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The Chemistry of Natural Waters Josh Hull 11/9/05 Experiment #10 Meredith Hudak Mike Hinman Tanner Gokec Tyler Hall

I.

Introduction: Hard water is water that has a high mineral content (water that contains a low mineral content is said to be soft). This content usually consist of high levels of metal ions, mainly calcium (Ca2+) and magnesium (Mg2+) in the form of carbonates1. Other metals as well as bicarbonates and sulfates may also be included.2 Earlier generations coined the phrase hard water because it made cleaning extremely difficult. Hardness is caused by compounds of calcium and magnesium, and a variety of other metals. All freshwater sources of water contain calcium and magnesium in varying quantities. Water tends to suspend, dissolve, and exchange certain trace elements and compounds from many objects that it contacts on its travels.3 For example, lime will harden water and peat will soften it. Total water hardness which includes both Ca2+ and Mg2+ is reported in parts per million (ppm) of calcium carbonate (CaCO3). Water hardness usually measures the total concentration of calcium and magnesium, the two most prevalent divalent metal ions.4 In some geographical locations, iron, aluminum, and manganese may also be present at elevated levels. Calcium usually enters the water from either CaCO3, limestone or from mineral deposits of CaSO4.5 Magnesium predominantly comes from dolomite, CaMg(CO3)2.6 Temporary hardness pertains to hardness which can be removed by boiling or by the addition of lime (calcium hydroxide). It is caused by dissolved calcium bicarbonate in the water. Calcium bicarbonate is less soluble in hot water than in cold water, so boiling (which promotes the formation of

carbonate) will precipitate calcium carbonate out of the solution, leaving water that is less hard on cooling.7 On the other end of the spectrum, permanent hardness is mineral content that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates in the water, which are more soluble as the temperature rises. The mixture of minerals dissolved in the water, together with the waters acidity or alkalinity, as well as the temperature, will determine the behavior of the hardness.8 Descriptions of hardness correspond roughly with the ranges of mineral concentrations:9 Hardness Soft Moderately Soft Slightly Hard Moderately Hard Hard Very Hard Concentration of Calcium (mg/L Ca) 0-20 20-40 40-60 60-80 80-120 >120

Water hardness does not present a health hazard, however it can cause many potentially costly problems. Hard water also causes scaling, which is the precipitation of minerals to form a rock-hard deposit called lime scale.10 Scale has the potential of clogging pipes and decreasing the life of toilet flushing units by 70% and water faucets by 40%.11 It may also coat the insides of tea pots and coffee pots, and clog and destroy hot water heaters.

In the household environment, hard water requires more soap and synthetic detergents for laundry and washing. It takes half as much soap for cleaning with soft water. Hard water and soap often combine with one another to form soap scum which cannot be rinsed off. This scum forms bathtub rings and unwanted spots on your dishes. Using soap on your body in hard water can cause the formation of scum which is often referred to as curd.12 The formation of scum and curd is caused when calcium and magnesium form insoluble salts with anions.13 This curd remains on the skin even after rinsing. The curd may then clog pores and coat body hair. This can serve as the origin for bacterial growth, causing diaper rash, minor skin irritation and skin that constantly itches.14 In commercial industry, hard water contributes to scaling in boilers, cooling towers, and other industrial equipment. When hard water is heated or evaporated, rocklike deposits consisting mainly of calcite crystals form on the surface of pipes, boiler walls, tubes, and evaporator surfaces.15 Scale is one of the banes of industry. It blocks jets and tubes, and narrows pipes. The hard layer interferes with heat transfer in boilers, leading to gross energy inefficiencies, and can often lead to metal corrosion and structurally weakness. In these settings, water hardness must be under constant review to avoid costly breakdowns. Hardness is controlled by the addition of chemicals and by large scale softening with zeolite resins. A water softener works on the principle of cation or ion exchange in which ions of the hardness minerals are exchanged for sodium or potassium

