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Water Analysis

Key Concepts

Water is an essential resource for living systems, industrial processes, agricultural production and domestic use.

The principal factors that are taken into consideration when determining water quality are:

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Example: Water Quality of a Typical Natural Aquatic System

Note that the concentration of ions is usually given in parts per million (ppm) and sometimes parts per billion (ppb).

Substance or Quality River Water Sea Water
pH 6.8 8.0

Dissolved Oxygen 6-8 ppm 6-8 ppm

Na+ 6.7 ppm 1.1 x 104 ppm

K+ 1.5 ppm 380 ppm

Ca2+ 17.5 ppm 400 ppm

Mg2+ 4.8 ppm 1.3 x 103 ppm

Cl- 4.2 ppm 1.9 x 104 ppm

SO42-/HSO4- 17.5 ppm 2.6 x 103

CO32-/HCO3- 33.0 ppm 142 ppm

Hg2+ < 1 ppb 0.03 ppb

Cd2+ < 1 ppb 0.1 ppb

Pb2+ < 1 ppb 4-5 ppb

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Tests for Water Quality

Method Reason for testing
Temperature Test
Use an alcohol thermometer in a hard plastic cover Temperature influences the amount of dissolved oxygen in water which in turn influences the survival of aquatic organisms (raising the temperature of a freshwater stream from 20 to 30oC will decrease the dissolved oxygen saturation level from about 9.2 ppm to 7.6 ppm.).
Increasing temperature also increases the rates of chemical reactions taking place in the water.
Increases in temperature are often associated with hot water discharge from power stations and industries that use water as a coolant.

Method Reason for testing
pH Test
Use a pH meter in a hard plastic cover, pH paper or Universal Indicator solution. pH measures the acidity or alkalinity of water.
pH of rain water is about 5.5-6.0.
Typically, natural water has pH 6.5-8.5.
A pH < 5 (acidic water) is most damaging to eggs and larvae of aquatic organisms.
Most aquatic life (except for some bacteria and algae) cannot survive pH < 4.
Natural alkalinity is due to CO2(g), HCO3-, CO32- and OH-, carbonate rocks such as limestone and dolomite increase alkalinity.
Alkalinity is increased by caustic substances from industry (KOH, NaOH), soil additives in agriculture such as lime Ca(OH)2, superphosphate which is mixture of Ca(H2PO4)2 and CaSO4, and soaps and detergents.
Natural acidity is due to CO2(g), HPO42-, H2PO4-, H2S, Fe3+, other acidic metal ions, proteins and organic acids.
Increases in acidity can be due to acids used in industry, acid mine drainage, acid rain.

Method Reason for testing
Turbidity Test
Use a Sechi disc or 500 mL of water in a measuring cylinder standing on paper marked with a black cross. Turbidity is a measure of water clarity.
Suspended solids in water can stop light reaching submerged plants and can raise water temperature.
Suspended solids often present in water are mud, clay, algae, bacteria and minerals such as silica, calcium carbonate and ochre (iron oxide).
Suspended solids can be increased by the discharge of wastes (domestic sewage, industrial and agricultural effluents), leaching of wastes (from mines), and agitation (dredging or shipping).

Method Reason for testing
Total Dissolved Solids (TDS) Test
Use an appropriate TDS meter.
Freshwater meters: 0-1990 ppm (parts per million).
Dual range brackish water meters: 0-19,900 ppm.
Salt-water meters: to above 35,000 ppm.
This is a conductivity test of available ions in the water, including Ca2+, Na+, K+, Fe2+, Fe3+, HCO3- and ions containing P, S & N.
High levels of Na+ is associated with excessive salinity and is found in many minerals.
Potassium is incorporated into plant material and is released into water systems when plant matter is decayed or burnt.

