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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:

turbidity
acidity & alkalinity
trace elements and nutrients such as nitrogen, phosphorus, halogens (chloride and fluoride ions), alkali metals (sodium and potassium ions), calcium and magnesium ions.
microorganisms
dissolved oxygen content (DO)

Saturated Dissolved Oxygen (DO) Levels

Temperature (^oC) Saturated level of DO (ppm)

Freshwater Sea water

10 10.9 9.0

20 8.8 7.4

30 7.5 6.1

40 6.6 5.0

Example: Water Quality of a Typical Natural Aquatic System

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

SO₄^2-/HSO₄^-	17.5 ppm	2.6 x 10³

CO₃^2-/HCO₃^-	33.0 ppm	142 ppm

Hg²⁺	< 1 ppb	0.03 ppb

Cd²⁺	< 1 ppb	0.1 ppb

Pb²⁺	< 1 ppb	4-5 ppb

Tests for Water Quality

Test	Method	Reason for testing
Temperature	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 30^oC 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.

pH	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^-, CO₃^2- 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), HPO₄^2-, H₂PO₄^-, H₂S, Fe³⁺, other acidic metal ions, proteins & organic acids. Increases in acidity can be due to acids used in industry, acid mine drainage, acid rain.

Turbidity	Use a Sechi disc or 500mL 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).

Total Dissolved Solids (TDS)	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.

Dissolved Oxygen (DO)	Winkler Titration Method -Halve the water sample. Place one sample in the dark (for BOD analysis). -To the other sample add 2mL MnCl₂(aq) (4g MnCl₂ dissloved in 10mL distilled water) + 2mL alkaline iodide solution (3.3g NaOH + 2.0g KI dissolved in 10mL distilled water). Shake sample. -Add 2 mL concentrated HCl. Shake. The iodine formed is directly proportional to the dissolved oxygen. -Titrate 50.0mL of the above solution with 0.0125M 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.

Biochemical Oxygen Demand (BOD)	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). Unpolluted natural waters have BOD<5mg/L. Treated sewage can have BOD 20-30 mg/L.	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

Salinity	Titrate a known volume of the water sample with silver nitrate solution (2.73g AgNO₃ per 100mL 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.	Many aquatic organisms can only survive in a narrow range of salt concentrations since salt controls their osmotic pressure.

Total Phosphate Test	-acid digestion using concentrated H₂SO₄ and ammonium persulfate. -Titrate using NaOH and phenolphthalein as indicator -Use a few drops of H₂SO₄ to turn the solution clear again. -Add ammonium molybdate solution then solid ascorbic acid. -An intense blue complex of molybdenum blue is formed which can be measured colorimetrically. Absorbency is measured at 882nm. (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 0.2mg/L are common. Concentrations of 0.05mg/L indicate the possibility of eutrophication (increased nutrient concentrations) and algal blooms are likely. Natural phosphate is due to decayed organic matter and phosphate minerals.

Total Nitrogen test	Kjeldahl digestion -digestion with concentrated sulfuric acid, converting the nitrogen into ammonia sulfate -Solution is then made alkaline -liberated ammonia is distilled, and the amount determined by titration with standard acid. distillation titration method for samples containing >1mg/L Add Nessler's reagent (100g mercuric iodide + 70g KI in 100mL distilled water, then add 160g NaOH in 700mL distilled water, then dilute to 1L) for samples containing <1mg/L and measure absorbancy colorimetrically at 425nm.	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.

Hardness	Calcium Ions, Ca²⁺ -complexometric titration using EDTA (ethylenediaminetetraacetic acid) at a pH of 12-13 (at this pH Mg²⁺ is precipitated and not complexed with EDTA) -OR potentiometric techniques using selective electrodes -OR Atomic Absorption Spectroscopy (AAS) -OR Gravimetric Method - measure the amount of CaCO₃(s) precipitated by a known volume of 0.02M Na₂CO₃ -OR by Flame Test - Ca²⁺ flame test a brick-red colour in a non-luminous Bunsen flame Mg²⁺ -complexometric titration using EDTA at pH=10 (both Ca²⁺ and Mg²⁺ will complex with EDTA at this pH, [Mg²⁺] can be found by subtracting the results of this titration from the results of the first titration.) -OR potentiometric techniques using selective electrodes -OR Atomic Absorption Spectroscopy (AAS)	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 SO₄^2- 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⁺.

Microorganisms	microorganisms in a water sample are counted under a microscope Method for finding the number of coliform organisms in a water sample: Filter a known volume of water sample through a filter that retains microorgnisms Transfer the filter to a sterile petri dish containing appropriate agar and incubate at 35^oC for 20-40 hours. Also incubate a control plate with agar only. Colonies will develop on the filter wherever bacteria are retained. Either visually, or using a microscope, count the number of coliform colonies. Express these values as CFU/100mL.	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

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) 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 for Cu²⁺, and 2.0 mg/L for Zn²⁺ and Mn²⁺.

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 (100g mercuric iodide + 70g KI in 100mL distilled water, then add 160g NaOH in 700mL distilled water, then dilute to 1L) -----> yellow-brown NO₃^- + conc H₂SO₄ + FeSO₄ -----> brown ring forms at junction S^2- + lead acetate solution -----> black deposit of PbS SO₄^2- + BaCl₂(aq) -----> white precipitate of BaSO₄(s)