) and/or dolomite (CaMg(CO 3

Transcription

1 Environmental Chemistry I Week 4 Alkalinity 1) Alkalinity Alkalinity is defined as the capacity of water to accept H + ions (protons). It can also be defined as the capacity of water to neutralize acids whereas acidity is the capacity of water to neutralize bases. Alkalinity and ph are different parameters. The ph of a solution is a measure of the concentration of H + ions in the water. On the other hand, alkalinity is a measure of the water s capacity to neutralize H + ions. 2) Alkalinity Sources Major species responsible for alkalinity in water are bicarbonate, carbonate, and hydroxide ions. These ions are effective by removing protons from water as given below. Ammonia, borates, organic bases, phosphates, and silicates can also be minor contributors to alkalinity. Alkalinity in natural waters is usually due to the carbonate and bicarbonate salts of calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K). Among those, carbonate is the most important alkalinity sources in natural water. Limestone produces high amount of carbonate ions through dissolving in water while flowing through geology. Limestone is composed of calcium carbonate (CaCO 3 ) and/or dolomite (CaMg(CO 3 ) 2 ) mineral. Alkalinity can increase the ph, when the alkalinity comes from a mineral source such as calcium carbonate (CaCO 3 ). When CaCO 3 dissolves in water, the carbonate (CO 2 3 ) can react with water to form bicarbonate (HCO - 3 ), which produces hydroxide (OH-): 1

2 CaCO 3 (s) Ca CO CO 3 + H2 O HCO 3 + OH The hydroxide ion (OH - ) is a strong base. An increase in OH - concentration will cause the ph to increase. In addition to rocks and soils, the alkalinity of streams can be influenced by salts, plant activity and wastewater. Wastewater comes from industrial sources, such as detergents and soap based products, contains higher amount of OH- which is a major contributory factor to alkalinity. On the other hand especially seawater contains minor alkalinity components such as borates, silicates, and phosphates. Carbonic acid also contributes to the alkalinity through the combination of carbon dioxide with water. CO 2 + H 2 O H 2 CO 3 Then, carbonic acid is converted to bicarbonate and carbonate as below. - + H 2 CO 3 HCO 3 + H HCO 3 CO3 + H While conversion of carbon dioxide to carbonic acid produces ions capable of buffering ph, it also causes a decrease in ph by producing (H + ) that CaCO 3 doesn t. 2- During the reaction carbonic acid (H 2 CO 3 ) reacts to form a carbonate ion (CO 3 ), 2 hydrogen ions (H + ) are released into the water. On the other hand, according the equations above, removal of CO 2 from the sample cause to decrease in H + and increase in alkalinity. 3) Importance of alkalinity Alkalinity is important in water treatment and in the chemistry and biology of natural waters. Highly alkaline water often has a high ph and generally contains elevated levels of dissolved solids. These characteristics may be detrimental for water to be used in boilers, food processing, and municipal water systems. Alkalinity is closely related to ph and alkalinity in water acts as a buffer that tends to stabilize and prevent fluctuations in ph. Without buffering capacity, any acid addition to water body would immediately change ph. Water that is a good buffer contains 2

3 compounds, such as bicarbonates, carbonates, and hydroxides, which combine with H + ions from the water and stabilize the shifts in ph. For example, CO 2 concentrations result in only minor changes in ph throughout the course of a day and alkalinity capacity of lake stabilize those fluctuations of ph. Removal of CO 2 during aeration decreases H + and increases the alkalinity. Alkalinity serves as a reservoir for inorganic carbon, thus helping to determine the ability of water to support algal growth and other aquatic life. Alkalinity is significant in the treatment of wastewater and drinking water. Especially in anaerobic treatment plants, formation of volatile acids increase H + ion concentration in the reactor and will stop anaerobic activity if there is insufficient alkalinity to neutralize the H + ions. Higher amount of acidity cause corrosion on pipes and alkalinity can protect water distribution system from the corrosive effects. The amount of acid to be used in the neutralization process depends upon the respective amount of alkalinity. Alkalinity is a major item that must be considered in calculation the lime and sodaash requirements in softening water by precipitation methods. Alkalinity measurements are made as a means of evaluating the buffering capacity of wastewaters and sludges. They can also be used to assess a natural waters ability to resist the effects of acid rain. 4) Alkalinity measurement Alkalinity is measured by titration using strong acids such as H 2 SO 4 or HCl and is reported in terms of equivalent CaCO 3. Total alkalinity can be directly measured by titrating water sample with acid until ph 4.5. At ph 4.5, all carbonate and bicarbonate ions are converted to CO 2. If water sample has ph above 8.3, the titration is made in two steps. In the first step, titration is conducted until ph is lowered top H 8.3. At this point, phenolphthalein indicator turns from pink to colorless. Phenolphthalein alkalinity primarily measures the amount of carbonate ion (CO -3 2 ) present. All carbonate present in water is converted to bicarbonate according to the following reaction; 3

