Water

ENVIRONMENTAL CHEMISTRY: Water

HARD WATER

  • Definition: Water that will not easily form a lather with soap.
  • Causes: Presence of Ca²⁺ or Mg²⁺ ions.
  • Result: Formation of a grey precipitate (scum) instead of lather.
  • Soft Water: Forms a lather easily due to Na⁺ ions.

TYPES OF HARDNESS

  • Temporary Hardness
  • Permanent Hardness
  • Measurement of Hardness: Measured in ppm (mg/L).

TEMPORARY HARDNESS

  • Cause: Calcium hydrogencarbonate Ca(HCO₃)₂, magnesium hydrogencarbonate Mg(HCO₃)₂.
  • Removal: Can be removed by boiling the water.

CAUSES OF TEMPORARY HARDNESS

  • Reaction: Carbon Dioxide and Water react to form carbonic Acid.
  • This carbonic Acid reacts with limestone (calcium carbonate) in the soil to form calcium hydrogen carbonate, which causes temporary hardness.

PERMANENT HARDNESS

  • Definition: Hardness that cannot be removed by boiling.
  • Causes:
    • Calcium sulfate
    • Magnesium sulfate
    • Calcium chloride
    • Magnesium chloride.

METHODS OF REMOVING HARDNESS

  1. Distillation
  2. Using Washing Soda
  3. Ion exchange resin
  • Note: Deionised water is not as pure as distilled water.

DISTILLATION

  • Process: Involves boiling the water and then cooling the vapor to separate it from the sulfate salts.
  • Limitation: Expensive due to the large quantities of water that need to be evaporated; not feasible on a large scale.

SOAPS (e.g. SODIUM STEARATE)

  • Function: Soap softens hard water by removing Ca²⁺ ions from the water.
  • Reaction:
    2C{17}H{35}COONa + Ca^{2+}
    ightarrow (C{17}H{35}COO)_{2}Ca ext{(s)} + 2Na^{+} (scum formed).
  • Only when all Ca²⁺ ions are removed can water form a lather with soap.

ION EXCHANGE RESIN

  • Process:
    • Ca²⁺ ions from hard water are exchanged for Na⁺ ions on a cation exchange resin.
    • Each calcium ion replaced by two sodium ions:
      Ca^{2+} + 2RNa
      ightarrow R_{2}Ca + 2Na^{+}.
    • To remove calcium ions, a solution of sodium chloride is passed through the resin to replace sodium ions with calcium ions.

DEIONISED WATER AND DISTILLED WATER

  • Definition: Deionised water has all dissolved ions removed, while distilled water has all dissolved ions, organic solids, and gases removed.

DEIONISED WATER

  • Method: Ion exchange resins use a mixture of cation and anion exchangers.
  • Process:
    • All cations replaced by H⁺ ions, and all anions replaced with OH⁻ ions, forming water.

EXAMPLE TEMPORARY HARDNESS – CA(HCO₃)₂ IN WATER

  1. Two hydrogen ions (H⁺) from the cation-exchange resin (RH) replace a Ca²⁺ in the water:
    2RH + Ca^{2+}
    ightarrow R_{2}Ca + 2H^{+}.
  2. A hydroxide ion (OH⁻) from the anion-exchange resin (ROH) replaces HCO₃⁻ in the water:
    ROH + HCO₃^{-}
    ightarrow RHCO₃ + OH^{-}.
  3. H⁺ and OH⁻ combine to form water:
    H^{+} + OH^{-}
    ightarrow H₂O.

ADVANTAGES OF HARD WATER

  1. Provides calcium for teeth and bones.
  2. Nicer taste.
  3. Good for brewing and tanning leather.

DISADVANTAGES OF HARD WATER

  1. Blocks pipes and leaves scale on kettles and boilers.
  2. Wastes soap.
  3. Produces scum.

HARD VS SOFT WATER RECAP

  • Hardness: Describes the presence of certain ions in water.
  • Temporary Hardness: Caused by temporary compounds like Ca(HCO₃)₂.
  • Permanent Hardness: Caused by permanent compounds such as sulfates and chlorides.

TOTAL HARDNESS

  • Total Hardness = Temporary Hardness + Permanent Hardness.
  • Measurement method: Titration against EDTA solution with the addition of Eriochrome Black T to check calcium and magnesium levels.
  • Calcium and magnesium ions react with EDTA, resulting in a color change from wine red to blue.

QUESTIONS RELATING TO TOTAL HARDNESS EXPERIMENT

  1. Importance of quick reaction and complete reaction between EDTA and metal ions.
  2. Reliability affected if other metal ions are present. Other ions: Lead (Pb²⁺), Cadmium (Cd²⁺).
  3. No distinction between temporary and permanent hardness.
  4. Methods to establish permanent hardness.
  5. Function of buffer solution in maintaining pH during titration.

SUSPENDED SOLIDS

  • Definition: Insoluble substances dispersed throughout a water sample.
  • Result: Causes turbidity (cloudiness).
  • Measurement: Levels measured by filtration.

DISSOLVED SOLIDS

  • Definition: Substances that are soluble in water.
  • Measurement: Levels measured by evaporation.

WATER TREATMENT

  • Strategies:
    1. Screening
    2. Flocculation
    3. Settlement (sedimentation)
    4. Filtration
    5. Chlorination
    6. Fluoridation
    7. pH adjustment

SCREENING

  • Process: Water passed through a wire mesh to remove floating debris and large objects.

