Introduction Nothing is more vital to the survival of living organism than water. In fact 60% of the human body is water; the brain is composed of 70% water and the lungs nearly 90% water. Also about 83% of our blood is water, which helps digest our food, transport waste and control body temperature. It is estimated that each day humans must have about 2.5 Liters of drinking water to maintain a reasonable quality of life. As the earth’s population continues to grow an adequate supply of clean water becomes a challenge. Water may be everywhere one looks and cover approximately 2/3 of the world but only a small amount of that water is considered “drinkable”. Undrinkable water is caused by both chemical and biological impurities. The impurities found in water sources such as rivers and lakes include but are not limited to bacteria, algae, viruses, fungi, minerals and pollutants. Many of these contaminants can be dangerous making it necessary to convert natural undrinkable water sources into drinkable water sources. Impurities found in undrinkable water are not always dangerous but are removed to improve the water’s smell, taste and appearance.
The water molecule itself is extremely important. Water is a polar molecule meaning that is has an uneven distribution of electron density. Water has a partial negative charge near the oxygen atom due to the unshared pair of electrons, and a partial positive charge near the hydrogen atoms. The polarity of water allows for ions and other molecules to dissolve in it causing contaminations. For example gases such as O2. and CO2 are easily dissolved in water. Fish breathe the dissolved oxygen and CO2 remains in large quantities to produce sparkling water. Water Contamination Biological impurities found in water include many dissolve salts. Ions such as potassium, calcium, magnesium, iron, sulfate, and carbonate become dissolved in water as rain and groundwater pass over rocks, which slowly dissolve into the water. Since rocks present in any given area varies so does the concentration of these ions in the water source. High concentration of ions such as calcium and magnesium also lead to hard water. Chemical impurities are also caused by various man-made sources. Two common chemical impurities are nitrite (NO2-) and phosphate (PO43-). Nitrite contamination is caused by fertilizer run-off from farms and agricultural lands. While phosphate contamination is caused by fertilizers, wastewater treatment plants, soaps, detergents and industrial processes. Being able to test for the presence of both biological and chemical impurities in water is important. This experiment is designed to give you experience in the testing and purification processes commonly performed in a water quality lab or water treatment plant. BEFORE your lab period you MUST collect ~ 1L of water from a natural water source in the area. For example a lake, river, or water run off. Once in lab the following techniques will be done to test for the concentration of phosphate in the water sample and purify the water: a test of pH, spectroscopic determination of concentration, serial dilutions to prepare standards and filtration to remove sediment and other solids. pH The pH of water determines the solubility and biological availability of chemical constituents such as nutrients and metals. pH is a good indicator of water contamination. The pH of a neutral liquid should be near 7, while an acidic solution is less than 7 and a basic solution is greater than 7. Distilled water should have a pH value of 7. The pH of the natural source water is going to vary because of several factors. The pH of a sample may be more than 7 because of photosynthesis. Photosynthesis uses dissolved CO2 which acts like carbonic acid in water. The loss of CO2 in the water reduces its' acidity, and increases the pH. Also the presence of large quantities of bacteria or other organic contaminants in water can produce ammonia that will raise the pH of the water. Acidic water can indicate the presence of many industrial wastes including sulfur and nitrogen oxides which become sulfuric and nitric acids when delivered by rain or runoff into lakes or streams. The pH is normally recorded in the field at the water sources but as long as the water sample is refrigerated to keep any biological entities from changing the pH it should be reasonably the same when taken in the lab. For this lab the pH will be recorded using an Orion® pH meter. The pH will be recorded twice, before and after the purification process. Determining Phosphate Concentration The phosphate concentration will be measured using a technique called spectroscopy. Spectroscopy is the study of the interaction of electromagnetic radiation with matter. Through the many spectroscopic techniques available, the shape, composition and the way a compound or molecule reacts can be determined. In order to understand how spectroscopy works one must first understand the basic principles of light and energy. When an electron encounters a source of energy, i.e. light, its energy increases moving the electron from one energy level to the next, usually from the ground state to the excited state. As the reverse process takes place and the energy is lost, an electron drops from the excited state back to the ground state and a photon is emitted. If this photon has a wavelength in the visible region a color is observed, and the spectrum emitted by the substance is called an emission spectrum. Complementary to an emission spectrum is an absorption spectrum, both spectrums can be added together to obtain a continuous spectrum. An absorption spectrum is produced when continuous electromagnetic radiation (every wavelength, all colors) passes through a substance and certain wavelengths are absorbed. In other words, an observed absorption spectrum is, in reality, made up of the wavelengths of light that are not being absorbed by the substance. A molecule can absorb some of the light only if it can accommodate that additional energy by promoting electrons to higher energy levels. The energy of the light being absorbed must match the energy required to promote the electron. Therefore, not all wavelengths of light are absorbed equally by a sample. An absorption spectrum depicts what wavelengths of light are absorbed by a sample. Spectroscopy Spectroscopy is studied using various instrumentation. In this experiment we will be using a Genesys-20 digital spectrophotometer. The Genesys-20 is one of the easiest types of spectrometers to use. A sample is placed into a cuvette, then placed into the instrument. Inside the spectrometer light is then passed through the sample cell at the selected wavelength. The light can then hit a molecule within the sample cell and scatter, or be absorbed by a molecule and re-emitted in any direction or pass clean through the sample cell without interacting with the sample at all. Thus the amount of light absorbed and emitted is concentration dependent. The amount of light that passes through the sample is then measured by the detector. Below is a schematic diagram of a spectrometer:
