Chemistry of Natural Waters
Thursday November 12, 2009
Water is such an important substance needed by all living species for survival. Freshwater is needed for agriculture, hydroelectric power, and drinking water. The United States uses 9 X 1011 liters of freshwater everyday.1 Freshwater is not completely pure. Its composition is based on dissolved solutes ranging from gases in the atmosphere to minerals from rocks and soils. The cations Ca2+ and Mg2+ are the most abundant species found in natural waters. These ions come from soluble sedimentary rocks such as limestone, dolomite, and gypsum.2
The presence of these dissolved ions in the water gives it a chemical property that is called water hardness. Water that is hard has a very high concentration of Ca2+ and Mg2+, as well as other dissolved cations and anions. Likewise, water that has a very low concentration of these ions is said to be soft. The hardness of water can be measured as concentrations in molarity (M), parts per million (ppm) which is also equal to mg/L, and grains per gallon. The accepted ranges of water hardness is as follows: Soft water - less than 17mg/l
Slightly hard - 17- 60 mg/l
Moderately hard - 60-120 mg/l
Hard -120-180 mg/l
Very hard 180 + mg/l
It is important to study the hardness of water because of the problems it can cause when left untreated. Hard water can cause “scale,” or build-up in plumbing fixtures and water heaters.3 Scale occurs when calcium carbonate, CaCO3 , is deposited as calcite crystals on the inner surfaces of pipes, and evaporator surfaces. This occurs when water with a high
hardness value is evaporated or heated. This buildup can block pipes, make heat transfers in boilers inefficient, and eventually corrode the metal of the pipes. Scale is considered to be the bane of industry because of the expense of replacing equipment and dealing with energy inefficiencies.2
The hardness of water can be determined by several techniques. Water can be evaporated, which leaves a residue of solids present in the water. This residue can be analyzed for the total dissolved solids (TDS) when a known amount of water is evaporated.
Another technique for quantitative analysis is ethylenediaminetetracetic acid or EDTA titration analysis. An indicator, eriochrome black T (EBT) is added to a known volume of the water sample. Magnesium ions react with this indicator, turning the solution a red color. EDTA solution is then added with increasing degree. The EDTA first reacts with calcium ions, then the magnesium indicator chelate. When all the magnesium ions have reacted with the EDTA, the solution turns blue. In order for EDTA to be successful, magnesium ions must be present in the water sample in order to be chelated when EBT is added. If magnesium ions are not present, the solution will remain blue and it will be impossible to determine at which point all of the ions will react with a certain amount of EDTA.
Atomic Absorption Spectrophotometry (AA) is another process used for the quantitative analysis of water hardness. AA works on the principle of the Beer-Lambert Law. Atoms of different elements have specific energy levels that must match the energy of light falling on these atoms in order for the atoms to be excited to the next energy level.
An AA spectrophotometer can measure the amount of certain ions present in the water by shining monochromatic light on the atoms. Only the atoms whose change in energy equals the wavelength of the light will absorb energy. The water sample is converted into an aerosol, which is heated by a flame. This high temperature atomizes the substances in the aerosol. The light hits the atoms and is passed through a monochrometer that only accepts a certain wavelength of light. This wavelength is specific only to calcium or magnesium. The absorbance value of the Ca2+ and Mg 2+...
References: Saddle River, NJ, 2009.
PSU Chemtrek; Keiser, Joseph T., Ed.; Prentice Hall: Englewood Cliffs, NJ, 2009.
Survey, 2007. Print.
Environmental Resources: Harrisburg, PA, 1978
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