CHEM 111 Lab Writeup
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Transcript of CHEM 111 Lab Writeup
The Chemistry of Natural WatersMichael MartoglioApril 6, 2009 Chem 111 Section 104 TA: Shradda Surve Group: Jun Lee, Hayley Miller, Skyler Marzewski
I. Introduction In this lab, we were instructed to set out to explore different ways to test characteristics of a sample of water and its water hardness. Hard water is evaluated as the type of water that contains high concentrations of the divalent cations Ca2+ and Mg2+. These two cations enter streams, rivers, lakes and other water bodies by the dissolution of minerals as the water flows through rocks and soil.(2) The Calcium comes from sedimentary rocks such as limestone and dolomite, while Magnesium comes from rocks like olivine and pyroxenes. (11) A water sample with high concentrations of both of these are said to be hard, while lower concentrations are soft. Because hard water is natural, it can be found basically anywhere and its uses are endless. Hard water can be used in showers, garden hoses, and even drinking water. Although soft water is preferred as a drinking water, hard water will not harm you. (12) Hard water is important because of its use in industrial boilers and evaporators. Also untreated waters that are still hard are better in cleaning. When used with soaps and detergents, they help reduce the chemical bonds between dirt and the surface make it easier to be cleaned. (11) Water hardness is measured in units of parts per million (ppm). The higher the ppm, the harder the water is considered. (5) In this lab we explored two primary ways of testing water hardness, EDTA and AA. The EDTA is a titration can only be used when there are significant amounts of Ca2+ and Mg2+. The EDTA is an inexpensive lab experiment. To use this experiment you take a known amount of natural water and bring its pH to 10 by use of a buffer. Next you add EBT indicator which turns the solution blue because of the high pH. This indicator will soon turn a red wine color with the presence of Mg2+ but does not test for the presence of Ca2+.(11) Then a known concentration of EDTA solution is added which reacts first with Calcium and then the Magnesium. Once all of the Magnesium
has reacted to form a colorless chelate and is removed from the indicator, it will return to its blue color. The end of the titration comes when the color is now a definite sky blue.(11) The use of an Atomic Absorption Spectrophotometer is a large machine that although is easier to use it works in a more complicated manner. In region 1 of the machine, you select what you are testing for (Ca2+ or Mg2+). A beam of light leaves region 1 with the same energy level required to dissociate a molecule of what youre testing for. In region 2, about 10% of your sample is picked up and turned into an aerosol. The aerosol is then released in the form of a flame. The light passes through the flame and since it has the energy to dissociate the element youve been selected it is absorbed by the aerosol in the flame.(1) The light passes into region 3 through a slit which only allows the wavelength for the element you selected in region 1. Region 3 contains a photomultiplier tube (PMT) which reads and determines how much of Ca2+ and Mg2+ is in the light. This amount is proportional to the amount in your water sample. (11) These two methods differ because the titration is a very inexpensive experiment that takes a few minutes to perform. The use of AA is a very large, expensive machine which takes merely seconds. Also the AA is only capable of calculating the absorption of Ca2+ and Mg2+. The titration can calculate all polyatomics such as Chloride and Sodium. Water softening techniques work by using softeners such as sodium carbonate to reduce the dissolved calcium and magnesium ions in the water. Large scale softening techniques use negatively charged resin to attract the positive Ca and Mg ions. When the ions attach to the resin, potassium, hydrogen or sodium ions are released into the water. (5) I expect all of the water samples to, at some level, be considered hard. I think the water sample from Whitmore to be the hardest because it is tap water from the bathroom. The sample from the apartment on Pugh street is also tap water but because it is not on campus I think it will be
softer. The samples from the HUB and the on-campus dorm are from water fountains. Because these are supposed to be suitable drinking waters, I think they should be softer. II. Procedure The procedure for this experiment came step-by-step from the PSU Chemtrek book. (1) In the first section we used the AA method to determine the level of hardness for our water samples. Because this is a machine that uses extremely advanced technology, the work on our part was limited and its process was mentioned above. The intensity of the light from region 3 in the spectrophotometer can be found using the Beer-Lambert Law. Our results will be discussed later. Next, we dissolved a small amount of our sample and compared it to a drop of distilled water and 1x10-3 M Ca2+. My water sample left a dark circle where it previously was while the distilled water left no trace behind and the Calcium left a light ring. This is because the white solids that remain are the original nonvolatile salts present in the water.(7) The amount of nonvolatile salts is known as the total dissolved solids (TDS). (11) We then performed a serial EDTA titration with Calcium, EBT indicator, buffer, and EDTA solution. After close observation to find the well where the color changed we used the formula MEDTA VEDTA = MCa2+ VCa2+. In this equation M is the molarity of the substances and V is the volume. For our experiment, the volume was represented by the number of drops in the well where the initial color change took place. This equation, when solved for MCa2+ will give us the concentration of the original Ca2+. (11) Again, our results will be discussed later. Using the same concept as before, we found the hardness of a mixture of 20 mg of a commercial conditioning product in our water sample. We then compared our results. III. Results Table 1 shows the absorbance rates for the water samples obtained by using the spectrophotometer.
