Ch. 5 - Waterways

Question 1

Test for sulfate

Answer 1

QUALITATIVE
- Sample is acidified with nitric acid to remove any carbonate or hydrogen carbonate
- Addition of barium nitrate solution forms white precipitate BaSO4
 Ba2+ (aq) + SO42- (aq) -> BaSO4 (s)

QUANTITATIVE
- Gravimetric analysis can be carried out by weighing the dried precipitate

Question 2

Test for chloride

Answer 2

QUALITATIVE
- Sample is acidified with nitric acid to remove any carbonate or hydrogen carbonate
- Addition of silver nitrate solution forms white precipitate AgCl
 Ag+ (aq) + Cl- (aq) -> AgCl (s)

QUANTITATIVE

• Gravimetric analysis can be carried out by weighing the dried precipitate
• If the concentration of the ion is low in the water sample, it may need to be initially concentrated by evaporation prior to analysis

Question 3

Total dissolved solids

Answer 3

QUANTITATIVE
Filtration & evaporation – suspended solids in water are firstly filtered off. The filtrate is evaporated to dryness and the dry residue is weighed

Question 4

Hardness

Answer 4

QUALITATIVE
Lathering – samples of water can be shaken in stoppered test tubes with soap solution, and then left to stand. The height of the froth indicates the softness of the water.
- Soft water produces significant lather whereas hard water only produces a grey floating scum.
- The soap (sodium stearate) in hard water forms a scum consisting of calcium and magnesium stearates

QUANTITATIVE

• EDTA titration – titration with a standard solution of EDTA, which reacts with metal ions (M2+) such as magnesium and/or calcium ions to form a metal-EDTA complex.
• AAS

Question 5

Turbidity

Answer 5

QUANTITATIVE

Turbidity tube - turbidity measured in NTUs

Question 6

Dissolved oxygen

Answer 6

Winkler titration

• The dissolved oxygen in a water sample (no air bubbles or sediments) is firstly reacted with manganese (II) ions in alkaline solution
• Acidified potassium iodide solution is added to form molecular iodine
• The amount of iodine is determined by titration with standard sodium thiosulfate solution, Na2S2O3, using a starch indicator which forms an intensely blue colour with iodine which disappears at end point.

Polarographic oxygen probes

Question 7

Biochemical oxygen demand

Answer 7

5 day BOD for unpolluted water

• The DO of a water sample is measured at the time of collection
• A second sample is incubated in the dark at 20oC in a sealed container for five days, and the DO is measured again
• The difference between the dissolved oxygen readings before and after incubation gives BOD

5 day BOD for polluted water

• A polluted waterway will readily consume dissolved oxygen, and will require different methods
• The sample may have to be systematically diluted with a standard nutrient solution and the 5 day BOD test is carried out
• The sample can also be periodically re-aerating with oxygen so that dissolved oxygen cannot reach zero. Similarly, a respirator can be used.

Question 8

Identify factors that affect the concentrations of a range of ions in solution in natural bodies of water such as rivers and oceans

Answer 8

Pathway of rain to the water body

pH of rainwater
- water from acid rain is better able to leach certain cations such as Ca2+ and Mg2+ from soils,
leading to an increased concentration of these ions

Human activity

• Land clearing leads to more water running across disturbed land and collecting sediments into streams - > increased conc. of Na+, K+, Ca2+ and Mg2+
• Fertilisers (NO3-, NH4+) and animal faeces can also be carried into water bodies
• Leeching from rubbish dumps - dangerous ions Cd2+ from batteries can enter streams

Discharged effluents
- Sewage runoff and stormwater runoff increases the concentrations of many ions (e.g. NO3-, PO43-), while industrial effluents can discharge heavy metal ions (e.g. Pb2+, Cu2+)

Question 9

Describe the design and composition of microscopic membrane filters and explain how they purify contaminated water

Answer 9

 Microscopic membrane filters are thin films made usually from synthetic polymers (e.g. polypropylene)
with pores of fairly uniform size, and they can be designed in sheet or capillary form
 Water is run across the surface of the membrane rather than perpendicular to it, to avoid clogging up of the fine pores.
 The membrane consists of either fine pores (sheet membrane) or capillary like fibres (capillary membrane)
 Pressure, developed through either a pump, gravity, or even a vacuum, forces the water through the filter, and into the inner tube of clean water
 Particles, bacteria, and even viruses are trapped outside the filter, since the pores are much smaller than those of sand filters. However, dissolved substances cannot be filtered.
 Many membrane filters are back-flushable and can be cleaned by blowing air from the clean side to dislodge trapped particles which are then washed away by the dirty water on the outside.
 By the size of their pores, they can also be classified into microfiltration, ultrafiltration and nanofiltration
 Sheet membranes are folded by pleating or spirally wound around a central rigid porous core. They are usually housed in an outer plastic container to form a removable, flushable cartridge, and altogether mounted in the water pipe to filter water.
 Capillary membranes are composed of hollow fibres with inside diameter around 200 μm, outside diameter 500 μm, and a pore size of 0.2 – 0.5 μm. Dirty water flows around each capillary, and the filtered water flows through the capillary walls. Many capillaries are bundled together to make a filtering unit with a very large surface area.