Method Reason for testing
Dissolved Oxygen (DO) Test

  • Winkler Titration Method :
    (i) Halve the water sample.
    Place one sample in the dark (for BOD analysis).
    (ii) To the other sample add 2 mL MnCl2(aq) (4g MnCl2 dissloved in 10 mL distilled water) + 2 mL alkaline iodide solution (3.3 g NaOH + 2.0g KI dissolved in 10 mL distilled water).
    Shake sample.
    (iii) Add 2 mL concentrated HCl.
    Shake.
    The iodine formed is directly proportional to the dissolved oxygen.
    (iv) Titrate 50.0 mL of the above solution with 0.0125 mol L-1 sodium thiosulfate solution using starch as an indicator.
    The end-point is reached when the blue-black colour disappears.
  • Colorimetric Method (A field kit is available using a 'Smart' colorimeter)

Collect 2 water samples, 1 for DO test, 1 for BOD test.
Sample must be collected under water to ensure there are no trapped air bubbles.
The Dissolved Oxygen test measures the current oxygen levels in the water.
The DO level varies with temperature.
DO levels are highest in the afternoon due to photosynthesis and lowest just before dawn.
DO is lowered by an increase in temperature (as from a discharge of hot water form a power station), increases in aerobic oxidation (due to increases in organic matter from sewage or due to inorganic fertilisers such as phosphates and nitrate with overstimulate algal growth).
Water with DO < 1ppm is dead.

Method Reason for testing
Biochemical Oxygen Demand (BOD) Test
The first water sample from above is kept in the dark for 5 days at the temperature at which the sample was collected.
Then the dissolved oxygen is determined using the Winkler titration method as above.
Subtract the mass of oxygen obtained an day 5 from mass of oxygen on day 1 to determine the BOD (mg L-1).
Unpolluted natural waters have BOD < 5mg L-1.
Treated sewage can have BOD 20-30 mg L-1.
BOD measures the rate of consumption of oxygen by organisms in the water over a 5 day period.
Increases in BOD can be due to animal and crop wastes and domestic sewage.
Untreated domestic sewage BOD ≈ 350 ppm
Waste water from breweries BOD ≈ 550 ppm
Waste water from petroleum refineries BOD ≈ 850 ppm
Abattoir wastes BOD ≈ 2,600 ppm
Pulpmill wastes BOD ≈ 25,000 ppm

Method Reason for testing
Salinity Test
Titrate a known volume of the water sample with silver nitrate solution (2.73 g AgNO3 per 100 mL distilled water) using K2CrO4 as indicator.
The end-point of the titration is given by the reddening of the silver chloride precipitate (AgCl(s)).
Volume of AgNO3 used = chloride content in g L-1.
Many aquatic organisms can only survive in a narrow range of salt concentrations since salt controls their osmotic pressure.

Method Reason for testing
Total Phosphate Test
(i) acid digestion using concentrated H2SO4 and ammonium persulfate.
(ii) Titrate using NaOH and phenolphthalein as indicator

(iii) Use a few drops of H2SO4 to turn the solution clear again.

(iv) Add ammonium molybdate solution then solid ascorbic acid.

(v) An intense blue complex of molybdenum blue is formed which can be measured colorimetrically.

Absorbency is measured at 882 nm.
(A field kit is available using a 'Smart' colorimeter)
Total Phosphate is used as an indicator of pollution from run-off in agricultural areas or domestic sewage.
Concentrations of 0.2 mg L-1 are common.
Concentrations of 0.05 mg L-1 indicate the possibility of eutrophication (increased nutrient concentrations) and algal blooms are likely.
Natural phosphate is due to decayed organic matter and phosphate minerals.

