ppm * 4.12 = ppm CaCO3 In this experiment, we used two different methods of water softening, one being adding baking soda to a water sample and the other was adding resin to the sample. Prior to any method of water softening being done to my water sample, I determined that there was 80 parts per million of CaCO3 in my water sample. In my sample, the baking soda did not lower the hardness level, although the sample may have been slightly softened, but not enough to determine it by looking at the EDTA titration after adding the baking soda. After this first type of softening was done, I determined that there was still 80 parts per million of CaCO3 in my sample. The resin being 8

9 added to the water sample significantly lowered the hardness level in my sample. After the resin was added, I determine via EDTA that there was only 20 parts per million of CaCO3 found in my water sample. Looking at the data from the other members of my group, both methods of water softening lowered the hardness level of their samples, with the method using the resin being a higher decrease in the amount of CaCO3 being found in the samples. Again, all of the data for my water sample along with the data for the other group members water samples for the softening portion of this experiment can be found in table 2. Graph 1 Graph 1 shows the AA calibration graph for calcium. This graph can be used to determine the unknown concentration of calcium in parts per million when the absorbance value is determined for the sample after AA analysis. Graph 2 9

10 Graph 1 shows the AA calibration graph for magnesium. This graph can be used to determine the unknown concentration of magnesium in parts per million when the absorbance value is determined for the sample after AA analysis. Table 2 Greg Wiltsie Gene Williams Madhu Yennawar Kyle Witmer Dave Zatezalo Binyang Zhang Allegheny River Synder Hall (filtered) State College (softened) Packer Hall Meridian Hall East Commons AA Ca Absorbance ppm Ca ppm CaCO AA Mg Absorbance ppm Mg ppm CaCO EDTA unsoftened Molarity 8X10^-4 3.2X10^- 3 6X10^-4 7.2X10^-3 2X10^-3 2.8X10^-3 ppm EDTA softened with baking soda EDTA softened with resin CaCO X10^- 2.4X10^- Molarity 8X10^-4 3 2X10^-4 2X10^ X10^-3 ppm CaCO Molarity 2X10^-4 8X10^ X10^-4 2X10^-3 ppm CaCO

11 Table 2 lists the data for the six members of the group for each of their water samples. The data listed is from the AA analysis for calcium and magnesium, the data for unsoftened water in EDTA analysis and the data for water after softening with baking soda and resin. Discussion My results showed (Table 2) that the water sample from the Allegheny contained the least amount of CaCO3 molecules in it, with the exception of the heavily filtered drinking water. Excluding the heavily filtered water, the water samples from the other group members had considerably higher values in all the categories, including AA analysis for calcium and magnesium, and EDTA titration with unsoftened water. In the AA analysis, the AA machine found that the samples all had higher absorbance values for both ions, which leads to having more parts per million of both calcium and magnesium, as well as the corresponding parts per million of CaCO3. As far as the EDTA titration goes, the other samples again came out with higher parts per million of CaCO3, and although the table does not list it, it could be noted that this would also correspond to a higher number of grains of CaCO3 in the samples. As I stated before, using the baking soda to soften my water sample did not make a difference, although the samples from the other group members did soften with the addition of the baking soda, so therefore the addition of baking soda to hard water is an effective method of softening water. The addition of resin to the water samples had a large effect on the hardness level in all the samples when comparing them to the unsoftened samples. Some of the samples dropped hardness levels of about 25%, with one sample, Kyle s, going from 720 ppm of CaCO3 in his unsoftened sample to determining that there was 0 ppm of CaCO3 in the sample after mixing in the resin. It is with results like this that show that adding resin to waters of high hardness levels would be very effective in reducing the levels of calcium, magnesium, and CaCO3 in these hard waters. By reducing the causes of hard water, the problems that are associated with the hard water would also disappear. As it was stated before, the addition of resin is the most expensive method that was tested to soften water. It would need to be looked at if the amount of hard water that is softened and the problems that would be eliminated with the softening of the water would warrant the expense of using the expensive resin. 11

12 To restate my hypothesis, I believed that my water sample from the Allegheny River would have a lower hardness level than water samples taken from residential drinking water sources. I thought this because during my research, I found that cations of calcium and magnesium were added to a water source after water flows into the groundwater. Most of the water that enters into the Allegheny River comes in directly from precipitation, so there will be no calcium or magnesium in the water because the water had not been in the groundwater to pick up the cations. Water gathered from residential water sources would have first seeped from the ground level down into the groundwater, where it would pick up the cations. These cations would stay with the water when it was put into the drinking water system. Home water softening systems would remove these cations from the water, but unfiltered water samples will still have these cations attached. According to the results gathered from my water sample as well as the other samples from my group, besides one residential source that had been filtered to almost no calcium or magnesium being found in it, my water sample from the Allegheny River had the least amounts of both calcium and magnesium compared to the drinking water samples taken from the Penn State campus. This matches up with my hypothesis that the waters from the State College area would be higher in cation concentrations since these waters move through the limestone rich area on the way to the groundwater. Because not much water moved into the Allegheny River from groundwater, the amount of these cations were less. In my sample, using the AA analysis, I determined there to be about 63 parts per million of CaCO3 in my sample. Using the same water sample but instead using the EDTA titration method, I came up with my sample having 80 parts per million of CaCO3 in it. I would guess that the amount determined from the AA analysis is the more accurate number, since this method is going on more than seeing which well the liquid changed color in. With the AA, the water is being tested by a high-tech machine that is testing for just that metal and giving a very precise number for the absorbance value, which can then be translated into the parts per million of that metal or of CaCO3. With the EDTA titration, the only way to determine the parts per million of CaCO3 in the water is by comparing the position of color change in a well tray of your water sample with the position of color change in a solution of a known concentration. I think that this method would be more effected by error than the AA is, just based on the fact that it is determined by someone looking at color change instead of a machine analyzing the sample. 12

