Water Quality Analysis
Conductivity is a measureof the ability of an aqueous solution to carry an electric current. This ability depends on the presence of ions; on their total consentration, mobility and valence; and the temperature of measurement.
Most problems in obtaining good data in conductivity monitoring equipment are related to electode fouling and to inadequated sample circulation. Conductivities grater than 10.000 to 50.000µmhos/cm or less than about 10 µmhos/cm may be difficult to measure with usual measurement electronic and cell capacitance.
Laboratory conductivity measurements are used to :
• Establish degree of mineralization to assess the effect of the total concentration of ion on chemical equlibria, physiological effect on plants or animals, corrotion rates, etc.
• Assess degree of mineralization of distilled and deionized water.
• Estimate sample size to be used to common chemical determinations and to check results of a chemical analysis.
• Estimate total dissloved solid(mg/L) in a sample by multiplying conductivity(in µmhos/cm) by an empirical factor. This factor may vary from 0,55 – 0,9, depending on the soluble components of the water and on the temperature measurement.
• Approximate the miliequivalents per liter of cations or anions in some waters by multyplying conductivity in units of µmhos/cm by 0,01.
Measurement of pH is one of the most important and frequently used test in water chemistry. Practically every phase of water supply and wastewater treatment e.g., acid-base neutralization, water softening, precipitation, coagulation, disinfection, and corrosion control, is pH dependent pH is used in alkalinity and carbon dioxide measurements and many other acid-base equlibria. At a given temperature the intensity of the acidic or basic character of a solution is indicated by pH or hydrogen ion activity.
Acidity of a water is its quantitative capacity to react with a strong base to a designated pH. The measured value may vary significantly with the end point pH used in the determination.
Ideally the end point of the acidity titration should corespond to the stoichiometric equivalence point for neuralization of acids present. The pH at the equivalence point will depend on sample, the choice among multiple inflection points, and the intended use of the data.
Alkalinity is significant in many uses and treatments of natural waters and waste waters. Because the alkalinity of many surface waters is primarily a function of carbonate, bicarbonate and hydroxide content, it is taken as an indication of the concentration of these constitutents. Alkalinity measurement are used in the interpretation and control water and waste water treatment processes.
Ideally the end point of the alkalinity titration when the alkalinity is due entirely to carbonate or bicarbonate content, the pH at the equivalence point of the titration is determined the concentration of carbon dioxide(CO2) at the stage. The pH values are suggested as the equivalence point for corresponding alkalinity concentration as miligrams CaCO3 per liter. “Phenolphthalein alkalinity” is the term traditionally used for the equantity measured by titration to pH 8,3 irrespective of the colored indicator, if any, used in the determination.
The Biochemical Oxygen Demand (BOD) determination is an empirical test in which standardized laboratory procedures are used to determine the relative oxygen requirements of waste waters, effluents, and poluted waters. The test measures the molecular oxygen utilized during specified incubation period for the biochemical degradation of organic material (carbonaceous demand) and the oxygen used to oxidize inorganic material such as sulfides and ferrous iron. The seeding and dilution procedures provide an estimate of the BOD at pH 6,5-7,5.
The chemical oxygen demand (COD) is used as a measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant. COD can be related empirically to BOD, organic carbon, or organic matter. The test is useful for monitoring and control after correlation has been established. The dichromate reflux methode is preffered offer procedures using other oxidant because of superior oxidizing ability, applicability to a wide variety of samples and ease of manipulation. Oxidation of most organic compounds is 95 to 100% od the theoritical value.
Most types of organic matter are oxidized by a boiling mixture of chromic and sulfuric acids. A sample is refluxed in strongly acid solution with a known excess of potassium dichromate (K2Cr2O7). After digestion, the remaining unreduced K2Cr2O7 is titrated ferrous ammonium sulfate to determonate the amount of K2Cr2O7 consumed and the oxidizable organic matter is calculated in terms of oxygen equivalent.