Note: The following procedures for testing water quality are all QUANTITATIVE
TESTING FOR THE CONCENTRATIONS OF COMMON IONS
Electrical conductivity can be used to measure the concentration of dissolved salts present as ions, called
salinity. The units are microsiemens per centimetre (µS cm
-1). Because ionic compounds conduct electricity in solution, the more dissolved salts (ions), the higher the conductivity. Water with an electrical conductivity of less than 280 µm cm
-1 is suitable for irrigation. Concentration of ions can also be measured in ppm or mg L
-1.
Atomic Absorption Spectroscopy (AAS) can be used for determining the concentration of many metallic ions in water samples. It relies on the fact that when electrons of an atom absorb light of a specific wavelength, they gain enough energy to jump to a higher energy level. The atomic absorption spectrometer consists of:
- Cathode Lamp - which is made from the same metal as the one being tested
- Flame or Furnace - into which the sample solution is sprayed, the heat of the flame or furnace makes the metal atoms gaseous
- Filter or Monochromator - which selects the specific wavelength of light to be measured
- Detector - which measures the amount of light recieved (that is, the total amount of light given from the cathode lamp minus the amount of light absorbed by the metal atoms)
- Processor - which calculates the concentration of the metal ion in solution from the amount of light recieved.
Basically, when the sample solution is sprayed into the flame or furnace, the electrons in the metal atoms absorb some of the energy from the cathode lamp. The rest of the light passes through to the detector to be measured, and the concentration of the metal ion in the sample is calculated from the amount of light absorbed.
Ion-selective Electrodes (ISE) are also available for determining ion concentrations in water. Similar to pH electrodes which measure specifically H
+ in water, the ISE measures the concentration of any specific ion. The change in electrode voltage between a reference electrode and the indicator electrode is related to the concentration of the specific ion being measured. The ISE can detect ion concentrations as low as ppb. As with AAS, a series of dilute standards are used to establish a calibration graph.
TESTING FOR ACIDITY
Potable water should have a pH between 6.5-8.5. The acidity of water is tested using
acid-base indicators or
pH meters. The pH electrode must be calibrated against buffer solutions of known pH.
Once the pH is measured, the concentration of H
+ in the sample can be calculated by using
[H+ = 10-pH.
TESTING FOR TOTAL DISSOLVED SOLIDS (TDS)
By evaporation: Natural water samples contain a wide range of dissolved inorganic salts and organic materials. Salt water in the oceans contain very high levels of mineral ions mainly in the form of sodium and chloride ions. Reservoir water supplies contain variable amounts of dissolved solids. To measure the TDS of water, a sample must first be filtered to remove undissolved particles. A known volume of filtrate is then evaporated and the mass of solid residue remaining is determined gravimentrically.
By using a conductivity meter: Most dissolved solids such as NaCl are ionic salts. The greater the concentration of dissolved salt, the higher the electrical conductivity. Electrical conductivity measurements can be made using a small meter.
TESTING FOR WATER HARDNESS
Hard water contains high levels of calcium, magnesium and aluminium ions. See my previous post for the relationship between water quality and the concentration in ppm of these ions.
To test water hardness, soap is used. While soft water forms a lather with soap, hard water does not, because in the presence of calcium or magnesium ions, the soap precipitates out as a "scum":
Ca2+ + 2Na stearate (soap) --> Ca (stearate) + 2Na+
"Stearate" is used to make the equation more simple. Soap is composed of sodium or potassium salts of long chain alkanoic acids such as stearic acid.
TESTING FOR TURBIDITY
Turbidity is measured in
nephelometric turbidity units (NTU).
Nephelometry: A meter is used to measure the percentage of light transmitted through a standard depth of water. As light is passed through the water sample, some is scattered by suspended particles. The remaining light is transmitted. The intensity of the light scattered at 90º allows the turbidity of the water sample to be determined. The nephelometer must also be calibrated using a series of standards.
Gravimetric methods: A measured volume of water is filtered through a preweighed filter paper, which is then dried and reweighed and the concentration of solid calculated.
Secchi Disc: A disc which contains a cross at the bottom. It is lowered into the water being tested, and the moment the cross can no longer be seen, the depth of water is recorded. Similarly, a measuring cylinder with a cross at the bottom can be filled with the water sample until the cross can no longer be seen.
TESTING FOR DISSOLVED OXYGEN (DO)
Oxygen is usually present in concentrations of 6-9 ppm. At concentrations of lower than 5 ppm, aquatic organisms begin to suffocate.
Oxygen-sensitive Electrode: Dissolved oxygen meters are based on electrochemical cells. The most common type of probe uses a gold or platinum electrode and a silver electrode in a KCl electrolyte solution. The voltage applied between the gold/platinum and silver electrodes does not cause electrolysis until oxygen reaches the electrolyte solution. The electrodes and electrolyte are separated from the water being sampled by a plastic membrane that oxygen can diffuse through. The amount of electrolysis is proportional to the amount of oxygen. The DO level is measured by a milliammeter.
Winkler Method: (This may look a little confusing, and I still can't write the equations from memory. Don't worry, we don't need it.
As long as we know the oxygen-sensitive electrode method, who gives a toss about Winkler.) The Winkler method is a way of fixing the amount of dissolved oxygen in a sample and determining the DO by titration at a later time.
Manganese (II) ions and hydroxide ions are added to the water sample. The amount of insoluble brown manganese (IV) oxide produced depends on the amount of DO:
2Mn2+ + 4OH- + O2(aq) --> 2MnO2(s) + 2H2O(l)
Acidified iodide solution reacts with the MnO
2, producing a yellow iodine solution.
2MnO2(s) + 8H+ + 4I- --> 2Mn2+ + 4H2O(l) + 2I2(aq)
The iodine released is titrated against a standard sodium thiosulphate solution from a burette containing starch indicator. The starch indicator forms a blue colour with iodine, and the blue colour disappears at the end point.
2I2(aq) + 4S2O32-(aq) --> 4I- + 4S2O32-(aq)
It can be seen that each dissolved O
2 molecule gives 2MnO
2 which gives 2I
2 which reacts with 4S
2O
32-(aq). Thus for each molecule of thiosulphate used at the end in the titration there was
1/
4 mole of dissolved oxygen in the original sample.
When water samples are collected from different locations, identical containers, identical sealing systems and the same collection procedures must be used. It is important that the containers are completely filled with water so there is no air space, and that they are kept out of light so algae present cannot add to the oxygen level by photosynthesis.
TESTING FOR BIOCHEMICAL OXYGEN DEMAND (BOD)
BOD is the quantity of oxygen needed by aerobic bacteria to break down all the organic matter in a water sample.
The higher the BOD, the greater the pollution in water. This is because organic waste requires oxygen for aerobic decomposition.
Oxygen Probe: Saturate a measured sample of water with oxygen, measure the dissolved oxygen concentration using the oxygen-sensitive electrode as mentioned before, seal and incubate at 20ºC for 5 days in the dark, measure residual dissolved oxygen concentration, and then calculate BOD.
Winkler Method: (My notes said "Wrinkler", but I'm pretty sure that "r" isn't supposed to be there).
