ignored.....
Hardness
Hard water contains significant concentration of
Mg and Ca cations and forms a grey scum with soap, whilst soft water forms a lather with soap. Hard water which forms a grey scum dirties clothes and hands and reduces water quality. When soap is added to hard water, the precipiates of calcium and magnesium
stearates will form.
QUALITATIVE TEST for hardness: a sample of water is added to a test tube and flakes of soap are added. The test tube is agitated and left for a few minutes. If a grey scum forms which floats on top, then the water is hard but if the soap lathers, then it is not hard water and instead is soft water.
QUANTITATIVE TEST for hardness: There's 2:
- AAS is a sensitive analytical technique which selectively measures concentrations of cations to accurate measurements such as ppm or even ppb using a graphite furnace for lower ppb concentrations instead of the flame. If testing for Calcium concentration, use a calcium lamp and briefly explain what happens - should be in your AAS notes.
- Titration with EDTA (ethylene-diamine-tetra-acetic acid) - apparently you don't need to know the method but it's something like: get 250 ml of the hard water and add 3 drops of NH4Cl buffered solution to it. Add a few drops of erichrome black T indicator and titrate it with the EDTA in order to determine the concentration.
DISSOLVED OXYGEN
Disssolved oxygen is the concentration of oxygen dissolved in 1 litre of water at 20oC. Dissolved oxygen must be measure (it's concentration) because it must be between 6-8ppm in water for fish to breathe. If it drops to below 5ppm, fish experience asphyxiation and their gills close up and they can't breathe. DO high level is also required for photosynthesis of plants at the surface of the water.
QUALITATIVE TEST: Methylene blue indicator - add some drops to the water and if the colour changes to blue then oxygen is present dissolved on water but if the indicator makes the water yellow then no oxygen present.
QUANTITATIVE TEST: Use Polarographic oxygen probe which has a silver ring anode and a platinum disk cathode and also a buffered KCl solution with a porous membrane which allows O2 to diffuse through. A constant voltage is established between anode and cathode and as the O2 diffuses into it, a current is generated and measuring the current allows the o2 concentration to be determined.
BIOCHEMICAL OXYGEN DEMANDIs the measure of oxygen needed in water for the complete breakdown of organic matter in the water by AEROBIC bacteria (they require oxygen). High BOD indicates a high amount of pollution via sewage and stuff which would drain the DO out of the water --> asphyxiation and fish die etc so this is related to dissolved oxygen from above.
QUALITTATIVE TEST = NONE!
QUANTITATIVE TEST = Use the same procedure as the dissolved oxygen method but BOD is undertaken over 5 day period with the first measurement on the first day and the last measurement on the 5th day to determine the change in dissolved oxygen concentration to determine the BOD of the water (how much o2 is used up over 5 days). ALso put the water sample in the dark to prevent photosynthesis or anything from occurring which could provide an inaccurate concentration of O2.