13 Any errors that I could see in this experiment would come from not using the correct number in the right place in a formula, or by using the incorrect formula to try and get what you are looking for. For example, by placing the absorbance value number for magnesium in the best fit line formula for calcium would result in the wrong number and would possibly effect the outcome of the experiment. Another source of error would come during the EDTA titration, when you are looking at the well that the color changed in. By putting the wrong number of the well in the formula, or as I proved before by just seeing the incorrect well, the data for this would be incorrect as well, causing problems for the rest of the data and the experiment. I feel that the more accurate and precise method of water analysis is with using the AA method. I feel that this is the more accurate and precise method of analysis because you are not basing the data off looking at the color change of a liquid, instead you are using data given to you by a machine that has analyzed the water sample and that the machine is set up to analyze only one metal at a time, thus giving better results than trying to determine the concentration of all the metals that you are looking for at one time. Although my outside sources did not state directly what hardness levels for river water would be, since every river would be different, they did state that river water would have lower levels of hardness in them, due to the fact that they have less amounts of water entering them through the groundwater. (Frank) My data from this experiment proved this fact that river water is softer than residential drinking water. Conclusions In this experiment, the idea was to test a water sample and determine the hardness of the sample by determining the parts per million of calcium, magnesium, and CaCO3 in the samples. This was done by using two different methods of water analysis, atomic absorption spectrophotometry (AA) and ethylenediaminetetracetic acid (EDTA). Both of these methods gave us the hardness levels of the water samples somewhat close to each other, but they were different because of the methods used in them. Also done in this experiment was to test the effectiveness of different methods of water softening. The two methods of water softening tested was the addition of baking soda to the water and the addition of a cation exchange resin to the water. In general, both methods worked in softening the hardness level of the water samples, with the resin doing a better job of softening the water. 13

14 My data from the experiment agreed with my hypothesis that the water sample taken from the Allegheny River is softer than residential drinking waters taken from the State College area, mainly because the river water lacked water from the groundwater, whereas the drinking water in State College has been filtered through the groundwater, picking up calcium and magnesium as it does, causing the water to be hard. As I stated prior, hard water is both an inconvenience and can also cause problems and can cost money to treat. Hard water clogs pipes, causes soaps and detergents to not work as well, causes a film to cover everything, and can slow the efficiency of water using machinery and boilers. There are many methods of softening hard water, although it seems as if the most effective method of softening water, resin, is also the most expensive method. Filters attached to water sources before the enter a house or factory may stop the effects of hard water. I have shown in my report that my water sample from the Allegheny River is softer than water samples gathered from drinking water sources from the State College area. The main reason that my river water is softer is because of the lack of groundwater, where the water picks up calcium and magnesium ions, that enters the river, causing the river to stay somewhat soft compared to waters that get most of the water through the groundwater. References Anderson, T.W., L.C. Neri, G.B. Schreiber, F.D. Talbot, and A. Zdrojewski. "Ischemic heart disease, water hardness and myocardial magnesium." Canadian Medical Association Journal (1975): Print. Brown, Theodore L., Eugene LeMay, Bruce E. Bursten, and Catherine J. Murphy. Chemistry: The Central Science. 11th ed. Upper Saddle River, NJ: Pearson Prentice Hall, Print. Frank, Larry. "Water Hardness." Web. 4 Oct < "Hard Water." Water Quality Association Web. 3 Nov < "Hardwater, Water Hardness." Wilkes University Center for Environmental Quality. Wilkes University. Web. 3 Nov < "Household Water Quality - Water Hardness." Virginia Tech Cooperative Extension. Web. 3 Nov < 14

15 PSU Chemtrek, August 2009-July Thompson, Stephen. Ed. Joseph T. Keiser. Plymouth, MI: Hayden McNeil, Print. Pages Water Hardness and Alkalinity. United States Geological Survey. Web. 3 Nov < Williams, Gene. Chemistry 111 Laboratory Notebook. Wiltsie, Greg. Chemistry 111 Laboratory Notebook. Witmer, Kyle. Chemistry 111 Laboratory Notebook. Wurts, William. "Understanding Water Hardness." Kentucky State University. Web. 3 Nov < Yennawar, Madhu. Chemistry 111 Laboratory Notebook. Zatezalo, Dave. Chemistry 111 Laboratory Notebook. Zhang, Binyang. Chemistry 111 Laboratory Notebook. 15