Method Reason for testing
Total Nitrogen Test

  • Kjeldahl digestion
    (i) digestion with concentrated sulfuric acid, converting the nitrogen into ammonia sulfate
    (ii) Solution is then made alkaline
    (iii) liberated ammonia is distilled, and the amount determined by titration with standard acid.
  • distillation titration method for samples containing > 1mg L-1
  • Add Nessler's reagent (100 g mercuric iodide + 70 g KI in 100 mL distilled water, then add 160 g NaOH in 700 mL distilled water, then dilute to 1 L) for samples containing < 1mg L-1 and measure absorbancy colorimetrically at 425 nm.
Total Nitrogen is an important indicator of eutrophic waters, especially for those contaminated by animal wastes, fertiliser run-off and domestic sewage.
Aquatic nitrogen is essential for the growth of organisms and is produced in natural processes including decay of proteins, the action of lightning, and the action of nitrogen-fixing bacteria on ammonia.
(also see Nitrogen Cycle)

Method Reason for testing
Hardness Test
Calcium ions are a major contributor to water hardness and are due to water running through rocks containing minerals such as gypsum (CaSO4.2H2O), calcite (CaCO3), dolomite (CaMg(CO3)2).
Hard water has a noticeable taste, produces precipitates with soaps which inhibits lathering and forms precipitates (scale) in boilers, hot water systems and kettles.
Temporary hardness (or 'bicarbonate hardness') is due to Ca(HCO3)2 which deposits CaCO3(s) as scale on boiling the water.
Magnesium ion levels are often high in irrigation water and can cause scouring in stock.
Ca2+ and Mg2+ can combine with Cl- and/or SO42- causing permanent hardness which can't be removed by boiling.
Water can be softened by an ion exchange process using a solid material such as a resin or clay that is capable of exchanging Na+ or H+ for Ca2+ and Mg2+.

Method Reason for testing
Test for Microorganisms
Microorganisms in a water sample are counted under a microscope
Method for finding the number of coliform organisms in a water sample:

  1. Filter a known volume of water sample through a filter that retains microorgnisms
  2. Transfer the filter to a sterile petri dish containing appropriate agar and incubate at 35oC for 20-40 hours.
    Also incubate a control plate with agar only.
    Colonies will develop on the filter wherever bacteria are retained.
  3. Either visually, or using a microscope, count the number of coliform colonies.
    Express these values as CFU/100 mL.
Many protozoa, bacteria, viruses, algae and fungi are found in natural water systems.
Some are pathagenic (typhoid, cholera and amoebic dysentry can result from water-borne pathogens).
The excessive growth of algae (called 'algal bloom') can degrade water quality because it lowers dissolved oxygen levels thereby killing other living things.
The level of bacterial contamination of water due to animal waste is measured by determining the number of coliform organisms such as E. coli

Method Reason for testing
Test for Heavy Metals

  • Ni2+ + dimethylglyoxime in ethanol turns pink-red
  • Fe3+ + ammonium thoicyanate turns blood-red
  • Cu2+ + dithizone in 1,1,1-trichloroethane turns yellow-brown
  • Cd2+ + dithizone in 1,1,1-trichloroethane turns blue-violet
  • Pb2+ + dithizone in 1,1,1-trichloroethane turns brick-red
    Pb2+ + 2KI(aq) → yellow precipitate of PbI2(s)
  • Zn2+ + dithizone in 1,1,1-trichloroethane turns pink
Heavy metals in concentrations above trace amounts are generally toxic to living things.
Trace amounts ( < 0.05 mg L-1) of Zn, Cu and Mn are present in most natural waters.
Zn and Cu may be present in higher levels in irrigation areas due to the use of galvanised iron, copper and brass in in plumbing fixtures and for water storage.
In irrigation areas, acceptable levels are:
  • 0.2 mg L-1 for Cu2+

  • 2.0 mg L-1 for Zn2+

  • 2.0 mg L-1 for Mn2+

Tests for Other Ions

  • Al3+ + aluminon → pink-red
  • Mg2+ + magneson I → light blue
  • Na+ flame test → yellow flame
  • K+ flame test → lilac flame
  • NH4+ & NH3 + Nessler's reagent (100 g mercuric iodide + 70g KI in 100 mL distilled water, then add 160 g NaOH in 700 mL distilled water, then dilute to 1 L) → yellow-brown
  • NO3- + conc H2SO4 + FeSO4 → brown ring forms at junction
  • S2- + lead acetate solution → black deposit of PbS
  • SO42- + BaCl2(aq) → white precipitate of BaSO4(s)

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