تحاليل المياه

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 9.0 10.9
20 7.4 8.8
30 6.1 7.5
40 5.0 6.1
Example: Water Quality of a Typical Natural Aquatic System
ITEM RIVER WATER SEA WATER

PH 6.8 8.0
DISSOLVED OXYGEN 6—8 6—8
SODIUM 6.7PPM 11000PPM
POTASSIUM 1.5PPM 350PPM
CALCIUM 17.5PPM 400PPM
MAGNESIUM 4.8PPM 1300PPM
CHLORIDE 4.2PPM 1900PPM
SULPHATE 17.5PPM 2600PPM
CARBONATE 33PPM 140PPM
MERCURY 1.0PPM 0.03PPM
CADMIUM 1.0PPM 0.1PPM
LEAD 1.0PPM 4-5PPM

Test

TEMPERATURE

METHOD
Use an alcohol thermometer in a hard plastic cover

Reason for testing

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

Ph

METHOD

Use a pH meter in a hard plastic cover, pH paper or Universal Indicator solution

Reason for testing

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 & organic acids. Increases in acidity can be due to acids used in industry, acid mine drainage, acid rain


Turbidity


METHOD

Use a Sechi disc or 500mL of water in a measuring cylinder standing on paper marked with a black cross

Reason for testing

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


METHOD

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

Reason for testing

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


METHOD

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

Reason for testing

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


METHOD

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

Reason for testing

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


METHOD

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


Reason for testing

. Many aquatic organisms can only survive in a narrow range of salt concentrations since salt controls their osmotic pressure


Total Phosphate Test


METHOD

-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
Reason for testing

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


METHOD

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

Reason for testing

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

METHOD

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).

Reason for testing

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+.




Microorganisms


METHOD

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 35oC 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

Reason for testing

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


METHOD

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

Reason for testing

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+..