Lect. 3- Chemical Water Quality- Continue

Transcription

1 The Islamic University of Gaza Faculty of Engineering Civil Engineering Department M.Sc. Water Resources Water Quality Management (ENGC 6304) Lect. 3- Chemical Water Quality- Continue ١

2 Alkalinity Alkalinity is a measure of the substances (ions) in water that react to neutralize acid (hydrogen ions) and resist changes in ph Sources Natural water systems include CO 3, HCO 3, OH -, HPO 4, H 2 PO 4-, HS -, NH 3 CO 3, HCO 3, OH - ( dissolution of mineral substances, from CO 2 ), HPO 4, H 2 PO 4-. ( detergents in wastewater, fertilizers and insecticides) HS -, NH 3 (products of microbial decomposition of organic material). Groundwater has higher alkalinity than surface water ٢

3 CO 2 influences the carbonate system in water as follows: Carbon dioxide dissolves in water and produces carbonic acid CO 2 + H 2 O H 2 CO 3 (dissolved CO 2 and H 2 CO 3 ) Carbonic acid dissociates producing H + 1. H 2 CO 3 H + + HCO 3- (HCO 3- can absorb another H + to become H 2 CO 3 ) 2. HCO 3- H + + CO 3 (CO -2 3 can absorb one H+ to become HCO -) 3 The ability of water to absorb H + ions (anions) without a change in ph is known as its alkalinity. In freshwater, alkalinity typically is due to the presence of excess carbonate anion (from the weathering of silicate or carbonate rocks) that when hydrolyzed produces OH - (and neutralizes H + ) as follows: Hydrolysis of carbonate and carbonate produces OH - CO 3 + H 2 O = HCO 3- + OH - ٣

4 The relative quantities of the alkalinity species are ph dependent. See the next figure ٤

5 Alkalinity species at various ph levels, (values calculated for water with a total alkalinity of 100 mg/l at 25 o C. alkaline CO 2 is acidic and lowers ph as the amount of CO 2 in the water increases. As ph increases from 7, HCO 3 - is formed and the water becomes slightly alkaline. acidic alkaline As ph continues to raise, CO 3 becomes the dominant source of alkalinity in most waters ph increases when CO 2 is removed from the water by photosynthesis and decreases when CO 2 is added to the water by respiration, especially at night when photosynthesis has stopped. ٥

6 Remember ph is a measure of acidity (hydrogen ion concentration) in water or soil. or ph is a measure of the acidic or basic characteristics of water Specifically, ph = -log [H+] So a ph of 7 means the [H+]=10-7 Waters with a low ph below 7 have a high hydrogen ion concentration and are termed acid and water with a high ph above 7 has a low hydrogen ion concentration and are termed alkaline ph = - log [ H + ] ٦ acid neutral alkaline ٦

7 Impacts Bitter taste to water, Reaction can occur between the alkalinity species and cations, resulting in precipitated substances that can foul pipes. Measurement By titration the water with an acid and determining the hydrogen equivalent. Expressed as mg/l of CaCO 3 For example: Each ml of 0.02 N H 2 SO 4 will neutralize 1mg of alkalinity as CaCo 3 Normality is the number of gram equivalents per liter = nm ٧

8 Hydrogen ions from the acid react with the alkalinity according to: H + + OH - H 2 O CO 3 + H + HCO 3 - HCO 3- + H + H 2 CO 3 If the acid is added slowly to water and the ph is recorded for each addition, a titration curve is obtained as shown: OH - + (1/2 ) CO 3 (½)CO 3 + HCO 3 - P M ٨

9 If P is the amount of acid required to reach ph 8.3, M is the total quantity of acid required to reached ph 4.5, The following generalizations can be made: P = M Alkalinity = OH - P = M/2 Alkalinity = CO 3 P = 0 Alkalinity = HCO - 3 (Initial ph is below 8.3) P < M/2 Alkalinity = HCO 3- +CO 3 P > M/2 Alkalinity = OH - + CO 3 ٩

10 Example A 200 ml sample of water has an initial ph of ml of 0.02 N H 2 SO 4 is required to titrate the sample to ph 4.5. What is the total alkalinity of the water in mg/l as CaCO 3? Solution: 1 ml of 0.02 N H 2 SO 4 will neutralize 1mg as CaCO 3 of alkalinity, So, 30 ml of 0.02 N H2SO4 will neutralize 30 mg as CaCO 3 of alkalinity. The concentration of alkalinity = 30 mg / 200mL = 150mg/L as CaCO 3