The Genesys-20 can be read in two different units, either absorbance or percent transmission. We normally record absorbance because this can be put directly into Beer-Lambert Law. The Beer-Lambert Law indicates that there is a correlation between the absorbance of a sample, its concentration, and its thickness. The law can be written as: A = ebc where e is the molar absorptivity (this is a constant which depends on the nature of the absorbing system and the wavelength passing through), b is the path length (the width of the sample cell or cuvette usually 1 cm), c is the concentration of the sample and A is the absorbance of the sample. If the molar absorptivity and the path length are held constant, the relationship between absorbance and concentration can be studied. If there is a direct relationship, a plot of different concentrations of the solution will generate a straight line. Then the system is said to obey the Beer-Lambert Law. When a curved plot is obtained, the system deviates from the law. When the plot is not linear, no correlation between concentration and absorbance can be easily established. (This can happen at particularly high concentrations.) Colorimetric Methods Since the phosphate ion in water is colorless and therefore not easily detected using typical spectrophotometric techniques (which determines concentration from color intensity). In order to observe an absorbance that can be related to concentration, phosphate must be reacted with a coloring agent. In this experiment we will be reacting phosphate with ammonium molybdate (NH4)6Mo7O24:4H2O and stannous chloride (SnCl2:2H2>O) in an acidic solution. When combined with phosphate, this coloring agent produces a blue compound whose absorption can be observed at 650 nm. The coloring agent will be added to your water sample so the absorbance can be read. The absorbance value of your water sample can then be used to determine the concentration of phosphate in your water. But this can only be done if the molar absorptivity of the system at 650nm is known. The molar absorptivity must be determined using a serious of standards with known phosphorous concentrations. In this experiment this will be done with serial dilutions. Serial Dilution Serial dilutions are a way to make solutions of varying concentrations very easily in succession. In this experiment there will be a stock solution with a concentration of 10 ppm phosphate solution, from that 10 ppm solution you will need to make phosphate solutions of 8 ppm, 6 ppm, 4 ppm and 2 ppm. It is extremely important to understand how to make serial dilutions before coming to lab. For example, we have 25 mL of a stock solution with a concentration of 15 ppm and want to make a serious of solutions with concentrations of 10 ppm, 7 ppm and 3 ppm. We also want each solution to initially be 25 mL in total volume. First we would need to make 25 mL of the 10 ppm solution, to do this we would need to use some volume of the 15 ppm solution and some volume of water to dilute the solution to 10 ppm, but how much of each? Using the equation M1V1= M2V2, where M1 is concentration of solution one, V1 is the volume of solution one, M2 is concentration of solution two and V2 is volume of solution two, we can determine the volume of 15 ppm solution needed to make a 10 ppm solution. We know that we want to make 25 mL of a 10 ppm solution, so let’s assign these values a V1 and M1 respectively, we also know that we have a 15 ppm solution let’s assign that to M2, now we can plug into the equation and solve for V2.
The results tell us that we need 16.7 mL of 15 ppm solution to make a 10 ppm solution, but we want a total of 25mL so we need to determine how much water to add. Simply subtract 16.7 from 25 (25 mL-16.7 mL) leaving us with 8.3 mL of water. Next we want to make a 7 ppm solution from the 10 ppm solution, to do this we will use the same formula and steps but this time we will use 7 ppm as M1 and 10ppm as M2. The equation should be set up as follows; (7ppm)(25mL) = (10ppm) (V2). By solving this equation you will find that you need 17.5 mL of 10 ppm solution and 7.5 mL of water to make the 7 ppm solution. Following the same procedure how much of the 7 ppm solution will you need to make the 3 ppm solution? How much water will you add? [See Answer] Below are four sample beakers showing the contents of each dilutions, notice as the dilutions progress the color intensity of the liquid decreases.
15ppm 10ppm 7ppm 3ppm Mohr Pipets All of the measurements done in making the serial dilutions must be done using a pipet. Mohr pipets are special volumetric glassware that is used to deliver an accurate volume of liquid. We will be using a Mohr pipet. A Mohr graduated pipet is marked similar to a graduated cylinder and is used by allowing the liquid to drop from one line to another, and the volume delivered is the difference between the two lines. Phosphate Concentration After the solutions are prepared by serial dilution and the coloring complete, the absorbance of each concentration must be determined. This is done use of the Genesys-20 Spectrometers. The concentrations and absorbance values of the serial dilutions are then used to determine the molar absorptivity. The molar absorptivity is the slope of the line when absorbance is plotted versus concentration, a type of calibration graph. Once the molar absorptivity of the system is known, the concentration of phosphate in the water sample you collected can be determined. In order to determine the concentration we must have the absorbance of the sample. The absorbance value is obtained in the same manner absorbances were obtained for the standards. The Beer-Lambert Law is then rearranged and solved for the sample's concentration (c). For example if we measure the absorbance to be 0.253, and know the molar absorptivity is 1.45 x104 M-1 cm-1 and the path length is 1 cm, we can solve for the concentration of the sample.
Water Purification Lastly we want to make the natural water source you collected “drinkable”, to do this we must filter the water to eliminate the chemical and biological impurities. Filtration is going to be done several times. The first step is to filter the water through filter paper, this we remove large particles from your sample. In the remaining filtration steps we will try to remove the organic impurities through the addition of chemical reagents and various filtrations medium. Once the filtration process is complete we will measure the pH and take the absorbance again to determine if the natural water source can be considered “drinkable” The processes that you are doing to your water sample are the same processes water treatment plants all over the world are using. The goal of these treatments is to turn the worlds “undrinkable” water into “drinkable” water. Standards and protocols are always changing, but one can be sure that they are regulated.
|
 |
Solutions and Colloids in the Purification of Water
