ENVE 201 Environmental Engineering Chemistry 1

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ENVE 201Environmental Engineering Chemistry 1

• CHLORIDES
• (Chapter 21)
• Dr. Aslihan Kerç

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Chlorides

• As mineral content ? Cl- ?
• Upland, Mountain supplies low in Cl-
• High in rivers and groundwater
• Sea-Ocean ?

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Sources of Chloride

• Surface waters dissolve chlorides from top soil.
• Spray from the ocean is carried in land as
  droplets.
• Ocean and sea waters invade the rivers that drain
  into them.
• Intermixing between freshwater and saltwater
  layers

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Sources of Chloride

• There is a hydrostatic balance between seawater
  and groundwater in areas adjacent to the ocean.
  Over pumping of groundwater disturbs the balance.
• ? Salt water intrusion into well

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Hydrostatic BalanceSalt water intrusion
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Hydrostatic BalanceSalt water intrusion
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Effect of Evapotranspiration

• Evapotranspiration of water used for irrigation
• Lost to the atmosphere
• Leaving the salt in the soil
• Destruction of soils crop-growing potential
• Highly saline irrigation water return to the
  surface waters
• Normal evaporation also increases salt content
  of surface water.

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Chloride in Wastewaters

• Human urine contain chloride ? originating from
  consumed food and water
• 6 gr Cl- / person / day
• Cl- in municipal wastewater 15 mg/L ? discharge
  to the surface water
• Industrial wastes also contain Cl-

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Chloride in Drinking Water

• Concentration above 250 mg/L give a salty taste
• For public use limit? 250 mg/L
• In arid regions ? 2000 mg/L can also be used ,
  human system become adapted.

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• Before development of bacteriologic test,
• Cl- and nitrogen determination ? used for
  detecting contamination of groundwater by
  wastewater
• Cl- measurement and total salinity ? important
  for irrigation water
• Chlorides can be used as tracers in Env. Eng. ?
  in groundwater studies.

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• Methods of Determination
• Mohr method (Argentometric) ? Employing silver
  nitrate as the titrant
• Potassium chromate as the indicator
• Mercuric nitrate method
• Mercuric nitrate ? titrant
• Diphenylcarbazone ? indicator

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• Instrumental Methods
• Potentiometric titration ? change in the
  potential between a reference electrode and
  silver-silver chloride electrode
• Titration w/silver nitrate solution
• Chloride present complexes with silver ions
• When all chloride complexes ? silver ion
  concentration increases
• Sudden increase in voltage

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• Instrumental Methods
• Automated ferricyanide method?Colorimetric
• Ion chromatography

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Mohr Method

• 0.0141 N silver nitrate ( N / 71 )
• Each ml 0.5 mg Cl-
• Cl- is precipated as silver chloride
• Ag Cl- ? AgCl (white precipitate)
• End point ? presence of excess Ag
• 2 Ag CrO-24 ? Ag2CrO4 (reddish-brown
  ppt)

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Mohr Method

• Sample size 100 ml
• pH 7-8 Ag ppt as AgOH at high pH levels and
  CrO-24 is converted to CrO-27 at low pH.
• A definite amount of indicator
• Cl- (mg/L) ( ml AgNO3 blank)0.5 1000
• ml sample
• 0.0141 35.45 0.5

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• Mercuric Nitrate Method ? Reading assignment