11 Example Determine the species and the quantity of each specie, of the alkalinity in Example 6 if the 8.3 equivalence point is reached at 11 ml of acid. Solution: From example 6: Initial ph = 10 [H + ] = mole /L, [OH - ] = 10-4 mole /L = (10-4 *1)*50,000 = 5 mg/l as CaCo 3 So, 5 ml of 0.02 N H 2 SO 4 will neutralize 1 mg as CaCO 3 of OH - in 1-L sample. The given sample of 200 ml will require 5 *(200/1000) = 1 ml of acid to neutralize total OH -. ١١

12 Solution (cont.): Then, the remained 10 ml of acid measures (neutralizes) one half of CO 3 and the same volume (10 ml) of acid is required of the remaining one half CO 3, So, 20 ml of acid measures 20 mg of CO 3 in a sample of 200 ml the concentration of CO 3 = 20 mg * (1000/200) = 100 mg/l as CaCo 3 This will leave 9 ml of acid to measure HCO3-, The concentration of HCO 3- = 9 mg * (1000/200) = 45 mg/l as CaCo 3 ١٢

13 Hardness Hardness is the concentration of multivalent metallic cations in solution (mg/l), which includes mainly Ca 2+ and Mg 2+ The units are, like alkalinity, mg/l as CaCO 3 Hardness is classified as carbonate hardness and non-carbonate hardness, depending upon the anion with which it associated. Anions of alkalinity (e.g., CO 3- ) and cations of hardness (e.g., Ca 2+ ) are normally derived from the same carbonate minerals and this is the reason for the observed general association between alkalinity and hardness ١٣

14 1. Carbonate hardness - the portion of total hardness that is chemically equivalent to the CO3 and HCO 3- alkalinity present in the water. 2. Non-carbonate hardness - that hardness which is in excess of carbonate hardness; will only occur in water where Total Hardness > alkalinity ١٤

15 Sources Caused by the presence of multivalent cations, mostly Ca 2+ and Mg 2+ ; (Fe 2+, Mn 2+, Sr 2+, Al 3+ may be present in much smaller amounts). Impacts Hardness determines how hard or easy it is to lather soap Hard water is water that requires considerable amounts of soap to produce foam or lather; the precipitates formed by the hardness and soap adheres to surfaces of tubes, sinks and dishwashers, produces scale in hot water pipes, etc. Not a health concern, but have economic concern ١٥

16 Measurement Use Hardness can be measured using chemical titration to determine the quantity of calcium and magnesium ions. Analysis for hardness is commonly made on natural waters and on waters for potable supplies and for certain industrial uses. Soft < 50 mg/l as CaCO 3 Moderately hard mg/l as CaCO 3 Hard mg/l as CaCO 3 Very hard >300 mg/l as CaCO 3 Maximum hardness of 500 mg/l in drinking water (puplic health service standards) ١٦

17 Fluoride Associated in nature with few types of sedimentary or igneous rocks. Fluoride is seldom found in surface waters, Fluoride appears in groundwater in only few geographical regions, Toxic to humans and animals in large quantities, Beneficial with small concentrations, 1 mg/l concentration in drinking water help to prevent dental cavities in children harder, stronger teeth. Added to water supplies for good dental formation Excessive fluoride can result in discoloration of teeth (< 2mg/L) ١٧

18 Metals 1. Nontoxic Metals Include calcium, magnesium, sodium, iron, manganese, aluminum, copper and zinc. Sodium: - in natural waters, earth crust. - reactive, soluble in water. - corrosive to metal surfaces, - Toxic to plants in large concentrations. Iron and manganese : - in natural waters, - (0.3 mg/l and 0.05 mg/l respec.) concentrations cause color problems and may cause taste and odor problems in the presence of some bacteria. ١٨

19 Iron associate with chloride or bicarbonate or sulfate, In the presence of oxygen, Fe 2+ is oxidized to Fe 3+ and forms an insoluble compounds with hydroxide (Fe(OH) 3 ). Manganese (Mn 2+ ) associated with chloride or nitrate or sulfate are soluble, while oxidized (Mn 3+, Mn 5+ ) are insoluble. Toxic Metals Are harmful to humans and other organisms in small quantities include arsenic, barium, cadimuim, chromium, lead, mercury, and silver ١٩