Human Activity Impact on Waterways

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5. Human activity also impacts on waterways. Chemical monitoring and management assists in providing safe water for human use and to protect the habitats of other organisms Identify that water quality can be determined by considering: * concentrations of common ions

* total dissolved solids
* hardness
* turbidity
* acidity
* dissolved oxygen and biochemical oxygen demand

Water quality is commonly defined by its physical, chemical, biological and aesthetic (appearance and smell) characteristics. A healthy environment is one in which the water quality supports a rich and varied community of organisms and protects public health.  Water quality is monitored to protect natural ecosystems, for aesthetic reasons, and to guarantee quality of food derived from water sources. Water quality can be determined by a number of factors, all of which are important in monitoring polluted water, and determining whether a water source is potable. CONCENTRATION OF COMMON IONS

* Potable water typically has low concentrations of common ions, whereas sea water has high ion concentrations * The table below shows the typical and maximum concentrations of common ions: Ions| Concentration (Sydney water supply) (ppm)| Recommended maximum (ppm)| Chloride| 20| 400|

Sulfate| 8| 400|
Nitrate| 0.4| 10|
Fluoride| 1| 1.7|
Sodium| 10| 300|
Calcium| 9| 200|
Aluminium| 0.2| 0.2|

* The concentration of sodium (Na+) and chloride (Cl-) ions are important indicators of the salinity of a water system, and also affect the taste of water * Magnesium and calcium ions are indicators of water hardness * Phosphate and nitrate ions need to be monitored to prevent eutrophication of a water system * Cations

* Atomic Absorption Spectrometry (AAS) is the preferred technique used to identify concentration of metal ions * Sodium [Na+], magnesium [Mg2+], calcium [Ca2+], potassium [K+], iron (III) [Fe3+] * Aluminium [Al3+], which is mobilised in soil by increasing acidity * Toxic heavy metals, such as mercury [Hg2+], lead [Pb2+], cadmium [Cd2+] and arsenic [As] in its various forms

* Anions
* Chloride [Cl-]
* Gravimetric analysis: chloride ions precipitated and weighed as silver chloride [AgCl] * Volumetric Analysis: Mohr Method
* Titration with AgNO3 solution
AgNO3(aq) + Cl-(aq) AgCl(s) + NO3-(aq)
* Yellow Potassium chromate K2CrO4 indicator
* As AgNO3 is added, AgCl is produced, forming a white precipitate (solution becomes cloudy) * Excess silver Ag+ ions after the equivalence point and any indicator chromate ions CrO42- react to form a red-coloured silver chromate Ag2CrO4 precipitate 2 Ag+(aq) + CrO42-(aq) ↔ Ag2CrO4(s)

Note that Ag2CrO4 is more soluble than AgCl. This is why Ag2CrO4 will not precipitate until all the Cl- has precipitated * Colour change from yellow to persistent red

* Sulfate [SO42-]
* Gravimetric analysis: sulfate ions precipitated and weighed as barium sulfate [BaSO4]

* Hydrogencarbonate [HCO3-]
* Volumetric analysis: titration with standard hydrochloric acid HCl + NaHCO3 H2CO3 + NaCl

* Fluoride [F-]

TESTS
* Qualitative tests – precipitation tests and flame tests * Quantitative tests – gravimetric analysis, volumetric analysis (e.g. titration), and AAS * Chloride levels can be measured by titrating the sample against silver nitrate, with potassium chromate as the indicator PURPOSE

* Excess calcium or magnesium ions makes water hard, and affects taste of water * Excess nitrates or phosphates may result in algal blooms and eutrophication ⇒ Eutrophication causes fish death, formation of scum and unpleasant odours, and release of toxins produced by some algae * Heavy metals may be toxic when ingested, and damage the nervous system ⇒ Heavy metals may bioaccumulate and increase in...
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