Table 1- AA Absorption Rates (6,7,8,9) Sample Name Ca2+ Absorbance HUB Apartment (Pugh St) Dorm (Packer Hall) Whitmore Skyler Hayley Mike Jun 0.2926 0.2051 0.3186 0.4834 Mg2+ Absorbance 0.3214 0.1671 0.3222 0.5419 1:1 1:1 1:1 1:1 Dilution
Graph 1 shows the absorbance value versus the concentration of calcium. These numbers were presented to us when conducting the AA experiment.
Graph 2 shows the absorbance value versus the magnesium of calcium. These numbers were presented to us when conducting the AA experiment. .
Also for section A, after producing these graphs and returning with our Calcium and MagGraph 2nesium data we are able to determine the hardness. By using the equations of
the trendline on the graphs and plugging in our values for Ca2+ and Mg2+ respectively we can find the light absorbance values. For Calcium, my trendline equation was: y = 0.0111x + 0.0513 Now when I plug in the Calcium concentration from the AA for the y value and solve the equation for x, I get x= 24.08 ppm Ca2+. I then use the following equation to convert to water hardness. (11) 1 ppm Ca2+ x 100 g CaCO3 1 mole _ = 2.5 ppm CaCO3 = 2.5 ppm hardness 2+ 40.0 g Ca 1 mole
For example my conversion would be: 24.08 ppm Ca2+ x 100 g CaCO3 1 mole _ = 60.2 ppm CaCO3 = 60.2 ppm hardness 2+ 40.0 g Ca 1 mole
Table 2 shows the results doing the same with my groups numbers. Table 2- AA Water Hardness (Ca2+ ) (6,7,8,9) Sample HUB Apartment (Pugh St) Dorm (Packer Hall) Whitmore Name Skyler Hayley Mike Jun Hardness (ppm) 54.3 34.6 60.2 97.3
We would do the same of the Magnesium value with the Magnesium trendline equation. My magnesium equation was: y = 0.0182x + 0.0635 Now when I plug in my Mg2+ concentration into the equation for y and solve for x I get
14.2 ppm Mg2+ . We then must convert this number into water hardness using the following equation: 1 ppm Mg x2+
100 g CaCO3 1 mole _ = 4.12 ppm CaCO3 = 4.12 ppm hardness 24.3 g Mg2+ 1 mole
For example, my water hardness would be 14.2 ppm Mg2+ x 100 g CaCO3 1 mole = 58.4 ppm CaCO3 = 58.4 ppm hardness 2+ 24.3 g Mg 1 mole
Table 3 shows my groups data going through those same steps. Table 3- AA Water Hardness (Mg2+ ) (6,7,8,9) Sample HUB Apartment (Pugh St) Dorm (Packer Hall) Whitmore Name Skyler Hayley Mike Jun Hardness (ppm) 58.4 23.4 58.4 108.2
The total hardness is shown in Table 4.
Table 4- Total Hardness (6,7,8,9) Sample HUB Apartment (Pugh St) Name Skyler Hayley Hardness (ppm) 112.7 58.0
Sample Dorm (Packer Hall) Whitmore
Name Mike Jun
Hardness (ppm) 118.6 205.5
Table 5 shows the absorption rates for my groups water samples obtained by using the EDTA titration Table 5- EDTA Titration (6,7,8,9) Sample HUB Apartment (Pugh St) Dorm (Packer Hall) Whitmore Name Skyler Hayley Mike Jun Drops of Sample 1 1 1 1 Drops of EDTA 39 24 23 12 Dilution none none none none
In section D, we had to determine the hardness of our water samples by counting the number of drops of EDTA it took to change the color of the well. We use the following equation with the following variables: MEDTA: (2.0x10-4 M) VEDTA: number of drops of EDTA (differs for each sample) MCa2+: What we are solving the equation for VCa2+: 1 For example, the equation for my water sample would be: (2.0x10-4 M) (23) = (MCa2+) (1) MCa2+ = 4.6x10-3 M Table 6 shows the concentration of the samples after plugging the number of drops in. Table 6- Calcium Concentration (6,7,8,9) MEDTA VEDTA = MCa2+ VCa2+
Sample HUB Apartment (Pugh St) Dorm (Packer Hall) Whitmore
Name Skyler Hayley Mike Jun
Concentration 7.8x10-3 M 4.8x10-3 M 4.6x10-3 M 2.4x10-3 M
Instead of expressing the concentrations i