ions.16 The most economical way to soften household water is with an ion exchange water softener. This unit uses sodium chloride (table salt) to recharge beads made of ion exchange resin the exchanges hardness minerals for sodium17. Artificial or natural zeolites can also be used. As the hard water passes through and around the beads, the hardness minerals attach themselves to it, dislodging the sodium ions. This process is called ion exchange18. When the beads or sodium zeolite has no sodium ions left, it is exhausted and can no longer soften the water. The resin is recharged by flushing with saltwater. The excess of sodium ions force the hardness ions off the resin beads. The excess sodium is rinsed away and the resin is ready to start the process all over again. According to the US Geologic survey, 85% of US homes have hard water.19 The softest water occurs in parts of New England, South AtlanticGulf, Pacific Northwest, and Hawaii regions.20 Moderately hard waters are common in many of the rivers of Tennessee, Great lakes, Pacific Northwest, and Alaska regions.21 Hard and very hard waters are found in some of the streams in most of the regions throughout the country22. Hardest waters are found in the streams of Texas, New Mexico, Kansas, Arizona and Southern California23. EDTA titration is used to determine the concentration of divalent cations (hardness) in water (for example the concentration of calcium and magnesium).24 1. A known volume of water is taken and the pH is adjusted to 10 by a NH3/NH4 buffer. 2. EBT indicator is added to the solution. At the

high pH the indicator is in the HD2- form, which is blue. 3. If magnesium is present in the water sample then it will react with the indicator to form a wine red chelate. Calcium does not react with the indicator. Therefore, at the start of the titration, the solution is wine red in color. 4. EDTA solution is now added to the solution from a microburet. It first reacts with calcium and forms a colorless chelate. As soon as enough EDTA has been added, it begins to react with the magnesium indicator chelate to produce a MgEDTA chelate. When the magnesium is removed from the indicator, it returns to its blue form. 5. The end point of titration is a definite change from a wine red color to a blue sky color.25 Titration is difficult to do if there is little or no magnesium in the water sample. If there is no magnesium in the sample, then the color of the solution at the beginning would be the same at the end of the titration, in other words there would be no end point. In order to ensure that the sample contains enough magnesium, it is usually spiked with a solution which contains the MgEDTA chelate.26 After the titrations were complete in the experiment, the following equation was used to determine the concentration of the calcium solution. Moles of EDTA=moles of Ca2+ MEDTAVEDTA=MCa2+VCa2+

Atomic absorption spectrophotometry (AA) is a technique which is used to determine metals that are dissolved or suspended in a solution.27 These metals can consist of alkalis, alkaline earth, and even transition metals. In order for the atom of interest to be excited, the energy of light falling on the atom must match the energy separation between two electronic energy levels.28 This principle is used in the operation of AA. Monochromatic light having the energy corresponding to the change in energy of the atoms of interest is shined through the sample which is to be analyzed.29 Atoms which have electronic energy separation will absorb the light. The amount of absorbance is proportional to the concentration of the metal atoms in the sample. The Beer-Lambert law is used to calculate the unknown metal concentration in the sample30. A typical atomic absorption spectrophotometer functions in a systematic way. Voltage across the electrodes excites the calcium and magnesium inside the lamp. When the excited Mg or Ca atoms relax, a monochromatic light is produced which equals the energy separation of the two electronic levels. The emitted monochromatic light will then be absorbed by Mg or Ca atoms in the water sample.31 The liquid water sample is then aspirated into the sample chamber where it is converted from a liquid to a fine aerosol which is introduced into a flame. The flame is composed of air-acetylene mixture which reaches 2300 degrees Celsius. This temperature is capable of atomizing everything in the liquid sample. The light from the hallow cathode lamp passes through the flame where the sample is atomized. The light will

only be absorbed if there is a matching energy separation of energy levels.32 A grating in the monochrometer is adjusted so that only the wavelength of light corresponding to the energy change of the metal of interest is allowed to pass through a narrow slit. This light then falls on the detector which is a photomultiplier tube (PMT). Since the metal atoms absorb some light passing through the flame from the lamp, a decrease in initial signal is detected by the PMT. This decrease is proportional to the concentration of metal in the sample.33 The concentration of a metal sample is determined by a calibration graph that is based on the light absorbance of known concentrations of the metal of interest. The goal of this experiment is to determine the hardness of five different water samples: Aquafina