Hsc Chemistry Water Notes

Topics: Water, Water pollution, Oxygen Pages: 9 (2327 words) Published: August 1, 2013
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

Equipment, testing procedures and other information about concentrations of common ions: * Test with ion sensitive electrodes (ISEs).
* For metal ions, atomic absorption spectroscopy (AAS).
* For anions, gravimetric analysis.
* Common guidelines for potable water include:
[ Fe 2+] less than 0.3 ppm
[ Al 3+] less than 0.2 ppm
[ Mn 2+] less than 0.1 ppm
[ F -] less than 1.7 ppm

Methods to determine concentrations of common ions:
* Atomic Absorption Spectrometry (AAS) is a common technique used to identify the concentration of metal ions. In testing water quality the concentration of the following cations are usually determined: sodium, magnesium, calcium and potassium.  * Gravimetric analysis can be used to determine the quantities of both cations and anions, e.g. for chloride ions precipitated and weighed as silver chloride. Common anion concentrations measured include chloride, sulfate, bicarbonate and fluoride ions.

Total dissolved solids (TDS):
Total dissolved solids (TDS) are determined by evaporation to dryness of a known volume of a filtered sample. The value is converted to parts per million (ppm) and expressed in mass per volume units, ppm (m/v). Since most of the dissolved solids are ionic, their presence can be determined by data loggers that have the electrical conductivity probe attached. They can be set up to record continuously if needed. Hardness:

Hardness is due to the presence of calcium and magnesium ions in the water. These form insoluble compounds with soap ions, resulting in a scum on the water surface and around sink basins. This removal of soap ions from solution reduces the ability of soap to lather. The test for hardness involves precipitating the calcium and magnesium ions from a known volume of the water sample with a solution of sodium carbonate (of known concentration), followed by filtering and drying of the precipitate. Most of the insoluble salt is assumed to be calcium and the concentration of calcium ions is calculated and reported in parts per million (ppm).

Turbidity:
This can be one measure of the ability of the water to support life. Turbidity results from the presence of suspended solids in the water. Water with a high turbidity reduces penetration of light and decreases photosynthesis, which in turn reduces the oxygen concentration. The test for turbidity is conducted using a turbidity tube standing on a white tile. The tube has a black cross marked on the base. The water sample is poured into the tube until the cross just disappears when looking from above. The use of a turbidity tube is less reliable than a transmittance of light test done with a colorimeter. The lower reliability is due to variations in human eyesight and the intensity of background light during testing.

Acidity (pH):
A pH reading below 7 would be expected where there are acid sulfate soils or where there is acid produced by decomposition of organic matter in stagnant situations. The test can be conducted with a data logger and pH probe, universal indicator solution or paper, or a pH meter. If using the universal indicator, comparison with a coloured pH scale provides the pH value. If it is less than 7, the solution is acidic.

Dissolved oxygen (DO):
There are several tests for determining the DO in a water sample. The Winkler methodfixes the amount of dissolved oxygen, which is later determined by titration. The amount of manganese dioxide produced by adding manganese(II) ions and hydroxide ions is a measure of the DO. Acidified iodide ions are added to cause the manganese dioxide to...
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