FLOCCULATION

  • Definition: Coagulation or precipitation.
  • Process: Small suspended particles form larger particles by adding flocculating agents like aluminum sulfate. Too much can affect taste.

SETTLEMENT (SEDIMENTATION)

  • Process: Water pumped into sedimentation tanks allowing suspended particles to settle at the bottom.

FILTRATION

  • Process: Remaining suspended particles removed by passing water through sand and gravel filter beds.

CHLORINATION

  • Purpose: Adding chlorine compounds to kill microorganisms and prevent reinfection.
  • Limitation: Excess chlorine can result in odour and affect taste.

FLUORIDATION

  • Definition: Addition of fluoride compounds (1 ppm) to drinking water to prevent tooth decay.
  • Caution: Excess can be poisonous and leads to teeth staining.

PH ADJUSTMENT

  • Importance: Adjust pH before treatment completion. Optimal pH for tap water is between 6 – 8 (ideal 7.2).
  • Methods:
    • If too acidic: Add Lime (Ca(OH)₂) to raise pH.
    • If too basic: Add sulfuric acid.
  • Consequences of pH: Acidic water can corrode pipes, while basic water causes hardness issues.

EXPERIMENT 19.2: MEASURING SUSPENDED AND DISSOLVED SOLIDS

  • Part (a): Measure suspended solids by filtering water through weighed filter paper and calculating the mass increase.
  • Part (b): Measure dissolved solids by evaporating filtered water and weighing the residue.
  • Part (c): Measure pH using a pH sensor or pH paper.

DISSOLVED SOLIDS CALCULATION EXAMPLE

  • Given Data:

    • Suspended solids: 0.68 g/500 cm³ → Convert to ppm:

    • 0.68 imes 2 = 1.36 g/L = 1360 ppm.

    • Dissolved solids: 0.13 g/100 cm³ → Convert to ppm:

    • 0.13 imes 10 = 1.3 g/L = 1300 ppm.

EUTROPHICATION

  • Definition: Enrichment of water with nutrients leading to excessive growth of algae and other plants.
  • Effects of Algae Blooms:
    • High pH
    • Low oxygen levels
    • Death of aquatic plants and animals
    • Low light penetration
    • Presence of toxic blue-green algae.

DISSOLVED OXYGEN IMPORTANCE

  • Need for aquatic life: Essential for fish survival.
  • Low solubility in water (oxygen's non-polarity); affected by temperature:
    • Low temperatures yield higher solubility, high temperatures yield lower solubility.
    • Example: Heating water (as in a kettle) leads to the escape of dissolved gases.

POLLUTION

  • Definition: Release of damaging substances into the environment.
  • Eutrophication Effects: Excessive plant growth leading to low transparency, oxygen depletion, fish kills, and impaired water quality.

BOD (BIOCHEMICAL OXYGEN DEMAND)

  • Test Conditions:
    • Dark for prevention of photosynthesis
    • 20°C for biological enzyme activity
    • Duration: 5 days for oxygen consumption measurement.

B.O.D TEST

  • Method: Two bottles filled with water, measuring dissolved oxygen using the Winkler Method. One bottled incubated without light at 20°C for 5 days. Difference in D.O levels indicates oxygen consumed biologically.

PRECAUTIONS IN B.O.D TEST

  • Reasons:
    • Dilute with distilled water if effluent levels are high, as high levels can use all oxygen.
    • Ensure no atmospheric oxygen is trapped while filling the bottle.

WINKLER METHOD

  1. Add manganese sulfate (MnSO₄) and alkaline potassium iodide (NaOH and KI) to water.
  2. Under alkaline conditions, manganese (II) sulfate produces manganese hydroxide, leading to precipitate reactions with dissolved oxygen.
  3. Titration with thiosulfate determines oxygen demand.

SEWAGE TREATMENT

  1. Primary Treatment: Physical/Mechanical process involving screening and settling.
  2. Secondary Treatment: Biological process where bacteria reduce organic materials.
  3. Tertiary Treatment: Chemical and physical treatment to remove nutrients like phosphates and nitrates.

POLLUTION BY HEAVY METALS

  • Cause: Industrial waste or battery disposal (heavy metals such as mercury, cadmium, and lead).
  • Characteristics: Cumulative poisons that build concentration in the body causing damage.
  • Removal: Use of precipitation methods for removal.

HEAVY METAL POLLUTION

  • Examples: Lead (Pb²⁺), Mercury (Hg²⁺), Cadmium (Cd²⁺).
  • Effects: Lead poisoning, caused especially by lead pipes; mercury leads to significant intestinal and kidney damage.

ATOMIC ABSORPTION SPECTROMETRY (AAS)

  • Functionality: Analyzes heavy metals in water. Atoms in ground state absorb characteristic light wavelengths for measurement.

COLORIMETRY

  • Application: Measures absorbance of light in colored solutions.
  • Principle: Absorbance correlates with solution concentration.

WATER ANALYSIS BY INSTRUMENTAL ANALYSIS

  • Techniques:
    • pH meter, pH paper, or universal indicators for acidity.
    • AAS for heavy metal concentration.
    • Colorimetry for colored solution concentration assessment.