3) chemical analysis of drinking water – pH-range, oxidizability, chloride and sulfate
-all natural water contains a range of inorganic and organic chemicals, chemical contaminants may originate from treatment processes, storage or distribution systems
-contaminants: metals (copper, lead, cadmium); asbestos; polynuclear aromatic hydrocarbons; vinyl chloride; organic chemicals; radionuclides
-> those which could affects human health: nitrate, arsenic, mercury and fluoride
pH-RANGE
-hydrogen ion concentration (moles of hydrogen ions per dm³)
-scale from 0-14 (0-6 acidic, 7 neutral, 8-14 alkaline)
-affects treatment process, disinfection with chlorine-based chemicals or coagulation
- according BGN instruction No#9 for drinking water, the acceptable range for pH in drinking water is from 6,5 to 9,5
-measurement: with litmus paper or acid-base indicators, pH-meter
OXIDIZABILITY OF WATER
-oxygen needed for the oxidation of organic substance in 1 liter of water (mg of oxygen per L)
-permanganate method is most used, permanganate index is for determination of the content of organic and inorganic water constituents that are oxidized by permanganate under test conditions
- according BGN instruction No#9 the water oxidazibility should be no more than 5mgO₂/L
-determination with potassium permanganate (KMnO₄): 100ml of water in Erlenmeyer flask -> add 5ml sulfuric acid -> add 10ml KMnO₄ and fast heating -> keep boiling for exact 10min -> add rapidly 10ml sodium oxalate -> discolouration -> titrate warm solution of KMnO₄ until a slight pink colour remains
-can be calculated with formula
DETERMINATION OF AMMONIA
-higher concentration in water polluted by agricultural wastes, fertilisers, sewage containing organic nitrogen, free ammonia or ammonium salts
-> ammonia is an indicator for FRESH organic pollution of drinking water
- according BGN instruction No#9, ammonium ion in drinking water should be no more than 0,5 mg/L
-determination: nessler’s reagent -> colour change in water (yellow/brown/orange) -> ammonia presence; no colour change -> no ammonia
DETERMINATION OF NITRITE (NO₂)
-normally only in low concentrations present in drinking water
-> indicator for RECENT organic pollution
- according BGN instruction No#9 nitrite in drinking water should be no more than 0,5 mg/L
-determination: with reagent of Griess-Illosvay -> if nitrite is present -> red-pink colour change
DETERMINATION OF NITRATE (NO₃)
-> indicator for OLD organic pollutions
-is the endproduct of aerobic decomposition of organic nitrogenous matter
-responsible for temporary blood disorder in infants (methhemoglobinemia)
- according BGN instruction No#9 nitrate in drinking water should be no more than 50mg/L
-determination: express colimetric method with Merck test (reaction zones into water for 1 sec and wait 1min, colour fields on the stripes change colour