4 The second phase of the titration is conducted until the ph is lowered to about 4.5. At this point methyl orange turns to from yellow to red. Bicarbonate ions are converted to carbon dioxide, and then, carbon dioxide combines with water to form a weak carbonic acid according to the following reaction until ph 4.5. The sum of phenolphthalein and methyl orange alkalinities gives the total alkalinity. When the ph of sample is less than 8.3, a single step titration is made to lower it until 4.5. Below ph 4.5, the water is less able to neutralize the sulfuric acid and there is a direct relationship between the amount of sulfuric acid added to the sample and the change in the ph of the sample OH alkalinity is absent if the ph of the water is below ph 10, and carbonate alkalinity is absent if the ph is below ph 8.3. If each concentration of alkalinity components is known, total alkalinity can be calculated as : Sources Bairr, M. Cann. Environmental Chemistry. W. H. FREEMAN AND COMPANY E.R. Weiner, Applications of Environmental Aquatic Chemistry, CRC Pres F.R. Spellman. The Science of Water, Concepts and Applications, CRC Press J.A. Salvato, N.L. Nemerow, F.J. Agardy. Environmental Engineering. John Willey and Sons Inc. J.P. Reynolds, J.S. Jeris, L. Theodore. Handbook of Chemical and Environmental Engineering Calculations. John Willey and Sons Inc. K. Addy, L. Green, E. Herron, Lecture Notes, University of Rhode Island 4

5 L.K. Wang, Y.T Hung, N.K Shammas. Physicochemical Treatment Processes, Humana Press Inc. N. P. Cheremisinoff, Handbook of Water and Wastewater Treatment Technologıes. Butterworth-Heinemann S. E. Manahan. Environmental Chemistry. Lewish Publishers 5

6 Environmental Chemistry Week 4 Alkalinity

7 Alkalinity Definition capacity of water to accept H+ ions (protons) capacity of water to neutralize acids major alkalinity components OH (hydroxyl ions) CO -2 3 (carbonate ions) HCO 3 (bicarbonate ions) minor alkalinity components Ammonia, borates, organic bases, phosphates, silicates

8 Alkalinity components In natural waters Bicarbonate and carbonate salts of Ca, Mg, Na, K Limestone CaCO 3 + CaMg(CO 3 ) 2 CaCO 3 (s) Ca 2+ + CO 3 2- CO H 2 O HCO 3- + OH - Carbonic acid CO 2 + H 2 O H 2 CO 3 HCO 3- + H + HCO 3- CO H +

9 Alkalinity components in sea (from "Chemical Oceanography" by Frank Millero; 1996)

10 Importance of alkalinity Without alkalinity, any acid addition immediately changes ph in water!!!! reef tank

11 Importance of alkalinity

12 Importance of alkalinity in anaerobic treatment process, ph decreases due to the productin of volatile acids neutralization is needed to keep ph at proper range decrease the amount of acid to be used for neutralization

13 Importance of alkalinity reservoir for inorganic carbon and support algal growth and other aquatic life protect pipes from corrosion

14 Alkalinity measurement Strong acid:h 2 SO 4 or HCl İndicator: phenolphatelein and methy orange ph measurement Total alkalinity=phenolphthalein alkalinity +methyl orange alkalinity

15 Determination of alkalinity components A-based on only alkalinity measurement B-pH analysis+alkalinity measurement C-equilibrium calculation

16 (A) only alkalinity measurement For water having ph>9, the result is approximate First phenolphtalein and methyl orange alkalinities are measured Then make assumptions only OH - only CO - 3 OH - +CO - 3 OH - +CO - 3 only HCO - 3

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18 Determination based on assumptions only OH - ph of water is ph >10 Water is titrated until ph 8.3 OH - alkalinity = phenolphatelein alkalinity only CO 3 - ph 8.5 Water is titrated until ph 8.3 Total alkalinity = carbonate alkalinity = 2x phenolphatelein alkalinity

19 Determination based on assumptions OH - + CO 3 - ph>10 Water is titrated A-until ph 4.5 B- from ph 8.3 to ph 4.5 Total alkalinity= alkalinity obtained from A CO3 alkalinity = 2x alkalinity obtained from B OH alkalinity=total alkalinity-carbonate alkalinity

20 Determination based on assumptions CO 3- and HCO 3 - ph>8.3 and ph <11 Water is titrated A-until ph 4.5 B- until ph 8.3 Total alkalinity= alkalinity obtained from A CO3 alkalinity = 2x alkalinity obtained from B HCO3 alkalinity=total alkalinity-carbonate alkalinity

21 Determination based on assumptions only HCO3- ph 8.3 Water is titrated until ph 4.5 HCO3 alkalinity=total alkalinity

22 (B) ph + alkalinity measurement ph measurement Phenolphatelin alkalinity measurement Methyl orange alkalinity measurement OH alkalinity is calculated from ph measurement 1 mol/l OH =50,000 CaCO 3

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24 CO 3 alkalinity Calculation of OH from ph measurement Titration until ph 8.3 Phenolphatelein alkalinity=oh alkalinity+1/2 carbonate alkalinity HCO 3 alkalinity Calculation of OH from ph measurement Titration until ph 8.3, Titration from ph 8.3 to ph 4.5 HCO3 alkalinity=total alkalinity ( CO 3 alkalinity+ OH alkalinity)

25 (C)Determination from equilibrium equations ph measurement